Equine coronavirus: Investigating seroconversion and faecal shedding patterns in naturally occurring field infections

Summary Background Equine coronavirus (ECoV) is an increasingly recognised cause of pyrexia and enteric disease in the horse. Objectives To (1) evaluate if horses seroconvert after clinical ECoV infection and (2) investigate the pattern of faecal excretion of ECoV to determine the optimal testing moment with the overall aim of helping veterinarians in the field diagnose ECoV infections. Study design Observational study of naturally occurring ECoV infection in 25 horses. Horses were included if they had compatible clinical signs and a positive faecal polymerase chain reaction (PCR) sample or had clinical signs and at least one other horse on the premises had a positive faecal PCR. Methods A dry format ELISA was used to test seroconversion on paired serum samples. The percentage of horses that seroconverted or showed a significant increase in sample to positive ratio (S/P) was calculated. PCR for ECoV was performed on faecal swabs submitted on Days (D) D1, D3, D7, D10, D13 and D16 of clinical signs. The percentage of positive PCR results on each day was calculated to determine the best sampling moment. Results In this study, 52% (13/25) of horses seroconverted or had a significant increase in S/P units. Between days 1 and 4 of clinical signs, horses were most likely to test positive for ECoV on PCR. Faecal shedding of ECoV can be intermittent and 4/23 (17% CI 95% 19%–33%) horses were shedding ECoV for longer than 14 days. Main limitations A delay in sampling at the start of clinical signs in some horses means positive PCRs and seroconversion could have been missed. Conclusions Faecal PCR samples early in the disease course are most useful for detection of ECoV and submitting samples from multiple days may aid in the detection of ECoV. Serological testing can be used as an adjunct test to faecal PCR testing to aid in the diagnosis of ECoV.