When ten cats were infected orally with undiluted or a 10−1 dilution of virulent feline infectious enteritis (panleucopaenia) virus, all developed severe leucopaenia followed by the development of demonstrable antibody, but none died. Eighteen of 29 cats given a 10−2 dilution of virus died of the disease. Three of the survivors had white blood cell counts of less than 4000 and three had counts between 4000 and 6000 cells. Although the remaining five animals never had individual counts of less than 6000 cells, the geometric means of these counts showed that a marked depression in the leucocyte counts had occurred. All surviving cats developed antibody.

Among the ten cats dosed with either 10−3 or 10−4 dilution of virus, four died of feline infectious enteritis and three developed antibody after falls in the leucocyte counts. It is suspected that low dilutions of feline infectious enteritis virulent virus in cats produce a phenomenon similar to that reported by von Magnus (1954) with influenza virus in eggs.

Leucopaenia is commonly defined as less than 4000 white blood cells/mm.3 of blood. Counts lower than this are usual in cats which either die of the disease or have received large doses of virus; they are less common in cats surviving after administration of diluted virus. Challenge of cats with pre-existing antibody did not provoke a depression in the leucocyte counts.