A disease in pigeons and doves gained infamy in 2006 after it decimated the UK’s greenfinch bird population by half a million within that year. In 2012, the bird’s population had plummeted to 2.8 million from an estimated 4.3 million. A new study has now found a protozoan parasite that was implicated in the disease in common mynas in Faisalabad, Pakistan.
The disease, avian trichomonosis, is an emerging infection more colloquially called frounce or canker. It wreaks havoc on both endangered and common wildlife populations. The parasite responsible for causing it, Trichomonas gallinae, is known to normally affect pigeons, doves and the birds of prey that eat them.
But in 2005, it jumped species and devastated the UK’s garden songbird populations of greenfinches and chaffinches. Some earlier studies have pointed out that they probably had a major role to play in driving the passenger pigeon to extinction, and that they are causing populations of the endangered Mauritian pink pigeon to fall. (Interesting aside: a T. gallinae-like protozoan had commonly infected the dinosaurs as well.)
Trichomonosis affects the upper respiratory and gastrointestinal tract of birds. Affected birds develop lesions in the mouth and/or oesophagus and show signs of lethargy and laboured breathing. Some of them drool saliva and have wet plumage around the bill. T. gallinae can’t survive outside the host and is vulnerable to dehydration. The parasite is transmitted when infected birds share their food and water with others.
Common mynas roost with the blue rock pigeon, the latter a species that hosts T. gallinae in their bodies without harm to themselves. Since mynas are also known to eat pigeon and dove fledglings, researchers already suspected that the disease had jumped species.
Mynas are highly adaptable and live and breed in a wide range of climates. They are also known to evict other resident birds from their nests, earning them the sobriquet ‘kalahapriya’, Sanskrit for ‘quarrel-lover’. Though native to Asia, the myna has thrived in new environments to which it has been introduced.
In fact, according to the International Union for the Conservation of Nature (IUCN), it’s one of the three bird species on the ‘world’s 100 worst invasive species’ list. Given its runaway success in occupying new territory, the myna’s ability to spread diseases to other susceptible domestic and wild avian species in other parts of the world is a matter of grave concern.
The global distribution of the common myna. Blue indicates its native range, red the regions in which it was introduced. Credit: Biatch/Wikimedia Commons
The researchers, from the University of Agriculture in Faisalabad, the King Saud University in Riyadh and the University of East Anglia, Norwich, captured 167 mynas across eight sites in Faisalabad to screen for T. gallinae. They used a binomial sequence-based genotyping system, where genetic variation is measured in predetermined DNA sequences, and found that about 20% of the birds were infected with the disease.
The infection rates were lowest in places where the contact between mynas and blue rock pigeons was limited. At the same time, the infection itself was widespread: there were infected mynahs in all the surveyed sites.
The study’s authors have inferred that the disease in mynahs was endemic, i.e. restricted to a particular region. Farah Ishtiaq, a disease ecologist at the Indian Institute of Science, Bengaluru, who was not involved in the study, thinks this is a bold claim to make because it has not been explored elsewhere and because there has been no evidence thus far for this claim.
According to Kevin Tyler, a cellular microbiologist at the University of East Anglia and one of the authors of the study, the genetic subtype, or strain, affecting the mynas was different from the one affecting European finches.
He also said in response to Ishtiaq’s comment that the subtype had been found at all sites at a similar level at different times. Together with the fact that T. gallinae did not appear to be causing severe disease, “our interpretation … is that this is consistent with the disease being endemic in this region rather than a recent outbreak.”
Spencer Greenwood, a professor at the University of Prince Edward Island, Charlottetown, who was not part of the study, also found the endemism claim reasonable given they had only found one genetic subtype and that most birds had not shown evident signs of the disease.
Then again, only 11 out of the 33 infected mynas had exhibited signs of poor health, suggesting they were able to carry the infection without fatal consequences to themselves while potentially being able to spread the disease to other species. However, more studies will have to be conducted before we’re sure whether mynahs are a reservoir or a dead-end host. This is because the mynas have the potential to be a reservoir for this disease. Though the authors have found infection, the study does not address transmission to and from mynas as hosts.
The infection incidence was higher among male mynas than female ones. Nicholas Clark, a postdoctoral fellow at the University of Queensland, Brisbane, who has studied avian malaria in mynas, said, “Perhaps the males are more likely to come into close contact with pigeons and doves.” He added that researchers would have to study the behavioural ecology of males and females in the nest to know more. He was not involved in the study.
However, Ishtiaq said that mynas have no sexual dimorphism – so sexes can’t be told apart without surgical or molecular testing. So it would be easy to over- or under-estimate the number of birds of either sex.
But Tyler is confident about being to use subtle differences in morphometric measurements to their advantage. In this study, the researchers were familiar with the captured mynas and could confidently ascribe sex. Nonetheless, he and his colleagues remain unsure as to why mostly male mynas were infected with trichomonosis.
Greenwood said other aspects, such as a myna’s roosting behaviour, weather conditions, availability of food, closeness to breeding season, etc. need to be considered to figure out why more males seem to be infected.
He also thinks that sampling the pigeons that share habitat with the mynas and determining what type of strains they carry would be important. “It would be interesting to see if they have the same subtype and if they do, at what prevalence do they occur compared to other subtypes.”
Protozoan parasites cause some of the world’s worst diseases. Because a lot of them are restricted to the developing world, studies such as this demonstrate the importance of studying emerging infectious diseases and their social and economic repercussions. “Mynas have already been implicated in the spread of bird flu through contact with poultry,” Tyler said, “so this could be of concern to poultry farmers.”
His team plans to work with groups in India and other places where mynas thrive.
Ishtiaq thinks that they have made a good start in understanding the host and geographical range of trichomonosis, and feels it would be good to test the common rosefinch for T. gallinae. This is the only finch species that migrates to South Asia and comes in contact with other resident birds. “I have seen many shared avian malaria parasites between resident finches and common rosefinch,” she said.
The study was published in the journal Parasitology on April 23, 2018.
Rashmi Bhat is a wildlife researcher with an overarching interest in applied conservation biology. She is based in Bengaluru, India.