Fatal encephalomyelitis outbreak among dairy cows caused by rabies virus phylogroup-1 with co-occurrence of Chlamydia abortus.

Rabies is a fatal viral zoonotic disease caused by a neurotropic Rabies virus (Lyssavirus rabies) of the genus Lyssavirus of the family Rhabdoviridae. Lyssavirus rabies primarily affects the central nervous system, while, Chlamydia spp. are intracellular bacterial pathogens and occasionally infect nervous system in livestock. In this reported sporadic outbreak, seven cows in an organised dairy farm manifested acute nervous symptoms, hypersalivation, muscle tremors and death along with encephalomyelitis as the predominant post-mortem lesion. Considering the nervous signs in the absence of visible bite marks, diseased animals were initially screened for rabies, bovine herpesvirus type 5 (BoHV-5), listeriosis, chlamydiosis and plant toxicities. Brain samples of all cows were found positive for rabies virus using N gene RT-PCR. The brain samples of morbid animals also showed typical necropsy and histopathological changes indicative of rabies. The virus was further confirmed by nucleotide sequencing of the partial N gene. The phylogenetic analysis indicated involvement of the Lyssavirus rabies strain belonging to genotype I of phylogroup 1, like other Indian rabies viruses, and closely related to the strains detected from the dogs from north India. Interestingly, Chlamydia abortus was also detected by PCR in the brain tissue and pericardial fluid of two animals. However, no specific histopathological lesions attributable to chlamydial infection were observed in brains tissues. The pathological significance of this co-detection remains uncertain. This study is the first report of the co-occurrence of rabies and chlamydiosis in cattle, with unknown clinical significance, and highlights the importance of rabies surveillance in livestock and canine population.