“Diseases like flu and herpesvirus are always going to be problems for horse owners, and they are not going away,” said Tom Chambers, PhD, of the University of Kentucky’s Gluck Equine Research Center. “The secret to effective vaccines for flu is effective surveillance.” This surveillance allows researchers and vaccine manufacturers to stay abreast of which strains are causing illness.

Chambers heads one of the three world-wide OIE (Office International des Epizooties) reference laboratories for equine influenza. (OIE is the international advisory body that establishes guidelines and standards regarding animal health testing, monitoring, and trade. It also collects and disseminates information on occurrence and treatment of animal diseases.) The others are headed by Dr. Jenny Mumford of the Animal Health Trust in Newmarket, England, and Dr. Werner Eichhorn in Munich, Germany.

Veterinarians are being asked to help track influenza to determine if there are new strains of flu and to help in surveillance, said Rob Holland, DVM, PhD, who works for Intervet. The first manufacturer of a modified live intranasal flu vaccine was Heska, which is being marketed by Intervet. That modified live intranasal flu vaccine was tested and proven effective in challenge studies in the late 1990s. Holland, who did his PhD work at the Gluck Equine Research Center in Kentucky, said that vaccines are only as good as the surveillance of disease in explaining why it is so important that reference laboratories keep abreast of circulating viruses.

“The various influenza vaccine manufacturers are helping pay for the surveillance testing, and in return, they can use the viruses from the reference laboratories to update existing vaccines,” said Chambers.

Holland said the most common influenza viruses haven’t changed much in the United States since the mid-1990s, but they could be changing more than researchers know because of the limitations of surveillance. “We don’t know (if strains are changing) because we’re not getting as broad a surveillance as is needed,” noted Holland.
The European and Asian (called Eurasian) strains of influenza have not yet been detected in the United States. The reason for this is unknown, but could be because most respiratory disease outbreaks don’t get proper diagnosis, said Chambers.

The U.S. influenza strains have become common throughout Europe, said Holland. It isn’t known if the influenza viruses used in making U.S. vaccines are offering protection against the Eurasian strains since those viruses can’t be brought into this country for challenge testing of vaccinated horses. It would be very expensive for American-based vaccine companies to do vaccine challenges in Europe just to see if those vaccines offered cross-protection against Eurasian strains of influenza.

“But we’re worried that if one or more of those Eurasian strains came over here that our vaccines wouldn’t be protective,” said Chambers.

The OIE reference laboratories would like to have “harmonization” between United States and European regulatory bodies who license vaccines. This means that the way that vaccines are tested and licensed isn’t the same in the United States as in Europe, and that makes it difficult for researchers and manufacturers to get information and licensing cross-over.

Harmonization (European EU and USDA licensing agreement) is an elusive goal, said Chambers. However, new vaccine technologies have necessitated new approaches to licensing. “The ultimate test is a challenge: Does the vaccine protect horses that are exposed to the disease?” Chambers added. “But the design and interpretation of challenge experiments can be done with varying levels of stringency. No vaccine is absolutely fail-safe (as it relates to efficacy).”

A study funded by the American Quarter Horse Association looked at vaccination of foals and maternal antibody interference (which is when the dam’s antibodies passed to the foal in colostrum actually inactivate the vaccine if it is given to a foal at too young an age). The research showed that maternal antibodies did indeed interfere with influenza vaccine in foals vaccinated at three and four months of age. But, it did not keep the vaccine from being effective when the foals were vaccinated at nine months or beyond.

This is important, because some vaccines might be rendered ineffective or less effective if the foals are vaccinated too young. (Research has shown that duration of immunity for influenza is about three months with an intramuscular injection vaccination, and about six months with an intranasal application.)

“We’re lucky because we’ve gone through many years without a major nationwide epidemic of flu,” said Chambers, stressing the need for veterinarians to participate in surveillance. “Some day there will be new strains appearing, and if we know what’s going on, we can update vaccines quicker and more effectively.”
Veterinarians who suspect a case of influenza should take a nasal swab while the horse has a fever, and at the same time take a serum sample. He/she should send these promptly to a diagnostic laboratory. Then, three to four weeks later, the veterinarian should take another serum sample so that antibody levels can be compared with the original, and any rise in antibody titers can be determined.

“The swab allows the laboratories to make an initial diagnosis. If it turns out to be flu, the lab can sequence the virus and see what strains are out there causing disease,” explained Holland. “Veterinarians can send those samples to most state diagnostic labs, or they can get the lab to send it to the Kentucky Livestock Disease Diagnostic Center in Lexington for virus isolation.”

“We need to do a better job of educating horse owners and veterinarians on the need to swab, and where to send the swab once they have it,” said Chambers. “The serum samples are a back-up in case there is a problem with the swab. Using the sera, the diagnostic lab can still usually determine the general cause of a disease, but this information by itself is typically too little and too late.”

Anyone with questions can contact the influenza reference laboratory at the University of Kentucky’s Gluck Equine Research Center in Lexington (859/257-4757).

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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