After being experimentally infected with a virulent
strain, the cattle lacking any copies of the PSR gene were so ill they had to be euthanized. Cattle with 1 copy of the gene were susceptible, but resisted illness and death for twice as long. Black cattle with 2 copies of the gene survived for
up to 10 days. Non-black cattle with 2 copies of the gene were signifi cantly more resistant; the researchers could not produce illness (nor any shedding in feces) in these animals, even using 10 times the typical chal- lenge dose of bacteria. “The non-black genotype and phenotype are con-
ferred through a gene called MCR. Non-black is a re- cessive phenotype; you must have 2 recessive alleles (1 from each parent) for the animal to be non-black,” says Carlson. Black is always dominant. The dominant MCR gene encodes a receptor needed
for black pigmentation, and has other functions. The recessive MCR/MCR cattle are not only non-black, but research suggests that they metabolize antibiot- ics faster (shorter withdrawal time needed) and also have an elevated pain threshold — less likely to go off feed when ill. Cattle with less resistance may have the bacterium
in their intestines and shed it, or may have intermit- tent diarrhea.
“The company also looked at less virulent strains
of salmonella and lower doses of the pathogen. They found that the non-black cattle with 2 copies of the PSR gene do not shed salmonella when infected, and had very few bacteria colonize in the intestine,” Carl- son says. Therefore, these resistant cattle don’t pass the disease to susceptible cattle. The few salmonella bacteria found in the intestinal tract of resistant cattle tend to be inactive and unable to cause disease.
Genetic test “The next step was to develop a genetic test that
could be licensed, to offer cattle producers a food-safety tool to minimize these pathogens,” he says. This might be of interest to producers trying to raise natural beef. “The company now offers this test to producers. It’s
been a struggle, however, because of the popularity of black cattle. Thus it will probably be a niche market for other breeds and for producers who want to market animals on their own,” says Carlson. This test might be of interest to producers of non-black cattle to see if their animals are resistant to 2 important food safety pathogens, which could be a marketing tool. PSR is now offering a test in which the prevalence
of the PSR SNP can be determined in a herd or in a breed. Hair samples from multiple cattle can be sub- mitted — up to 100 animals at a time. “We are not yet offering individual animal testing, but hope to make individual tests available sometime this year,” says Carlson. Carlson also says recent research has shown these
Resistant cattle don’t pass the disease to susceptible cattle.
resistant cattle are healthier and perform better in the feedlot. “Our research with these cattle in a closed herd reveals that these animals are the healthiest ones in the herd. They have the best average daily gains, fertil- ity, longevity and marbling scores. By identifying and propagating resistant cattle, the beef industry could produce
healthier cattle and also dramatically reduce the in- cidence of beef-associated outbreaks of salmonella and E. coli O157:H7.” This type of natural disease resistance does not re-
quire immunization or prior exposure, so the animals don’t require handling and vaccination to instill this resistance. “This natural resistance is not compromised by stress, unlike immunity which can be compromised during weaning, shipping, commingling of cattle, etc.” says Carlson. This resistance is simply part of the ani- mal’s genotype and general makeup.
76 The Cattleman August 2014
thecattlemanmagazine.com
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