Abstract: Background- Pathogenic Flaviviridae affecting Australian horses include Murray Valley encephalitis, Japanese encephalitis and Kunjin strain of West Nile viruses. Hypothesis/Objective - Maternally-derived Flavivirus antibodies in foals will be present at 24 hours of age but will have declined by 4 months of age. Most foals will be naturally exposed to Flaviviruses within their first year of life. Animals- 23 mare/ foal pairs from a teaching herd. Methods- A Pan-Flavivirus blocking ELISA was performed on serum obtained from: (a) mares 2 months prior to foaling and when their foals were 24 hours and 16 weeks of age; (b) foals at birth (pre-suckle), and then at 24 hours, 1, 2 and 3 weeks, then 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13 months of age. The lab defined > 40% inhibition as positive. The percentage of mares and foals with positive Flavivirus results at each time point were calculated. Results- 95.6 % of mares had positive Pan-Flavivirus ELISAs. All foals were negative on pre-suckle samples, while 18/23 (78%) were positive for Flavivirus maternal antibodies at 24 hours of age. Maternal antibody persisted in 9/23 (40%) foals at 4 months of age. Natural infection occurred in 7/23 (30.4%) foals with positive results occurring as a second peak after decline of maternal antibody. Conclusions and clinical importance- In endemic areas, persistence of maternal antibody may confuse the diagnosis of Flavivirus infection in foals. This study occurred during drought conditions resulting in reduced infection rates in foals.
