An Outbreak of Type A(2) Influenza Among Horses.

The clinical diagnosis of equine influenza was first based on the spectacular contagiousness of the disease, the general clinical resemblances to human influenza and the almost complete absence of complications usually observed in infectious viral arteritis, viral rhinopneumonitis or in other respiratory infections of the horses. The specific viral etiology of the epizootic was ascertained through the isolation of a type A influenza virus and further substantiated by evaluation of the immunological response of the sick horses, as demonstrated by complement fixation and hemagglutination-inhibition tests, using normal and convalescent sera. The agent isolated was typed and proved to be similar to a type A(2) virus isolated from humans this year. Because of the widespread nature of this epizootic, one cannot exclude the possibility that was an expression of an attack by the virus on an unprotected population of horses without previous infectious experience with the influenza virus. Even though mortality and serious complications or sequelae were negligible in this present outbreak, heavy financial losses were suffered by owners, track operators and others. Future epizootics may well be of an even more severe nature. Attention must be focused on the control of this disease, not only on account of its veterinary and economic aspects but also because of the possibility that horses might be a reservoir of infection for humans.