Journal of Infection (2008) 56, 83e98
www.elsevierhealth.com/journals/jinf
REVIEW
Human infections associated with wild birds
Sotirios Tsiodras a,*,g, Theodoros Kelesidis b,g, Iosif Kelesidis b,
Ulf Bauchinger c,f, Matthew E. Falagas d,e
a
University of Athens Medical School, 1 Rimini Street, Xaidari, 12462 Athens, Greece
Beth Israel Deaconess Medical Center, Harvard University Medical School, Boston, MA, USA
c
University of Munich (LMU), Planegg-Martinsried, Germany
d
Alfa Institute of Biomedical Sciences, Athens, Greece
e
Department of Medicine, Tufts University School of Medicine, Boston, MA, USA
f
Mitrani Department of Desert Ecology, Ben-Gurion University of the Negev, Ben-Gurion, Israel
b
Accepted 1 November 2007
Available online 21 December 2007
KEYWORDS
Communicable
diseases;
Avian infection;
Wild birds;
Infectious diseases;
Influenza;
Lyme disease;
Arbovirus;
West Nile encephalitis;
Enteric infection;
Antimicrobial
resistance
Summary Introduction: Wild birds and especially migratory species can become long-distance
vectors for a wide range of microorganisms. The objective of the current paper is to summarize
available literature on pathogens causing human disease that have been associated with wild
bird species.
Methods: A systematic literature search was performed to identify specific pathogens known to
be associated with wild and migratory birds. The evidence for direct transmission of an avian
borne pathogen to a human was assessed. Transmission to humans was classified as direct if there
is published evidence for such transmission from the avian species to a person or indirect if the
transmission requires a vector other than the avian species.
Results: Several wild and migratory birds serve as reservoirs and/or mechanical vectors (simply
carrying a pathogen or dispersing infected arthropod vectors) for numerous infectious agents. An
association with transmission from birds to humans was identified for 10 pathogens. Wild birds
including migratory species may play a significant role in the epidemiology of influenza A virus,
arboviruses such as West Nile virus and enteric bacterial pathogens. Nevertheless only one case
of direct transmission from wild birds to humans was found.
Conclusion: The available evidence suggests wild birds play a limited role in human infectious
diseases. Direct transmission of an infectious agent from wild birds to humans is rarely identified.
Potential factors and mechanisms involved in the transmission of infectious agents from birds to
humans need further elucidation.
ª 2007 The British Infection Society. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ30 210 5831989, þ30 6932 665820; fax: þ30 210 5326446.
E-mail address: tsiodras@med.uoa.gr (S. Tsiodras).
g
The first two authors contributed equally to this work.
0163-4453/$30 ª 2007 The British Infection Society. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.jinf.2007.11.001
84
Introduction
Free-living birds, including migratory species, can become
long-distance vectors for a wide range of microorganisms
that can be transmissible to humans.1 This creates the
potential for establishment of novel foci of emerging or
re-emerging communicable diseases along bird migration
routes.2 Certain pathogens are more often isolated in migratory birds in comparison to other animal species3,4 and
the potential for transport and dissemination of these
pathogens by wild birds is of increasing public health concern stimulated by the recent spread of diseases like
highly pathogenic Avian influenza A (HPAI H5N1 Asian
lineage) and West Nile virus (WNV) infection.3,5 Avian
influenza A (HPAI H5N1 Asian lineage) and West Nile virus
infection, well known to affect birds for decades, have
been recently observed to affect areas far away from
the locations where they were originally identified, generating the hypothesis that migratory birds transported
these pathogens to new geographical locations.6 However
as is the case with the highly pathogenic avian influenza,
scientific data do not always support such hypotheses.7
Several factors affecting wild bird species including migratory species such as increasing stress levels and crowding
potentially promote infectious disease transmission among
birds but available data supporting this are scarce or nonexistent.
The objective of this paper is to summarize available
literature on pathogens causing human disease that have
been associated with wild birds including wild migratory
bird species. Although wild bird borne infections can occur
at any spatial scale, from very localized, to short and long
distance, from an epidemiologic point of view the transmission of pathogens from wild birds to humans over a long
distance is most important. Therefore, in the current
manuscript we focused more on the role of wild migratory
birds in the spread of certain pathogens. The paper
focuses on available evidence of transmission of avian
borne pathogens to humans. We speculated that such
evidence would originate from enhanced animal and
human surveillance and the application of advanced
molecular diagnostic testing during the recent years.
Furthermore, we attempted to identify factors potentially
contributing to such transmission from the available body
of science.
Methods
Two reviewers (TK and IK) independently performed the
literature search. The following terms were used in
searches of the PubMed database: ‘‘wild birds’’, ‘‘migratory birds’’, ‘‘infection’’, and specific pathogens known to
be associated with wild and migratory birds e.g. ‘‘West
Nile virus’’, ‘‘avian influenza’’, ‘‘influenza A’’, ‘‘Lyme disease’’ and ‘‘arbovirus’’. We also screened articles related
to the initially identified publications to expand our data
sources. Despite the availability of scientific data on this
issue even before 19668,9 we focused in the modern area
where molecular diagnostics might enhance our ability to
study such interactions between birds and humans. Similar
searches were conducted for each individual migratory
S. Tsiodras et al.
bird species identified through a list provided by the
Royal Ornithological Society of Great Britain and World
bird databases (Avibase World List).10,11 We also used the
widely used Sibley and Monroe Classification for birds.12,13
To evaluate the role of recent diagnostic developments,
we also performed an additional search of the literature
by using the term polymerase chain reaction (PCR) and
‘‘migratory birds’’. Additional epidemiologic information
for the identified pathogens-diseases was obtained from
the websites of the United States Centers for Disease Control (CDC), World Health Organization (WHO), FAO, and
OIE.14,15
Study selection and data extraction
The role of wild and migratory birds in the transmission of
an infectious disease to humans was discussed in consensus
meetings where all authors participated. Transmission to
humans was classified as direct if there was evidence for
direct transmission of the pathogen from the avian species
to humans through direct contact with an infected bird and
genetic/serological evidence of the presence of a particular
pathogen in both the avian species and humans. Transmission to humans was classified as indirect if there was
evidence for transmission of the pathogen from the avian
species to humans through indirect contact with an infected bird and genetic/serological evidence of the presence of the particular pathogen in both the avian species
and humans. We considered indirect ways of transmission,
those through contaminated water from feces of waterfowls and through vectors that are carried by wild birds
such as mosquitoes and ticks (Table 1). Finally, we classified pathogens to be associated with a ‘‘theoretical risk
for transmission’’ when in the literature there were reports that these pathogens were isolated both from humans
and wild birds, using microbiological, genetic or serological
methods, but there were no reports of actual direct/indirect transmission of these pathogens from wild birds to
humans. Despite the lack of actual evidence in such cases,
the risk exists in theory e.g. through ingestion of water
contaminated from feces of wild birds or exposure to
inanimate surfaces contaminated by bird secretions or
droppings.
Compiled relevant bird species data (with formal avian
family names) are presented in the appendix. This appendix
further includes data on pathogens that are borne by wild
avian species that have not yet been associated with human
infection in published reports.
Results
Evidence for direct transmission
The systematic review of the literature review identified no
real evidence for direct wild bird to human transmission
with the only exception being the cluster of H5N1 human
cases in Azerbaijan where the affected patients were
plucking feathers from mute swans that had succumbed
to H5N1 infection.16
Wild birds and human infections
Table 1
85
Pathogens that have been reported to be indirectly transmitted from wild birds including migratory species to humans
Microorganism(s)
(I) Bacteria
Chlamydiaceae
Chlamydophila
psittaci
Enterobacteriaceae
Escherichia coli
Salmonella
(enterica
typhimurium)
Mycobacteriaceae
Mycobacterium
(avium,
ulcerans)
Spirochaetaceae
Borrelia
burgdorferi
sensu lato
genomic species
Reported transmission to human
(indirect transmission) (n Z 10)
Migratory bird species (formal family
names for each bird species can be
found in the appendix)
Geographic
area
Ornithosis17e22
Egrets (Ardea Alba), grackles
(Quiscalus), gulls (Larus), migratory
waterfowl species (Anatidae),
passerines (Passeriformes), pigeons
(Columbidae), psittacine birds
(Psittaciformes), raptors (North
American raptors), shorebirds (North
American shorebirds), wild ducks
(Anatidae), and others
Worldwide
Bloody diarrhea [Vero cytotoxinproducing E. coli O157, Shiga toxin stx2fcontaining E. coli O128 strain)23,24]25,26
Salmonellosis (enteritis)27e30
Finches (Fringillidae), gulls (Larus),
pigeons (Columbidae), sparrows
(Passeridae), starlings (Sturnidae)
Wild crows (Corvidae), ducks
(Anatidae), gulls (Larus), passerines
(Passeriformes), raptorial birds (North
American raptors), songbirds
(Passeriformes), terns (Sternidae),
waterfowls (Anatidae)
Worldwide
Regarding M. avium it is generally
believed and occasionally reported that
man (especially immunocompromised,
elderly) can contract the disease from
birds, but this has not been fully
clarified.31e33
Possible transmission of M. ulcerans to
humans through contaminated water
from feces of waterfowls (Anatidae)34
Lyme disease30,35e41
Crows (Corvidae), raptors (North
American raptors), rooks (Corvus
frugilegus), wild ducks (Anatidae),
wild pigeons (Columbidae)
Worldwide
American Robins (Turdus migratorius),
cardinals, songbirds (Passeriformes),
sparrows (Passeridae), thrushes
(Turdidae) and other ground foraging
birds, waterfowl (Anatidae)
North America,
Europe
Europe, South
America, Asia
Worldwide
(II) Fungi
Cryptococcus
Yes (wild pigeons)42e46
Psittacine birds (Psittaciformes),
starling (Sturnidae), wild pigeons
(Columbidae)
(III) Viruses
Flaviviridae
West Nile virus
Yes3,30,47e49
North American shorebirds, common
Africa Europe,
grackles (Quiscalus quiscula), doves,
Asia, America
hawks, house finches (Carpodacus
mexicanus), and house sparrows
(Passer domesticus), songbirds
(Passeriformes), raptors (North
American raptors), owls (Strigidae),
and various corvids (crows, jays,
Corvidae)
(continued on next page)
86
S. Tsiodras et al.
Table 1 (continued)
Microorganism(s)
Reported transmission to human
(indirect transmission) (n Z 10)
Migratory bird species (formal family
names for each bird species can be
found in the appendix)
Geographic
area
St. Louis
encephalitis
virus (SLEV)
Yes3,49e51
America
Western Equine
Encephalitis
virus (WEEV)
Yes49
North American shorebirds, common
grackles (Quiscalus), doves, hawks,
house finches (Carpodacus mexicanus),
and house sparrows (Passer
domesticus), songbirds
(Passeriformes), owls (Strigidae), and
various corvids (crows, jays, magpies)
North American shorebirds, quails
(Coturnix)
Orthomyxoviridae
Influenza A virus
To date, only domestic poultry are
known to have played a major role in the
transmission cycle of the H5N1 virus from
animals to humans.52 However, there is
also the potential contribution of other
hosts like carnivores e.g cats to both
virus transmission and adaptation to
mammals.53,54 Dead or moribund cats
were found to be infected with H5N1
virus soon after the virus was detected in
wild birds in Germany.53 This suggests
that H5N1 virus can be transmitted from
wild birds to cats53 whereas in another
report avian influenza A virus subtype
H5N1 was transmitted to domestic cats
by close contact with infected birds.54
However, there has been no documented
case with wild migratory bird to human
transmission although the theoretical
risk exists.55
Serologic evidence of avian influenza
infection in 1 duck hunter and 2 wildlife
professionals with extensive histories of
wild waterfowl (Anatidae) and game bird
exposure has been reported.56
There is an association (not necessarily
causal) between recreational contact
with H5N1 contaminated water and the
onset of confirmed human H5N1 disease
in 3 cases.53,57,58 In one of these cases
asymptomatic ducks may have shed virus
into the pond.53
Possible direct transmission of highly
pathogenic avian influenza in family
cluster in Azerbaijan.16 Occupational
exposure to avian species may increase
veterinarians’ risk of avian influenza
virus infection.59
Transmission can cause: Respiratory
infection, keratoconjuctivitis, diarrhea,
encephalitis30,60e66
Dabbling ducks (e.g common MallardAnas platyrhynchos), geese
(Anserinae), gulls (Larus), swans
(Cygninae), guillemots (Uria aalge),
mountain hawk eagles (Spizaetus
nipalensis) North American Bluewinged Teal (Spatula discors),
shearwaters (Procellariidae), terns
(Sternidae). Wild aquatic birds are
regarded as the principal reservoir of
influenza viruses, and migrating ducks
(Anatidae) disseminate influenza
viruses worldwide
America
Worldwide
Wild birds and human infections
Evidence for indirect transmission or a theoretical
risk for transmission
Although a large number of avian borne pathogens have
been identified in the literature, we found relatively scarce
evidence for indirect transmission of avian borne pathogens
to humans (Table 1). Unfortunately, in the vast majority of
the reports reviewed herein, data were unavailable to further characterize the way of transmission of certain pathogens beyond the stage of a speculative argument. This
would be expected for zoonoses which usually require amplification in an animal species cycle before spill-over to
humans. Nevertheless and based on our criteria several
avian borne bacterial, fungal, viral pathogens could be indirectly transmitted or associated with a theoretical risk for
transmission to humans (Table 1). We identified 58 such
pathogens for which wild birds can serve as reservoirs, mechanical vectors, or both (Tables 1 and 2). However, the
paucity of available data did not allow us to make the distinction whether the involved species serve as reservoir or
vector in most of the cases.
Scarce microbiological, serological and epidemiological
data supported indirect transmission from wild birds to
human for 10 of these pathogens (Table 1). Application of
advanced molecular diagnostic testing during the recent
years has led to the isolation of these microbial agents
known to affect humans in birds. The examples include bacterial spp. like Escherichia coli,24,25 Borrelia Burgdorferi,37
Anaplasma phagocytophilum,87 Salmonella typhimurium,28
Campylobacter spp.,79 and Mycobacterium spp.,31e33 viruses like Influenza virus,56,60,61,64,65 West Nile virus,126
St. Louis encephalitis virus3,50,51 and Western Equine Encephalitis virus49 and fungi like Cryptococcus spp..43,44,46
These have been isolated from many wild birds using standard serological3,30,47,48,50,51,56,60,61,64,65,79 and microbiological techniques.28,31e33,37,43,44,46,79,126,127 Moreover
vectors with the ability to carry pathogens have also been
isolated from wild birds.3,37,85,87 For example, ornithophilic
mosquitoes and ticks are the principal vectors of pathogens
like West Nile virus in the Old World, and B. burgdorferi, respectively, and birds of several species, chiefly migrants,
appear to be the major introductory or amplifying hosts
of these vectors.3,37,85,87
Methods that have been used to confirm association of
microbial agents isolated from wild birds with infection in
humans include molecular methods like sequence analysis
for Ehrlichia85 and Mycobacterium species,32,33 phylogenetic analysis,25 pulsed-field gel electrophoresis,26 polymerase chain reaction,26 immunomagnetic separation
(IMS) for E. coli,25,26 serological methods for influenza
virus56,59 and psittacosis,17 and epidemiological methods
for Salmonella spp.,28,29 Borrelia spp.,36 West Nile virus,30,48,49,126 St. Louis encephalitis virus,49,51 and Western
Equine Encephalitis virus.49
However, in most scientific literature, there is no detailed data regarding the detection and characterization of
pathogens and their relation to wild birds. In most of the
cases, it seems that wild birds serve as vectors of the
pathogen. In these cases, the indirect role of wild birds in
transmission of the infectious agents can be only speculated and the implicated pathogens are classified as having
87
the theoretical risk of transmission from wild birds to
humans (Table 2).
Twenty-one wild avian family species were identified
that are reservoirs, mechanical vectors or both for infectious agents that may affect humans (Listed with their
formal family names in the appendix according to the Sibley
and Monroe Classification for birds). A short description of
pathogens that may be transmitted from wild birds to
humans is outlined below.
Types of microorganisms carried by wild birds that
could affect humans (indirect transmission or
theoretical risk)
Bacteria
A range of bacterial pathogens affecting humans has been
associated with wild and migratory birds. An indirect
transmission to humans has been reported for some of
these such as the enteric pathogens E. coli24 and Salmonella spp.28,29 Tick-borne pathogens such as Borrelia burgdorferi sensu lato species have been also associated with
human infection from wild migratory birds.35e38,85,87 A theoretical risk for transmission to humans has been reported
for other bacterial pathogens such as Yersinia spp.,76,128
Campylobacter jejuni77 and both cholera and non-cholera
Vibrio spp.92
Fungi
Yeasts and yeast-like fungi have been isolated from wild
and migratory birds such as Candida spp.,129,130 and hyphomycetes e.g. Aspergillus spp., Microsporum spp., Trichophyton spp.,112 and cryptococci.43 A theoretical risk for
transmission to humans exists but scientific data to support
this are extremely scarce. Cryptococci that are quite ubiquitous in nature have been reported to be transmitted to
humans indirectly from wild pigeons (Columbidae), occasionally causing clinical infection, especially in immunocompromised patients.42
Viruses
Important viral species have been isolated from wild
migratory birds and can affect humans indirectly including
influenza A viruses,62,131 the West Nile virus (WNV),3,47 the
St. Louis encephalitis virus (SLEV).3,50,51 Several other viral
species can theoretically be transmitted from wild birds to
humans (Table 2).
Parasites
Wild and migratory birds can disperse in nature a diverse
number of protozoa such as Babesia and other haemoparasites. The potential for transmission exists for some parasitic species (Table 2).
Factors potentially contributing in transmission
The issue of the transmissibility of various pathogens from
wild birds including migratory species to humans is fairly
complex. Several factors determine the possibility of such
a spread. Some factors relate to the affected species
including the birds themselves (e.g. the avian species
involved, susceptible local vertebrate recipients or
Pathogens with theoretical risk for transmission (but no reports of actual direct/indirect transmission) from wild birds including migratory species to humans
(I) Bacteria
Gram-positive cocci
Enterococcus
Staphylococcus
Gram-positive rods
Clostridium perfringens
Listeria monocytogenes
Enterobacteriaceae
Yersinia species
Campylobacteraceae
Campylobacter jejuni
Helicobacter spp.
Anaplasmataceae
Anaplasma
phagocytophilum
Migratory bird species
Geographic area
Possible spread through polluted water67,68;
transmission has been reported from other birds69e71
Ducks (Anatidae), seagulls (Larus), waterfowls
(Anatidae) and other migratory birds such as quails
(Coturnix)
Ducks (Anatidae), mallards (Anas platyrhynchos),
passerines (Passeriformes), seagulls (Larus), and
other migratory birds including quails (Coturnix),
raptors (North American raptors)
Worldwide
Crows (Corvidae), ducks (Anatidae), gulls(Larus),
Pelicans (Pelecanus) and marine birds, raptors (North
American raptors), shorebirds (North American
shorebirds), waterfowls (Anatidae)
Crows (corvus), gulls (Larus), rooks (Corvus
frugilegus) and other migratory birds
Crows (corvus), ducks (Anatidae), gulls (Larus),
magpies, (Corvidae) pigeons (Columbidae),
pheasants, starlings (Sturnidae), terns (Sternidae),
wagtails (Motacilla), waterfowls (Anatidae) and other
migratory species
Europe, Asia
Migrating ducks (Anatidae), passerine birds e.g.
crows (corvus), pigeons (Columbidae) and seagulls
(Larus), sparrows (Passeridae)
Europe, North America,
Asia
Geese (Anserinae), gulls (Larus), passerines
(Passeriformes), terns (Sternidae), various wild birds
North America, Europe,
Australia
Geese (Anserinae), gulls (Larus)
Worldwide
Geese (Anserinae), seagulls (Larus), swans
(Cygninae), wild ducks (Anatidae)
Passerine birds (Passeriformes) American Robins
(Turdus migratorius), robins, songbirds
(Passeriformes) veery (Catharus fuscescens),
American warbler
Worldwide
Possible through faecal pollution of environmental
water samples72
Possible through accidental ingestion of
contaminated water73; food-borne enteritis has been
reported from non-migratory birds74
Possible through accidental ingestion of
contaminated water75
Enteritis30,76
Intestinal campylobacteriosis.30,77,78 Whether
waterfowl (Anatidae) have a role in the dissemination
of Campylobacter spp. that results in increased
human disease is likely to be elucidated through
development and greater use of typing methods.79
Typing might allow links to be established between
isolates of avian, environmental, and human origin.79
Enteritis (Helicobacter canadensis).80,81
Possible transmission of H. pylori by contaminated
water from feces of waterfowls (Anatidae)82
Possible through faecal pollution of environmental
water samples72,83
Possible through faecal pollution of environmental
water samples e.g. gulls (Larus)84
Human granulocytic ehrlichiosis85e87
Worldwide
America, Asia
Worldwide
North America, Europe,
Asia
S. Tsiodras et al.
Other gram negative
bacilli (Pseudomonas,
Aeromonas, etc.)
Anaerobic bacteria
Potential for transmission to humans exists (n Z 50)
88
Table 2
Microorganism(s)
Rickettsiaceae
Coxiella burnetii
Vibrionaceae
Vibrio cholerae
(II) Viruses
Bunyaviridae
Nairoviruses: CrimeanCongo haemorrhagic
fever (CCHF)
Coronaviridae
Avian infectious
bronchitis virus, other
coronaviruses
Flaviviridae
Japanese encephalitis
virus (JEV)
Other flaviviruses
Murray Valley
encephalitis virus
(MVEV), Usutu virus
(USUV)
Sindbis virus
Tick-borne Encephalitis
virus (TBE)
Herpesviridae
Anatid herpesvirus 1,
(duck plague virus),
Marek virus
Paramyxoviridae
Newcastle disease virus
(NDV, avian
parainfluenza virus 1,
paramyxovirus-1)
Tuberculosis.88 Possible transmission of
mycobacterium from humans to birds has been
reported through close contact between humans and
pet birds but it is not known if humans can acquire
the infection from birds.88
Possible through ticks 90,91
Green-winged macaw, psittacines
(Psittaciformes)88,89
Pigeons (Columbidae)
Europe, Asia
Cholera, non-cholera Vibrio infections92,93
Wild aquatic birds (Anatidae), gulls (Larus)
North America
Possible transmission through ticks and transmission
has been reported for other birds94,95
Crows (Corvidae), wild aquatic birds (Anatidae),
passerines (Passeriformes), rooks, (Corvus frugilegus)
Europe, Asia, Africa
Serological evidence in humans exposed to birds has
been reported96
Passerines (Passeriformes), pheasants (Phasianidae)
Worldwide
Yes97e99
Colonial ardeids (Ardeidae), herons (Ardeidae),
marsh birds, quails (Coturnix)
Blackbirds (Turdus merula), wading birds, crows and
magpies (Corvidae) (Usutu virus), Pelecaniformes
(MVE virus)
Wild birds and human infections
Mycobacterium species
M. tuberculosis
Worldwide
Yes (MVEV)100,101
NR (USUV)
Ockelbo disease,102,103 Pogosta disease,104 plus
possible transmission to humans as migratory birds
are hosts of mosquitoes which are vectors for these
viruses
Possible through ticks 105e108
Marek’s virus (transported by wild birds) has been
associated with multiple sclerosis in humans.109,110
Serological evidence in humans exposed to migratory
birds has been reported.96 Can cause self-limiting
conjunctivitis as occupational exposure to affected
poultry
Blackbird (Turdus merula), carrion crow (Corvus
corone), passerine birds (Passeriformes) wild grouse
(Tetraonidae), wild ducks (Anatidae)
Blackbirds (Turdus merula), sandpipers
(Scolopacidae), wild mallards (Anas platyrhynchos),
wild grouse (Tetraonidae), other wild birds
Japanese quails (Coturnix coturnic japonica),
passerines (Passeriformes), pigeons (Columbidae),
raptors (North American raptors), wild anseriforms
(Anatidae), geese (Anserinae), swans (Cygninae)
Cormorants (Phalacrocoracidae), gulls (Larus),
passerines (Passeriformes), pelicans (Pelecanus),
raptors (North American raptors), waterfowls
(Anatidae)
Europe, America
Europe, Asia, North
America, and Africa
Worldwide
89
(continued on next page)
90
Table 2 (continued)
Microorganism(s)
Potential for transmission to humans exists (n Z 50)
Migratory bird species
Geographic area
Other Paramyxoviridae
(pneumoviruses)
NR
Gulls (Larus), waterfowl (Anatidae)
Europe, Africa, Asia
Possible through faecal pollution of environmental
water samples with wildfowl droppings111,112
Coots (Fulica), grebes (Podicipedidae), herring gulls
(Larus argentatus), migratory ducks (Anatidae), owls
(Strigidae), storks (Ciconiidae), swans (Cygninae)
House-sparrows (Passer domesticus), seagulls
(Laridae), starlings (Sturnidae)
Wild geese (Anserinae), wild woodcocks (Scolopax)
Worldwide
Picornaviridae
Egg drop syndrome virus
Foot-and-mouth disease
virus
Reoviridae
Avian rotavirus, orbivirus
and other spp.
Togaviridae
Eastern (EEE ) and
Western (WEE ) equine
encephalitis viruses
Venezuelan equine
encephalitis virus
(VEE)
(III) Parasites
Coccidia (Eimeria)
Cryptosporidium
Helminths parasites
Sarcocystis
NR but according to some studies birds do not have an
important role in the transmission of enteroviruses113
Not reported but evidence for transmission to
mammals111,114e116
Possible through mosquitoes that are vectors for
these viruses117,118
Possible through mosquitoes that are vectors for
these viruses 119,120
Possible through contamination with faecal
material121
Has been reported for other non-migratory birds122
Possible food-borne through eating small water
fish.123
Cercarial dermatitis (swimmer’s itch) due to
exposure to marine schistosomes124
Possible through contaminated water125
Cliff swallows (Petrochelidon pyrrhonota), finches
(Fringillidae), American Robins (Turdus migratorius,
smaller species of Passeriformes, several trans-Gulf
migrant starlings (Sturnidae), waterbirds (Anatidae)
Nestling birds such as Cliff swallows, North American
shorebirds, songbirds (Passeriformes), wild ducks
(Anatidae)
Europe
Asia, Africa, Europe,
America
America
South to Central America
Cranes (Gruidae), owls (Strigidae), wild pigeons
(Columbidae), waterfowls (Anatidae)
Cranes (Gruidae), exotic seagulls (Larus), wild
anseriforms: ducks (Anatidae), geese (Anserinae),
swans (Cygninae) and wild birds (order
Passeriformes, Phasianidae, Fringillidae, and
Icteridae), waterfowl species (Anatidae)
Gulls (Larus), ducks (Anatidae), passerines
(Passeriformes), waterfowl species (Anatidae)
North America,
Asia, Africa
America, Africa, Asia
Cowbirds (Molothrus), exotic birds, mallards (Anas
platyrhynchos), passerines (Passeriformes), wading
birds, wild anseriforms (Anatidae), geese
(Anserinae), swans (Cygninae)
America, Africa, Europe
Australia, Europe,
Africa, Asia, America
S. Tsiodras et al.
Wild birds and human infections
invertebrate vectors), others to the pathogen itself (e.g.
stability of the agent in the environment), and lastly some
factors relate to the environment (e.g. temperature,
humidity). Studies of certain pathogens like influenza virus
illustrate the interaction of factors that limit the transmission and subsequent establishment of an infection in
a novel host species and may help us in understanding how
and why some pathogens become capable of crossing host
species barriers.132
Factors relating to the implicated pathogen and the
affected species
Pathogens associated with wild and migratory birds may be
transmitted to humans via several routes. Generation of
contaminated aerosols by waterfowl flocks may result in
respiratory infections through inhalation of dust or fine
water droplets generated from infected bird feces or
respiratory secretions in the environment (e.g. Newcastle
Disease or chlamydiosis).30 Birds can contaminate water
with feces, nasal discharges, and respiratory secretions
(e.g. influenza A virus, Enterobacteriaceae) resulting in
a waterborne human infection after direct contact with
aquatic environments.30 Recently, the European CDC concluded that the bathing risk in the case of waters contaminated with the H5N1 virus cannot be excluded and should
be assessed on a case by case basis even though the chance
of such an event is highly unlikely.133 Food-borne infections
may result after consumption of infected carcasses of wild
birds or raw or undercooked blood, organs, or meat, e.g.,
WNV, avian influenza A (H5N1), M. avium, Clostridium
spp., Sarcocystis, Frenkelia.52,63,134 Infections may lastly
result after direct contact with the skin, feathers, external
lesions or droppings of infected wild birds (e.g. avian pox,
WNV encephalitis, H5N1, mycoplasmal conjunctivitis). A
major source of wild birdehuman contact is hunting and
the cleaning of killed birds. Often birds are field-dressed
by hunters with minimal protection bringing them in contact with blood, organs and feces.30 Serologic evidence of
avian influenza infection in hunters and wildlife professionals has been reported.56 In addition, occupational exposure to avian species (e.g veterinarians) may increase risk
of infections like avian influenza virus infection. Indirect
infection may occur through the same routes if wild birds
transmit the infection to domestic animals, e.g. poultry
or via exposure to inanimate surfaces contaminated by
bird secretions or droppings. Transfer of infected material
can happen with shoes, clothing or other inanimate objects.
Wild birds when serving as reservoirs exhibit multiplication of the pathogen within their organism. Aggregations of
bird species that occur during certain periods within the
avian annual cycle may enable transmission of pathogens
between individuals. Extreme examples for such aggregations can be found at moulting and staging areas of eared
grebes Podiceps nigricollis,135,136 at roosting sites for European starlings Sturnus vulgaris, at landbridges between
continents (e.g. Gibraltar, Bosporus) widely used by soaring
and gliding species like larger birds of prey and white storks
Ciconia ciconia and at breeding sites of many seabirds. In
terms of numbers, the vast amount of migratory birds do
migrate solitarily in ‘broad front’ and therefore do not encounter an increased risk of pathogen transmission, while
91
some species travel hundreds to thousands of kilometres
from their breeding grounds and re-fuel at distinct stopover
sites.137 These ‘‘staging areas’’ provide the opportunity for
close intermingling of species that are otherwise widely
separated during the major part of the year.35,138 Thus,
the theoretical opportunity for exchange of pathogens is increased among avian species, which make use of the same
stopover sites. In such instances duration and concentration
of the agent in the blood or the gastrointestinal tract of migrating birds are important for the subsequent infection of
another competent vector that feeds or gets exposed in
crowding situations or during stopover e.g. a tick. Several
studies have recorded infections e.g. B. burgdorferi and
human granulocytic ehrlichiosis (HGE) in ticks removed
from birds.36,37,87 Ticks commonly infest a wide range of
avian species, especially, sparrows (Passeridae), thrushes
and other ground foraging birds.30,36,37,139,140 Although
a wide range of tick species has been reported to parasitize
wild birds, Ixodes spp. are the most likely ones to carry infections (e.g. B. burgdorferi) especially in Europe and
North America. Ixodid ticks often attach to hosts for 24e
48 hours while acquiring a blood meal. In tick-borne viruses, bacteria, and protozoa, the infectious larval or
nymphal tick may remain attached to the body of a migratory bird for several days and then deposited during migration in a new geographic area. During migration, there is
sufficient time for some birds to travel hundreds or even
a few thousand miles before ticks complete feeding and
drop off. Even if these birds have small tick burdens, their
large numbers could result in substantial contributions to
local tick populations in coastal areas.40 There is even evidence of transhemispheric exchange of spirochete-infected
ticks by seabirds indicating the capacity for wild birds to
carry infected ticks for long distances.141 Moreover, birds
can carry infections in their bloodstream which is introduced to local population of ticks at other sites. Therefore,
birds play an important role not only in maintaining infections
such as B. burgdorferi sensu lato in areas of endemicity, in addition some of them, through their migration, also play a role
by spreading ticks within and between continents.36,139,142,143
Exposure to tick-borne diseases is primarily peridomestic,
so the contribution to tick related human infection of avian
ticks relative to mammalian ticks around dwellings is critical.38 Birds that are implicated in peridomestic transmission
of tick related infections to humans, especially in North
America, include American robins (Turdus migratorius),
northern cardinals (Cardinalis cardinalis), and song sparrows
(Melospiza melodia) that frequently use backyard environments and some of which are commonly seen at bird feeders.
Therefore, they are likely to drop engorged larvae in peridomestic environments like lawns and gardens,40 where ticks
are less common than in woods and at wood edges but
more likely to encounter people.38,144 Even though the survival of nymphs is low in open habitats, the contribution of
birds to human infection in the peridomestic environment
could be substantial and deserves further study.40
An additional factor is the physiologic stress that wild
migratory birds suffer with migration, a risk factor for
immunosuppression and increased susceptibility to infectious diseases. Avian species may exhibit an increased
susceptibility to certain pathogens (e.g. West Nile virus)
compared to other vertebrate groups.3,4 Changes and
92
adaptations occur in migratory birds during long-distance
migration.63 For some birds, the stress of migration can
lead to reactivation of otherwise latent infections.145
West Nile virus was isolated from migrating birds that
were under migratory stress.146 However, an opposing
argument is that infected migratory birds could not survive
long-distance travel; thus their role in transmitting communicable diseases is of less importance.147 For example, in
the case of avian influenza most outbreaks in wild birds
seem to reflect local acquisition of infection from a contaminated source, followed by rapid death nearby.148 There is
only limited evidence that some wild birds can carry the virus
asymptomatically, and no evidence from wild bird outbreaks
that they have done so over long distances during seasonal
migration.148
Understanding the balance between the changes and
adaptations that occur in migratory birds during longdistance migration is important to comprehend susceptibility of certain migratory birds to develop infections. Similar
factors e.g. age and bird gender may in addition influence
migratory patterns leading to spread of diseases in novel
geographical areas.3
Factors relating to the implicated pathogen and the
environment
Migrants of most bird species in the New World seldom use
the same stopover sites on northward, spring migration as
they do on southward, fall migration. This is because
migration routes are determined by complex interactions
of environmental factors such as direction of prevailing
winds, weather patterns, location of available food resources and geographical barriers (e.g. large bodies of
water, deserts and mountains). These factors seldom
combine to favour the same route in different seasons.3
Seasonality is a significant factor influencing both, wild
birds (wild resident and migratory species) and other vectors e.g. mosquitoes, ticks leading to changes in transmission dynamics.149e151 For mosquitoes, a spring population
peak in Europe and North America occurs during the spring
migration of birds.146e148 The effect of seasonality in the
flyway patterns of major migratory birds was observed for
certain diseases such as West Nile virus encephalitis. The
incidence of West Nile virus disease is seasonal in the temperate zones of North America, Europe, and the Mediterranean Basin, with peak activity from July through
October.152 Both avian and human infection rates drop to
near zero as winter approaches and mosquitoes become
dormant.153 Season is important for some non-vector-borne
pathogens, as well. For example, influenza A viruses remain
infectious in water at lower ambient temperatures and at
the same time major congregations of migratory waterfowl
occur, increasing the likelihood of transmission among
birds. Furthermore, numerous bird species (e.g. crows
and gulls) are attracted to untreated sewage, garbage
dumps, manure, and other sources of enteric pathogens
that can then be transmitted to humans. These areas
should be appropriately covered and not open to the access
of wild migratory birds.
Migratory bird flyways and transmission
Long-distance migration is one of the most demanding
activities in the animal world and several studies demonstrate
S. Tsiodras et al.
that such prolonged, intense exercise leads to immunosuppression exacerbating the possibility of spreading infections. On the other hand, infected symptomatic wild birds
may act as vectors over shorter distances.154 Understanding
bird migration, avian migration patterns and infectious diseases of birds would be useful in helping to predict future
outbreaks of infections due to emerging zoonotic pathogens
and can provide important information that could explain
the pattern of spread of certain infectious agents. Numerous variations in flyways exist. For some ocean migratory
wild birds, a nomadic wandering that can appear random
is probably related to poorly understood weather or ocean
conditions.155,156 Major migratory flyways, especially between continents are known to be used by migratory birds
when commuting between breeding and wintering areas
and vice versa. Nevertheless, these flyways are only used
by a fraction of the existing species on the move, predominately by waterfowl and soaring and gliding migrants like
large raptors and storks which aggregate and follow fairly
easily defined routes.
The complex overlapping of major flyways and the lack
of information on migratory species potentially involved in
the spread of disease make simple association of wild
migratory flyways with outbreaks of certain infections
extremely difficult despite the significant amount of literature on the subject. For example, in Alaska, there is
a notable overlap between the Pacific and East Asia/
Australasia flyways through which scientists believe avianflu-infected migrating birds, such as the bar-tailed godwit
(Limosa lapponica), dunlin (Calidris alpina), and red knot
(Calidris canutus), will transfer the Asian strains of H5N1 influenza virus to North American birds over the next few
months14 although this was not confirmed in a recent
study.157 On the other hand, other more local migratory
bird routes have been described in association with West
Nile virus outbreaks.3
Societal factors
Furthermore, societal factors like captivation of wild birds
in zoos and importation and sale of wild birds as pets should
also be considered as important factors which can enhance
the spread of pathogens from wild birds to humans. Cryptosporidium has been reported to be transmitted from some
non-migratory birds in zoos to humans.122 A theoretical similar risk for avian influenza exists as avian influenza was recently isolated from a wild swan in the Dresden zoo in
Germany.157 Similar risk can be encountered in bird parks
since outbreaks of infections related to birds like psittacosis have occurred.17 Finally, the international trade of
exotic pet birds carrying influenza A viruses enhances the
risk of worldwide dissemination of potentially virulent influenza A virus and may pose a serious health threat to
humans.158
Limitations of the current literature review
There are several limitations of this work and clearly
further work is necessary. Some of the identified agents
are quite ubiquitous in the environment raising the question
about how to quantify the additional impact wild resident
Wild birds and human infections
and migratory birds may have on transmission. There is still
scientific debate over the actual role migratory birds might
play in the transmission of certain communicable diseases.
In support of this argument we did not find any evidence for
direct transmission from wild and migratory birds to
humans for any of the identified pathogens the only
exception being the cluster of H5N1 human cases in
Azerbaijan.16 In addition, in many cases, there was no further available information that would allow further elucidation of the real epidemiological role played by wild birds in
the ecology of the considered infections, especially in underdeveloped countries. Many reports do not exactly clarify
how the birds are implicated in the transmission of these infections and in the majority of cases this transmission could
not be established by adequate scientific methods. Thus, in
many of the reports reviewed herein, there were no data
regarding serologic assays or molecular diagnostic techniques used to detect and characterize pathogens and identify birds as vectors of disease. In these cases only
associations of these infections with migratory birds could
be made (Table 2).
The evidence reviewed herein suggests that many
pathogens can infect multiple host bird species and that
these pathogens in theory could be responsible for emerging infectious disease outbreaks in humans and wildlife.
However, the ecologic and evolutionary factors that constrain or facilitate such emergences are poorly understood.
In the literature, a different terminology is used to describe
the interaction between hosts, including wild birds, and
pathogens. Terms such as multihost pathogens, reservoir
hosts, and spill-overs are frequently used, but often such
different terms are used to describe the same phenomenon. There is a need for a single, standardised comprehensive framework that characterizes disease outcomes based
on biologically meaningful processes. An example of such
conceptual framework is based on the pathogen’s betweenand within-species transmission rates and can be used to
describe possible configurations of a multihostepathogen
community.159 In particular, the much-overused terms reservoir and spill-over can be seen to have explicit definitions, depending on whether the pathogen can be
sustained within the target host population.159 However,
the paucity of available published data did not allow us
to determine whether the involved species of certain wild
birds serve as reservoir or spill-over. Finally, only few studies have reviewed the role of migratory birds in transmission of all different infections and these studies remain
descriptive.112
Migratory birds cannot be blamed for recurrent outbreaks at the same geographical location over subsequent
years unless there is in an introduction of the pathogen to
known or novel avian or other animal reservoir hosts and
vectors. Furthermore, for some viruses that are considered
to be transmitted by wild migratory birds (e.g. West Nile virus), duration of high levels of viremia for most species
tested has been found to be limited and usually less than
24 hours. However, exceptions to that rule exist. The house
sparrow (Passer domesticus) has demonstrated WNV viremia of sufficient duration to indicate its ability to serve
as a competent host for WNV.3
Furthermore, other modes of transmission such as the
import of infected products may be of equal importance in
93
the spread of diseases like avian influenza and scientists
are still debating the evidence of the role of migratory birds
in the wide geographical spread of the influenza A (H5N1)
virus. Highly pathogenic avian influenza viruses have been
isolated rarely from wild birds and, apart from a single
case in common terns in South Africa,160 when they have,
it has usually been in the vicinity of outbreaks of highly
pathogenic avian influenza virus in poultry or geographically and chronologically close to known outbreaks in poultry. In fact the de novo generation of highly pathogenic
avian influenza virus strains (restricted to subtypes H5
and H7) so far has been described to have occurred only
in domestic poultry and the occurrence of highly pathogenic avian Influenza viruses in wild birds is most likely
the result from spill-overs from the poultry population.
Another important limitation is that there is no way to
predict whether the comprehensive lists presented in this
paper may expand in the near future. Moreover, the fact
that a lot of the pathogens carried by wild and migratory
birds that are presented in Table 2 have not been associated with human infection does not mean that these pathogens cannot cause human infection through routes
presented for other pathogens in the same table.
Future directions
Identifying links between environmental factors and infectious disease risk is essential to understanding how
human-induced environmental changes will affect the
dynamics of human and wildlife diseases. Studying large
wetland areas, and by extension, intact wetland bird
communities, may represent a valuable ecosystem-based
approach for controlling infections caused by migratory
birds including WNV.161 Recent evaluations suggesting links
between high biodiversity among wild birds and reduced
vector-borne disease risk, such as WNV, may lead to a better
understanding of distribution patterns of such diseases.48
Recent findings on the origin of the WNV infections suggest
a single species to act as a super spreader and the transmission of WNV appears in new light.162 These recent findings
demonstrate imposingly how important detailed studies
on contact rates between vectors and host species are
and how careful interpretations need to be made before
drawing any conclusions. Estimation of the infection rate
of wild bird populations with human pathogens or with
other vectors carrying pathogens is clearly an indicated future challenge required to judge the possibilities of bird to
human transmission of pathogens. The same accounts for
the transmission between and within bird species. Recent
investigations indicate the influence of social and sexual
behaviours and their seasonal components on intra-specific
transmission,163,164 while the inter-specific transmission
rate remains speculative. Birds are considered to show behavioural changes due to pathogen infection, which will
considerably influence transmission rates.163 Furthermore,
accurate data on the speed and direction of migratory birds
may enable us to predict the timing of bird migration in
more detail; this will assist in monitoring the risk of infections that may be caused by wild birds. While this knowledge is available for larger bird species due to the use of
satellite tracking, only limited data are available on the
94
individual level for some North American songbirds with the
use of radio-telemetry tracking.165 Producing maps depicting the ecology of the vectors including mosquitoes and
ticks ecology in combination with maps of migratory routes
of wild birds along with access to real time climatic data
could be the key for developing a real time early warning
system for forecasting vector-borne disease outbreaks.166
The spatial and temporal pattern of migration of wild
birds as well the spatial distribution throughout the annual
cycle can provide further insight. Application of stable
isotope analysis has already resulted in new insights where
bird populations spend the time between the seasonally
reoccurring breeding events,167 a knowledge which can be
of great importance for future predictability of disease
outbreaks.
Human medicine often does not delve deeply into the
role of animals in the transmission of zoonotic agents and
veterinary medicine does not cover the clinical aspects of
human disease. However, to effectively and completely
cover the area of infections associated with wild birds
would require involvement of both physicians and veterinarians especially those dealing with avian species.168
Unfortunately, one recent study demonstrated that communication between physicians and veterinarians about
zoonotic diseases is largely absent.168 Therefore, one important factor that could potentially explain the paucity
of available data regarding the transmission of pathogens
from wild birds to humans could be the lack of communication between physicians and ornithologists. To most effectively decrease the risk of infections associated with wild
birds, the public health and animal health sectors must collaborate in developing strategies to decrease human exposure to pathogens carried by wild birds.
An effective public educational campaign could also put
in perspective and clarify myths and realities about the risk
of acquiring infections associated with wild birds. Activities
near geographical areas with extensive wild bird activity
really carry minimal risk especially if people take personal
protective measures for high risk activities such as handling
dead wild waterfowl. Normal behaviour that complies with
general hygienic standards should suffice.
Conclusions
We attempted to summarize the published scientific evidence regarding the direct and indirect roles of wild birds in
transmission of certain infections to humans. Although we
could not fully define this role and it appears that further
research is necessary, several conclusions can be made.
First, there is no real evidence for direct wild birdehuman
transmission besides rare examples occurring under exceptional circumstances. Several human infections can theoretically be transmitted from wild and migratory birds
although the scientific base for most of the associations
remains speculative. These findings are expected for
zoonoses, which usually require the amplification in an
animal species cycle before spill-over to humans. Wild and
migrant birds are most important in seeding pathogens into
these amplification systems. This explains why most of the
association with transmission from bird to human may
only occur indirectly. On the other hand, there is strong
S. Tsiodras et al.
evidence for the dispersal of pathogens to new geographical locations by migrating birds but it is largely unknown
how this will affect transmission to humans. The recent
emergence of infections like West Nile virus and influenza
A in various parts of the world is a prominent example of
how rapidly and widely a migratory bird associated disease
can spread. The potential factors and mechanisms involved
in the transmission of such infectious agents from birds to
humans need further elucidation. An in-depth comprehension of avian migration routes as well as further research
using advanced molecular testing of the prevalence, pathogenesis, and clinical associations of several pathogens that
are transmitted to humans from the various migratory bird
species would lead to a better understanding of the transmission dynamics of diseases carried by avian species helping to
predict future outbreaks of relevant human infections.
Conflict of interest
None.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jinf.2007.
11.001.
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