Age distribution of naturally occurring acute babesiosis in cattle in Sweden.

The age distribution of clinical cases of babesiosis reported by local veterinary practitioners was investigated in 1976 and 1981. The first study was based on material collected primarily for identifying the Babesia species, the second on computerized reports from a part of one county in Sweden. The results were similar. Most diseased animals, 135/165 (82%) and 145/161 (90%) respectively, were more than 2.5 years old (cows), while 27/165 (169b) and 12/161 (8%) respectively, were 1–2.5 years old and only 3–4 (2–3%) animals were calves less than 1 year old. This age group, however, probably did not meet the same infection risk as did older animals. Among cows (>2.5 years old) there seemed to be no influence of age on the distribution of clinical babesiosis. Calculation based upon the entire animal population of the county investigated in 1981 revealed that clinical babesiosis was reported about 11 times more often among cows than among heifers and steers. The comparatively high resistance among 1–2.5 year–old cattle may be a function of an inverse age resistance, and/or may be influenced by vaccination against babesiosis on »high risk« farms in Sweden and a lower risk of infection on other farms such that animals tend to escape infection prior to adultness. Babesia organisms were found in 156 of 165 cases (95%) reported as clinical babesiosis.