Circulation of classical swine influenza virus in Europe between the wars?
In 1957 the World Health Organization arranged a survey of horse and swine sera in a number of countries in order to gain information on the role and importance of animals in the epidemiology of influenza. The veterinary services of the countries concerned were requested to obtain blood specimens from these animals, if possible both before and after the human pandemic of Asian influenza. This paper reports on the results of haemagglutination-inhibition and complement-fixation tests performed on these sera in WHO Influenza Centres and other collaborating laboratories.It is apparent from these results that the Asian (A2) strain can cause natural inapparent infection in horses and swine. Equine influenza caused by the A-equi strain is also present in many countries from which it had not been previously reported, and infection in pigs with the A-swine strain, long known in the USA, has now been recorded in at least two European countries.In a concluding section, the findings in this survey are related to other observations on the position of animals in influenza epidemiology, and future research needs are outlined.