05/04/2005 - Gene-transfer technology used to produce dairy cows capable of resisting a widespread bacterial infection could help dairy producers increase yields.
US scientists have used the technology to produce dairy cows capable of resisting mastitis, a widespread bacterial infection.
The infection is the most costly disease of the dairy industry and affects both the quantity and quality of the milk produced.
"This research is an important first step in understanding how genes can be used to protect animals from disease," said Edward B. Knipling, administrator of the US department of agriculture (USDA)'s agricultural research service (ARS).
This scientific discovery, published in the current edition of Nature Biotechnology, demonstrates the potential of biotechnology for developing cattle with resistance to mastitis. Currently, vaccines, antibiotics and a cow's own immune system cannot effectively fight the bacterium Staphylococcus aureus, a major cause of mastitis.
The disease is a major problem for dairy producers, who face tough economic conditions and ever-tighter margins. A study into the effect of clinical mastitis on milk yield, which was published in the Journal of Dairy Science, found that the daily loss during the first two weeks after the occurrence of mastitis varied from 1.0 to 2.5 kg, and the total loss over the entire lactation varied from 110 to 552 kg and depended on parity and the time of mastitis occurrence.
Regardless of the time of occurrence during the lactation, mastitis had a long-lasting effect on milk yield; cows with mastitis did not reach their premastitis milk yields during the remainder of the lactation after onset of the disease.
In an attempt to solve this issue, the ARS scientific team led by Robert J. Wall, an animal physiologist with the ARS Biotechnology and Germplasm Laboratory in Beltsville, built a transgene - genetic material produced using recombinant DNA technology - that included the genetic code for producing a naturally occurring, antimicrobial protein called lysostaphin.
Research shows that the gene for secreting lysostaphin comes from a non-pathogenic species of Staphylococcus that uses the protein to repel its cousin, S. aureus. The scientists introduced this transgene into Jersey cows. The lysostaphin is secreted into milk, where it kills S. aureus, thus protecting cows from becoming infected.
"The three genetically engineered cows that have been tested so far are expressing lysostaphin in their milk and are resistant to S. aureus intramammary infection," said Wall.
*All three transgenic cows showed little or no sign of infection after repeated exposures to S. aureus - and one, named GEM, never became infected, indicating complete protection."
Overall, the researchers found that in tests, 71 per cent of the mammary glands that were exposed to S. aureus from nontransgenic animals became infected - compared to only 14 per cent for the transgenic animals.