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Extreme Weather Conditions Spark Potomac Horse Fever Epidemic Throughout U.S.

Horse Health - Press Release

Heavy rains, standing water and flooding have led to increasing levels of Neoriketssia risticii (previously known as Ehrlichia risticii) cases - more commonly known as Potomac Horse Fever - across the United States. Outbreaks have been seen throughout the East Coast and extremely wet weather has made conditions perfect for the disease in the Midwest as well, affecting states such as Oklahoma. Equine media outlets are currently tracking a severe outbreak in New York with fatalities already recorded.

Vaccination against Potomac Horse Fever (PHF) - including a fall booster shot in this year's epidemic - is a critical component in the fight to save horses from the deadly disease. PHF-Gard(TM), from Pfizer Animal Health, has been shown in scientific studies to be 80% effective in preventing infection by Neoriketssia risticii bacteria. Horses that have been vaccinated may become infected, but they may also experience a less severe form of PHF, which increases their chances for survival.

"I have no doubt that vaccination and a booster shot is absolutely needed to save horses' lives," said Robert Holland, DVM, PhD, Senior Equine Technical Services Veterinarian at Pfizer Animal Health. "Horse owners and veterinarians are concerned about potentially fatal diseases like West Nile Virus, but the truth is that as many, or even more horses in certain regions, may contract, or even be killed, by Potomac Horse Fever this year."

Neoriketssia risticii is carried by fresh water snails, which can transmit the disease to 17 different species of aquatic insects. The horse then acquires the disease by ingesting these bugs, which can spread out onto pastures and throughout water supplies. The disease often requires nine to twelve days of incubation before symptoms appear.

"Owners need to understand that this is a very serious disease," said Dr. Holland. "Horses are dying because the symptoms of infection aren't easily recognized and the vaccine isn't being used widely enough. There also seems to be the impression, perhaps because of the name Potomac Horse Fever, that the disease is restricted to a particular area of the East Coast. This is simply not true. Neoriketssia risticii is an extremely widespread organism and it's seen virtually everywhere in this country."

Potomac Horse Fever is extremely difficult to diagnose as its clinical symptoms mimic many other equine bacterial infections. Depression and a transient high fever are among the first signs of infection. After seven to ten days, owners may notice a loss of appetite, colic, toxemia, diarrhea and even life-threatening laminitis.

In many cases, the animal's stool is loose and profuse and dehydration may result. Edema of the legs, abdomen and head indicate poor circulation caused by the disease, as well as a protein imbalance. Mares infected with Potomac Horse Fever may abort pregnancies late in gestation.

PHF-Gard is supplied to veterinarians in convenient ten-dose vials. Each dose is in a small, one-milliliter amount and is administered via an intramuscular injection to healthy horses one year of age and older. The vaccine uses an established antigen purification system to produce an immunogenic response. Annual revaccination is recommended. The AAEP Vaccination Guidelines suggest that horses in endemic areas be vaccinated more frequently due to increased risk of exposure.

For more information on Pfizer Animal Health's complete line of equine health care products, visit

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