Seasonality, natality and herd immunity in feline panleukopenia.

Feline panleukopenia (FPL) was diagnosed in 185 of 7043 feline admissions (2.63%) at a university veterinary hospital over an eight-year period. FLP has a distinct seasonal pattern, occurring during July, August and September. Seasonal peaks were noted in all the years studied. Cats less than one year of age accounted for 70% of the total morbidity. The birth of felines in the United States also assumes a distinctly seasonal pattern. Analysis of 47,786 purebred litters born during 1970-1972 revealed a peak during April, May and June with a national median of May 29. A unifying hypothesis is presented to account for the seasonal occurrence of FPL. An influx of susceptible cats occurs annually following the birth of large numbers of kittens each spring, and disappearance of maternal immunity during the next two to three months. The addition of a large number of susceptible kittens leads to the development of summertime epidemics and serves to exemplify the principles of "herd immunity."