Anaplasmosis of ruminants is an infectious and transmissible disease of cattle, sheep, goats, and other domestic ruminants, characterized by progressive anemia associated with the presence of intraerythrocytic inclusion bodies designated as anaplasma (75). There are three known species: A. marginale, A. centrale, and A. ovis. A. marginale is the representative species of this group, and its susceptible host is cattle. A. centrale also causes bovine anaplasmosis, and A. ovis is the cause of ovine and caprine anaplasmosis. They are not thought to be zoonotic diseases at this time. Once an agent of anaplasmosis is transmitted to a host animal, it invades the mature erythrocytes. Sequential replication, release and new invasion (acute phase) result in the rapid onset of clinical signs that are attributable to severe hemolytic anemia in A. marginale infection
(75). They include depression, weakness, fever, labored breathing, inappetence, dehydration, constipation, jaundice, abortion, and death. The pathogenic roles of A. centrale and A. ovis are comparatively weaker than that of A. marginale. Thus, little is known about the vectors and reservoirs of A. centrale and A. ovis.
Although A. marginale can be transmitted among cattle mechanically by blood-contaminated mouthparts of biting flies and mosquitoes, it is transmitted biologically by ticks
(76). On the basis of experimental and epidemiological data, several insects, including horse flies (Diptera: Tabanidae), stable flies (Stomoxys), deer flies (Chrysops), horn flies (Siphona), and mosquitoes (genus Psorophora) have been incriminated as potential vectors (75). About 20 tick species have been shown to transmit anaplasmosis, although field evidence indicating the tick as the principal disease vector is lacking (76,77). D. andersoni, D. occidentalis, and D. variabilis are vector ticks in the United States (78). The ultrastructure and development of the pathogenic agent in D. andersoni and D. variabilis have been well documented by the series of studies by Kocan. The tropical cattle tick, Boophilus microplus, has also been demonstrated to be vector of A. marginale (79,80).
Carrier cattle are the main reservoirs of infection. Early serological studies suggested that wild cervids, including white-tailed deer and mule deer (O. hemionus), might be reservoirs of A. marginale (81-83). Elk (C. elaphus) were experimentally infected with A. marginale. The pathogen was detected in the blood of these elk and caused disease in splenectomized domestic bovine calves after subinoculation of blood from the elk (84). Experimental infection of A. marginale was also demonstrated in splenectomized mule deer (85). Recently, molecular biological techniques were used to detect the agent from reservoir animals, including Iberian red deer (C. elaphus hispanicus) (86).
A. bovis is phylogenetically more closely related to A. phagocytophilum than to A. marginale or A. centrale. Morulae are found in monocytes of infected cattle (87). A. bovis infection in cattle is seldom reported and is usually associated with subclinical infection (87). Thus, little is known about the epidemiology of this agent. Hyalomma spp., Rhipicephalus appendiculatus, and A. variegatum have been proven to be vectors of A. bovis (88-90).
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