West Nile virus infection of Thoroughbred horses in South Africa (2000-2001).
Authors: Guthrie A J, Howell P G, Gardner I A, Swanepoel R E, Nurton J P, Harper C K, Pardini A, Groenewald D, Visage C W, Hedges J F, Balasuriya U B, Cornel A J, MacLachlan N J
Journal: Equine veterinary journal
Summary
# West Nile Virus Infection in South African Thoroughbreds: A Benign Endemic Pattern West Nile virus circulates widely amongst South African horses, yet causes no apparent clinical disease — a finding that contrasts sharply with the neurological complications reported from Europe, North America and Asia. Researchers tracked 488 Thoroughbred yearlings over a 12-month period following the 2001 National Yearling Sales, with paired serology samples tested by serum neutralisation assays; maternal serum samples from 243 dams were also analysed to assess population exposure levels. Approximately 11% of yearlings seroconverted to WNV during the study interval, whilst 75% of their dams showed prior exposure, indicating endemic circulation across geographically dispersed studfarms throughout the country. Experimental inoculation of two horses with a recent South African isolate (lineage 2) produced neither clinical disease nor viraemia, supporting field observations that endemic southern African strains differ substantially in their pathogenic properties. For equine practitioners, these findings suggest that WNV seropositivity alone should not trigger assumptions of neurological risk, and that geographical strain variation may be crucial to understanding the virus's clinical manifestations — an important distinction as WNV surveillance expands globally.
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Practical Takeaways
- •South African horse practitioners should recognize that WNV exposure is common but benign in local populations, contrasting with the neurological disease risk in other geographic regions
- •Young horses showing WNV seropositivity without clinical signs may simply reflect endemic exposure rather than active disease requiring treatment
- •Awareness of geographic variation in WNV pathogenicity is important when considering disease risk and diagnostic interpretation in international horse movements
Key Findings
- •Approximately 11% of South African Thoroughbred yearlings seroconverted to WNV over a 12-month period
- •75% of dams showed WNV seropositivity, indicating high prevalence of exposure in breeding populations
- •No neurological disease was observed in any naturally infected horses or in 2 horses experimentally inoculated with a South African WNV isolate
- •Endemic southern African WNV strains do not appear to cause neurological disease in horses, in contrast to strains from Europe, North America, and Asia