Pennsylvania Led the Nation in 2003 Cases of Equine West Nile Virus :: Pennyslvania Equestrian - News for the Horse owner
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Pennsylvania Led the Nation in 2003
Cases of Equine West Nile Virus

Pennsylvania's horse country has a new and chilling national distinction.

Lancaster and Chester Counties, the state's number one and number two counties in horse population, also had the nation's highest and second highest number of equine West Nile virus cases in 2003. Pennsylvania as a whole led the nation with 527 cases of equine West Nile virus reported in 2003, half of them in Lancaster and Chester Counties. The number of cases statewide exploded more than fivefold from 97 cases in 2002.

Lancaster County had 167 equine cases, Chester 97, and York (which ranks third in state horse population) 41. About a third of horses infected with WNV die or are euthanized.

Most other states saw the number of equine WNV cases drop last year. Texas, which led the nation in 2002 with 1,597 cases, saw its numbers drop to 384 cases in 2003. Illinois, which had 1,107 cases in 2002, had just 26 in 2003.

Nationwide, 14,717 cases of WNV were reported in 2002. That number dropped to 4,636 in 2003.

Renderers Swamped

Jay Smoker, Lancaster County Coordinator for the West Nile Virus Mosquito Control Program, said that so many horses died in Lancaster County last year that for a time, the renderers couldn't keep up. "We had calls about dead horses laying in fields for two, three, even five days before someone could get to them." So many Amish horses were stricken that last fall driving horses were selling at a $600 premium at the New Holland auction, he said. "I could tell you harrowing stories about the emotional and economic loss."

Smoker and other experts said a variety of factors contributed to the loss. The wet weather, the large concentration of horses in the region, and the reluctance of many owners to vaccinate their horses combined to make the region the nation's West Nile encephalitis epicenter in 2003.

Mosquito Borne

West Nile virus is a mosquito borne virus first seen in the US in 1999. Spread solely by mosquitoes which feed on birds, the virus causes encephalitis or inflammation of the brain.

Originally limited to one species of mosquito, it is now found in 20 or more. Though it can be transmitted to humans, it is most deadly to horses.

A vaccine for horses was developed and fully licensed more than a year ago. Protection follows two doses, given three to six weeks apart. The vaccine has been proven safe and 95 percent effective.

High Rainfall

A national map of where equine WNV increased rather than declined last year shows a strip along the east coast from Massachusetts to South Carolina, an area that experienced exceptionally high levels of rainfall last year. (Cases also increased in Montana and New Mexico.)

David Jackson, Chester County Environmental Health Director and WNV Coordinator, said "We had an unusual situation last year in which we went from record drought to record rainfall in 12 months. There was water in more areas than ever before. Mosquitoes were breeding in hoofprints in fields" at one Honey Brook farm they treated. "When mosquitoes are that widespread it's impossible to treat everywhere."

But wet weather alone does not make an epidemic. The crucial link in the chain was that, for a variety of reasons, people did not vaccinate their horses.

Dr. Nancy Winning, who has practiced large animal medicine in southern Lancaster and Chester Counties for 14 years, said none of the 65 WNV cases, both young and old, she saw last year were vaccinated.

"Basically, people thought it was not that serious a threat," Winning said. "They had the information, they just didn't realize the seriousness. One client said 'Somebody should have told us' (after his horse was diagnosed with WNV). The information was there, there were even ads on the radio last year. I don't know what that says about us in this area that it didn't sink in.

"One client lives next to a swamp and I urged him and urged him to vaccinate. He didn't and his horse died. It's not the way they were kept. After a positive diagnosis the state goes in and offers suggestions on how better to control the mosquitoes and most of the time owners were not doing anything wrong other than not vaccinating."

Plain Farmers

"The Plain people did not do a good job of vaccinating their horses," Smoker said. Penn State's Cooperative Extension service added WNV information to the educational programs offered to farmers over the winter of 2002/2003. "It takes time for them to catch onto something new and make the change," Smoker said. "And low milk prices early in the year created cash flow problems and made the $70 to $80 it costs to vaccinate a horse tough for many Amish farmers.

"Just three of the 168 Lancaster County horses who contracted the disease were vaccinated, and those may not have been vaccinated properly or early enough in the year," he said. "The disease ran almost concurrent with the mosquito breeding season, with the first case reported the second week of July."

Smoker and his staff visited about 120 of the owners of horses that contracted West Nile virus. Most of them vaccinated their remaining horses on the spot or shortly after, he said. No further educational programs were conducted this winter. His position is not a year round job, though "there was a concession the previous year to get the word out," he said.

Amish farmers were not alone in foregoing WNV vaccinations. Dr. Carlos Jimenez of Chester County said that of the 15 WNV cases he saw last year, only three were Amish horses. "All of them were age 19 or older, and none of them were vaccinated. We pushed the vaccine last year because of the rain and because the vaccine received a full license from the FDA. Before that, when the license was conditional, we were hesitant about recommending for every horse. We are leery about using every vaccine before all the facts are known. It seems it's a good vaccine. We are also using the new one (a live canarypox vaccine) which also seems to work well."

Rumors and the Internet

Rumors, misinformation and the internet played a role in owners' decisions not to vaccinate. "That goshdarn internet," Winning said. "People heard bad things about the vaccine that turned out to be lies. They chose to believe what they read on the internet instead of checking into it."

Dr. Tim Van Grouw with Smoketown Veterinary Hospital in eastern Lancaster County, whose practice treated about a hundred WNV positive horses last year, concurred. "A lot of people vaccinated in the fall when the disease was around and some of the horses got the virus within ten days of the vaccination. Vaccinating a horse that's been exposed is like getting the virus twice. It appears as a vaccine reaction or a failure but it's not. However, people talk to each other and they read things on the internet, and there's been a lot of misinformation circulating."

In the fall of 2002 and later, stories of horses dying from WNV inoculations flew between mailboxes and in chat rooms. "There were rumors of breakthroughs (transmission of the disease to properly vaccinated horses) among all levels of the horse community," Smoker said.

Last spring, stories about miscarriages and birth defects caused by vaccinating pregnant mares for WNV kept some breeders from vaccinating their horses. A 2003 article in The Denver Post reported mares were aborting their foals and foals were being born with congenital defects due to the West Nile virus vaccine.

The issue was a hot topic at the annual American Association of Equine Practitioners conference earlier this year. One Kentucky researcher commented that none of the supposedly affected foals were examined to find the cause of abortion, and that quotes taken from veterinarians at Fort Dodge Animal Health, manufacturers of the vaccine, were misconstrued by The Denver Post. Dr. John Madigan of the University of California, Davis concluded, "It does not appear that there's a relationship between WNV vaccination and reproductive problems."

Now that more than 13 million doses of West Nile Innovator have now been given, it has been proven as safe and efficacious as any other equine vaccine. The vaccine has been proven 95% effective when administered as labeled. Side effects have been in line with other vaccines.

Smoker said that ignorance of the vaccine and misinformation remains. "We still haven't gotten past that. I hope the word that the vaccine is safe and efficacious gets around."

Equine Vaccines

Fort Dodge now offers equine combination vaccines that combine Fort Dodge's WNV killed virus vaccine, West Nile Innovator, with protection against Eastern, Western, and Venezuelan encephalitis (EEE, WEE, and VEE), and tetanus, in a variety of combinations.

In January Merial introduced Recombitek WNV vaccine, which uses a modified canarypox vector to carry a small amount of WNV DNA into cells.

Don't Wait To Vaccinate!

Equine West Nile encephalitis is a deadly disease—every unvaccinated equine is at risk. WNV-positive mosquitoes have overwintered in Pennsylvania. Vaccines are safe and effective. To protect your horses:

  • Make an appointment NOW to vaccinate against WNV. Two doses administered three to six weeks apart are needed and, according to the label, full immunity is not achieved for a month after the second dose. Mosquito season varies from year to year, so the time to vaccinate is now.
  • Give both inoculations, not less than three or more than six weeks apart. Infection can still occur in horses not vaccinated according to label directions.
  • Horses previously vaccinated for WNV should receive an annual booster. It is not yet clear how long immunity continues.
  • If you have previously vaccinated with WNV Innovator and switch to the Merial canarypox vaccine, two doses are required for immunity.
  • If you switch, be aware that in limited testing, the Merial vaccine has an 88 percent effectiveness rate compared to 95 percent for Fort Dodge's WNV Innovator, which has been used in six million horses.
  • Don't let mosquitoes breed on your property. Dispose of any water-holding containers—discarded tires, wading pools, wheelbarrows, birdbaths. Mosquitoes can breed in any puddle of water standing for more than four days. People get WNV too, and the disease is expected to spread further to other species of mammals for which no vaccine exists.