Blood cells infected with Babesia bigemina

The initial spread of tick fever in the late 1800s had a devastating effect on the cattle industry. In Queensland alone, it caused cattle numbers to decrease from 5.5 million to 2.5 million in little more than a decade. Since that time, improved methods of control have done much to limit the effect of the disease, but tick fever is still costing as much as A$28 million each year in lost production.

Cause

Bovine babesiosis is caused by two organisms in Australia: Babesia bovis and Babesia bigemina. Of the two species, B bovis is by far the most important, causing about 80% of outbreaks and an even higher percentage of deaths. Both Babesia species are single cell organisms that develop in the red blood cells of cattle and are transmitted in Australia by the cattle tick Boophilus microplus.

Transmission of B bovis takes place when engorging adult female ticks pick up the infection. They pass it on to their progeny via their eggs. Larvae (or seed ticks) then pass it on in turn when feeding on another animal. B bigemina is also passed from one generation of ticks to the next. Engorging adult ticks pick up the infection and nymphal and adult stages (not larval stages) of the next generation pass it on to other cattle.

Occurrence and spread

Boveine babesiosis is only found in eastern and northern parts of Australia where the cattle tick is present. One infected tick is sufficient to transmit the infection, but only a very small number of ticks actually carry the disease. In the case of B bovis, as few as 1 in 5000 ticks may be infected compared with 1 in 500 with B bigemina. As a result, B bigemina organisms are usually more prevalent in infected herds.

Animal susceptibility

British and European breeds are very susceptible and mortality after infection with Babesia species can be high. Bos indicus breeds such as Brahman, Sahiwal and, to a lesser extent, crossbred cattle show resistance to the disease. Despite this, nearly 1 in 5 outbreaks of tick fever involves these breeds.

Calves from immune mothers receive temporary protection (maternal antibody) from the colostrum (first milk), which prevents clinical disease in the short term. However, the calves may still be infected at this time. This protection lasts about three months and, in most cases, is followed by an age resistance that lasts until the animals are about nine months old. Calves exposed to infection when the maternal or age resistance is high rarely show clinical symptoms but develop a solid, long-lasting immunity.

It is therefore possible to have both Babesia organisms and cattle ticks present on a property without animal losses or clinical disease. This situation is known as endemic stability. On the other hand, cattle not exposed to babesiosis as calves will become susceptible to infection as the age resistance gradually wanes with time.

If exposed to the disease later in life, they may well develop a severe, life-threatening infection. Losses are likely when tick numbers on a property increase or when susceptible cattle are brought onto a tick-infested property. Endemic stability is less likely to develop to B bovis than to B bigemina because B bovis has lower infection rates in ticks than B Bigemina.

Clinical signs

Disease caused by B bovis is normally severe and large numbers of cattle can get sick and die. Symptoms include:

• fever (higher than 40^oC) for several days before other signs become obvious
• loss of appetite
• depression
• weakness and a reluctance to move
• red urine (haemoglobinuria) followed by anaemia and jaundice in the latter stages of infection
• diarrhoea
• abortion in cows
• death within days of the onset of fever
• nervous signs caused by a condition known as cerebral babesiosis.

When the nervous symptoms of cerebral babesiosis develop, the outcome is almost always fatal. Symptoms include:

• hypersensitivity
• circling
• head pressing
• aggression
• convulsions
• paralysis.

Disease caused by B bigemina is usually less severe but can develop very rapidly:

• anaemia occurs suddenly and severely, jaundice and death can occur with little warning
• red urine is present earlier and more consistently than in B bovis infections
• nervous signs are not seen.

Cattle that recover from either infection may take several weeks to regain conditions but recovery is usually complete. Surviving bulls may have reduced fertility for several weeks or months. In mild infections, signs are less obvious and sometimes even difficult to detect. Calves usually show no symptoms after infection.

Post-mortem examination

In severe infections, congestion of most organs is intense with haemorrhages under the membranes of many internal organs. Prolonged cases may show signs of anaemia and jaundice. Light-red to dark-red discolouration of the urine is often seen. The spleen is enlarged, sometimes several times its normal size, and the cut surface resembles raspberry jam. The liver is swollen and may be yellowish-brown, with the gall bladder distended with large amounts of thick granular bile. The kidneys and lymph nodes are also enlarged.

In cerebral babesiosis, the grey matter in the brain has a characteristic cherry-pink colour. Microscopically, B bovis infections cause massive numbers of infected red blood cells to accumulate in the small blood vessels of the animal. This is not the case with B bigemina.

Diagnosis

The history of the cattle, clinical signs and post-mortem lesions are often suggestive of babesiosis but it is impossible to make an accurate diagnosis based on these findings alone. The simplest method to confirm the infection is by laboratory examination of stained blood smears from sick animals. Even blood smears and organ smears from dead animals can be useful, particularly brain smears.

Laboratory examination will also distinguish between B bovis and B bigemina infections, an important feature when considering prognosis and control.

Treatment and control of outbreaks

The only drug available for use in the treatment of babesiosis in Australia is Imidocarb (Imizol^® or Imidox^®).

Note: Imidocarb has a 28-day withholding period in cattle and restrictions apply to its use in lactating dairy cattle in Australia.

Recovery is the rule if treatment is given early in the course of the infection. If treatment is delayed, supportive therapy may be essential if the animal is to survive. This includes the use of blood transfusions, intravenous administration of fluids, good nutrition, shade and perhaps other supportive therapies.

Prevention

There are several options for the prevention of babesiosis in Australia, ranging from keeping animals tick-free and use of Imidocarb for short-term control, to vaccinating with tick fever vaccine.