SHEEP FARMERS in Australia and New Zealand have a problem. Their charges are susceptible to intestinal nematode worms which cause them to waste away and can, in severe cases, kill them.Over the years, the worms have evolved resistance to drugs once used to attack them. But a second approach is to breed countermeasures into the sheep themselves. This can be done by counting the number of worm eggs in animals’ droppings and selecting as sires and dams those that have the fewest, on the assumption that these are the sheep with the least worm-friendly guts, and that this property will show up in their offspring as well. That works. But how, has been obscure. A study just published in Animal Microbiome, by Erwin Paz of the University of Western Australia and his colleagues, throws light on the matter. It suggests that what is being bred for is a propensity to develop a worm-hostile gut microbiome.Gut microbial health, once a fringe medical interest, is now mainstream for human beings, as bugs in the alimentary canal are linked with conditions ranging from obesity to arthritis. But what is true of people is true of other animals, too. And, in the case of sheep, Dr Paz and his collaborators seem to have hit the jackpot.Dr Paz knew from past work that sheep with severe worm infestations tend to have abnormal bacterial populations in their faeces. This led him to wonder whether the bugs in the guts of resistant sheep were somehow hampering the worms’ activities. He and his team therefore looked at a flock of 200 sheep at a farm in Western Australia and selected ten that had particularly high loads of worm eggs in their faeces (1,940 per gram, on average) and ten that had low loads (410 per gram). These unfortunate sacrificial beasts were then slaughtered and their entrails examined—not for auguries of the future but for their microbial populations.As they had hoped to, the researchers did indeed find systematic differences between the microbiomes of the two groups. These differences were particularly notable in the small intestine, where the worms live, with resistant sheep having richer and more diverse bacterial populations in this part of the gut than vulnerable animals did. In particular, the team noticed that populations of bacteria which ferment carbohydrates such as cellulose and turn them into short-chain fatty acids were especially abundant in the resistant animals.It is, of course, possible, that the worms are affecting the sheep’s microbiomes, rather than the other way around. But Dr Paz thinks this is unlikely, because all of the sheep looked at had at least some worms. Rather, he suspects, genetically resistant sheep are providing an intestinal environment conducive to bacteria which either produce compounds that repel the worms or consume resources which the worms depend upon. If that is correct, it opens up a third approach to worm control, which is to employ probiotics to shape the gut microbiomes of flocks directly, rather than relying on selective breeding to do it at one remove.To enjoy more of our mind-expanding science coverage, sign up to Simply Science, our weekly newsletter.