Bird or avian malaria caused by parasites Plasmodium and Haemoproteus is one of the most common and widespread diseases in birds. Avian malaria leads to decrease in reproductive success, anaemia, changes in body condition and even mortality in extreme cases. Mosquitoes transmit Plasmodium, while biting midges (Culicoides sp.) are responsible for Haemoproteus transmission in birds.
Previous studies from the temperate regions have shown that temperature plays an important role in the transmission of the parasites and also regulates the presence of these blood-feeding insect vectors. But a new study by scientists from Indian Institute of Science, (IISc) Bengaluru, and Wildlife Institute of India (WII), Dehradun, showed that seasonal variation had no influence on prevalence of avian malarial parasites in India. The results were recently published in Ecology and Evolution.
Though human malaria follows a seasonal pattern in the Himalayan foothills with peak transmission from July to October, the avian malaria parasites showed no peak and were found to stay in blood throughout the year
Temperature effects
The team conducted year-round (December 2008 to December 2009) sampling inside WII campus using mist-nets. They collected a small quantity of blood from the resident Himalayan birds before releasing them. A systematic record on avian abundance was also maintained. They also sampled mosquitoes in the same habitat to understand the influence of seasons on mosquito species and their abundance with change in temperature.
Among the 413 birds screened, 153 i.e. 38% (37 Plasmodium, 97 Haemoproteus, 21 co-infections) showed infection with blood parasite. They examined the relationship between monthly temperature, mosquito abundance and parasite prevalence. Though the mosquito abundance increased with temperature, the Plasmodium infection decreased. This could probably be due to a temperature threshold above which the parasite cannot develop into an infective stage inside the mosquito.
Bimodal patterns
“In U.K., we see a bimodal pattern where the infection peaks during spring and autumn and subsides during winter. In spite of well-defined summer–winter conditions in Dehradun, the mosquito abundance did not vary much between seasons,” explains Dr. Farah Ishtiaq from IISc and first author of the paper. But the composition of mosquito species changed with season — Culex mosquito dominated in summer while Uranotenia mosquito was prevalent in winter leading to infection throughout the year.
Dehradun is a wintering ground for many high elevation Himalayan birds as well as small European perching birds.
The team sampled migrants to understand parasite prevalence. Molecular analysis on the parasites revealed that most parasites found in resident birds were locally transmitted and not from the migrant birds. This confirmed that there was no exchange of parasite from migrant to resident birds.
“This study [carried out on birds on the Himalayan foothill] provides a basis for furthering our understanding on the ecology and epidemiology of avian malaria and the spread of disease across the resident high-elevation Himalayan bird communities. These high elevation birds are probably not exposed to parasites throughout the year,” adds Dr. Ishtiaq, commenting on the significance of the work.
