Transmission
Babesia is spread through the saliva of a tick when it bites. At its nymphal stage, a tick will bite into the skin for a blood meal. The tick, if not removed, will stay attached for three to four days, with longer periods of feeding associated with a higher probability of acquiring the parasite. The parasite can survive in the tick as it molts through its various developmental stages, resulting in all stages being potentially infectious. Some species of Babesia can be transmitted from a female tick to its offspring before migrating to salivary glands for feeding. B. microti, the most common species of Babesia in humans, though, has not been shown to transmit transovarially. Ticks of domestic animals that transmit Babesia, causing much disease, include the very widespread cattle ticks, Rhipicephalus (Boophilus) microplus, and R.(B.) decoloratus. These ticks have a strict one-host feeding cycle on cattle, so the Babesia can only be transmitted by the transovarial route.
In the Americas, Ixodes scapularis is the most common vector. This hard tick, commonly known as a deer tick, is also the vector for other tick-associated illnesses, such as Lyme disease. Many species of Babesia only infect nonhuman mammalian hosts, most commonly cattle, horses, and sheep. B. microti and B. divergens are the two main pathogenic species in humans. Their reservoirs are theorized to be the white-footed mouse (Peromyscus leucopus Rafinesque), microtus voles (Microtus spp.), and the white-tailed deer (Odocoileus virginianus). These woodland species are hypothesized reservoirs because although they are known to harbor the disease, complete reservoir competence has not yet been shown.
Most cases of transmission between humans are attributed to a tick vector. As of 2003, though, the Centers for Disease Control and Prevention (CDC) acknowledged more than 40 cases of babesiosis contracted from packed red blood cell (PRBC) transfusions and two infections documented from organ transplantation. PRBC transfusions that cause infections were identified through testing of the blood donor for B. microti antibodies. The occurrence of PRBC transfusions as a mechanism of Babesia transmission puts pressure on governmental organizations, such as the CDC, to heighten standard measures for screening blood donations.
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