Resistant parasites: update on the threat to livestock

A recent report in the Australian Veterinary Journal (Kotze & Hunt, 2023) reviewed the industry and found that resistance to chemicals used against all the major parasites of economic and welfare importance to ruminant producers (sheep, cattle, and goats) has been found in Australia and is increasing. This is a serious threat to Australian animal health, welfare, and the future of livestock production.

Parasite resistance to a chemical begins when the dose of chemical given to the host animal does not kill all the parasites.The survivors breed and over generations the ability to survive that chemical gets more common. This isn’t really a surprise – ‘survival of the fittest’ is how evolution works – except this is survival of the nastiest.  

Resistance might mean that a product will kill less parasites (e.g., many single-active drenches are useless against barber’s pole worm). It can also mean that a ‘long-lasting’ product doesn’t last as long as it used to (e.g., ~12 weeks against sheep blowfly when you used to get ~24 weeks protection). A completely clear picture is not possible due to limited use or availability of resistance testing; however, the threat is worsening for all.:

• severe in many regions for barber’s pole worm and scour worms in sheep and goats,
• worsening for sheep blowfly, cattle ticks, liver fluke (sheep and cattle), buffalo fly
• low for cattle lice.

It has never been more important to know what works on your property (where possible), and to work integrated pest management (IPM) into your animal management. Frustratingly, testing for resistance in parasites isn’t widely done in Australia and crucially for many parasites it isn’t even available – meaning that resistance may already be even worse than we think – and limits our ability to ‘know what works’.

Table 1 provides a quick summary of the methods available at present (if any) for resistance testing in each parasite group.

Table 1. Testing for resistance can be done in 2 ways: measuring parasite burden of your animals before and after a treatment; or, collecting live parasites and sending them to a lab for direct analysis

*Do not rely on new chemicals to become marketed. They take a long time to be developed and in the long run, resistance will develop to them too.

We can’t completely remove the need for chemicals, but non-chemical management reduces their workload on your property. Kotze and Hunt’s paper also summarises the strategies that help reduce reliance on chemical control; and graded the “readiness” of alternative methods, using a rating scale.

Non-chemical measures often work to interrupt a parasite’s life at various stages (e.g. in the environment), and so these can ‘catch’ parasites that may be able to survive a certain chemical – e.g. an adult fluke living in a sheep’s liver may be able to survive a dose of flukicide, but if the sheep aren’t grazing marshy areas, they will be less likely to meet fluke in the first place.

The measures below, used together with careful and accurate chemical treatments, slow down the development and spread of resistant parasites. It’s becoming vital that producers take up non-chemical management, to extend the remaining life of the chemicals we already have.

The ‘readiness’ ratings used in the paper ranged on a scale from:

• At a basic research stage – (i.e., years away, if ever!) e.g.:
  • scour worm and flystrike vaccines.
  • biological control of parasites.
  • regional parasite suppression (especially for highly mobile insects like flies);
  • reduction in scouring via preventing hypersensitivity scours or certain plant species.

• Effective and available now for commercial use – see below

All topics below and many more are covered in dedicated ParaBoss articles for the livestock species and parasite, under the ‘Prevent’ and ‘Treat’ tabs, some examples are: Manage Pesticide Resistance for Cattle (FlyBoss), How can drench-resistant worms be kept out of your property? (WormBoss), Managing Chemical Resistance (FlyBoss), Control on the Property for Cattle – TickBoss.

CAN DO IT NOW:  Know what works – invest in testing (where possible, see Table 1) to know which chemicals are effective against the parasites on your property. This pays back in not wasting your money on products that don’t work and which will make future resistance worse. As always, ensure good technique, accurate dosing, etc.

CAN DO IT NOW: Management – e.g.:

• quarantine new animals and ensure treatment has worked to stop them carrying resistant parasites onto your property.
• ‘Hacking’ parasite life cycles to reduce their numbers or stop them from ever infesting your animals (e.g., ‘smart grazing’; fencing off marshy areas where fluke thrives; providing browse for goats; shearing/ crutching ahead of peak fly times; introducing dung beetles to reduce faecal pats where flies breed, etc).
• Maintain fences and good relations with neighbours to stop animals straying.

CAN DO IT NOW (for some parasites:) Vaccines – Vaccines work by boosting the animal’s own ability to fight off the parasite. A vaccine exists for the barber’s pole worm (sheep, with off-label use in goats possible under a veterinary prescription) and ticks (cattle – has not been available in Australia for some time). Other possible vaccines are a long way off (e.g., against other scour worms, liver fluke, sheep blowfly).

CAN START NOW (for long-term payback): Breeding resistant livestock– just like parasites select themselves for resistance to drugs, you can select breeding animals for their resistance to parasites. This is a long-term strategy, and may need compromise on other production traits, but as chemical resistance will only get worse it should be a good investment in future-proofing your lines.

Reference – you can read this for free as it is an open-access article, and we encourage you to do so:

Kotze, AC and Hunt, PW “The current status and outlook for insecticide, acaricide and anthelmintic resistances across the Australian ruminant livestock industries: assessing the threat these resistances pose to the livestock sector” Australian Vet Journal (2023), online open access.

CSU – Charles Sturt University

NSW VDL – New South Wales Veterinary Diagnostic Laboratory, Menangle

QLD DAF – Queensland Department of Agriculture cattle tick testing laboratory