The mosquito-borne Getah virus may have circulated interactively between horses and pigs during a regional outbreak in Japan around three years ago, researchers believe.
“The outbreak among racehorses in 2014 and 2015 was likely preceded by viral amplification in pigs,” Hiroshi Bannai and his colleagues reported in the journal BMC Veterinary Research.
“However, the virus might not be transferred in a one-way manner from pigs to horses; instead, it might circulate interactively between the two species,” they reported, noting that the high levels of virus in the blood required for mosquito transmission has been seen in horses and pigs.
Getah virus is widespread from Eurasia to Australasia. It causes fever, skin eruptions, and limb swelling in horses, while in pigs it can trigger fetal death and reproductive disorders.
Both pigs and horses can play important roles in the amplification and circulation of the virus.
Outbreaks of Getah virus infection in horses occurred in autumn 2014 and 2015 at the Miho training center of the Japan Racing Association after 31 years of no reported outbreaks in the country. There were 33 horses affected in 2014 and 30 in 2015.
The Miho training center is in Ibaraki Prefecture in eastern Japan. Surrounding the center, in Ibaraki and the neighboring Chiba Prefecture, are more than 30 private racehorse farms.
Previous research has shown the virus was not only affecting the training center in 2014 and 2015, but also the private horse farms surrounding the center.
Because the two prefectures are major producers of pigs, the involvement of pigs in Getah virus circulation in the area and their association with outbreaks in horses have been suspected.
The study team checked for virus seropositivity among pigs in the two prefectures around the training center and compared its genomic sequence with those of previous equine isolates.
They found that Getah virus was prevalent among pigs in south Ibaraki and north Chiba in autumn 2014 and 2015, consistent with the outbreak among racehorses. A high seropositivity rate was also found among pigs in 2013, when there was no detectable outbreak in horses.
Most of the infections in pigs seemed to be sub-clinical or not reported.
The strain isolated from a pig in 2015 was closely related to the strains isolated from horses in 2014 and 2015, they reported.
“This finding, together with the serological data, indicates this was the first convincing case that Getah virus circulation among horses and pigs occurred interactively in a specific area.
“On the basis of the theory that pigs are natural hosts of Getah virus, the outbreak among racehorses in 2014 and 2015 was likely preceded by viral amplification in pigs,” the study team suggested.
However, they said the virus might not necessarily have been transferred in a one-way manner from pigs to horses, but may have circulated interactively between them.
They said they could not conclude on the evidence that the outbreaks in 2014 and 2015 were necessarily initiated from infected pigs. It was possible, they wrote, that the virus came from infected horses transferred in from other areas.
That said, the data suggested that pigs were important natural hosts and amplifiers.
“Periodic surveillance of pigs over a larger area would be useful for estimating the risk of epizootics in horses.”
The study team comprised Bannai, Manabu Nemoto, Hidekazu Niwa, Koji Tsujimura, Takashi Yamanaka and Takashi Kondo, all with the Equine Research Institute of the Japan Racing Association; and Satoshi Murakami, from Thermo Fisher Scientific, Life Technologies Japan Ltd.
Geospatial and temporal associations of Getah virus circulation among pigs and horses around the perimeter of outbreaks in Japanese racehorses in 2014 and 2015
Hiroshi Bannai, Manabu Nemoto, Hidekazu Niwa, Satoshi Murakami, Koji Tsujimura, Takashi Yamanaka and Takashi Kondo.
BMC Veterinary Research 2017 13:187 DOI: 10.1186/s12917-017-1112-6