Experts study genetic parasite resistance

Ability to resist invaders | Researchers hope to find animals with chromosomes that offer nematode resistance

VANCOUVER — The growing problem of parasites resistant to chemical treatments might have a genetic solution. 


Farmers need a long-term strategy to enhance an animal’s ability to tolerate the negative effects of worms and eventually develop resistance through genetic selection or vaccines, says French researcher Nathalie Mandonnet of Institut National de la Recherche Agronomique (INRA).


An animal has two mechanisms to defend itself against parasites: tolerating the infestation or either preventing its entry or expelling it once it gets inside. 


“There is a lot of knowledge, but depending on the population of parasites, the mechanism of resistance is not always the same,” Mandonnet told the World Congress on Genetics Applied to Livestock Production, which was held in Vancouver Aug. 17-22.


Early animals could live with parasites, but the balance of nature was disrupted once they were domesticated and selected for better growth and other economically valuable traits. Researchers think that genetics could restore the equilibrium between host and parasite.


Certain tropical breeds of goats and sheep are known to have parasite resistance. 


However, genomic information is limited, especially for goats. As well, a single animal may have to fight off a number of different worm species. 


Nevertheless, farmers could look for resistant strains of animals and introduce integrated control that could include drugs to fight parasites. 


“The farmer has to combine two or three solutions,” Mandonnet said.


Producers could also try grazing two different species or breeds of animals in a single pasture as well as offer improved nutrition. This helps reduce the effects of parasite loads because the animals are consequently healthier and more productive.


Resistance has also been found against gastrointestinal parasites in certain breeds of sheep, said Valentina Riggio of the Roslin Institute and the University of Edinburgh.


European researchers have found some spots on sheep DNA that appear to offer resistance to parasites. They found markers near chromosome 13 as well as other chromosomes that may offer resistance to parasites that sheep pick up when they graze pastures.


“We did identify regions of interest for resistance to nematodes in Scottish Blackface sheep,” Riggio said.


“We were curious to see if we could identify chromosomal regions across populations.” 


Selection for nematode resistance has mainly been based on the use of fecal egg counts. However, this is costly and time consuming and may not indicate which worms the animals picked up. A better approach would be to select animals that carry resistance. 


Data was collected on 750 Scottish Blackface lambs, 2,370 Sarda-Lacaune backcross ewes, 1,000 Martinik Blackbelly-Romane cross lambs and 64 Texel lambs. The animals came from different environments, which was done to see if animals might develop resistance on a regional basis. 


All animals were genotyped using a 50k SNP chip. 


Some SNPs were found to be im-portant for more than one trait within the same parasite species, although the significance varied with the trait. Researchers found eight SNPS associated with strongyles and nematodes, common worms that infest livestock. 


Research showed it was easier to predict resistance to parasites when the animals were closely related, but with current technology it was less successful in finding resistance genes among crossbreds.

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