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Cattle tuberculosis is on the increase

March 20, 2002

Foot and mouth may have masked the spread of another cattle killer

Cattle being slaughtered. Restrictions on animal movements. Farmers facing ruin. It all sounds horribly familiar to anyone who has lived through Britain`s foot and mouth (FMD) epidemic. This time, though, the culprit is bovine tuberculosis. The disease is on the increase and FMD may have helped it to spread.
        
In 2000, nearly 9000 cattle tested positive for TB and were slaughtered to control the disease. This pales in comparison with the 600,000 cattle destroyed during last year`s FMD crisis. But TB has been endemic in British herds for decades. It is a chronic problem for farmers in disease hot spots-notably south-west England and south Wales.
        
Now New Scientist has learned that the foot and mouth outbreak has left scientists with virtually no idea how bad the TB problem is. This is because TB testing stopped during the FMD epidemic, because of fears that vets would spread the FMD virus between farms. Consequently, some 26,000 TB tests on suspect cattle are overdue. And because no cattle have been culled in the meantime, TB is almost certainly spreading faster in infected areas. "We anticipate there will be an increase," says John Bourne, chairman of the Independent Scientific Group on Cattle TB, "but it is difficult to know until the tests are completed." The ISG is an independent panel of scientists that advises the British government.
        
One consequence of the backlog is that farmers whose herds were destroyed because of FMD may be unwittingly restocking with TB-infected cattle. It`s already happened in Cumbria and Scotland and more cases will probably emerge as testing catches up. Earlier this year, the government reacted to the problem by banning the movement of animals from the 1000 farms most at risk from TB until they have been tested. But it could well be too little too late, as farmers have been restocking since October.
        
The FMD epidemic may also have compromised the government`s attempt to get to the bottom of cattle TB. Badgers are the main suspects-some scientists think they could be a reservoir for the disease. Many farmers are also convinced that badgers spread the infection between farms, pointing out that the number of infected cattle has risen since legislation to protect badgers came into force in 1991. To resolve these questions, a massive field trial was launched in 1998. The five-year trial is designed to find out if culling badgers in TB hot spots curbs the disease in cattle.
        
Dubbed the Krebs trial, after the scientist who designed it, the experiment involves 30 plots of 100 square kilometres grouped into 10 triplets. In the first plot of each triplet, badgers are culled from the start. In the second, culling is carried out only if there is a TB outbreak, while in the third-a control area-badger numbers are monitored with no culling. The aim is to find out if fewer TB infections occur in the culled areas. But all fieldwork came to a halt during the FMD epidemic and 21,000 cattle in the trial areas were destroyed in the effort to halt the disease.
        
The trial has now restarted. But Steven Harris, an expert on TB in badgers at Bristol University, says it`s becoming a farce. Farmers have already been illegally culling badgers in some of the control areas, says Harris, and he doesn`t believe the surveys are accurately counting badger numbers. "With foot and mouth we`ve got yet one more variable on top of that."
        
Bourne admits the FMD crisis will make it harder to find out if badgers are spreading TB between cattle. But he says if farms are restocked with uninfected cattle, scientists will have the perfect opportunity to study how the disease passes between herds, and what steps farmers can take to reduce the risk to their cattle.

Author: James Randerson

New Scientist