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7-26-2002

Some further information for anyone interested: Thus far in 2002, 59 horses have been reported infected (thru 23 July 02). These are primarily inFlorida (15), Texas (20) and Louisiana (16) with scatterings in otherstates. If you look at infected bird distributions as an overlay, the horsecases are a subset of that distribution clearly relating bird viremia andhorse disease.  The number of case positives was a low percent of totalhorses in the affected area with hundreds of thousands of horses in Florida,Texas, Louisiana, Kentucky, Mississippi, North Dakota and Tennessee (theother states with equine cases). However, the disease is much more prevalentthis year than last as we see in birds. The "new" states reporting WNVactivity for 2002 are MN, TX, NE, OK, ND and WV. While the horse so far isthe most sensitive mammalian host, several other wild mammals have been shown positive for WNV. The most recently reported is black bears.

The total known clinical cases of equine WNV reported by USDA during 2001 was 738, with cases concentrated Florida, Georgia, New Jersey and Long Island, NY, with lesser numbers in New England, southern Georgia, Alabama, Louisiana. The outcome was reported on only 470 of these, with mortality or euthanasia of 156 (33.2%). The figures were 36% (9/25) in 1999 and 38%(23/60) in 2000. So we can fairly say that about a third of clinically affected horses either die or are euthanatized. Remember, though,performance horses are common in Florida and elsewhere in the south. A horse that cannot live up to its potential, particularly if insured, is more likely to be euthanatized for reasons other than moribund condition.

Therefore these figures somewhat (to what degree I cannot say) overestimate the mortality that might be expected from WNV in feral horses. In an epidemiological review of New England cases in 2000, horse disease was mildly correlated with distance to migratory waterfowl and blackbird roosts. However, the number tested (about 1800 horses) suggested a low attack rate for WNV in horses. Once clinical disease occurred, however, mortality was similar to that reported by others (about one third).

Unlike Eastern or Western Equine Encephalomyelitis, horses that survive the first 2-3 weeks of WNV infection are likely to make full recoveries. In experimental infections conducted by CDC (Bunning et al 2002), 12 horses were inoculated with NY99 from a horse brain and crow brain isolates. The tests were run because previous reports in Europe and Africa suggested that most equine infections were either mild or inapparent with only occasional severe disease. But during 1999 there was a focus on Long Island where severe disease was observed (the numbers suggest about 8 of 83 in at the site, 36 which were seropositive, died). The CDC experiments used mosquito transmission to allow any extrinsic incubation to run its course. Only 1 horse of the 12 became clinically ill with WNV although all 12 were demonstrably infected based on viral neutralizing antibody. Transmission studies to mosquitoes failed. They concluded horses infected with NY99 developed low viremias of short duration and were not important amplifying hosts. Their results were consistent with overseas studies that find less than 10% clinical attack rate (the rate at which clinical disease occurs in known infected animals). Several other similar studies reported similar findings on viremias but are unreliable because of low sample size (generally 4-7 horses per study).

The data are pretty consistent that a low number of exposed horses get infected, a low number of infected horses develop clinical disease, but a fair number of clinically ill horses either die or are euthanatized (about one third). EEE and WEE are considerably more threatening to feral horses than WNV.

On the vaccine, August 2002 is suppose to be the end date for trials to demonstrate efficacy of the vaccine. I haven't seen any efficacy data yet. It is a killed vaccine so safety is not likely to be an issue. Adverse reactions reported so far are about 7 per 100,000 doses with about half of those likely unrelated to WNV vaccine.