Greenfinch and Trichomoniasis

 

Trichomoniasis is becoming a larger threat to British songbirds on a national level but is still not really prevalent in the North and East Yorkshire areas. In July five greenfinch were found dead in close proximity to one another in the Lockton/Levisham area, three of these birds had been rung in Dalby less than 5 miles east. The sudden localised increase in mortality rates, is suggestive of this or a similar disease. However I am unsure whether any testing was conducted on these birds and have not seen them, so other possibilities exist. Regardless this raises the subject of Trichomoniasis as a possible negative factor on greenfinch populations in the locality.

What is Trichomoniasis?

Trichomoniasis is caused by a protozoan parasite called Trichomonas gallinae. It typically causes a yellow plaque like coating and necrosis on the walls of the throat, mouth, oesophagus and nasal cavities. Occasionally in severe infections it may spread around the host and affect organs like the liver. Infected birds are characterized in the later stages by dribbling, regurgitation of food, labored breathing and have difficulty swallowing. Encountered birds are often found fluffed up and relatively unresponsive. Where the infection is weak birds may recover, however typically birds fail to recover and the disease which can last over several weeks in some cases is eventually fatal. Trichomonas cannot survive for more than a few minutes outside its host.

Trichomoniasis commonly occurs in pigeons, however it has been infrequently in other birds like passerines and raptors, until the recent increase in finch populations. It is suggested that Trichomonosis was a key factor contributing to the extinction of the passenger pigeon. In pigeons it usually exists high up in the throat close to the mouth in the pharyngeal region, whereas finches differ slightly in having it lower down closer to the oesophagus. It is believed to have been passed to finches from pigeons, which are a significant reservoir for the disease.

Recent history in British finch populations.

Trichomonosis emerged as a none specific fatal disease of finches in Britain around 2005 and rapidly became epidemic within greenfinch, and to a lesser extent chaffinch, populations a year later in 2006. Originally it was identified in British Finch in April 2005, this was followed by a further low mortality of British finches throughout the rest of the year, with a peak in cases between September and November. During the summer of 2006 the number of confirmed cases increased dramatically, more so in greenfinch than other species. Approximately 6300 mortalities were reported, primarily from garden feeding stations during 2006, this compares with 84 incidents in 2005. By 2007 breeding populations of both greenfinch and chaffinch had decreased by 35% and 21% respectively in some areas, according to one study. This represents a mortality of over half a million birds. The disease continues to persist in Britain and is particularly prevalent in the west midlands, Wales and western Britain. Data collected from BTO and RSPB garden bird studies, contributed to by the public have shown a marked decline in greenfinch numbers, more so than other species of "garden" bird.

Relevance to the British greenfinch population

The British greenfinch population increased steadily between the 1980's and mid 2000's. During 2007 a sudden significant decrease occurred. This is suggested to have been largely driven by trichomonosis. It is estimated that a loss of half a million birds occurred around the 2007 period. It is worrying that no similar rates of mortality caused by infectious diseases have occurred in wild British bird populations, other records of trichomonaisis from passerines globally rarely show such a negative impact in a wild population. In the years prior to 2007 the disease has remained a significant negative factor on greenfinch populations.

Why are greenfinch so susceptible to Trichomoniasis?

The reasons as to why greenfinch are so susceptible are still largely unclear. Greenfinch are a species, which could be classed as a highly vulnerable to infectious diseases. Similar diseases like salmonellosis and colibacillosis are regularly reported in greenfinch populations. As greenfinch are highly gregarious, diseases can quickly pass between individuals and remain viable within the population. Being granivores means that large numbers gather a feeding stations which further increase contact rates. However trichomoniasis is not especially prevalent in other birds which gather in high numbers at feeding stations like tits and sparrows. This suggests that foraging behaviour is unlikely to be a key factor in making greenfinch so vulnerable alone.

The significance of feeding stations has increased over the past 50 years, in which land uses have changed in the wider countryside increasing the species dependence on them therefore increasing the likelihood of birds coming into contact with each other. In addition by feeding throughout the summer, the risk of young birds becomes significantly increased due to feeding adults using feeding sites then returning to the nest.