First molecular study in tropical Himalayan birds shows no correlation between seasonal variations &malarial parasite prevalence
10 Sep 2017
by Dr Farah Ishtiaq, Intermediate Fellow
Indian Institute of Science, Bangalore
Avian malaria is very widely distributed disease and have been reported from virtually every part of the world except Antarctica. It is primarily caused by blood parasites that belong to Plasmodium species, however, Haemoproteus parasites do infect blood cells as well. Infections from both parasite group are sometimes collectively referred as avian malaria. Being vector-transmitted parasites, both Plasmodium and Haemoproteus species need specific insect vectors for their transmission between birds, which are usually mosquitoes for Plasmodium and biting midges for Haemoproteus transmission.
Our team has shown for the first time that avian Plasmodium and Haemoproteus parasites remain in the blood throughout the year without undergoing dormant phase in a tropical system where parasites survive in the tissues of the avian hosts. These findings are in stark contrast to temperate regions where temperature regulates the presence of vectors and transmission of parasites with a bimodal pattern of spring and autumn peaks with absence of infection in winter. This study was recently published in the journal of Ecology and Evolution.
Avian malaria was found in 38% of the birds examined
For a year in tropical Himalayan foothills in Dehradun, our team conducted seasonal bird sampling using mist-nets to record morphometric (wing length, weight), blood samples and released the birds on the site. In addition, mosquitoes sampling was conducted around the same habitat.
Among 413 bird blood samples representing 49 species from 19 families that were screened for infection with Plasmodium spp. or Haemoproteus spp., 153 birds (37.68%) in 24 species (49%) were infected. Of these 37 were infected with Plasmodium, 95 Haemoproteus and 21 were co-infections.
The parasite infections varied across avian hosts; among resident foothills birds, Oriental white-eye (Zosterops orientalis) showed high Haemoproteus infections (62.35%) followed by a long distance migrant Blyth’s reed warbler (Acrocephalus dumetorum) (45%).
The authors found no effect of malaria parasites on body condition; however, the large body mass birds were more infected than birds with small body mass. Large birds’ surface area provides more biting opportunities and emits more carbon dioxide which attracts host-seeking vectors.
Fifteen species representing six groups (genera) were identified from 588 specimens of mosquitoes. Of these, Culex quinquefasciatus was the prevalent species which is a known vector of avian Plasmodium. As expected, the mosquito abundance increased with temperature as well as change in mosquito species composition. However, the infection levels of Plasmodium decreased with increase in temperature. This implied that there is a temperature threshold above which parasite cannot develop into infective stage inside mosquito and Plasmodium transmission declines with increased temperature.
The team sampled birds across a gradient of altitude in breeding season (April-May) to understand the parasite prevalence patterns in migrant and resident birds. Of 119 bird samples representing 50 species from 17 families that were screened for Plasmodium spp. and Haemoproteus spp. infections, only seasonal migrants were marginally more infected than resident birds.
Parasite diversity and cross-species infections
Using molecular methods by amplifying mitochondrial DNA of the parasites, 37 unique DNA sequences of parasite were isolated. Haemoproteus parasite exhibited a high proportion of infections representing 27 lineages whereas Plasmodium had just 10 lineages. Most of the parasite lineages found in the resident foothill birds were locally-transmitted, not recovered in migrant birds and based on their presence in young birds sampled on site as well as the presence of lineages in the birds throughout the year.
This is the first seasonal study in the tropics which captures the absence of seasonal pattern in prevalence of avian Plasmodium despite a strong correlation between mosquito abundance and temperature.
This study provides a basis for furthering our understanding on the ecology and epidemiology of avian malaria and the spread of disease across Himalayan bird communities especially in high altitude, which may not generally be exposed to vector and parasites throughout the year.
Seasonal dynamics in mosquito abundance and temperature do not influence avian malaria prevalence in the Himalayan foothills. Farah Ishtiaq, Christopher G. R. Bowden, Yadvendradev V. Jhala. Ecology and Evolution. Wiley. September 2017
Banner image credit: Dr Farah Ishtiaq. Rufous-gorgeted Flycatcher (Ficedula strophiata) breeds in high-altitude but winters in valley in Dehradun, India