IMPACT OF PERSISTENT ANAPLASMA-MARGINALE RICKETTSEMIA ON TICK INFECTION AND TRANSMISSION

Anaplasma marginale, an intraerythrocytic rickettsia of cattle, is tra nsmitted biologically by ticks. Because of the brevity of acute A. mar ginale infection, transmission may rely on the tick's ability to acqui re the organism from persistently infected cattle with low rickettsemi a levels. By using a nucleic acid probe to quantitate low-level infect ion, we found that rickettsemia levels in persistently infected cattle fluctuated at approximately 5-week intervals during a 24-week period, from <10(4) infected erythrocytes per ml of blood to high levels of a pproximately 10(7) infected erythrocytes per ml of blood. Cattle maint ained very low rickettsemia levels (<10(4.3) infected erythrocytes per ml of blood) for approximately 4 to 8 days of every 5-week cycle. The effect of fluctuations in rickettsemia in persistently infected cattl e on acquisition by Dermacentor andersoni nymphal and adult male ticks was examined. A positive correlation was observed between rickettsemi a levels in cattle and the resulting infection rates of ticks. At high rickettsemia levels, up to 80% of ticks acquired infection, but even at extremely low rickettsemia levels, 27% of adult male ticks became i nfected. Moreover, once ticks acquired infection, biological replicati on of the organism within the ticks appeared to make up for initial di fferences in the infecting dose. The high infection rates in adult mal es, combined with their intermittent feeding behavior and the observat ion that only a few infected ticks were required for transmission to a susceptible host, suggest that adult male D. andersoni ticks are epid emiologically important in A. marginale transmission. Because cattle w ith all levels of rickettsemia were capable of efficient transmission to ticks, population control efforts must include decreasing transmiss ion from persistently infected individuals.