Israeli study charts rise of West Nile Virus among horses

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The spatial distribution of West Nile Virus seroprevalence in horses in Israel during 1997, 2002 and 2013. Circles inside the dashed orange marking represent farms located in the Great Rift Valley area. Image: PLoS ONE
The spatial distribution of West Nile Virus seroprevalence in horses in Israel during 1997, 2002 and 2013. Circles inside the dashed orange marking represent farms located in the Great Rift Valley area. Image: PLoS ONE

Researchers have cast a spotlight on the rise of West Nile Virus, showing growing levels of exposure to the virus among horses in Israel over a 16-year period.

Blood samples taken from 179 horses last year revealed that 85.5 percent had been exposed to the virus.

By comparison, 1997 testing indicated that 39 percent of 290 horses tested had been exposed, while in 2002 the level of exposure among 827 tested horses was found to be 66.1 per cent.

They noted that blood testing of horses in Israel during previous research in 1959–1960 showed a 35 percent seroprevalence to the virus.

The evidence, they said, pointed to a “true surge” in virus occurrence among horses in the region.

Amir Steinman, from the Hebrew University of Jerusalem, set out with his colleagues to undertake a thorough serological investigation of West Nile Virus in horses in Israel, which is considered a longstanding endemic region for the virus.

Their analysis straddled a massive West Nile fever outbreak in humans and horses in the region in 2000.

Electron microscopy of West Nile virus.
Electron microscopy of West Nile virus.

The researchers, whose findings have been published in the open-access journal, PLoS ONE, found that risk factors included the age and breed of the horse.

Significantly lower spring rain was observed during years with increased human rates of West Nile that occurred between 1997 and 2007, they noted.

West Nile virus was first isolated from the blood of a sick woman in Uganda in 1937. It has since become recognized as one of the most widely distributed flaviviruses in humans, as well as in horses.

Birds carry the virus. Mosquitoes that bite infected birds are then capable of infecting horses and humans.

West Nile virus infection was first reported in the Mediterranean basin, in both Egypt and Israel, in the early 1950s, and cases with severe neurological signs in humans were reported for the first time in 1957 in Israel.

The dramatic appearance of the virus in the New York City area in 1999 and its subsequent spread across North America significantly raised the amount of attention the virus received.

In the decade following the discovery of the virus in New York, 37,008 cases of West Nile fever and 16,196 cases of neuroinvasive West Nile disease were reported in humans in the US.

The incidence of West Nile in horses in the US is even higher – about 700 per 100,000 animals – and the disease seems to be more severe.

More than 25,000 horses in the US have been affected since 1999, with a third of cases proving fatal. In humans, 4 to 9 percent of cases prove fatal.

“West Nile virus imposes a great threat on human and animal health not only by emerging in new geographical areas but also as it causes epidemics in endemic areas,” Steinman and his colleagues reported.

“Currently, West Nile Virus appears to be expanding its geographical range in Europe and causing increasing numbers of outbreaks associated with human morbidity and mortality.”

The virus strain has been spreading within Hungary and to the eastern part of Austria, and thereafter through the Balkan states to Greece. In the last three years, large human epidemics have been detected in Italy, Hungry, the Russian Federation and the Ukraine.

“Israel serves as a crossroad for bird migration between Africa and Euroasia, and therefore a major focus of attention during the global spread of West Nile fever,” the researchers wrote.

Israel, they said, has been considered endemic to West Nile Virus since the first recognized human outbreak in 1951.

Thereafter, small, intermittent outbreaks were reported in 1952, 1953, 1957 and 1980. However, in 2000, Israel experienced its largest recorded outbreak affecting hundreds of people, dozens of horses and several flocks of geese.

During the following decade, between 2000 and 2012, nearly 1400 cases in humans were reported.

Identified cases were fewer between 2001 and 2004 but greatly increased in 2005 and have stayed consistently high since then.

The researchers said ambient temperature was constantly shown to play an important role in WNV transmission by affecting the growth rates of insect vector populations, the interval between blood meals, and viral replication rates.

During the outbreak in Israel in 2000, the minimum temperature was found to be the most important climatic factor that encouraged earlier appearance of disease.

“The contribution of precipitation to the transmission and spread of WNV is more complex and controversial,” they said.

“Heavy rainfall during spring may increase standing water resources at the beginning of the hot season and therefore increase the vectors natural habitats.

“In contrast, during drought conditions, the reduced water flow creates stagnant water pools which become richer in the organic material ideal for breeding mosquitoes.

“This may encourage birds to circulate around small water holes and thus increase the interactions with mosquitoes.

“These discrepancies regarding the role of precipitation in WNV transmission raised the assumption that the patterns of disease incidence may change over large geographic regions, depending on differences in the ecology of mosquito vectors.”

Numerous studies had described the association of drought conditions with West Nile Virus human infection in the US, they said.

The researchers found lower rates of exposure to the virus among ponies and warmblood horses, as compared to other breeds.

The reason, they said, was unknown, but might indicate different preference of mosquitoes to different breeds of horses, due to variables such as density of hair and differences in sweat composition.

“The higher seroprevalence detected in the quarter horses in our study is in accordance with a study performed in Florida, in which quarter horses were the most commonly affected breed, and with a study performed in Saskatchewan, Canada, which indicated that most of the West Nile Virus clinical cases were light horse breeds and most were used for pleasure riding.”

However, as opposed to the above study, in which dark-coloured horses were most commonly affected with the virus, analysis in the current study showed higher seroprevalence in the lighter colours (grey/light appaloosa; 70.4 percent) as compared to the darker colours (black/dark brown/dark grey; 52.9 percent).

In conclusion, the researcher said: “Demographic variables such as age and breed were found to significantly associate with seroprevalence to West Nile Virus.

“But more prominently, environmental factors (geographical location and precipitation amounts), and more specifically the geographical location of the horse along the Great Rift Valley, the main route of birds migration in Israel, was found to be of great impact on the likelihood of WNV exposure throughout the years investigated.

“Interestingly, weather conditions, and more specifically rainfall conditions, such as periods of spring drought, together with lower incidence rate in the population, were shown to precede transition from endemic state to epidemic state of West Nile Virus observed in humans in Israel during the years 2000 and 2005.”

Steinman was joined in the research by Karin Aharonson-Raz, Anat Lichter-Peled, Shlomit Tal, Boris Gelman, Daniel Cohen and Eyal Klement.

Aharonson-Raz K, Lichter-Peled A, Tal S, Gelman B, Cohen D, et al. (2014) Spatial and Temporal Distribution of West Nile Virus in Horses in Israel (1997–2013) – from Endemic to Epidemics. PLoS ONE 9(11): e113149. doi:10.1371/journal.pone.0113149
The full study can be read here.

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