Jan. 13, 2000 MADISON - University of Wisconsin-Madison animal scientist Mark Cook is finding ways producers can raise chickens economically with fewer antibiotics. "I believe that our new tools and strategies will increase both animal and human health," he says.
The research could slow the development of antibiotic-resistant bacterial strains, preserve the useful life of antibiotics, and allow scientists to breed chickens with strong immune systems, according to Cook, a researcher at the UW-Madison College of Agricultural and Life Sciences.
There have been growing concerns about the development of new bacterial strains resistant to antibiotics. The increasing number of antibiotic-resistant strains means doctors are losing their ability to cure human infections.
One controversial aspect of the antibiotic-resistance issue has been whether the routine feeding of antibiotics to agricultural animals contributes to the increase in antibiotic-resistant strains. European countries have banned the routine use of antibiotics in chicken feed. However, Cook believes that banning antibiotics would dramatically increase food costs, leave animal producers with few strategies to control diseases and pathogens, and greatly reduce investment in new pharmaceuticals to fight infection in both animals and people.
"To keep birds healthy, we've come to rely on antibiotics, which promote growth by warding off infections and their growth-reducing effects," Cook says. "We need other ways that poultry producers can keep the birds growing rapidly without feeding antibiotics routinely in the diet."
In the 1950s, Cook says, researchers found that animals fed antibiotics grew faster than those not fed the compounds. Using antibiotics in feed gave producers an economic advantage in raising animals to market weight quickly and at low cost. The routine addition of growth promoting antibiotics to animal feed soon became commonplace in agriculture.
When the practice began, researchers didn't fully understand why antibiotics improved animal growth, according to Cook. "You might expect that infectious microbes themselves decrease animal growth. Instead, the animal's own immune system is responsible for the growth depression that occurs when an animal is infected."
"Chickens respond to immune stimulants much the way we do to the flu," Cook says. "They don't want to eat, their muscles begin to break down and they lose weight."
The reason, Cook explains, is that when faced with a foreign substance -- such as an infectious microbe -- the immune system's white blood cells release cytokines. These chemical messengers cause the gut to produce peptides that result in animals not eating. The cytokines also trigger a response in muscles that promotes wasting.
Cook has tested two compounds that allow birds to grow just as fast as birds fed antibiotics. The compounds don't kill microbes but work by blocking the pathways that lead to weight loss.
The first is an antibody -- not an antibiotic -- that Cook identified. The antibody binds to the gut peptides, keeping them from causing the birds to lose their appetites. When fed the antibody, chickens retain their appetites, even when their immune systems are attacking microorganisms.
Cook has patented a method for producing the antibodies in eggs, a rich source of antibodies. Dried egg powder from these birds can be fed to large flocks. The egg antibodies improve feed conversion and growth, but should not lead to drug-resistant bacteria. The company with the exclusive rights to the production method is scaling up for mass production with the help of many other companies.
The other compound Cook has tested is conjugated linoleic acid (CLA), which was originally identified as a compound in ground beef that inhibits some forms of cancer. Cook believes that CLA inhibits the message that triggers the breakdown of muscle cells.
Based on his analysis of studies of domestic poultry, Cook believes that chickens today have weaker immune systems than birds grown on farms 50 years ago. He and several colleagues at the UW-Madison have begun a long-term project to test this idea.
"I believe that by selecting for animals that grow rapidly, we've bred animals with weakened immune systems," he says. "Animals that respond strongly to diseases, vaccinations and other immune system stimulants eat less and lose muscle mass. Breeders can't now select for birds with strong immune systems because that means selecting for slower-growing animals, which would put them out of competition in the industry."
Feeding animals antibodies and CLA, rather than antibiotics, will allow breeders to select for animals with stronger immune systems, according to Cook. That would lead to animals better able to fight off infections on their own, thus further decreasing the need for antibiotics to treat sick birds.
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