Maximizing Benefits of Influenza Vaccines

"Beware the vaccination paradox!" began J. Richard Newton, BVSc, MSc, PhD, FRCVS, DLSHTM, DipECVPH, of the Animal Health Trust in Suffolk, United Kingdom, during his session on influenza vaccination at the 43rd annual British Equine Veterinary Association (BEVA) Congress, held Sept. 15-18 in Birmingham.

He described the vaccination paradox as follows:

  • A disease is highly prevalent.
  • People vaccinate against it.
  • Disease incidence is notably reduced.
  • People stop vaccinating because it doesn't appear to be a common threat.
  • The number of susceptible individuals increases.
  • The disease reappears!

As an example of this process, Newton recalled a large equine influenza outbreak in South Africa in late 2003, not long after the South African Jockey Club went from mandatory to optional flu vaccinations. At one point in the outbreak, 1,000 horses were affected, one was dead, and racing was cancelled in several areas.

Most outbreaks occur in non-vaccinated animals, he said, although vaccine 'breakdown' does occur periodically, mostly in young Thoroughbreds (usually because of frequent exposure to other horses, but also possibly due to travel and exercise stress). 'Breakdown' in vaccination protection (infection and clinical signs in properly vaccinated animals) happens when the virus that causes the disease mutates, becoming less susceptible to the immune response produced by vaccines containing older strains of virus.

In other words, the viruses in the vaccine and those a horse encounters in the field are different enough that the vaccine no longer effectively confers complete protection.

"This confirms the need for a potent vaccine with epidemiologically relevant strains," Newton stated.

He discussed several studies on flu vaccines as well. Based on these studies, he made several recommendations:

  • Use a potent vaccine with epidemiologically relevant strains of virus (i.e., the viruses in the vaccine should be closely related to the strains circulating in different parts of the world at present).
  • Give foals their first vaccination at six months or older.
  • Extend the primary course interval in primed horses (the time between the first two doses of the year in previously vaccinated horses).
  • Vaccinate before high-risk periods (sales, races, start of training, etc.).
  • Decrease booster intervals to around six months in young horses.
  • Decrease intervals between primary and other courses to around three to four months.

"Some data suggest that a three- to four-month booster interval might be beneficial in providing more complete protection by decreasing the period at which the horse is at most risk (when his antibody titer is low)," he commented.

"In conclusion, flu is a serious disease, vaccines prevent it, the virus evolves over time, and vaccines get out of date, so surveillance is necessary and is being conducted. Consequently, vaccines and protocols are having to be continually refined," he summarized. (For more information, see. www.TheHorse.com/ViewArticle.aspx?id=1983, and the BEVA Congress report in the February 2005 issue.)

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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