Molecular epidemiological studies of Leucocytozoon caulleryi in commercial layer flocks in Southern peninsular India reveal the presence of new subclusters.

The Leucocytozoon parasites are one of the important protozoa species affecting domestic poultry and wild birds. Though there are frequent reports about the incidence of Leucocytozoon in wild birds, the information regarding the occurrence of commercial poultry is underestimated. From October 2015 to August 2022, ailing and dead birds from 39 commercial layer flocks were submitted to the Poultry Disease Diagnosis and Surveillance Laboratory, Namakkal, with a history of brief illness, followed by mortality with oozing of blood from oro-nasal orifice. The presence of Leucocytozoon caulleryi was confirmed in all the flocks by laboratory examination and PCR. All the Leucocytozoon-positive cases were reported after heavy rainfall and the affected farms were inhabited with Culicoides flies. The infected birds were dull and depressed, exhibited labored breathing, and blood-tinted ropy mucous from the oro-nasal cavity. Necropsy examination revealed pale-anaemic comb and wattles, massive haemorrhagic clot over the surface of the kidney, peritoneum, and trachea, and scattering of greyish-white/reddish military megaloschizonts throughout the serosa of viscera and skeletal muscles. Histopathological examination revealed many spherical cysts or megaloschizonts in the intestine, pancreas, gizzard, spleen, liver, kidney, heart, and breast muscles. The cysts were deeply seated within the above organ's parenchyma or embedded in the serosa. There was extensive connective tissue proliferation with mononuclear cell infiltration around the cyst. Leucocytozoon-specific DNA was detected in all the flocks by PCR. Phylogenetic analysis of the cytochrome b gene of L. caulleryi reported from various parts of the world revealed that L. caulleryi is grouped into two major clusters (Cluster I & II). Further, Indian L. caulleryi sequences fall under sub-cluster I and II along with L. caulleryi reported from Taiwan, Egypt, China, Myanmar, Japan, South Korea, and Thailand.