Equine West Nile Virus: An International Perspective

Most American owners are aware of the increased number of West Nile virus (WNV) cases confirmed in the U.S. horse population this year. What they might not know is how the virus affects horses in other countries.

Australia and New Zealand

According to C.J. (Kate) Savage, BVSc (Hons), MS, PhD, Dipl. ACVIM, WEVA Oceanic delegate, a similar virus--Kunjin--is endemic in parts of Australia.

"(Kunjin) is antigenically and genetically similar to WNV and was reclassified as a subtype of WNV in 1999 (WNV/Kunjin)," she said.

Parts of Australia suffered a large equine WNV/Kunjin outbreak in 2011, she said, after which Frost et al. identified a new WNV strain as the cause.

"Results showed that most of the cases were caused by a new strain of WNV, which has been termed WNV(NSW2011)," Savage explained. "This is most closely related to WNV/Kunjin. However, the new strain appears to invade nervous tissue to a greater degree than the original Kunjin virus."

Neighbor New Zealand (NZ) remains WNV-free to date; however, risk of the disease spreading to the island nation is a concern. "In 2004, New Zealand scientists looked at the possibility of WNV getting into NZ and becoming endemic," Savage explained. "It was thought that vectors and hosts were present, so great awareness was necessary."

Italy

Sandro Barbacini, DVM, Italian WEVA board member, said that Italy experienced WNV outbreaks several years ago.

"The outbreaks were in Northern Italy staring from the Ferrara area, very close to the delta of the Po River--a very flat humid area with a lot of flies and mosquitoes," Barbacini explained. "(The disease) then moved west along the river."

Although Italy has not seen any outbreaks in recent years, Barbacini noted that most horses residing within 40-50 kilometers (roughly 24-31 miles) of the river have been vaccinated.

Switzerland

Swiss WEVA board member Vincent Gerber, PhD, DVM, Dipl. ACVIM, ECEIM, FVH, said that Switzerland remains WNV-free to date.

"Voluntary vaccination is permitted, but at present is only recommended for horses travelling to endemic areas," he noted.

Thailand

Siraya Chunekamrai, DVM, PhD, WEVA's Thai board member, relayed that a recent study of native ponies (performed in collaboration with the Maxwell H. Gluck Equine Research Center at the University of Kentucky) found positive WNV titers despite an absence of clinical signs.

Chunekamrai said that horses are not currently vaccinated against the disease in Thailand.

India

Finally, India-based Ravi Reddy, BVSc, MRCVS, WEVA president, described "an outbreak of strange neurological cases over the last six to seven months. The 16 affected horses were all 2-year-olds born at one farm in Haryana, located in North India.

"Most of these horses left the farm in North India in February 2012," Reddy said. "The horses have died at the premises of the Pune Race Course, Bangalore Turf Club, Mysore Race Club, and four other farms in the Bangalore area.

"All the horses start with very mild symptoms: slightly off color, slight incoordination," he continued. "They rapidly deteriorate showing torticollis (neck twisting), blindness, seizures, and death. Most prove fatal over a course of 48 hours. Three have lingered from a fortnight to a month. At present one case is still alive in Mysore with partial blindness."

Reddy said blood samples from horses previously residing at the Haryana farm were sent to Dubai for testing.

"Strangely 40 of the 41 2-year-olds sampled from this stud farm now stabled in the Pune area showed a seropositive West Nile titer," he said. "We believe this is an incidental finding and not the cause of this present disease," as other factors have been found in the disease investigation.

West Nile has been identified sporadically in humans; however it has not yet been reported in Indian horses, Reddy said. Horses are not currently vaccinated against the disease.

Take-Home Message

Although it's not prevalent in all parts of the world, veterinarians around the globe are keeping an eye on WNV and taking appropriate precautions when necessary.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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