"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," which allows researchers and vaccine manufactures 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 for 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 England and Dr. Werner Eichhorn in Germany. A meeting on influenza was held in conjunction with the convention.

Veterinarians are being asked to help track influenza to determine if there are new strains and to aid surveillance, said Rob Holland, DVM, PhD, who works for Intervet. Heska was the first manufacturer of a modified live intranasal flu vaccine, which is marketed by Intervet. Holland, who did his PhD work at the Gluck Equine Research Center, said vaccines are only as good as disease surveillance.

"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 limited surveillance.

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. He added, "We're worried that if one or more of those Eurasian strains came over here that our vaccines wouldn't be protective."

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 (USDA and European Union licensing agreement) is an elusive goal, said Chambers. 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 asked. "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 showed that maternal antibodies (the dam's antibodies passed to the foal in colostrum) interfered with influenza vaccine in foals vaccinated at three and four months of age. They did not keep the vaccine from being effective when foals were vaccinated at nine months or beyond.

This is important, because some vaccines might be ineffective or less effective if 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."

If influenza is suspected, the veterinarian should take a nasal swab while the horse has a fever, and at the same time take a serum sample--these should be sent promptly to a diagnostic lab. Then, three to four weeks later, the veterinarian should take another serum sample so 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 is flu, the lab can sequence the virus and see what strains are out there causing disease," explained Holland. "Veterinarians can send samples to most state diagnostic labs, or they can get the lab to send them 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," said Chambers.

Anyone with questions can contact the influenza reference laboratory at the University of Kentucky's Gluck Equine Research Center in Lexington (859/257-4757). (See complete article #4123 online.)


"At rest, (a horse) breathes about 86,000 liters of air a day, about 25,000 gallons, which may carry quite a bit of dust particles that can really injure the respiratory tract," said Laurent Couëtil, DVM, Dipl. ACVIM, of Purdue University in the In Depth: Inflammatory Airway Disease (IAD) session. The result is a chronic cough and mild exercise intolerance, seen in 11-65% of racehorses today depending on the diagnostic criteria used (endoscopy or cytology) and whether measurements were taken before or after exercise.

IAD typically occurs in younger horses, and its characteristics include:

  • Clinical signs of a chronic cough, mucopurulent exudates (small amounts of pus, in this case deposited in the airways), mild exercise intolerance, and chronic respiratory tract inflammation;
  • Many different causative factors;
  • Mild airway obstruction;
  • Non-recurrent; and
  • Bronchitis/bronchiolitis.

Typically IAD lasts seven weeks, but it can last anywhere from four to 22 weeks. IAD can affect any breed of horse, but, "It's certainly more common in Standardbred and Thoroughbred racehorses," said Couëtil. He added, "Racehorses use 100% of their cardio-respiratory potential (pulmonary gas exchanges are a major limiting factor and a mild degree of IAD might have a profound effect)." Training, racing, and frequent travel are stressful to the immune system and might result in lower airway disease. In racing, "Horses that finish last are more likely to have IAD," he said.

Mucopurulent exudates found in the the junction of the trachea and the bronchial tubes, and an increased percentage of neutrophils (a type of white blood cell) in bronchoalveolar lavage, have been associated with exercise intolerance and poor performance, said Couëtil. Clinicians can determine the type of IAD affecting a horse by examining the cells in a bronchoalveolar lavage, and they can differentiate IAD from recurrent airway obstruction.

Suspected causes for IAD include bacteria, viruses, and inhaled environmental pollutants. While bacteria are frequently isolated from tracheal wash samples, the role of bacteria in the pathogenesis of the disease is unclear. Couëtil said, "Contrary to a common belief, respiratory viruses do not appear to play an important role in IAD."

Travel and head elevation can affect IAD and can cause it to elevate into a more serious condition. "If you keep a horse's head elevated more than six hours, you start seeing increasing numbers of bacteria in the tracheal fluid and start seeing accumulation of mucus," he said. He explained that some horses have developed full-blown pneumonia after having their heads elevated for 24 hours.

No genetic data is available for IAD, so it is unclear what role genetics might play in IAD susceptibility, but genetic predisposition remains a possibility like it is with heaves in horses and asthma in people.

Horses with signs of IAD are more likely to perform poorly, but scientists still aren't sure of the mechanism that causes exercise intolerance in these horses. It could be a result of impaired gas exchanges due to airway obstruction. Airway hyperresponsiveness is thought to play an important role in triggering the cough.

A retrospective study performed at Purdue University on 163 horses presented for poor performance evaluation showed that 20% of the cases had a combination of respiratory disease and at least one problem in another body system. This emphasizes the importance of performing a comprehensive evaluation of poor performers. (See complete article #4124 online.)

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