The Good and the Bad- Resistance in Cattle, Goats and Sheep and their Parasites

Our livestock are constantly facing attack from lice, worms, ticks, and flies. Modern treatments have been provided that effectively kill these pests, with minimal risk to the health of the animals or the people that apply them. But these treatments won’t work forever. Here we examine the reasons why parasites develop resistance to chemicals, and what we can do to out-think them.

What is resistance?

When talking about parasites, the word ‘resistance’ is used in two different ways.

1. Resistance= immunity. This is the ‘good’ resistance.

Some populations of animals are naturally ‘resistant’, or immune, to infection with parasites. A good example is adult cattle that have been exposed to small intestinal worms (Cooperia) for several months. Their immune systems will act to expel adult worms, prevent juvenile worms from establishing in the gut and reduce the number of eggs each female worms lays.

In a typical mob of cattle, sheep or goats, there is a large amount of variation in the level of resistance (immunity) that each individual develops. Some animals are very susceptible to the parasites, while others appear to be bulletproof. This is the basis for selection for resistance, using tools such as MerinoSelect to breed worm-resistant sheep, or worm-resistant goats.

It is also the main reason why some breeds of cattle, such as tropical breeds, are naturally resistant to ticks. Crossbreeding allows these characteristics to be shared with other breeds.

The best thing about using selection and breeding for resistant stock is that the superior ‘immunity’ genes are passed down to the offspring, meaning that the benefits stay on the property and multiply ‘herd immunity’ in each generation.

Resistance to parasites may be ‘acquired’, such as the example with cattle Cooperia above, or ‘natural’, such as where a parasite will be rejected by the majority of stock in a mob, regardless of previous exposure, such as ticks in Brahman cattle.

Resistance (immunity) is similar to ‘resilience’ (ability to cope with a large number of parasites without losing production), but is governed by a separate set of genes. Selection for resilience is possible but the heritability is generally lower, so it takes a lot longer to achieve a herd or flock that is resilient, compared to one that is immune.

2. Resistance = treatments don’t work. This is the ‘bad’ resistance.

Every time we use a chemical treatment (e.g. drench or dip) on our stock, the parasites are doing their own genetic selection. The ‘susceptible’ parasites are allowed to die, while the ‘resistant’ or ‘fit’ individuals will survive.

The reason why some parasites survive a treatment and others are killed is the same as in our livestock. There is a wide variety of susceptibility to the chemicals due to natural variation caused by mutations in the genes.

Populations of parasites are very large. For example, if a mob of sheep have a typical background worm egg count of 300 eggs per gram, each one will drop 300, 000 eggs per day, based on a daily faecal output of 1 kilogram. A mob of 1,000 head will therefore be depositing 300 million eggs every day onto the pasture. Each of these eggs has potential to grow into an infective larva and infect the sheep. It is easy to see how over the space of a few weeks, worm larva populations present on the paddock will be similar to the number of people on earth.

Male and female parasites undergo sexual reproduction and their genome is also very large, meaning there is ample chance for gene mutations to survive. Some of these mutations give the parasites resistance against our chemical treatments.

It only takes a few individuals in this huge population to survive and they will become the ‘Adam and Eve’, giving birth to the next generation of partially or fully-resistant parasites.

Resistance may occur due to parasites:

1. Decreasing the penetration of the chemical into their body
2. Exporting the chemical out of their cells e.g. using the ABC transporters found in worm cells
3. Developing new metabolic pathways that aren’t targeted by the chemical
4. Detoxifying (metabolising) the chemical within their cells

See ParaBoss for more information about both types of resistance: the good resistance in goats, sheep and cattle and the bad resistance in their parasites.