DURATION OF PROTECTIVE EFFICACY OF EQUINE INFLUENZA IMMUNOSTIMULATINGCOMPLEX TETANUS VACCINES

Seven previously untreated five-month-old New Forest ponies received t wo doses of equine influenza immunostimulating complex vaccines, one w ith and one without an immunopurified tetanus toroid component, given by deep intramuscular injection six weeks apart, followed by a booster dose without tetanus toxoid five months later. Fifteen months after t he third dose of vaccine, the ponies were challenged by exposure to an aerosol of influenza A/Equine 2/Sussex/89 (H3N8), a virus isolated fr om a recent outbreak of influenza A/equine 2 in Britain. The challenge produced severe clinical signs of influenza (pyrexia and coughing) in five unvaccinated control ponies. Four of the vaccinated ponies were completely protected against clinical disease, and two of these were a lso protected against infection as demonstrated by their lack of an an tibody response after challenge. No coughing was recorded among the va ccinated ponies, and only three of the seven vaccinated ponies experie nced a transient mild pyrexia. The mean duration and severity of the p yrexia among the vaccinated ponies was significantly less (P<0.01) tha n among the controls, and the excretion of virus was almost eliminated , thus demonstrating the protective efficacy of the vaccines 15 months after vaccination. Monitoring of tetanus antitoxin antibodies showed that protective levels (greater than or equal to 0.01/iu/ml) were main tained for at least 20 months after vaccination.