Swallows are small, sparrow-like birds found worldwide. There are around 90 different species of swallows, including barn swallows, cliff swallows, tree swallows, bank swallows, and cave swallows. While swallows are beloved for their aerial acrobatics and cheerful chirping, they can potentially carry and transmit diseases to humans and other animals.
Avian Salmonellosis
One of the most common and concerning diseases carried by swallows is salmonellosis. Salmonella is a group of bacteria that can cause diarrheal illness in humans. Swallows and other bird species can be infected with Salmonella through contaminated food or water sources. Infected birds shed the bacteria in their feces, which can then spread to humans.
Here are some key facts about salmonellosis in swallows:
- Salmonella outbreaks linked to swallows have been documented worldwide, including in the United States, Canada, Europe, and Australia.
- The species of swallow most commonly implicated are barn swallows and cliff swallows.
- Salmonella transmission typically occurs when swallow feces contaminates food preparation surfaces, feed meant for livestock, or surfaces where children play.
- In one study, Salmonella was isolated from 17% of sampled cliff swallow nests.
- Outbreaks often peak during summer months when large numbers of swallows gather to nest and hatch young.
Proper hygiene and sanitation measures are important to prevent Salmonella spread from swallows to humans or domestic animals. Controlling and containing swallow nesting areas can also help reduce risk.
Histoplasmosis
Histoplasmosis is a fungal infection caused by breathing in spores of the fungus Histoplasma capsulatum. While histoplasmosis is not exclusive to swallows, bird droppings in swallow nesting sites or roosts can enrich soils with the fungus.
Key facts about histoplasmosis in swallows:
- Histoplasma grows best in soils enriched with nitrogen and phosphorus, such as found under swallow nests or bat guano deposits.
- Disturbing contaminated soils by sweeping or raking can aerosolize Histoplasma spores and spread the infection if inhaled.
- Most human cases occur when cleaning or demolishing old bridges, barns, attics, or other structures harboring swallow nests.
- Outbreaks have occurred across the midwestern and eastern U.S. as well as in Central and South America, Africa, and Asia.
- Infection can range from mild flu-like illness to severe pneumonia requiring hospitalization.
Protective equipment like respirators and dampening contaminated soil reduces risk when handling areas likely inhabited by swallows in the past. Soil fumigation may also help eliminate Histoplasma from nesting areas.
Ectoparasites
Ectoparasites are organisms that live on the external surface of the host’s body, such as lice, fleas, ticks, and mites. Swallows frequently host a number of ectoparasites, including:
- Swallow bugs (Oeciacus vicarius) – These are nest-based bugs that feed on cliff and barn swallow blood. They can accumulate in large numbers in nests and surrounding areas. Swallow bugs can transmit encephalitis viruses and possibly other pathogens.
- Mites – Mites like Ornithonyssus sylvarium feed on swallow blood and can cause mite dermatitis in humans.
- Lice – Chewing lice like Machaerilaemus malleus can spread to and infest humans who handle swallow nests and young.
- Fleas – Fleas rarely parasitize swallows but can use their nests and become a nuisance for humans.
Wearing protective clothing, gloves, and masks when cleaning areas near swallow nesting sites helps prevent ectoparasite exposure and bites.
West Nile Virus
West Nile Virus (WNV) is a mosquito-borne flavivirus that can infect and cause disease in swallows and other birds. Mosquitoes transmit the virus after feeding on infected birds then bite humans and other animals.
Key facts about WNV in swallows:
- WNV was first detected in North America in 1999 and has spread widely across the continent.
- Cliff swallows seem to be the most important WNV amplifying hosts, supporting viral replication and transmission.
- Infections peak in mid-summer and decline by fall migration.
- WNV does not appear to significantly impact swallow survival or populations.
- WNV-infected swallows develop sufficient viral levels in their blood to reinfect biting mosquitoes.
WNV control strategies include mosquito surveillance and control programs. Avoiding mosquito bites through repellents and protective clothing can prevent human WNV infections.
Avian Influenza
Avian influenza refers to infection with influenza type A viruses that circulate naturally in wild birds like swallows worldwide. Avian influenza viruses in swallows include:
- H3N8 subtype – Low pathogenic virus found in cliff swallows and house sparrows.
- H4N6 subtype – Low pathogenic virus isolated from swallows in Canada.
- H7N9 subtype – Low pathogenic strain carried by tree swallows migrating from China.
While low pathogenic avian influenza rarely causes clinical signs in wild birds, these viruses have the potential to mutate and cause outbreaks in poultry. Proper poultry biosecurity prevents contact between wild birds and domestic flocks.
Eastern Equine Encephalitis
Eastern equine encephalitis virus (EEEV) is an alphavirus that can cause neurological disease in humans, horses, and some bird species. EEEV is maintained in a cycle between Culiseta melanura mosquitoes and wild bird reservoirs like swallows.
Key EEEV facts:
- Tree swallows are susceptible to infection and die-offs from EEEV.
- Cliff swallows have tested positive for EEEV antibodies, indicating past exposure.
- EEEV transmission linked to swallows has been documented along the Atlantic and Gulf Coasts of the U.S.
- Human EEEV cases typically occur near freshwater hardwood swamps where swallow nesting happens.
Strategies to reduce EEEV transmission include eliminating mosquito breeding sites and protecting humans from mosquito bites.
Cryptosporidiosis
Cryptosporidium is a protozoan parasite that can cause gastrointestinal illness if swallowed. Birds like swallows can carry and shed Cryptosporidium oocysts in droppings.
Facts about cryptosporidiosis in swallows:
- A study in Spain detected Cryptosporidium oocysts in fecal samples from 11% of surveyed barn swallows.
- Subtypes found include C. baileyi, C. meleagridis, C. parvum, and C. hominis.
- Surface water contamination from swallow feces has caused cryptosporidiosis outbreaks linked to drinking untreated water.
- Livestock can also become infected by swallows roosting in barns and contaminating feed and water sources.
Preventive measures involve protecting drinking water supplies from contamination and proper handwashing hygiene when handling swallow nesting areas.
Conjunctivitis
Swallows can suffer from a severe conjunctivitis and ophthalmitis, particularly during cold weather events. The bacterium Mycoplasma gallisepticum is often implicated as the cause.
Key points about conjunctivitis in swallows:
- House sparrows may serve as a reservoir host for Mycoplasma and transmit infection to nearby swallows.
- Conjunctivitis outbreaks in swallows often coincide with cold snaps and storms during migration.
- Infected birds develop swollen, crusty eyes that eventually render them blind and unable to feed.
- Mortality rates upwards of 50% have been reported in some swallow conjunctivitis events.
There is no specific treatment for infected swallows in the wild. Providing supplemental feeding and housing during outbreaks may help some survive.
Conclusion
In summary, swallows carry a diverse array of pathogens transmissible to humans, pets, livestock, and other birds. Proper precautions are prudent when handling or nesting near these aerial acrobats. Being aware of the potential health hazards swallows present allows the public to continue enjoying the arrival of these harbingers of spring and summer.