Seasonal abundance and distribution of Parafilaria bovicola ovipositional blood spots on cattle in South Africa.

More than 23 000 cattle of both sexes and different ages were examined for blood spots caused by egg-laying females of P. bovicola. Although these studies extended over four years and involved 5 farms in different parts of the Transvaal Bushveld, the overall results were the same. Ovipositional bleeding was strongly seasonal with blood spots first appearing in winter (June), reaching a peak in spring (September-November) and thereafter declining rapidly as summer progressed. In a single year at Zoutpan Research Station up to 92,1% of the 1st year heifers had already bled by November and this proportion increased only slightly to 95,1% by the end of the bleeding season (May). The number of blood spots per animal showed a similar seasonal abundance except for a second peak of abundance in June for 1st year heifers and oxen. The prevalence of blood spots in cattle of different ages and sex varied markedly. At Mara Research Station half as many oxen bled in their 2nd year as in their 1st year, while at Zoutpan 19,2% fewer heifers bled in their 2nd year than in their 1st. Bulls bled the most, then 1st year oxen, 1st and 2nd year heifers and 2nd year oxen, with breeding cows bleeding the least. A high female hormone level appears to be associated with the development of immunity. The shortest period from birth to 1st blood spot (the apparent prepatent period) was 191 days, while 81,8% of oxen bled for the 1st time within 279 days after birth.(ABSTRACT TRUNCATED AT 250 WORDS)