THE MERCK VETERINARY MANUAL
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Theileriases

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Theileriases are a group of tickborne diseases caused by Theileria spp. A large number of Theileria spp are found in domestic and wild animals in tick-infested areas of the Old World. The most important species affecting cattle are T parva and T annulata, which cause widespread death in tropical and subtropical areas of the Old World. T lestoquardi, T lowenshuni, and T uilenbergi are important causes of mortality in sheep.

Both Theileria and Babesia are members of the suborder Piroplasmorina. Although Babesia are primarily parasites of RBCs, Theileria use, successively, WBCs and RBCs for completion of their life cycle in mammalian hosts. The infective sporozoite stage of the parasite is transmitted in the saliva of infected ticks as they feed. Sporozoites invade leukocytes and, within a few days, develop to schizonts. In the most pathogenic species of Theileria (eg, T parva and T annulata), parasite multiplication occurs predominantly within the host WBCs, whereas less pathogenic species multiply mainly in RBCs. Development of the schizont stage of pathogenic Theileria causes the host WBC to divide; at each cell division, the parasite also divides. Thus, the parasitized cell population expands and, through migration, becomes disseminated throughout the lymphoid system. Later in the infection, some of the schizonts undergo merogony, releasing merozoites that infect RBCs, giving rise to piroplasms. Uptake of piroplasm-infected RBCs by vector ticks feeding on infected animals is the prelude to a complex cycle of development, culminating in transmission of infection by ticks feeding in their next instar (trans-stadial transmission). There is no transovarial transmission as occurs in Babesia. Occurrence of disease is limited to the geographic distribution of the appropriate tick vectors. In some endemic areas, indigenous cattle have a degree of innate resistance. Mortality in such stock is relatively low, but introduced cattle are particularly vulnerable. Unlike in babesiosis, in theileriasis there is no evidence of increased resistance in calves <6 mo old.

East Coast Fever

East Coast fever, an acute disease of cattle, is usually characterized by high fever, swelling of the lymph nodes, dyspnea, and high mortality. Caused by Theileria parva, and transmitted by the tick vector Rhipicephalus appendiculatus, it is a serious problem in east and southern Africa.

The African buffalo (Syncerus caffer) is an important wildlife reservoir of T parva, but infection is asymptomatic in buffalo. T parva transmitted by ticks from either cattle or buffalo cause severe disease in cattle, but buffalo-derived parasites differentiate poorly to merozoites in cattle and generally are not transmitted by ticks. Hence, buffalo T parva are maintained as a separate population. Buffalo T parva were previously considered a separate subspecies (T parva lawrencei), but DNA typing indicate that the cattle and buffalo parasites are a single species. T parva is usually highly pathogenic, causing high levels of mortality, although some less pathogenic isolates have been identified.

T parva sporozoites are injected into cattle by infected vector ticks. An occult phase of 5–10 days follows before infected lymphocytes can be detected in Giemsa-stained smears of cells aspirated from the local draining lymph node. Subsequently, the number of parasitized cells increases rapidly throughout the lymphoid system, and from about day 14 onward, cells undergoing merogony are observed. This is associated with widespread lymphocytolysis, marked lymphoid depletion, and leukopenia. Piroplasms in RBCs infected by the resultant merozoites assume various forms, but typically they are small and rod-shaped or oval.

Clinical signs vary according to the level of challenge, and they range from inapparent or mild to severe and fatal. Typically, fever occurs 7–10 days after parasites are introduced by feeding ticks, continues throughout the course of infection, and may be >106°F (41°C). Lymph node swelling becomes pronounced and generalized. Lymphoblasts in Giemsa-stained smears of needle aspirates from lymph nodes contain multinuclear schizonts. Anorexia develops, and the animal rapidly loses condition; lacrimation and nasal discharge may occur. Terminally, dyspnea is common. Just before death, a sharp decrease in body temperature is usual, and pulmonary exudate pours from the nostrils. Death usually occurs 18–24 days after infection. The most striking postmortem lesions are lymph node enlargement and massive pulmonary edema and hyperemia. Hemorrhages are common on the serosal and mucosal surfaces of many organs, sometimes together with obvious areas of necrosis in the lymph nodes and thymus. Anemia is not a major diagnostic sign (as it is in babesiosis) because there is minimal division of the parasites in RBCs, and thus no massive destruction of them.

Animals that recover are immune to subsequent challenge with the same strains but may be susceptible to some heterologous strains. Most recovered or immunized animals remain carriers of the infection.

Treatment with parvaquone and its derivative buparvaquone is highly effective when administered in the early stages of clinical disease but is less effective in the advanced stages, in which there is extensive destruction of lymphoid and hematopoietic tissues. Immunization of cattle against T parva using an infection-and-treatment procedure is practical and continues to gain acceptance in some regions. The components for this procedure are a cryopreserved sporozoite stabilate of the appropriate strain(s) of Theileria derived from infected ticks and a single dose of long-acting oxytetracycline given simultaneously; although oxytetracycline has little therapeutic effect when administered after development of disease, it inhibits development of the parasite when given at the outset of infection. Cattle should be immunized 3–4 wk before being allowed on infected pasture. Parasitized bovine cells containing the schizont stage of T parva and T annulata can be cultivated in vitro as continuously growing cell lines. In the case of T annulata, cattle can be infected with a few thousand cultured cells. Attenuated strains produced by serial passage of such cultures form the basis of live vaccines used in several countries, including Israel, Iran, India, and the former USSR.

Incidence of East Coast fever can be reduced by rigid tick control, but this is not feasible in many areas because of cost and the high frequency of acaricidal treatment required.

Tropical Theileriosis

Theileria annulata, the causal agent of tropical theileriosis, is widely distributed in north Africa, the Mediterranean coastal area, the Middle East, India, the former USSR, and Asia. It is transmitted by several species of ticks of the genus Hyalomma. T annulata can cause mortality of up to 90%, but strains vary in their pathogenicity. The kinetics of infection and the main clinical findings are similar to those produced by T parva, but unlike in East Coast fever, anemia is often a feature of the disease. Characteristic signs include fever and swollen superficial lymph nodes. If the disease progresses, cattle rapidly lose condition. The schizonts and piroplasms are morphologically similar to those of T parva. Animals that recover from infection are immune to subsequent challenge. Treatment and control are as described for East Coast fever (see East Coast Fever).

Other Theileriases of Cattle

The Theileria orientalis group, consisting of the closely related parasites T orientalis, T buffeli, and T sergenti, has a worldwide distribution. These parasites are transmitted by ticks of the genus Haemaphysalis. The piroplasms are larger than those of T parva and T annulata, and they multiply principally by intraerythrocytic division. Mortality, particularly in indigenous cattle, is rare, but infection can sometimes result in progressive chronic anemia.

T mutans and T velifera are found in Africa, where they are transmitted by ticks of the genus Amblyomma. Multiplication occurs mainly by intraerythrocytic division. The piroplasms are morphologically indistinguishable from those of T orientalis and T taurotragi (an African parasite of eland and cattle), but the parasites can be differentiated by serologic tests such as indirect fluorescent antibody and by DNA typing. Some strains of T mutans are pathogenic as well. In addition, concurrent infection may add to the pathogenicity of T parva.

Ovine and Caprine Theileriases

Theileria lestoquardi (formerly T hirci) causes a disease in sheep and goats similar to that produced in cattle by T annulata, with which it is closely related. T lestoquardi is transmitted by ticks of the genus Hyalomma. The limited available epidemiologic data indicate that T lestoquardi has a more restricted geographic distribution than that of T annulata, being particularly prevalent in the Middle East and northeast Africa. Mortality can approach 100%. Schizonts can readily be demonstrated in Giemsa-stained smears of needle biopsies from swollen superficial lymph nodes.

Recently, two species of Theileria, T lewenshuni and T uilenbergi, have been identified as the causal agents of a severe disease in sheep in China. These species are morphologically indistinguishable and cause similar disease but can be distinguished by DNA typing methods. They are transmitted by ticks of the genus Haemaphysalis. Schizonts are detected in a range of tissues, but later and in smaller numbers than in other pathogenic Theileria spp. Piroplasms are consistently detected in RBCs. Morbidity and mortality rates of up to 65% (T lewenshuni) and 75% (T uilenbergi) have been seen in susceptible animals introduced into endemic areas. Affected animals show sustained fever and anemia.

Several other nonpathogenic Theileria spp (eg, T ovis) are also widely distributed. Piroplasms of these species are polymorphic.

Equine Theileriasis

Babesia equi was reclassified as T equi in 1998, based on DNA analysis and other biologic data (see Babesiosis).

Last full review/revision January 2015 by W. Ivan Morrison, PhD, BVMS

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