Abstract
BackgroundAs we reported previously, Getah virus infection occurred in horses at the Miho training center of the Japan Racing Association in 2014. This was the first outbreak after a 31-year absence in Japan. Here, we report a recurrent outbreak of Getah virus infection in 2015, sequential to the 2014 one at the same site, and we summarize its epizootiological aspects to estimate the risk of further outbreaks in upcoming years.ResultsThe outbreak occurred from mid-August to late October 2015, affecting 30 racehorses with a prevalence of 1.5 % of the whole population (1992 horses). Twenty-seven (90.0 %) of the 30 affected horses were 2-year-olds, and the prevalence in 2-year-olds (27/613 [4.4 %]) was significantly higher than that in horses aged 3 years or older (3/1379 [0.2 %], P < 0.01). Therefore, the horses newly introduced from other areas at this age were susceptible, whereas most horses aged 3 years or older, which had experienced the previous outbreak in 2014, were resistant. Among the 2-year-olds, the prevalence in horses that had been vaccinated once (10/45 [22.2 %]) was significantly higher than that in horses vaccinated twice or more (17/568 [3.0 %], P < 0.01). Horse anti-sera raised against an isolate in 2014 neutralized both the homologous strain and a 2015 isolate at almost the same titers (256 to 512), suggesting that these viruses were antigenically similar. Among horses entering the training center from private surrounding farms in 2015, the seropositivity rate to Getah virus increased gradually (11.8 % in August, 21.7 % in September, and 34.9 % in October). Thus, increased virus exposure due to the regional epizootic probably allowed the virus to spread in the center, similarly to the outbreak in 2014.ConclusionsThe 2015 outbreak was caused by a virus which was antigenically close to the 2014 isolate, affecting mostly 2-year-old susceptible horses under epizootiological circumstances similar to those in 2014. The existence of 2-year-olds introduced from regions free from Getah virus could continue to pose a potential risk of additional outbreaks in upcoming years. Vaccination on private farms and breeding farms would help to minimize the risk of outbreaks.
Highlights
As we reported previously, Getah virus infection occurred in horses at the Miho training center of the Japan Racing Association in 2014
We previously reported that an outbreak of Getah virus infection occurred in racehorses at the Miho training center of the Japan Racing Association in autumn 2014, affecting 33 horses [4]
As in the 2014 outbreak, in 2015 our results indicated that there was a high prevalence of Getah virus infection among horses on the private farms (Fig. 2)
Summary
Getah virus infection occurred in horses at the Miho training center of the Japan Racing Association in 2014 This was the first outbreak after a 31-year absence in Japan. Getah virus is classified in the genus Alphavirus in the family Togaviridae [1] It is mosquito borne and is widespread from Eurasia to Australasia. We previously reported that an outbreak of Getah virus infection occurred in racehorses at the Miho training center of the Japan Racing Association in autumn 2014, affecting 33 horses [4]. It was the first reported outbreak of infection with this virus among vaccinated horse populations worldwide, and the first one in Japan since 1983 [5]. The direct cause of the outbreak was still unclear, and the epizootic pattern of this re-emerging virus in upcoming years was unpredictable
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