Vaccination Table Topic (AAEP 2012)

Vaccination Table Topic (AAEP 2012)

Attendees discussed a variety of topics surrounding current a future equine vaccinations.

Photo: Erica Larson, News Editor

The first topic of discussion at the vaccination table topic at the 2012 American Association of Equine Practitioners' Convention, held Dec. 1-5 in Anaheim, Calif., was the equine rhinitis A virus vaccine that has been granted to Boehringer Ingelheim on a conditional license. To obtain a conditional license from the USDA, the manufacturer must provide data to demonstrate their product is safe, immunogenic (able to provoke an immune response in the animal's body), and has a reasonable expectation of efficacy. Additionally, there must be a demonstrated need for the vaccine in the target species based on disease risk, lack of available treatments, or other factors. Attendees briefly discussed the most likely equine populations to benefit from this vaccination: athletic horses that travel frequently or compete in venues with horses from multiple sources. The audience was asked if anyone had information about the conditionally licensed Rhodococcus equi vaccine from Harris vaccines, but no company representatives were present. John Prescott, MA, Vet. MB, PhD, a professor in the department of pathobiology at Ontario Veterinary College, might have a promising intrarectal R.equi vaccine in development.

For West Nile virus (WNV), there are currently four vaccines on the market. David Wilson, BVMS, director of the University of California, Davis, Veterinary Medical Teaching Hospital, commented that inactivated WNV virus vaccines appear to be less subject to maternal antibody interference than inactivated Eastern and Western equine encephalitis vaccines. He also expressed the opinion that the recombinant West Nile virus vaccine can overcome maternal antibodies, potentially giving it an advantage in at-risk foals.

Attendees next discussed equine influenza. Pfizer's flu outbreak model has demonstrated that in an unvaccinated population, 90% of the horses are susceptible to clinical disease. After one vaccination just 40% are susceptible. This number is reduced to 26% following a second vaccination. Antigenic shifts (in which the virus changes itself/produces new strains, undermining vaccine efficacy) in current equine influenza isolates are of interest. The H3N8 (Influenza A-equine-2) isolate from 1963 has branched into Eurasian and North American strains. In the North American strain, three branches include Florida (Clade 1 [C1] and Clade 2 [C2] variants), Kentucky, and South African lineages. All outbreaks in North America (and in most of the rest of the world) in recent years have involved Clade 1 viruses. David Wilson commented, however, that a recent case of influenza seen at the University of California, Davis, in a horse recently imported from Germany, had the C2 strain, highlighting the fact that C2 strains continue to be a threat and should be included in North American vaccines. The Ohio 2003 strain contained in some vaccines is the Florida C1 type. Cross protection between C1 and C2 strains is much weaker than protection against the homologous strain following vaccination. Julia Wilson, DVM, Dipl. ACVIM, Turner Wilson Equine Consulting LLC, in Stillwater, Minn., asked a pragmatic question about veterinarians' experience administering the intranasal vaccine intraorally in horses that are too difficult to pass the nasal tube. Attendees did not express strong opinions regarding the efficacy of intraoral administration, although it should be noted that this is not the approved route and the manufacturer makes no claims for efficacy for this route. Although some racing jurisdictions require influenza vaccination every 60-90 days, David Wilson believes this is too short an interval and that a six-month (minimum of four-month) interval is appropriate. Craig Barnett, DVM, a senior equine technical services veterinarian with Merck Animal Health, mentioned the World Animal Health Organization (OIE) meeting in January, and said that the Center for Veterinary Biologics has made updating vaccines easier for manufacturers, as only serologic (blood) studies are needed when new isolates are included in vaccines.

An audience member asked about rattlesnake vaccines. One of the plasma manufacturers has an anti-rattlesnake antibody, but the commentator was unsure of whether this product was effective for diamondbacks or was species-specific.

Veterinarians are using intrarectal Lawsonia intracellularis vaccination with a double dose of swine vaccine on a few farms with endemic problems with proliferative enteritis. Nicola Pusterla, DVM, PhD, Dipl. ACVIM, an associate professor in the Department of Medicine and Epidemiology at the University of California, Davis, said he knew of vaccine use in Kentucky, southern California, Australia, and South Africa. He said most are using monthly total plasma protein concentrations and serology to determine infection prevalence and to make vaccination decisions. A foal with normal protein levels and positive titer is not susceptible. The vaccine engenders a cell-mediated immune response but not a positive serology. Practitioners administer two intrarectal doses of 20-25 mL three weeks apart using a 16-inch soft catheter 30 days before the historical onset of disease. He recommended encouraging the foals to run around for 30 minutes before vaccine administration to promote defecation and to allow greater mucosal contact of the vaccine. He said the cost of the vaccine is $1.20-1.50 per mL, so it is relatively expensive to vaccinate. The infected foals might amplify the number of Lawsonia in the environment, but a rodent most likely brings it onto the farm.

Lastly, attendees discussed equine herpesvirus 1 vaccination. David Wilson, BVMS, commented that the virus does not stimulate immunity while latent, allowing recrudescence (a new outbreak after a period of inactivity) when the horse is stressed. The decrease in virus shedding induced by vaccination is unfortunately short lived and should be considered in vaccination planning.

This table topic was moderated by Julie Wilson, DVM, Dipl. ACVIM, of Turner Wilson Equine Consulting LLC, in Stillwater, Minn., and David Wilson, BVMS, director of the University of California, Davis, Veterinary Medical Teaching Hospital.

About the Author

Julie Wilson, DVM, Dipl. ACVIM

Julie Wilson, DVM, Dipl. ACVIM, is with Turner Wilson Equine Consulting LLC, in Stillwater, Minn.

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