Avian diseases caused by parasites have been on the rise in recent years, posing a serious threat to bird populations around the world. One parasite in particular, Trichomonas gallinae, has emerged as a significant cause of sickness and death for an increasing number of bird species.
Trichomonas gallinae is a protozoan parasite that primarily affects pigeons, doves, and raptors that prey on them. It causes a disease known as avian trichomonosis that often results in lesions in the mouth, esophagus, and crop of infected birds. While this parasite was previously thought to just impact columbids like pigeons and doves, it has now been detected in a wide range of species from raptors to passerines. The spread of this deadly parasite has raised alarm among ornithologists and conservationists concerned about its impacts on wild bird populations.
In this article, we will cover the key questions around this emerging avian health crisis:
What is Trichomonas gallinae and how does it kill birds?
Trichomonas gallinae is a microscopic, single-celled protozoan parasite that infects birds. It is an obligate extracellular parasite, meaning it can only survive for brief periods outside of a host. T. gallinae is transmitted between birds through direct contact with contaminated food or water sources. It can also be passed from parents to offspring.
Once a bird is infected, T. gallinae invades and damages the upper digestive tract. It causes lesions and ulcerations in the mouth, esophagus, and crop. This makes it difficult for birds to swallow food and water. The parasite also releases proteolytic enzymes that break down cell membranes and tissues.
Infected birds often exhibit symptoms like difficulty and uncoordinated swallowing, lethargy, feather ruffling, and emaciation. They may also regurgitate food. Without treatment, the lesions and systemic infection can eventually prevent the bird from eating or drinking enough to survive. Birds often die from starvation and dehydration.
Secondary infections are common and can also be fatal. The open sores caused by T. gallinae allow other pathogens like bacteria, viruses, or fungi to invade tissues. These secondary infections may accelerate a bird’s decline.
Which species are most at risk?
Trichomonas gallinae has traditionally been found in pigeons and doves worldwide. It is endemic in columbids and causes chronic infections, but rarely mass mortality events. However, in recent decades, T. gallinae has emerged as a more lethal threat to birds of prey.
Raptor species like Cooper’s hawks, red-tailed hawks, and great horned owls appear highly susceptible. These birds become infected when preying on carrier pigeons or doves. While columbids can tolerate low-level, chronic infections, raptors experience more acute disease that often turns fatal.
T. gallinae outbreaks have killed thousands of raptors across the United States, Canada, and Europe. Declining raptor populations in areas like the Canadian Prairies have been linked to trichomonosis epidemics. Beyond raptors, passerine species like finches, robins, and sparrows are also at risk as they increasingly share feeders with infected doves.
What regions have been most impacted?
Avian trichomonosis has emerged as a global threat, but some geographic hotspots have witnessed repeated large-scale mortality events:
United States: T. gallinae epidemics first emerged in the 1990s among mourning doves in Alabama. Outbreaks killing thousands of raptors, especially Cooper’s hawks, have since recurred in multiple states including California, Georgia, Kentucky, and Virginia.
Canada: Epidemics in the Canadian provinces of British Columbia, Alberta, Saskatchewan, Manitoba, Ontario and Nova Scotia have killed thousands of band-tailed pigeons, rock pigeons, and raptors since the 1990s. Declines in Ferruginous hawk populations have been linked to trichomonosis.
United Kingdom: Trichomonosis outbreaks emerged in the UK in the 2000s, disproportionately impacting Eurasian collared doves, European turtle doves, and greenfinches. Cases peak between May and September.
Continental Europe: Large die-offs linked to trichomonosis have been reported in recent decades in Spain, France, Germany, Switzerland, Austria and the Netherlands. Predators like red kites and goshawks have suffered significant mortality.
Australia: Trichomonosis was first detected in Tasmanian wildlife in 2011. It has since spread through the mainland, causing mass mortality events in rainbow lorikeets and other parrots.
How is Trichomonas gallinae transmitted between birds?
T. gallinae spreads between birds through both direct and indirect contact. Understanding transmission pathways is key to controlling outbreaks. The main routes of transmission include:
Shared food and water sources: Birds come into contact with T. gallinae by sharing contaminated bird feeders, bird baths, and natural water sources. Saliva with the parasite can contaminate surfaces.
Regurgitation: Infected birds often regurgitate food and crop liquids. Nearby birds may pick at this regurgitated material and become infected.
Crop-to-crop contact: During courtship, breeding, and feeding of young, many species mouth-to-mouth pass food. This facilitates crop-to-crop transmission.
Predation: When raptors or scavengers prey on infected birds, they ingest tissue with a high concentration of parasites. This route explains trichomonosis spread to predators.
Nest material: Parents may inadvertently introduce infected saliva or crop secretions into the nest through regurgitated food or during nest construction. Nestlings can then contract the parasite.
Fomites: T. gallinae may spread through contaminated cage surfaces, bird banding equipment, or other fomites contacting infected birds. However, the parasite likely persists for only brief periods outside a host.
Identifying major transmission pathways within an outbreak is key to deploying effective control measures. This often involves molecular epidemiological tracking of parasite strains and infection pathways.
What factors have enabled the parasite’s spread?
Scientists have proposed several factors that may explain the apparent expanded range and deadliness of avian trichomonosis in recent decades:
Increased reservoir hosts: Some columbids like Eurasian collared doves have exponentially grown their populations and range. These abundant and widely distributed birds provide increased reservoirs to sustain more T. gallinae transmission.
Urbanization: High densities of pigeons and doves in urban and suburban areas provide parasite reservoirs. Raptors may become infected when hunting these bird populations.
Backyard bird feeding: Feeders that attract both columbids and passerines provide opportunities for T. gallinae transmission across diverse species.
Climate change: Warming temperatures may support larger parasite populations and favor transmission. More migratory birds using a given location during longer nesting seasons also increases exposure.
Immunosuppression: Other environmental stressors like chemical pollution may be weakening the immune response of exposed birds. This increases their susceptibility to Trichomonas infections.
Virulent strains: New genetic strains of T. gallinae may have evolved that are more pathogenic in certain bird species. However, strain virulence factors remain poorly characterized.
Further research is still needed to quantify the relative contribution of each proposed factor. But the consensus is that humans have enabled T. gallinae to exploit new ecological niches through environmental changes.
How can Trichomonas gallinae outbreaks be controlled?
Controlling virulent Trichomonas gallinae outbreaks presents major challenges for wildlife managers and ornithologists. Key measures that may help mitigate parasite transmission and mortality include:
Targeted treatment: Orally dosing affected and at-risk birds with anti-parasitic drugs like dimetridazole can treat infections if caught early. Treatment may be feasible for small captive populations.
Separating food and water sources: Providing separate feeders for columbids versus passerines can help disrupt transmission across species. Using platforms rather than bird baths reduces contamination.
Improved sanitation: Frequently cleaning and disinfecting bird feeders, baths, cages, and equipment reduces parasite spread. Allowing periods of non-use also decreases transmission.
Temporary feeder removal: Bird feeding moratoriums during outbreaks may encourage dispersion and decrease contact. Feeders can be restored once epidemics subside.
Habitat management: Discouraging artificial aggregations of different bird species through plantings and habitat alterations can promote natural foraging and lower contagion risks.
Public education: Informing bird owners and wildlife enthusiasts about transmission risks and prevention encourages practices like routine disinfection that limit spread.
However, these measures are often labor-intensive, costly, and logistically challenging over large areas. In the end, avian trichomonosis control will likely require integrated approaches combining targeted medical intervention, wildlife management, and public engagement.
What is the prognosis for affected bird populations?
The long-term impacts of trichomonosis epidemics on wild bird populations remain concerning but uncertain. Outcomes likely depend on several key factors:
Virulence evolution: Continued evolution of T. gallinae strains towards lower or higher lethality could respectively alleviate or worsen future mortality impacts. Ongoing genetic monitoring is needed.
Development of immunity: Birds that survive infection may gain some protective immunity against future epidemics. But the duration of immunity remains poorly understood.
Species characteristics: Life history traits like clutch sizes and generations to maturity influence the resiliency of populations recovering from die-offs. Small endangered populations are at highest risk.
Conservation status: Common bird species with abundant global populations like pigeons and doves are less vulnerable compared to rare endemic birds with small fragmented populations.
Transmission dynamics: Determining whether T. gallinae becomes endemic in novel hosts and ecosystems or only causes occasional epidemics will dictate long-term impacts.
While Trichomonas gallinae is unlikely to ever be completely eradicated from bird populations, targeted conservation efforts facilitated by continued surveillance and research offer hope for containing its deadliest effects. But the parasite will likely remain an important selective pressure.
Conclusion
The protozoan Trichomonas gallinae has rapidly emerged as a detrimental parasitic disease across diverse bird groups worldwide. While columbids serve as common reservoirs, raptors and an expanding range of passerines are being impacted by the spread of more virulent parasite strains into new regions and hosts. Control measures like targeted medical treatment, separating bird resources, and habitat management may reduce transmission. But avian trichomonosis will likely remain a persistent threat magnified by human-altered environments. Continued mortality and selective pressure on highly susceptible species remain a serious concern requiring ongoing monitoring, research, and mitigation efforts. Only through proactive One Health approaches addressing avian health, human behavior, and the environment can the deadliest impacts of this pandemic parasite be ameliorated.