Tickborne Fever (Pasture fever) in Ruminants
Tickborne fever of ruminants is caused by ANAPLASMA PHAGOCYTOPHILUM that invade neutrophils and monocytes. Transmitted by the tick IXODES RICINUS and has been reported in Europe, India and South Africa. There can be fever and leukopenia with other signs absent or mild, but infection predisposes animals to other diseases. In-utero transmission of this disease to a calf was reported.
Tickborne fever (TBF) is a febrile disease of domestic and free-living ruminants in the temperate regions of Europe. TBF is prevalent in sheep and cattle in the UK, Ireland, Norway, Finland, The Netherlands, Austria, and Spain. It is transmitted by the hard tick Ixodes ricinus . A similar disease transmitted by other ticks has been described in India and South Africa. The main hosts are sheep and cattle, but goats and deer are also susceptible.
After infestation with infected ticks, the incubation period may be 5-14 days, but after injection with infected blood, the incubation period is 2-6 days. In sheep, the main clinical sign is a sudden fever (105-108°F [40.5-42.0°C]) for 4-10 days. Other signs are either absent or mild, but the animals generally appear dull and may lose weight. Respiratory and pulse rates are usually increased, and a cough often develops.
In cattle, the disease is known as pasture fever in many parts of Europe, including Finland, Norway, Austria, Spain, and Switzerland. The disease occurs as an annual minor epidemic when dairy heifers and cows are turned out to pasture in the spring and early summer. Within days, the cows are dull and depressed, with a marked loss of appetite and milk yield. Affected cows usually suffer from respiratory distress and coughing. Clinical signs are more obvious and last longer in newly purchased animals than in home-bred animals. Often, veterinary advice is sought after an abrupt fall in milk yield.
Abortions affect susceptible ewes and cows newly introduced onto tick-infested pastures during the last stages of gestation, with abortions occurring 2-8 days after the onset of fever. Except for aborting ewes, death due to TBF is rare. The quality of the semen of infected rams and bulls may be greatly reduced. Variations in severity of the clinical effects may be related to differences between strains of A phagocytophilum or in host susceptibility.
Perhaps the most significant effect of TBF infection is its serious impairment of humoral and cellular defense mechanisms, which results in increased susceptibility to secondary infections such as tick pyemia, pneumonic pasteurellosis, louping ill, and listeriosis.
In sheep, the onset of high fever in tick-infested areas during the spring and summer in association with hematologic changes and the presence of inclusions within granulocytes is diagnostic. The clinical disease usually is seen only in young lambs born in tick-infested areas or in older animals newly introduced to such areas. Blood smears should indicate the association of TBF and cases of tick pyemias and abortions, particularly when abortions occur after pregnant animals are moved from tick-free to tick-infested pastures. TBF could be established retrospectively as a cause of abortions by demonstrating a rise in antibody titers by indirect immunofluorescence.
In affected dairy cattle, the main clinical signs are abortions and a sudden drop in milk yield. The other clinical sign commonly associated with TBF in cattle is respiratory illness after a herd is introduced to tick-infested pastures. TBF must also be considered when abortions and stillbirths, particularly in heifers, occur soon after their introduction to tick-infested pastures. Therefore, in areas where TBF is enzootic, blood smears must be examined for the presence of organisms in all cases of abortion in sheep and cattle and when milk yield is suddenly reduced soon after the animals have returned to pasture.