Field evaluation of a commercial M-protein vaccine against Streptococcus equi infection in foals.

The research study identifies the effectiveness of a commercially available M-protein vaccine in protecting foals against Streptococcus equi infection, known as strangles. The double-blind trial revealed the vaccine significantly reduced certain symptoms of strangles in one group of foals.

Study Design

• The study involved a double-blind randomized clinical trial with 664 weaned mixed-breed foals situated in two adjacent feed-lots named A and B.
• Lot B consisted of 114 foals. They were vaccinated by intravenous injection with either the M-protein vaccine or a saline solution as a placebo. This was done in three biweekly intervals.
• In lot A, which included 450 foals, only one dose of the vaccine or placebo was delivered.

Evaluation Method

• A single observer, blinded to the treatment groups, carried out the clinical observations. They examined all the foals in lot B and a random sample of 120 foals in lot A.
• The observer examined for symptoms of strangles, including cervical lymphadenopathy (swelling of the lymph nodes in the neck), bilateral nasal discharge, and swelling at the vaccination injection sites.
• Bacteriologic culture of nasal swab specimens or lymph node aspirates from selected foals with clinical disease yielded S equi, confirming the presence of the disease in the population.

Results

• In lot B, only 29% of vaccinated foals developed cervical lymphadenopathy, compared to 71% of non-vaccinated controls. Meanwhile, in lot A, a slightly higher percentage of vaccinated foals (53%) developed the symptom compared to non-vaccinated ones (48%).
• The vaccinated foals in lot B also displayed a significantly lower prevalence of mucopurulent nasal discharge, another symptom of strangles, as confirmed by the chi-squared contingency analysis.
• However, in terms of side effects of the vaccination, a higher number of vaccinated foals in both lots experienced palpable swelling at the vaccine injection site compared to placebo controls.

Conclusions

• The M-protein vaccine demonstrated its effectiveness in reducing the incidence of neck lymph node swelling and nasal discharge associated with strangles in vaccinated foals, compared to non-vaccinated foals.
• However, the incidence of swelling at the vaccine injection site was considerably higher in vaccinated foals than in those who received the placebo, indicating a potential side effect of the vaccine.