Impact of persistent Anaplasma marginale rickettsemia on tick infection and transmission

Abstract

Anaplasma mapignak, an intraerythrocytic rickettsia of cattle, is transmitted biologically by ticks. Because ofthe brevity of acute A. marginake infection, transmission may rely on the tick's ability to acquire the organism from persistently infected cattle with low rickettsemia levels. By using a nucleic acid probe to quantitate low-level infection, we found that rickettsemia levels in persistently infected cattle fluctuated at approximately 5-week intervals during a 24-week period, from <104 infected erythrocytes per ml of blood to high levels of approximately 107 infected erythrocytes per ml of blood. Cattle maintained very low rickettsemia levels (<10-3 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 cattle on acquisition by Dermacentor andersoni nymphal and adult male ticks was examined. A positive correlation was observed between rickettsemia levels in cattle and the resulting infection rates of ticks. At high rickettsemia levels, up to 80%o of ticks acquired infection, but even at extremely low rickettsemia levels, 27% of adult male ticks became infected. Moreover, once ticks acquired infection, biological replication of the organism within the ticks appeared to make up for initial differences in the infecting dose. The high infection rates in adult males, combined with their intermittent feeding behavior and the observation that only a few infected ticks were required for transmission to a susceptible host, suggest that adult male D. andersoni ticks are epidemiologically important in A. mai nale transmission. Because cattle with all levels of rickettsemia were capable of efficient transmission to ticks, population control efforts must include decreasing transmission from persistently infected individuals.

Structural basis for segmental gene conversion in generation of Anaplasma marginale outer membrane protein variants

“…Males from the Reynolds Creek stock of Dermacentor andersoni (free of A. marginale and other pathogens) were selected from engorged nymphs that had molted to the adult stage following incubation for 48 h at 26∞C and 92% relative humidity. Tick transmission of the St. Maries strain to cattle using the Reynolds Creek D. andersoni has been previously reported (Eriks et al, 1993;Futse et al, 2003). Holstein calves (3-4 months of age), confirmed to be A. marginale negative by the MSP-5 CI-ELISA (Torioni de Echaide et al, 1998), were each infected by tick transmission with the St. Maries strain.…”

A Review of Bovine Anaplasmosis

“…Some tick species or genotypes can become infected with the appropriate strains of A. marginale when they feed on an acutely infected animal, but they are also efficient bio‐magnifiers in acquiring A. marginale when there are low levels of parasites in erythrocytes during the ruminant host’s chronic or carrier phase (Scoles et al., 2005a). The likelihood of a tick acquiring at least one organism in the acute phase is virtually 95–100% and around 27–84% in the chronic phase (Eriks et al., 1993). After development of A. marginale in tick gut cells, many other tick tissues become infected, including the salivary glands from where the rickettsiae are transmitted to vertebrates during feeding (Kocan, 1986; Kocan et al., 1992a,b; Ge et al., 1996).…”

Insights into Mechanisms of Bacterial Antigenic Variation Derived from the Complete Genome Sequence of Anaplasma marginale

“…maintain a persistent infection, thus maximizing the opportunity for the appropriate tick stage, which is present only during a limited period of the year, to acquire the pathogen. [4][5][6][7][8][9][10][11][12][13][14][15] Importantly, ticks are able to efficiently acquire Anaplasma during feeding on the low bacteremia levels associated with long-term persistence. 4,5,14…”