Tick fever is an infectious condition caused by one of the three protozoan parasites that target red blood cells of cattle (Babesia bovis, Babesia bigemina and Anaplasma marginale). The disease occurs in northern Australia where cattle tick (Boophilus microplus) is endemic.
Tick fever is transmitted from infected to non-infected cattle usually by tick bites but sometimes by veterinary instruments (Anaplasma). The organisms invade red blood cells causing destruction of many infected cells. The resultant anaemia, fever and weakness may kill the animal, especially older, Bos taurus animals. Survivors take weeks or months to recover.
Babesia bovis is responsible for about 80% of outbreaks. Bos taurus breeds are very susceptible while Bos indicus breeds and crossbreds show some resistance.
In the live export process, cattle may develop tick fever if exposed to cattle ticks in the previous one to four weeks. Cattle most at risk are those from non-tick areas introduced to tick areas in preparation for export.
Rapid loss of condition and dark urine are the most common signs first noticed. There is fever, anaemia, jaundice, brown urine from excreted liver pigments in anaplasmosis, red or black urine from haemoglobinuria in babesiosis, and often deaths. Nervous signs commonly occur with Babesia bovis resulting from parasitised red blood cells blocking capillaries in the brain.
Necropsy findings include thin watery blood, jaundice (yellowing of normally white tissues), enlarged spleen with a cut surface resembling raspberry jam, enlarged and yellow-brown liver, and the gall bladder is distended with thick black bile. Urine in the bladder is dark.
Laboratory confirmation in live animals requires unclotted blood in EDTA tubes submitted chilled for haematology and parasitology, and thin and thick air dried blood smears for parasitology. Dead animals additionally require impression smears from organs including the kidney, liver, spleen and brain (cerebral cortex) for parasitology. Sections of brain, kidney, liver, heart and spleen may also be submitted chilled for parasitology and in buffered formalin for histology.
Differential diagnoses include anthrax where there is sudden death and enlarged spleen, haemonchosis where there is profound anaemia, and lead poisoning where there are nervous signs and death.
Early treatment with imidocarb diproprionate usually results in a rapid recovery and can eliminate protozoal organisms from infected cattle. Treatment with oxytetracycline antibiotics may reduce severity of symptoms if imidocarb is unavailable.
Vaccines are available to stimulate immunity and acaracides to keep animals tick free. Some animals may react to vaccination with signs similar to tick fever that may persist for up to 60 days. It is suggested that animals not be vaccinated immediately before they are loaded onto an export vessel since the stress of export may predispose them to vaccine reactions. Veterinary instruments such as dehorners should be cleaned and disinfected between animals.