The genetics of worm resistance

Vet Sara Sutherland tries to sort the jargon around the genetics of resistance to parasitic worms.

Some breeds of sheep, and some individual sheep within a breed, will be more or less able to handle worms. Even though they are eating the same number of worm larvae in the grass, some will suffer from worms and some will be just fine.

By careful selection you can choose to breed sheep that aren’t affected by worms, reducing the need to drench. Genetics and immunology are the two areas with the most impenetrable jargon of all the scientific things farmers need to get their heads around. I’ll try to explain the jargon as simply as possible, without “dumbing it down”.

You can breed sheep to have lower worm egg count (WEC or faecal egg count FEC) and you can breed sheep to grow well despite a worm challenge.

What does low FEC mean?

A FEC measures the worm eggs present in the poos. Some poor sod looks at poos through a microscope and counts the eggs. You can have low FEC if you have fewer adult worms in the stomach or small intestine (depending on the worm species), adult worms are there but aren’t laying eggs, or immature worms are affecting the animal but not able to lay eggs yet.

Animals with diarrhoea will seem to have a lower FEC because of dilution. Animals that do more poos will seem to have a lower FEC. Also, different worm species lay different numbers of eggs. Barber’s Pole worm (Haemonchus contortus) lays thousands of eggs a day, where Teladorsagia (brown stomach worm) lays hundreds. So a FEC of 700 epg is more significant if it is mainly Teladorsagia than if it is mainly Haemonchus. We rely on FEC to reflect how many worms are affecting the animal but it’s not simple!

What other ways can we tell if an animal has worms?

Weight gain or BCS is the easiest to measure, FEC is the next easiest. Dag score is easy to measure but other things affect dag score, and some worms do not cause diarrhoea or dags.

It is possible to measure how many adult worms are in the stomach or intestines but you have to kill the animal and wash the worms out of the stomach or intestine and count them under a microscope. Which genes are being turned on or off in response to worm challenge is really hard to measure.

We call “resistance” the trait of having a lower FEC for a given worm challenge. Don’t confuse this with worms being resistant to drenches! The heritability of resistance is around 0.25 – in very simple terms that means a quarter of the difference in FEC in the next crop of lambs is because of genetics.

You can also select for resilience which is the ability of a sheep to grow well despite a high FEC. Resilience is the trait of not being affected by worms – in other words the sheep is eating the larvae, these develop into adults and lay eggs, but the lamb’s growth rate is not affected and they don’t develop ill thrift. When we look at genes, different areas of the genome are associated with resistance rather than resilience. This is good as it means a lamb can inherit both. And you can select for dag score independently of FEC and resilience!

Adult worms in resistant animals lay fewer eggs, and there are fewer adult worms (larvae don’t develop into adults and adults are expelled from the digestive tract). What happens in the sheep that makes this happen?

Antibodies are released from the body to respond to infection. Different types of antibodies have different names and do slightly different things. There are also cytokines which are the messengers that coordinate the immune system.

Sheep with resistance to worms produce more cytokines to release more antibodies. In the gut they “flush” larvae out by increasing gut movement and mucus production like a wormy slip ‘n slide.

Some of them can also kill adult worms. Others limit the establishment of adult worms in the gut. A lamb first limits how many eggs the worm lays, and later stops the larvae from developing into adults.

Resistant sheep release more cytokines and release more antibodies to fight off worms. Goats are less resistant to worms than sheep because they don’t produce as many of these antibodies.

Different genes are involved in these different aspects of immunity. A lot of genes are involved with resistance to worms. It wouldn’t make evolutionary sense for something so complicated and multi-dimensional to be controlled by only one or two genes. Also different factors turn genes on and off. A large number of genes each have a small effect rather than a few genes each having a large effect. More genes are involved with some causes of resistance (management factors that lead to resistance) than others.

Fewer antibodies around lambing

During the periparturient period (four weeks before to four weeks after lambing) sheep release fewer antibodies, so the adult worms that are there release more eggs. This is called the peri-parturient rise in egg counts or PPR.

The hormones oestrogen and prolactin are responsible for this decrease in antibodies. Leptin is a hormone that is higher in fat sheep, and a drop in leptin around lambing decreases the antibody response and cytokines. This explains why fat sheep have less increase in worm egg count around lambing than thin sheep (another reason to do your body condition scoring). Cortisol released during stress inhibits the release of cytokines and antibodies. Lots of different factors are involved in how antibodies are released, and lots we don’t yet know about.

Full immune response depends on exposure to larvae. In other words, lambs that are never exposed to larvae will take longer to develop immunity to larvae. That doesn’t mean you should avoid drenching lambs in summer to give this development a helping hand! The immune system of drenched lambs will do fine. Drenching doesn’t stop lambs from being exposed to larvae as they are still eating larvae after being drenched – the drench just kills the adult worms in the gut.

Change takes time

It took 20,000 years for sheep to become how we know them today. Genetic solutions are important in the fight against worms, but don’t expect miracles overnight. Instead, incorporate the genetics of worms into your ram selection each year along with other traits that are important to you. Talk to your ram breeder about what they are including in their selection, and how much they are focusing on this.

Apart from buying rams with a low worm FEC genotype, how can you influence immunity in your animals? High protein diets increase eosinophil release and the cells in the abomasum that directly protect the lining from worms.

Antibodies are made out of protein. Well-fed animals are also likely to show more resilience. All the changes I have talked about (antibody release etc…) don’t start the moment a lamb is born but develop slowly over the first 12-16 months of life. Target worm control measures at young animals.

Do a Wormwise course or talk to your vet about your parasite management plan – there are lots of tools you can use to reduce the impact of worms on your flock.

• Sara Sutherland is a veterinarian with Veterinary Services Wairarapa.