In a bid to produce cattle resistant to tuberculosis, scientists have produced genetically engineered cows containing a gene known to convey some protection against the disease. Although it did not render all of the animals impervious to the tuberculosis-causing pathogen, some were able to resist infection and others showed minimal disease symptoms. These encouraging results raise the possibility that in the future, this technique could be used in favor of current methods that have undesirable consequences, such as culling or antibiotics.
Bovine tuberculosis (TB) is a chronic infectious disease caused by a bacterium called Mycobacterium bovis, a close relative to the organism that causes human tuberculosis. The disease is widely distributed across the globe and affects a broad range of mammals, such as badgers, bison, elk and African buffalo, but cattle are the primary hosts. Bovine TB is a serious worldwide threat because no effective programs exist to eliminate or control it in less developed areas affected by it. Furthermore, it can also spread to humans, typically via the ingestion of unpasteurized milk, and then subsequently between humans.
The typical reactive control measure to a positive TB diagnosis is to cull the entire herd, which is obviously devastating to the farmer. In the U.K., culling of badgers is also being trialed since these animals are reservoirs of the disease. However, this has been met with strong opposition not only because it is extremely expensive, but also because studies have indicated that it is ineffective since it disrupts badger setts, encouraging them to disperse and hence contact more cattle.
It’s clear there is a need for more research into control strategies to curb this problem, which is why scientists from the Chinese agriculture ministry began exploring the possibility of producing TB-resistant cows. As described in Proceedings of the National Academy of Sciences, the researchers adopted a widely used genome modification tool that allows scientists to both insert and delete genes at a high level of precision. With this technique, the scientists were able to insert a mouse gene called SP110 into the DNA of Holstein-Friesian cows, a breed of large dairy cattle. This gene has been shown to help protect mice against infection with TB.
Of the 23 genetically modified (GM) calves produced, 13 survived to adulthood. When they grew cells from these cows in the lab, they were found to be more resistant to infection with M. bovis than cells taken from non-GM cows. Next, they introduced the pathogen into the lungs of three GM cows and three control cows. One of the GM cows completely resisted infection, whereas the other two showed significantly fewer signs of disease in their organs than the control animals.
Lastly, to mimic natural transmission scenarios, the scientists housed nine GM cows and nine controls with infected cattle. Impressively, while all of the controls became infected, six of the GM cattle resisted TB and the others displayed few disease symptoms. Furthermore, the scientists found that the gene did not appear to affect the rest of the cows’ genome, and could also be passed from parent to offspring.
Although this technique doesn’t produce completely resistant cattle, livestock expert Mike Coffey points out that it could help raise the general resistance, which could slow down disease transmission.