May 23, 2012
Parasite Proves To Be Troublesome For Diagnosing Bovine Tuberculosis
Brett Smith for RedOrbit.com
Researchers have uncovered new clues in the battle against bovine tuberculosis (bTB), which claimed the lives of thousands of U.K. cattle and wildlife last year.A parasitic flatworm often found in cattle, Fasciola hepatica - or common liver fluke, reduces the sensitivity of bTB skin tests and produces false negatives in the animals, according to a team of U.K. scientists at the Universities of Nottingham and Liverpool.
Many in the British beef and dairy industry fear bTB could be spreading across the countryside because this widely used skin test for the disease is rendered ineffective when cattle are infected with the liver parasite. In 2011, concerns of a bTB epidemic resulted in the slaughter of approximately 25,000 cattle in England, at a cost to the country of more than £90 million, or $140 million (U.S.). Solutions for eradicating the infection have included a massive culling of badgers, which carry the disease along with other wild animals such as deer and foxes.
According to the study published in Nature Communications, researchers tested milk from more than 3,000 dairy herds across England and Wales for signs of infection and added the data to an existing model of bTB transmission. Once they assumed that a fluke infection inhibited bTB detection, they achieved a closer match between the model and actual bTB detection rates.
“Everyone is aware that current methods aren´t detecting early enough or with enough sensitivity,” said the University of Liverpool´s Diana Williams, an author of the paper. “We need to look at better control of fluke.”
The authors suggest that the fluke may alter the production of key cells and proteins in the immune system, which are vital to achieving a genuine result in both the skin test and the second most common test for bTB, the IGRA blood test.
Common liver fluke live in the livers of mammals, as the name denotes, and produce eggs which are then passed on into the intestine. Anemia can result in animals that are heavily infected with the flatworm. Interestingly, several chemotherapy drugs have been developed using proteases secreted by the parasite.
"We have been very interested in the ability of Fasciola hepatica to modulate host immunity for some time and this study is a worrying example of when this occurs in nature given an estimated 70-80 per cent of dairy herds show signs of liver fluke infection demonstrates the scale of this problem,” said Robert Flynn, of the University of Nottingham and co-author of the study.
Previous studies have shown that bTB is spread primarily through the transmissions of respiratory secretions from infected to uninfected animals. This transmission usually happens when animals are in close contact with each other when the bTB-causing bacteria released into the air through coughing and sneezing. Research also suggests that the disease can also be contracted from ingesting contaminated feed.
The results of the study could have implications beyond cattle. Both human liver flukes and bTB infections are common in tropical and sub-tropical areas, particularly in Africa.
“We know that a similar immune mechanism exists in humans,” Dirk Werling, an immunologist at the Royal Veterinary College in Hatfield told Nature.
“The potential consequences of these observations could potentially be quite severe, not only for the farm animals, but also for people in third-world countries.”