Abstract:
Although OldWorld alphaviruses, Middelburg- (MIDV) and Sindbis virus (SINV), have
previously been detected in horses and wildlife with neurologic disease in South Africa, the pathogenesis
and clinical presentation of MIDV and SINV infections in animals are not well documented.
Clinical samples from horses across South Africa with acute or fatal neurologic and febrile infections
submitted between 2014–2018 were investigated. In total, 69/1084 (6.36%) and 11/1084 (1.01%)
horses tested positive for MIDV and SINV, respectively, by real-time reverse transcription (RT) PCR.
Main signs/outcomes for MIDV (n = 69): 73.91% neurological, 75.36% fever, 28.99% icterus and
anorexia, respectively, 8.70% fatalities; SINV (n = 11): 54.54% neurological, 72.73% fever, 36.36%
anorexia and 18.18% fatalities. MIDV cases peaked in the late summer/autumn across most South
African provinces while SINV cases did not show a clear seasonality and were detected in fewer South
African provinces. MIDV could still be detected in blood samples via RT-PCR for up to 71,417 and
21 days after onset of signs in 4 horses respectively, suggesting prolonged replication relative to SINV
which could only be detected in the initial sample. Phylogenetic analyses based on partial sequences
of the nsP4 (MIDV n = 59 and SINV n = 7) and E1 (MIDV n = 45) genes, as well as full genome
sequences (MIDV n = 6), clustered the MIDV and SINV strains from the present study with previously
detected strains. MIDV infection appears to be more prevalent in horses than SINV infection based on
RT-PCR results, however, prevalence estimates might be different when also considering serological
surveillance data.
