Strangles

Immunity in horses vaccinated with Strep­tococcus equi bacterin is satisfactory if the rec­ommended vaccination schedules are followed.¹⁶ The full 3-dose course elicited the most satis­factory serum antibody response. Challenge studies by other investigators and clinical ob­servations also indicate adequate protection against strangles using recommended vacci­nation programs.

Immunity elicited by the 3-dose vaccine course lasts for approximately 1 year, in con­trast to immunity following natural infection, which lasts for only 9 weeks.¹⁶ Immunity was thought to last "for years" following natural in­fection. The differences observed between re­sults of laboratory studies and field observations regarding duration of immunity following nat­ural infection are that in the field, continuous protection is attributable to immediate an-amnestic response following re-exposure in re­covered animals with low antibody titers. However, various practitioners and clinicians at institutions throughout the country do not always agree with the findings that commer­cial bacterins are effective. It is unknown how and why such vast differences in results can occur in the hands of "capable" individuals.

Equine Encephalomyelitis

The immune response to vaccination against VEE, EEE and WEE is good. Studies indicate that the duration of immunity generated by immunization against these viruses lasts for at least a year. Antibody titers following vacci­nation are adequate and protection against challenge virus is consistent among horses vaccinated up to 12 months prior to exposure to virulent virus. In some instances, horses without titers following vaccination were also protected against challenge virus, attesting perhaps to the importance of cell-mediated im­munity or well-primed antibody-producing cells.

Use of a modified-Iive-virus vaccine against WEE is effective and allows for less frequent administration intervals as compared with vaccination schedules involving killed-virus vaccines. However, the possibility of reversion of virus from a modified-live state to a virulent state has prevented its commercial production.

Heterologous stimulation by virulent virus among horses vaccinated with VEE-WEE-EEE vaccine has been reported.¹⁷ The amount of het-erologous stimulation not only indicates greater antigenic similarity between VEE and WEE viruses than between EEE and VEE or WEE, but the "mutual enhancement" probably in­creases protection against infection by virulent VEE, EEE or WEE viruses in vaccinates.

Bivalent (EEE-WEE) and trivalent (EEE-WEE-VEE) vaccines are effective and safe, provided the recommended vaccination sched­ules are followed. Minimal protective titers against VEE are considered to be 1:40, as de­termined by plaque-neutralization tests. Pro­tective titers against EEE and WEE are not known due to the apparent role of cell-me­diated immunity against these viruses and the difficulty in assessing this function.

Rabies

Not since 1966 have any experimental data been reported on the immune response to ra­bies vaccine in the horse. Although a challenge virus was not used in that investigation, 85% of the horses responded immunologically 40 days postinoculation with serum-neutralizing titers of 1:10 or better. A serum-neutralizing titer of 1:10 is considered the minimal protec-

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tive titer, based on several studies in labora­tory and other domestic animal species. Minimal protective titers in the horse have not been es­tablished. The duration of immunity to rabies in vaccinated horses lasts at least a year.

Tetanus

Vaccination of horses against tetanus usu­ally confers solid immunity. Several studies in­dicate that aluminum-based and water-in-oil emulsions of tetanus toxoid elicited excellent circulating antitoxin levels.^18-21 Adverse reac­tions to toxoid preparations were rare.

A tetanus antitoxin titer of O.O1 JU/ml is considered the minimal protective titer in hu­mans. Although studies of protective titers in horses have not been reported, it is generally accepted that a tetanus antitoxin titer of 0.01 lU/ml is also considered the minimal protec­tive titer in horses. Based on this assumption, vaccination of horses with tetanus toxoid should, in most cases, protect the animal against infec­tion by Clostridium tetani.

Studies in humans indicate that the dura­tion of immunity lasts up to 10 years. There­fore, "booster" immunization is recommended every 10 years in humans. The controversy as to how often horses must be revaccinated re­mains unresolved. Although available data in­dicate that horses are protected against tetanus for at least 31/2 years after booster injections and may not be protected beyond 5 years, the duration of immunity in adult horses must be accurately investigated.^{20 21} Some investigators recommend yearly vaccination intervals since severe adverse reactions from vaccination have not been reported in horses as have occurred in humans and the maintenance of more than ad­equate levels of serum antitoxin is economi­cally acceptable. Also, horses are more accessible to infection with Colostridium tetani and more susceptible to its toxin than are humans.