Intervet-Schering Plough Animal Health has recalled all one-dose and five-dose vials of PreveNile, and while company characterizes the move as “urgent,” specifics on what warrants that distinction remain unclear.

Veterinarians are being asked to check their inventory of the equine vaccine immediately, and contact distributors to return the product for credit.

In an April 28 letter to veterinarians, the company states: “This recall is being conducted because Intervet has identified increased incidence of adverse events related to currently marketed serials.”

For PreveNile One-Dose, those series include:

91669001
91669002
91669005
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91669016
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Serials for PreveNile Five-Dose include:

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91669010
91669011
91660003
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Intervet’s statement does not indicate numbers or types of adverse event reports tied to PreveNile or the incidents’ severity. Company representatives could not immediately be reached. Officials with the U.S. Department of Agriculture’s Center for Veterinary Biologics, which regulates vaccines, did not immediately respond to a VIN News Service query. The agency had not addressed the PreveNile recall on its website on Tuesday.

While there are other West Nile virus vaccines for equines on the market, PreveNile is the only vaccine that requires one dose for primary immunization. Released in 2006, PreveNile contains a lyophilized yellow fever chimera virus vaccine, with the human vaccine yellow fever virus used as a backbone. In its marketing materials, Intervet notes that it used chimera technology to create PreveNile, which means that two similar viral structures were combined to create one. The benefits of the technology include safety, Intervet reports.

In December, Intervet-Schering Plough Animal Health presented a study at the American Association of Equine Practitioners’ annual convention that evaluated how the vaccine stimulates a pregnant mare’s immune system as well as research showcasing the drug’s efficacy.

Three months later, a discussion surfaced on the Veterinary Information Network (VIN) that has since included several references linking PreveNile injections to inflammation in the necks of some horses, as well as fevers and depression, though others suggest that such responses are typical of vaccines in general.

Most recently, a handful of Internet reports have alluded to severe cases of anaphylactic reactions in horses in Oregon and the surrounding region. Following that lead, the VIN News Service contacted Dr. Donald Hansen, Oregon state veterinarian, who stated that he is unaware of any such cases. A representative with the Oregon Veterinary Medical Association echoed that statement, and officials with Oregon State University’s College of Veterinary Medicine did not immediately respond to a VIN News Service request for information.

Several VIN consultants have indicated a lack of knowledge concerning what prompted the recall. At least one VIN member has expressed disappointment that the drug is under recall.

“I’ve never had a problem with it, and I used 500 doses of PreveNile last year. It’s one of the least reactive vaccines that I’ve used,” says Dr. Jean Feldman, a practitioner in Hamburg, N.Y. "Now I have to return the 300 doses that I was shipped the day before the recall."

VIN consultant and equine practitioner Dr. Benjamin Buchanan says that despite not knowing specifics about the adverse reactions tied to PreveNile, he considers Intervet's response to be encouraging.

“Generally, I’m pleased that they’re keeping up with it enough that they know there is a problem,” he says.