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Bovine petechial fever is a rickettsiosis of cattle caused by Ehrlichia (Cytoecetes) ondiri. The disease occurs in the highlands of Kenya and Tanzania at altitudes of 1,500 to 3,000 m, although it is considered likely to occur in neighboring countries with similar topography. Characteristically, bovine petechial fever occurs in cattle that break out from fenced pastures and graze adjacent forest or bushland areas, or when they are grazed on these areas at the end of the dry season. It also occurs in cattle that have been recently introduced to these areas, and indigenous cattle appear to acquire resistance. Epidemics occur in cattle imported to infected areas and last 1 to 2 months, involving 60% to 80% of the group and resulting in significant losses. However, no outbreak has been reported for over a decade.
Infection can be experimentally transmitted to cattle, sheep, goats, wildebeest, impala, duiker, bushbuck, and other wild ruminants, but natural disease is seen only in cattle. E. ondiri infection is believed to be endemic in wild ruminants, especially bushbuck and duiker, and the disease sporadically spills into cattle grazing forest edges or scrubs. The vector is not known, although epidemiologic findings suggest a tick vector of restricted distribution.
The disease in cattle is characterized by high fever and widespread petechial hemorrhages in mucous membranes for periods up to 10 days; epistaxis, melena, and unilateral conjunctivitis occur in more severely affected animals. The eyeball is tense, protruding through swollen, everted conjunctival sacs as the so-called “poached-egg eye.” Pregnant animals may abort, and there is a fall in milk production for several weeks in lactating animals. Anemia may be severe enough to result in death 3 to 4 weeks after infection. There is a profound lymphocytopenia by the second day of infection, followed by leukopenia and thrombocytopenia. The organism can be demonstrated in granulocytes and monocytes in Giemsa-stained blood smears during the febrile period, but it cannot yet be cultured. Serology (indirect fluorescent antibody test) can be carried out to detect antibodies against E. ondiri.
Grossly, the main lesions of ondiri disease are widespread petechial hemorrhages and enlarged, congested lymph nodes. In severe cases, death is often a result of severe hemorrhages into the lungs and airways. Abomasal mucosa is edematous, and the contents of the ileum and colon are tarry.
Differential diagnosis includes other hemorrhagic diseases of cattle, such as acute trypanosomiasis, acute theileriosis, hemorrhagic septicemia, bracken fern poisoning, Rift valley fever, and heartwater.
Tetracyclines are effective in treating early experimental cases but are ineffective in advanced cases. Recovered animals may be latently infected and are immune to reinfection for at least 2 years. Control is by avoiding grazing cattle in forest edges and in paddocks with patches of thick scrub.