Medically important ixodid ticks often carry multiple pathogens, with individual ticks frequently coinfected and capable of transmitting multiple infections to hosts, including humans. Acquisition of multiple zoonotic pathogens by immature blacklegged ticks (Ixodes scapularis) is facilitated when they feed on small mammals, which are the most competent reservoir hosts for Anaplasma phagocytophilum (which causes anaplasmosis in humans), Babesia microti (babesiosis) and Borrelia burgdorferi (Lyme disease). Here, we used data from a large-scale, long-term experiment to ask whether patterns of single and multiple infections in questing nymphal I. scapularis ticks from residential neighbourhoods differed from those predicted by independent assortment of pathogens, and whether patterns of coinfection were affected by residential application of commercial acaricidal products. Quantitative polymerase chain reaction was used for pathogen detection in multiplex reactions. In control neighbourhoods and those treated with a fungus-based biopesticide deployed against host-seeking ticks (Met52), ticks having only single infections of either B. microti or B. burgdorferi were significantly less common than expected, whereas coinfections with these 2 pathogens were significantly more common. However, use of tick control system bait boxes, which kill ticks attempting to feed on small mammals, eliminated the bias towards coinfection. Although aimed at reducing the abundance of host-seeking ticks, control methods directed at ticks attached to small mammals may influence human exposure to coinfected ticks and the probability of exposure to multiple tick-borne infections.

The tick-borne intracellular pathogen Anaplasma marginale (Rickettsiales: Anaplasmataceae) develops persistent infections in cattle and tick hosts. While erythrocytes appear to be the only site of infection in cattle, A. marginale undergoes a complex developmental cycle in ticks and transmission occurs via salivary glands during feeding. Many geographic isolates occur that vary in genotype, antigenic composition, morphology and infectivity for ticks. In this chapter we review recent research on the host–vector–pathogen interactions of A. marginale. Major surface proteins (MSPs) play a crucial role in the interaction of A. marginale with host cells. The MSP1a protein, which is an adhesin for bovine erythrocytes and tick cells, is differentially regulated and affects infection and transmission of A. marginale by Dermacentor spp. ticks. MSP2 undergoes antigenic variation and selection in cattle and ticks, and contributes to the maintenance of persistent infections. Phylogenetic studies of A. marginale geographic isolates using msp4 and msp1α provide information about the biogeography and evolution of A. marginale: msp1α genotypes evolve under positive selection pressure. Isolates of A. marginale are maintained by independent transmission events and a mechanism of infection exclusion in cattle and ticks allows for only the infection of one isolate per animal. Prospects for development of control strategies by use of pathogen and tick-derived antigens are discussed. The A. marginale/vector/host studies described herein could serve as a model for research on other tick-borne rickettsiae.

Tick populations were monitored between October 1991 and September 1992 at monthly intervals on 10 Boran (Bos indicus) heifers at Didtuyura ranch, Yabelo district, Ethiopia. Twelve different tick species in four genera were identified from a total of 16 026 specimens collected. Of all the ticks collected, Rhipicephalus spp. constituted 84.7%, Amblyomma spp. 9.9%, Boophilus spp. 4.8% and Hyalomma spp. 0.24%. The ventral parts of cattle were found to be the most favoured feeding sites for most of the ticks collected. The size ranges of the standard ticks collected were 4–8 mm; 9–16 mm; 4–8 mm and 8–13 mm for R. pulchellus, A. variegatum, B. decoloratus and A. gemma, respectively, while their respective male to female sex ratios were 2:1; 3.2:1; 1:5.4 and 2.6:1. Packed cell volume (PCV) values of blood samples taken from 600 cattle were monitored monthly during the study period. There was a reduction in PCV during the dry season of the year but generally the values were within the normal range of 32–40%. The parasites causing anaplasmosis, babesiosis and theileriosis (T. mutans) were detected by microscopic examination of blood smears and serologically (ELISA) at seroprevalence rates of 94.68%, 77.19% and 30.9% respectively.