Breeding for disease resistance

By Nicola Dennis

I have been ill a lot this winter. That is the thing about living with a paramedic and a small child who licks everything. Every disease that is in circulation walks straight through the front door. Should Ebola hit our shores, I expect I will be one of the first to perish.

You will survive me in that case, but when it comes to livestock diseases, most of us are trapped in the same germy boat. The explosion of Mycoplasma bovis farms across the country has taught us that our internal biosecurity (i.e. between farms) has all the integrity of a toddler licking a toilet seat.

My email inbox has been overflowing with queries and speculation about the Mycoplasma bovis chaos, it has really captured the hearts and minds (and other parts of the anatomy) of the ag community. There are few things more alluring to a scientist than a juicy problem and the whiff of authority effing it all up. Putting that aside, there is no escaping that we farmers, as a collective, are to blame. Pretty much all kiwi farmers (not just cattle owners) rely heavily on stock trading and contract grazing to manage the ebbs and flows of pasture growth. We are moving god-knows-what around the country on a daily basis.

Where does breeding and genetics fit into this hot mess? There are two aspects. Firstly, if a disease comes through and kills (or entices the government to kill) a large quantity of animals, then this limits the higher merit options available for breeding replacement stock. For those that fall in with the crowd, this would be setback, but there will probably be enough variation in the animals left in NZ (or overseas) to recover what was lost. For those breeding niche traits or rare breeds, then all progress could be lost. If you value the effort you have put into your breeding program, then biosecurity should probably be a whole lot more than the latest buzz word!

My second point is a little less dismal. We can breed for disease resistance, so that our animals stand a fighting chance from the outset. There are overall disease resistant traits that can measure the strength of the animal’s general immunity, such as the antibody- and cell-mediated immune response tests pioneered by Bonnie Mallard in Canada. Less high-tech approaches, such as survival/longevity traits, can serve a similar role. General immunity is important because there is an inevitable trade off between production and health. If we push for a streamlined animal that converts nearly all its food into production, then we begin to throw away things that are needed in a crisis, such as the immune system and body fat.

We can also breed for resistance against specific traits. NZ Ram breeders are making good gains in breeding for Facial Eczema resistance in sheep. Overseas, the UK has a breeding index, TB advantage, to breed dairy cows that have better resistance to Mycobacterium bovis (aka “Bovine tuberculosis” or “the real M. bovis, thank you very much”). This approach can be powerful, but there is a caveat. Like all other traits, there needs to be good recording of the phenotype. Without good data there is no trait. The facial eczema trait works because animals can be given the disease (in a controlled way) and the resulting liver damage (or lack thereof) can be measured via a blood test. This means a lot of information is available for a single animal. The TB resistance trait is a bit more of a bucket approach and this works because the whole country is testing for the disease.

At this point, breeding for resistance to something like Mycoplasma bovis is a nonstarter. It is so difficult to determine if a single cow is infected, and clinical signs are so erratic, that those in charge are forced to cull whole herds in the quest for eradication. Diseases that can be accurately diagnosed or nonspecific symptoms (such as lameness) that can be recorded in large enough numbers are, however, ripe for the picking.