Sporadic bovine encephalopathy caused by Chlamydia pecorum secondary to bovine viral diarrhoea virus infection in calves in South Australia

Background Despite bovine viral diarrhoea virus and Chlamydia pecorum being important endemic diseases of cattle, there are limited reports of theirco-occurrence.Case report Several 12-18-week-old, weaned Hereford calves presented with ill-thriftiness and neurological signs on a mixed cattle and sheep farm in South Australia in July 2021. Immune suppression resulting from transient infection with bovine viral diarrhoea virus (BVDV) is implicated in predisposing to infection with Chlamydia pecorum, the causative agent of sporadic bovine encephalopathy (SBE). Chlamydia spp. are difficult to culture in vitro or definitively identify based on current standard molecular based tests. In this case, diagnosis was confirmed by immunohistochemistry.Conclusion To the authors' knowledge, this case report is the first to document BVDV transient infection occurring in conjunction with SBE. Given the current high prevalence of BVDV on Australian farms, such co-infections may have significant future clinical relevance. This case also highlights the need for appropriate tests, such as immunohistochemistry to demonstrate the causative organism in histological lesions and thus reduce the occurrence of false negative diagnosis.