Estimation Models for the Morbidity of the Horses Infected with Equine Influenza Virus

Estimation formulas for the morbidity of horses infected with equine influenza virus bylinear regression, logistic regression and probit transformation were developed, usingdata from the outbreak at the Sha Tin Racing Track in Hong Kong in 1992. Using theseformulas, we estimated the equine influenza virus morbidity rates at training centersbelonging to the Japan Racing Association (JRA) in October 1997 and in October 1998. In1998 JRA started a new vaccination program, and every horse must now be vaccinated twiceper year. At that time, the vaccine included two US lineage virus strains, theA/equine/Kentucky/81 strain and the A/equine/La Plata/93 (LP93) strain, against equinetype-2 influenza viruses; it did not include any EU lineage virus strains, such asA/equine/Suffolk/89 (SF89). Comparing the geometric mean (GM) values of hemagglutinationinhibition (HI) titers between the LP93 strain and the SF89 strain in 1997 and in 1998,they both rose significantly at every age (p<0.05) by Wilcoxon test. Calculations bythe simulation models show the morbidity rates for LP93 diminished from 0.439 (linear),0.423 (logistic) and 0.431 (probit) to 0.276 (linear), 0.265 (logistic) and 0.271(probit), respectively. On the other hand, the estimated morbidity rates for SF89diminished only slightly from 0.954 (linear), 0.932 (logistic) and 0.944 (probit) to 0.946(linear), 0.914 (logistic) and 0.927 (probit), respectively. Our simulation models couldestimate the effect of the vaccine on each of the equine virus strains represented by themorbidity of infected horses. Thus, they are useful for vaccine evaluation.