West Nile Roundup

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Many optimists and experts firmly believed the harsh New York winter would kill the West Nile Virus (WNV) that popped up in late summer 1999 and that would be the end of it. As we now know, those people had a rude awakening early in 2000.

With federal funding, Northeastern states geared up with WNV-surveillance efforts in early spring. At the same time, local health authorities tended to downplay the threat. In March, Dr. Clifford Johnson, a Maryland Department of Health veterinarian said, “I don’t think we should be overly concerned. It should be treated like any other disease that can be passed by mosquitoes.” He further stated the potential threat was “low.”

In late April, Dr. Stephen Ostroff of the Centers for Disease Control in Atlanta stated, “This virus is here, and we have to understand that it is not feasible to eliminate it from the Western Hemisphere.”

Not long after that, the first report of virus isolation from dead birds appeared. Two birds collected from Rockland County, N.Y., tested positive on June 9, early for a mosquito-transmitted infection to appear in the Northeast. The same day, word came that a dead crow found on May 30 in Bergen County, N.J., was also positive. Dead-bird counts and positive mosquito pools continued to mount steadily from there.

Six cases of West Nile in humans were announced before the first positive equine, which came August 29 in a 26-year-old horse from Staten Island. The next equine case was a shocker. Not only was it found in a young (two-year-old) horse, but it was the first evidence of WNV found in Rhode Island — without any previous positive bird or mosquito samples. But that then happened again in areas in New Jersey, Pennsylvania and Delaware.

Sadly, we believe this makes horses better sentinels of virus activity than the flocks of chickens most states and Canada were relying upon to detect virus activity (the chickens were set out and monitored for the virus). Only six sentinel chickens tested positive during the outbreak, all in areas already with WNV confirmed in wild birds. By mid-October, there were 29 equine and 18 human cases. However, by the end of November, the number had jumped to 64 horses. Of these, 22 (42%) died or were euthanized, similar to last year’s percentages and those reported worldwide.

International Scene
The U.S. horse population was not the only one dealing with West Nile in 2000. The Camargue region of France is an idyllic Mediterranean locale, the delta of the Rhone River, largely untouched by time. It is veined with waterways and brackish marshes where their famous black bulls and white horses of the Camargue are free to roam, sometimes up to their knees in the water. As picturesque as it may be, there is also a dark side. The marshes are also ideal mosquito breeding grounds.

Europe is no stranger to West Nile outbreaks, although they occur sporadically, tending not to recur in any predictable or sequential pattern. When it does hit, it is believed that migrating infected birds, coming from endemic regions of Africa, introduce the virus. This year it hit the horses of Camargue hard.

Between September 8 and October 15, 69 horses within a 15-kilometer radius were confirmed infected. Of those, 17 died. Authorities reacted quickly with larvicide applied to the area twice in September. An extensive serological survey was also done, with over 3,500 blood samples drawn in an effort to determine the extent of exposure and geographical area. The quick action taken by French authorities stopped the disease in its tracks.

It’s Not Just Humans, Horses And Birds
Last year, we were told West Nile was only a problem for birds and the occasional horse or human. Data from New York’s testing program, the most extensive in the country, proves this is not the case. A total of 25 mammals have tested positive for WNV, including bats, squirrels, cats, rabbits, raccoons and a chipmunk. The significance of this to the life cycle of the virus is unknown at this time, although at least one mammal, the lemur, is strongly suspected of being a reservoir for the virus.

West Nile Virus is here and here to stay. In just its second season, we feel it has clearly assumed the status of the most common type of encephalitis and potentially life-threatening infectious disease facing our horses.

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