What is Parasite Resistance?
Put simply, resistance is when a worm or parasite survives a treatment that is intended to kill them. Over time, parasites evolve to be able to survive the drenches/chemicals they are exposed to. They do this by changing their genetic makeup to build a tolerance for the chemical. When the parasites breed, their offspring also inherit this gene for chemical tolerance, creating generations and generations of resistant parasites. Making it far more difficult to keep your stock from being held back by parasite burdens.
Identifying chemical resistance in parasite populations on farms depends on the parasite species (e.g., worms vs flies vs ticks) and the type of product used. Formal diagnosis of chemical resistance requires formal testing. This is outlined in the following sections.
Gastrointestinal worms
Severe resistance to a short-acting worm drench (oral, injectable or pour-on) can be identified if the drench ‘didn’t work’ in the first week after administration of the treatment. E.g., the stock continued showing severe signs of worm burdens after drenching. However, most cases of chemical resistance in worms are not that severe (thankfully).
Resistance to long-acting worm drenches (mostly injectable) presents itself as a shortening of the protection period claimed on the product. E.g., a product may claim up to 4 months of protection against Barber’s Pole, but if your worm population is resistant you might see the worms returning much sooner than that.
The only way to accurately diagnose chemical resistant worms is to conduct drench resistance testing or a drench check. This involves conducting a worm test 1-2 days before drenching (test 1), drenching the stock, then conducting another worm test 11-12 days after drenching (test 2). If the egg counts are reduced by less than 95% from test 1 to test 2, then there are some resistant worms on your property. Using this method, once a drench reduces the counts by less than 80-90%, it would be advisable to speak with a parasite management advisor.
Flies
Controlling flies in Australia (Buffalo Fly or Sheep Blowfly) is generally centered around long-acting products. When flies develop a tolerance (resistance) to the chemical(s) in those products, it shows up as a shortening in the protection period claimed by the product. This is because the concentration of a long-acting chemical slowly tapers off over time, if there are resistant flies around, they will be able to survive the lower concentrations occurring in the later stages of the claimed protection period. As a result, you will see flies around your stock a bit sooner.
To formally diagnose resistance in sheep blowflies you need to order a maggot collection kit from the NSW DPI (Ph: 02 4640 6446). Once you collect and send the maggots to the laboratory, they will do testing and generate a resistance report specific to your fly populations. Unfortunately, there are no such services available for buffalo fly testing.
Cattle Tick
Cattle Ticks, specifically QLD Cattle Tick Rhipicephalus australis (microplus), are also quite clever at becoming resistant to chemicals. Resistance to short-acting or ‘knockdown’ products (injectable, pour-on, sprays, dips) can be identified if not all ticks die off the cattle within the first day or so after treatment.
Similarly to worms and flies, ticks that are resistant to the active ingredient/chemical in long-acting products will rear their heads again before the product’s expected protection period is over. So, you will see a reduction in the protection period.
This is how to identify a suspected case of resistance in the ticks on your property. To gain an accurate diagnosis, further resistance testing should be done by contacting your local QLD DPI staff (veterinarian or biosecurity officer) and arranging for a sample of engorged female ticks to be sent away for testing.
Lice
Detecting and diagnosing chemical resistance in lice is difficult, in both sheep and cattle. Product efficacy can be largely dependent on the accuracy of administration, and the structural integrity of your internal and boundary fences.
Parasite Resistance Summary
Parasite resistance presents itself differently depending on the species of parasite, and the length of action of the chemical they’re resistant to. If in doubt, get in touch with your trusted parasite advisor who can help get to the bottom of one of the industry’s biggest headaches.
Stay tuned for our next article which is about management and prevention of parasite resistance by using highly effective combination drenches. If you have any further questions regarding any topics in this article, don’t hesitate to give us a call on 1800 243 683.