Abstract:
Several conclusions of importance to studies on the epidemiology of heartwater were drawn from
an investigation in which the numbers of adult Amblyomma hebraeum ticks, to which a closed herd of
Hereford cattle were exposed over a period of 6½ years, were manipulated. With a tick Cowdria ruminantium infection rate of 3-5 %, an endemically stable situation was created by dipping the herd only when an average of 10 adult male and female A. hebraeum ticks were counted on 10 animals. When the average was increased to 15 during the calving period, 97 % of calves acquired a tick-mediated immunity at the age of 6 months.
Because only adult ticks confined to the hindquarters are counted, this procedure is recommended
as a feasible and practical guideline to stock owners wishing to determine a dipping programme that
would ensure endemic stability.
The indirect fluorescent antibody test gave a true reflection of the infection rate through ticks in
calves 3-6 months old, but not in older animals that had been re-infected more than once. This is
because on one hand antibody may persist for 2 years after withdrawal from tick exposure and on the
other the artificial re-infection of cattle with a tick acquired immunity is not always followed by a rise in
antibody titres and may even result in seronegativity.
Four cows infected and re-infected through licks, remained immune to challenge for 2 years after
withdrawal from tick exposure.
Within the confines of one farm 3 isolates of C. ruminantium that differed in pathogenicity and
immunogenicity were recovered from ticks. One of these isolates was almost non-pathogenic to cattle.
