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Acaricide resistance is a genetic change (mutation) in individual ticks within a parasite population that allows resistant ticks to survive treatment. Because it is genetically based, resistance to the same chemical group is passed on to the next generation.
There are a number of ways that resistance is expressed. For example, gene mutations that block the active have been shown to be responsible for most resistance to synthetic pyrethroids (SPs) in ticks and it is likely the main mechanism for resistance to amitraz. In contrast, detoxification mechanisms in ticks have been shown to be important for organophosphates (OPs) and to a lesser extent for SPs.
Resistance is heritable, meaning that resistance is passed on from one generation to the next. Repeated use of the same chemical group can cause ongoing selection of the more resistant parasites, as susceptible types are killed. If the relative proportion of resistant ticks in the population increases compared to susceptible ticks, and the resistant ticks come to dominate, then the effectiveness of the chemical to control the parasite will become compromised.
Importantly, resistance to different chemical groups occurs through different mutations within the ticks. As a result, resistance to one chemical group does not usually equate to resistance to other chemical groups. However, resistance against an active within a chemical group, will confer resistance (though not necessarily to the same extent) to all other actives within the same chemical group. Avoid repetitive use of actives within the same chemical group to reduce the build-up of resistant individuals within populations.
Wherever ticks are a potential problem for cattle production the application of acaricides is one of the most important components of a control program. In order to make sensible selections from a range of possible strategies for controlling ticks, often the most important question is which acaricide should be applied? The selection of acaricide then influences the frequency and method of application. Although resistance to commercially available acaricides is common, each property will have its own profile of acaricide resistance and in designing a cost effective control program it is important to know which acaricides can be expected to be useful, and which are unlikely to work.
Specific requirements for collecting and storing ticks for acaricide resistance testing in cattle ticks should be obtained from the laboratory that will do the testing. However, most commonly the preferred sample is adult female ticks that are fully fed and approximately 10 mm in length. 30 to 60 fully engorged ticks are required, depending on the type of test to be done and the number of acaricides to be tested. Where possible, ticks should be collected from several cattle, not just one animal. If possible collect ticks prior to treatment or a minimum of 49 days post treatment. For transport, ticks should be put into small non-crushable boxes, preferably made of cardboard, with a few small holes allowing air to circulate. Only a small amount of freshly cut, green grass is needed to provide moisture. More problems occur with samples that are too wet, rather than too dry, provided that transport times and temperatures are not excessive. Where large numbers of ticks are submitted for bioassay testing, the cardboard containers should be placed in a cool box with a cooler brick (wrapped in paper towels) and transported to the laboratory by the fastest route. Do not submit ticks in airtight containers, plastic bags or glass tubes. Do not place ticks in cotton wool. Do not expose ticks or containers to excessive heat/sunlight or chemicals. It is important to contact and discuss specific requirements with the laboratory that will do the testing.
Approaches for testing ticks for acaricide resistance include ‘Bioassays’ conducted on ticks of specified life-cycle stages in the laboratory to determine the proportion of ticks killed by varying concentrations of acaricide. For a limited number of acaricides it is possible to determine the proportion of a sample of ticks that carry a ‘specific DNA mutation’ that confers resistance through genetic testing.
Bioassays are the most common means of diagnosing resistance. Two tests are currently available for resistance testing in Australia; the Adult Immersion Test (AIT) is used for fluazuron resistance testing, and the Larval Packet Test (LPT) for organophosphate, synthetic pyrethroid, macrocyclic lactone and amitraz resistance detection. For all testing, fully fed, adult females are obtained as stated above.
The Adult Immersion Test (AIT) requires 30 – 60 fully engorged ticks for testing but send a larger sample if you can collect them. This allows for a minimum of 10 ticks each for two testing concentrations plus a control. For AIT testing, ticks are either collected prior to treatment, or a minimum of 49 days post treatment. The ticks must reach the laboratory in good condition, and before they have begun to lay eggs. In the AIT, engorged adult females are exposed to acaricide by immersion in solutions of different concentrations for a set amount of time. The adults are then left to lay eggs which are incubated until they hatch. The percentage of larvae that hatch from the submitted sample, compared to a laboratory control strain, provides the resistance percentage or resistance profile.
A larval packet test (LPT) determines the acaricide resistance status of the offspring of the submitted ticks. Adult females are incubated for generally two weeks until they lay eggs, the eggs are then incubated for around four weeks allowing the larvae to hatch and harden. Resistance testing is performed on the larvae. Each female produces some 2,000-3,000 larvae, so it is possible to test hundreds of larvae at a time, and to have replicated bioassays for each of several acaricides and at several concentrations. Bioassays quantify the proportions of a sample that survive or die at any given concentration of each acaricide. Resistance level is usually expressed as the proportion of survivors at a concentration that is expected to kill almost all susceptible ticks. These bioassays are time consuming and require specialist laboratory facilities for culturing and handling ticks. Careful technique is essential and the assays take around 8 weeks from the time of collection of samples until the delivery of results.
Pen trials for resistance testing are quite complicated because they require a large number of larvae and specialised facilities. For this reason they are mainly used for research on resistant strains, and in the development of new acaricides.
It is now possible to extract the DNA from ticks and determine whether they carry a mutation that makes them resistant to synthetic pyrethroids (SPs) and amitraz. Although such genetic testing uses standard molecular genetic techniques, genetic testing for acaricide resistance has not become routine. The main reason for this is that genetic testing can only reveal the status of a tick or a sample of ticks in relation to the specific mutations that have already been discovered. For example, the target site of SPs has been well characterised and three mutations have been discovered that confer resistance to SPs. Most known SP-resistant populations will have one or more of these mutations, but resistant populations may exist that don’t carry any of these mutations. It is quite possible that as-yet unknown mutations exist in the same gene, but in a place not examined using the current genetic tests. For this reason, a positive result (i.e. a resistance-conferring mutation is found to be present) confirms that there is resistance, but a negative result does not confirm that resistance is absent, it just indicates that the known mutations are absent. There is also the possibility that different mechanisms are responsible for resistance in a population, e.g. detoxification. The main advantage of genetic tests for resistance is that they are quick (only require a day or two) and they are quantitative (it is possible to state exactly the proportion of the population that carries a mutation).
Acaricide resistance is an inevitable consequence of using acaricides. The more often an acaricide is used, the greater the number of ticks exposed, and the quicker resistance is likely to develop.
There are several strategies to manage resistance, but by far the most effective measure is to reduce the number of times that acaricides are applied.
Strategies for delaying the emergence of acaricide resistance include:
These strategies are discussed in more detail below.
When acaricides are applied to a mob of cattle, not all ticks in the population associated with those cattle are exposed to the acaricide. Ticks that are living in the pasture, either as females that have dropped from the host and waiting to lay eggs, as eggs, or as larvae waiting to attach to a host, are not exposed. The non-exposed part of the population is said to be ‘in refugia’. The higher the proportion of a population that is in refugia at the time of an acaricide application, the lower the selection for acaricide resistance. It is possible to increase the refugia at the time of a treatment by only treating the most heavily infested cattle. Of course, this will not be an acceptable option if tick-borne diseases are a serious concern. The proportion of ticks in refugia is generally lowest at the very beginning of the tick season, in those areas in which the seasonality of ticks is pronounced. A higher proportion of the total tick population will be on cattle just after the ‘spring rise’ of ticks than at the end of the season. This is precisely why ‘strategic’ tick control programs concentrate treatments during spring and early summer – there is a greater effect on the population for a smaller number of treatments.
Under some circumstances there might be a benefit from rotation or alternation between acaricide chemical groups. There is no benefit at all to rotations that involve different brands or different actives from within the same chemical group. The infrastructure and available products will usually determine whether a rotation strategy is logistically possible. Clearly, when there is already evidence of resistance to the acaricide in use it makes no sense to continue using it and rotation onto another group of acaricides is advisable. In some cases it might be possible to return to the acaricide against which resistance has been noted, but for most acaricides it does not seem to be an option.
For example, experiments have shown the potential reversion of cattle tick susceptibility to amitraz following a three year rotation to a different chemical group. Conversely reversion to synthetic pyrethroid (SP) and organophosphate (OP) susceptibility appears to be more difficult to achieve, possibly due to their different modes of inheritance.
Whether combination products are a good idea for managing resistance is currently the subject of debate among scientists and policy makers. Theoretically, acaricide products that contain combinations of acaricides from different chemical groups to target the same parasite are a good idea if the chemical groups are new, and there is little or no resistance in the target populations. However, in practice, combinations are more likely to be introduced when acaricide resistance is already widespread. In such circumstances, combination chemicals are working as single actives and using them together is probably contributing to the acceleration of resistance against several acaricides at once.
Acaricides should always be used exactly as specified on the product label. Wide-spread under-dosing of a chemical (e.g. under-estimating the weight of animals being treated, poor application technique, uncalibrated dosing) has often been blamed for the rapid emergence of resistance. Read the product label to ensure the product isn’t unintentionally exposing non-target parasites to chemicals (e.g. products to treat ticks can also affect worms or lice).
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