Udder infections can hit dairy herds hard.© Alamy

Each year, the dairy industry loses billions of dollars to mastitis, an infection of cows' milk glands. Now researchers have succeeded in genetically engineering cows to resist this disease.

The bacterial infection causes inflammation and swelling, and a loss in milk production. Combined with the cost of treating the disease, this adds up to a loss of about $2 billion a year in the United States, and $200 million in Britain.

Technological advances that make it possible to collect more milk also make mastitis harder to contain. "We've increased their yield with the milking machine, which spreads infections from cow to cow," explains Andrew Biggs an expert on mastitis at the Vale Veterinary Centre in Tiverton, Devon, UK.

One bacterium that commonly causes mastitis, Staphylococcus aureus, is notoriously resistant to treatment: only 15% of infections are cleared up by antibiotics. The medications often fail to fully penetrate the mammary glands, leaving the surviving bacteria to wreak havoc yet again.

Milking the genome

To solve this problem, a team of US researchers turned to genetic engineering. They introduced a gene from the related bacterium S. simulans into the DNA of Jersey cows. This allows them to produce a protein, normally created by S. simulans, that kills S. aureus.

This is the first time that biologists have created transgenic cows that resist disease. Previous attempts to improve animals' disease-fighting abilities have used chickens and sheep.

Using genetic engineering to tackle mastitis represents a judicious use of the technology, claims Robert Wall of the US Department of Agriculture in Beltsville, Maryland, who helped to breed the cows. "This technology is very powerful, but so is selective breeding. So we did not want to 'waste' genetic engineering on a task that animal breeders could already achieve," he says. "That made mastitis an attractive target, because breeding for mastitis resistant cows has not been very successful."

Wall and his colleagues created five adult transgenic cows carrying the gene for the S. simulans protein lysostaphin. The three that underwent testing showed significantly better resistance to S. aureus infection than their non-transgenic counterparts, the researchers report in Nature Biotechnology¹.

Of the mammary glands infused with the mastitis bacteria cultures, only 14% became infected in the three transgenic cows compared with 71% in the control animals. The transgenic cow that produced the greatest amount of lysostaphin never became infected.

All white?

Does the lysostaphin protein present any problems if ingested by humans? The researchers do not believe so. "It seems not to attack any protein produced by mammals, and therefore is unlikely to be harmful to the cow or the consumer," says Wall.

However, Biggs is sceptical that farmers will buy cows altered by genetic engineering. Products such as beef have a chequered history because of mad cow disease, but "milk has a squeaky clean image", he says. "Genetic engineering may decrease mastitis incidence, but if consumers vote with their feet then those cows aren't going to have a future."

Biggs also points out that this advance does not cure all types of mastitis. "This is only targeted at one of the pathogens causing mastitis, so the benefits would be concentrated in only those herds that have problems with S. aureus," says Biggs.

"It's a contagious germ that can be controlled with good hygiene and good milking practices," says Leo Timms of Iowa State University in Ames. "This is a neat technology, but it's not a substitute for all of the other prevention practices."

Wall does not expect these transgenic cows to appear in the dairy industry in the immediate future. "These animals are only a first step in what will probably be a decade-long process," he says. 

US Department of Agriculture, Beltsville, Maryland

• References

  1. Wall R. J., et al. Nature Biotech., Advanced online publication, doi: 10.1038/nbt1078 (2005).