Swallows are small passerine birds in the family Hirundinidae. There are around 90 species of swallows worldwide, and they are found on every continent except Antarctica. Swallows are aerial insectivores, meaning they catch and eat insects in flight. Some of the most widespread swallow species include the barn swallow, tree swallow, bank swallow, and cliff swallow.
As with all wild bird populations, swallows are susceptible to a variety of infectious diseases and parasites. Their migratory nature means that swallows can pickup and transmit diseases across large geographic areas. Understanding swallow diseases is important for monitoring the health of wild bird populations and protecting human health, as some diseases are transmissible between birds and humans.
Major swallow diseases
Some of the major infectious diseases found in swallows populations include:
West Nile virus
West Nile virus is a mosquito-borne flavivirus that can infect and cause disease in birds, humans, horses and other mammals. West Nile virus was introduced to North America in 1999 and rapidly spread across the continent, causing peaks of outbreaks in wild birds and humans.
Many swallow species, including cliff swallows, barn swallows and tree swallows, are considered important hosts and reservoirs of West Nile virus. When bitten by an infected mosquito, swallows can develop West Nile fever or even more severe neuroinvasive disease leading to death. However, swallows are also amplifying hosts – they develop a sufficient viral load from mosquito bites to then infect other uninfected mosquitos when bitten. This makes swallows key players in the transmission cycle of West Nile virus.
Avian influenza
Avian influenza refers to infection of birds with influenza type A viruses. These viruses occur naturally in wild birds, but can be transmitted to domestic poultry and cause severe outbreaks. Many swallow species have been found to harbor avian influenza viruses and show evidence of past infection. However, most avian influenza virus strains do not cause severe illness in swallows. The role of swallows in transmitting avian influenza viruses is still not fully understood. More surveillance is needed to characterize the diversity of avian influenza viruses circulating in swallow populations and their ability to spread viruses to other birds or humans.
Trichomonosis
Trichomonosis is a parasitic disease caused by the protozoan Trichomonas gallinae. Trichomonosis has emerged as a significant cause of epidemic mortality events in finches and pigeons in Europe. Recently, outbreaks with high mortality have also been reported in swallows, primarily in juvenile barn swallows. The disease is spread between birds by direct contact during courtship and feeding of chicks. Lesions form in the upper digestive tract and affected birds show difficulty swallowing. Trichomonosis causes severe population impacts when epidemics occur, highlighting the need for more research on transmission risks.
Salmonella
As with many wild bird species, swallows can be infected with various strains of the bacterial pathogen Salmonella. Salmonella bacteria are passed in bird feces. Consuming contaminated food or water leads to Salmonella infection. Salmonellosis outbreaks in swallows can lead to rapid deaths of nestlings still in the nest. Salmonella strains found in swallows also have public health significance, as birds can transmit bacteria that cause foodborne illness in people. Monitoring swallows for Salmonella provides an early warning of strains that could spread more widely in the environment.
Parasites
In addition to viral and bacterial diseases, swallows host a variety of endoparasites (internal parasites) that may impact their health and survival:
Nematodes
Nematodes or roundworms are one of the most common parasites reported in swallows. Different swallow species have been found to harbor a variety of nematode species in their digestive tracts, including Acuaria, Dispharynx, and Capillaria worms. Heavy nematode infections may contribute to mortality of nestlings.
Flukes
Swallows frequently host intestinal flukes of the genus Brachylaima. These parasitic flatworms have complex life cycles involving snails as intermediate hosts. Swallows become infected when they consume infected snails. High fluke burdens have been associated with swallow deaths, particularly in cliff swallows.
Mites
Feather mites are microscopic arachnids that feed on the feathers and skin of birds. Swallows frequently harbor a diversity of feather mite species, which are generally commensal rather than parasitic. However, some mite species can attain burdens high enough to cause irritation, feather damage, and anemia in nestling barn swallows.
Lice
Chewing lice (Mallophaga) and feather lice (Ischnocera) are wingless insect parasites that live on the skin and feathers of swallows. Heavy lice infestations may contribute to overall poor health and grooming injuries in nestlings. Lice can spread rapidly between closely packed swallow nests.
Risk factors
A number of factors may influence the risk of disease infection and spread in swallow populations:
Migration
The long distance migrations undertaken by swallows promote contact and pathogen transmission between populations that would otherwise never interact. Migratory rest stops and overwintering grounds are hotspots where swallows congregate and amplify local transmission.
Nesting colonies
Swallows that breed in dense nesting colonies provide ideal conditions for pathogens to spread rapidly once introduced. Large numbers of immunologically naive fledglings are also highly susceptible to outbreaks.
Insectivorous diet
As aerial insectivores, swallows are exposed to any pathogens circulating in local insect populations.West Nile virus, for example, amplifies in mosquitos before being transmitted to swallows that feed on them.
Wetland association
Many swallow species rely on wetlands and aquatic habitats. This brings them into contact with the mosquitos and snails that transmit viruses, parasites, and other pathogens. Nesting around standing water may also increase exposure.
Anthropogenic changes
Habitat loss on wintering grounds, climate shifts, increased use of pesticides, and other human-driven changes have stressed swallow populations and potentially increased disease susceptibility in recent decades.
Research and surveillance
More active research and surveillance programs focused on swallow diseases would provide valuable information for supporting swallow conservation. Some priorities include:
– Monitoring swallow colonies and mortality events for evidence of pathogens and parasites
– Screening swallows for exposure to known viruses such as West Nile virus, avian influenza, etc.
– Conducting necropsies on dead swallows to determine causes of death and characterize disease conditions
– Analyzing swallow parasites and pathogens at the molecular level for insights on transmission patterns
– Banding swallows to track individual infection risks over migratory journeys
– Collecting mosquitos and other vectors at swallow nesting sites to test for endemic pathogens
– Using satellite tracking to identify migratory routes and connect pathogen spread between regions
– Evaluating environmental and landscape factors associated with increased disease prevalence
– Developing swallow-specific antibody assays to efficiently screen populations for exposure to pathogens
– Comparing health between stable and declining swallow populations to identify disease impacts
– Modeling climate change influences on patterns of vector-borne disease in swallows
Conclusions
In summary, swallows are susceptible to a range of viral, bacterial, and parasitic diseases that can impact population health and survival. However, our understanding of swallow diseases remains limited, and more surveillance is critically needed. Tracking swallow disease dynamics will provide key insights into the changing ecological pressures these aerial insectivores face. With populations of many swallow species in decline, a One Health approach that considers swallow health within the interconnected environment is essential. Improving knowledge of swallow diseases can support conservation efforts for these iconic migratory birds.
