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The general experience as well as my own one regarding supposed resistance problems in the field is that most cases of product failure are not due to resistance but to incorrect use of parasiticides or to a mistake in assessing the specific problem. It is often easier to blame resistance than to accept an own mistake. For this reason,the first thing to do in case of a product failure is to collect as much specific information as possible and to analyze it in order to exclude incorrect use.

The following recommendations regard mainly livestock parasites, not pet parasites. To find out whether you have a parasite resistance problem with some dog, cat or any other pet, the only reasonable thing to do for the time being is to ask a veterinary doctor.

Elements for determining whether there could be a resistance problem

In most cases, getting a precise diagnosis of parasite resistance in a given property requires to run some kind laboratory tests that in most cases cannot be done by the producer himself in the field or in his property.

There are already some biochemical or molecular tests that allow to detect some types of resistance directly with parasite samples from the field, but only for a few parasites and most are not yet commercially available: but it is currently a research topic intensively investigated.

In most cases, a precise diagnosis of resistance requires taking samples of the parasite (adults, larvae, eggs, faeces, etc.), bring them to a service lab, breed the individuals and then run specific tests in vitro (parasites off the host) and/or in vivo  (parasites on the host) comparing the results with those of a so-called susceptible strain of the parasite, i.e. a non-resistant population kept in the lab. 

In many countries such service labs are not easily available or affordable for most producers. Where they are available and affordable, the whole process needs time, in the best case 2-3 weeks, often several months. Thus, if it is always recommended to look for a precise diagnosis from a lab, in most cases producers or field vets will have to make decisions regarding how to treat a livestock population without such a diagnosis. In these cases several elements can help conclude that there is probably a resistance problem, or not. And to act correspondingly.

In the following several criteria are proposed to find out in the field and without sophisticated laboratory methods whether a case of product failure is likely to be due to resistance or not.

1. What was exactly done in the field and what has been observed afterwards on the animals?

It is necessary to check as precisely as possible what happened: it is not enough to simply say e.g. "the product didn't work", or "the animals still have ticks", or something similarly simple. It must be determined e.g. whether the problem affects all animals in the property, or only a certain group. If only a group is affected, which one, eg.: calves, mother cows, heifers... Or those in a particular paddock and other animals in other paddocks are not affected; or those that were treated in the morning but not those treated in the evening; or those treated by so-and-so, etc.

To find it out it is crucial to talk with all people directly involved: those that prepared the product, those who gathered the animals for treatment, those that actually did the treatment, etc. For a technician or sales representative that is dealing with a complaint it is not enough to ask the owner if he was not directly involved in the treatment and hasn't checked the animals personally. For the owner it is not enough to ask the foreman if other people actually did the treatments, etc.

It is also crucial to check whether the product used was spoiled: expired, contaminated, etc. and whether it was correctly prepared: dilution (if dilution was required), dosing, etc.

All used equipment has to be checked for proper functioning as well: pour-on, injection or drench guns, spray nozzles, etc.

The following recommendations apply mainly for cattle ticks and horn flies, partly also to red fowl mites, houseflies and to a lesser extent to blowfly strike: for these parasites the producer can recognise infestations or at least the damage caused by the naked eye and thus estimate whether a given treatment achieved the expected effect or not. This is not the case for other parasites, e.g. gastrointestinal worms or flukes.

2. Which parasite is the problem?

It is quite easy to diagnose the wrong parasite for a non specialist. In many countries there are ticks that are quite similar to cattle ticks. And it is not always easy to distinguish between stable flies, houseflies, horn flies, face flies and other flies that may occur in livestock operations. 

In most countries there are not that many external parasites for which there is a significant risk that product failure is due to resistance. The key ones are:

  • cattle ticks (Rhipiceiphalus=Boophilus microplus) worldwide in tropical and subtropical countries
  • horn and buffalo flies (Haematobia irritans) on cattle worldwide
  • red fowl mites (Dermanyssus gallinae) on poultry worldwide, in some countries northern fowl mites (Ornithonyssus sylviarum) too
  • houseflies (Musca domestica) worldwide in any livestock production facility, mainly poultry, pig and dairy operations
  • blowfly strike (Lucilia cuprina) in sheep farms mainly in Australia and New Zealand (elsewhere resistance less an issue)
  • liver flukes (Fasciola hepatica) mainly in sheep farms, worldwide
  • sheep lice (Damalinia=Bovicola ovis) mainly in Australia (elsewhere resistance less an issue)

Resistance has also been confirmed in some countries for other ticks (e.g. Amblyomma cajennense), for stable flies (Stomoxys calcitrans) or for sheep scab (Psoroptes ovis). But incidence is still rather low in most countries and thus so far unlikely to cause a sudden product failure due to resistance.

Regarding helminths, there are very serious resistance problems with:

Cases of resistance of the lungworm Dictyocaulus viviparus have also been reported in cattle. Unfortunately, determining which parasite is causing the problems is much more difficult for worms than for insects or ticks, simply because you cannot see them by the naked eye.

In any case, one of the first things to do in case of product failure is to try  to determine which parasite is causing the problem.

3. Are all animals in a property affected or only a certain group?

This is a key question, because resistant parasites do not discriminate between their hosts. And there are many situations that can explain why a group of animals shows a higher infestation (or an apparent worse control) than other groups even within the same property. In most cases of product failure, if it only affects a group of animals , it can be explained by unusual treatment mistakes, i.e. things that are usually done correctly, for whatever reason  were not done as they should with a particular group of animals.

The explanation may be trivial -e.g. not all the animals were treated) or more subtle. E.g. in plunge dips, the first animals to get through the dip may not have received enough product because the dip wash was not correctly stirred after adding the product for dilution. Using a spray race or a shower dip, the noozles may have not worked properly during a certain time (clogging, electric problems, etc.) causing some animals to be underdosed. During pour-on or spray-on administration short wind gust or sudden downpours may have affected a group of animals during or after treatment, etc.

A situation where only part of the animals in a property would show higher infestations (and thus raise suspicion of product resistance) without an application error can e.g. happen if some but not all cattle visited a spot in the property heavily infested with cattle tick larvae. Those animals will carry a much higher load of ticks, and after treatment they may give the impression that the product failed (see explanation below).

4. Is it the first time that the product fails, or comparable failures have been happening in the past?

Usually resistance does not appear suddenly, without "warning", but develops gradually. What is often observed in the case of field resistance is that product efficacy diminishes progressively over years.

Without resistance, after a treatment, animals remained free of parasites and remained clean for a more or less longer period of time depending on parasite and product.When resistance starts, treatments do not completely clean the animals of parasites, and/or the protection period becomes progressively shorter. Practically it means that the animals have to be treated more frequently to keep them "clean".

The progressive increase in the number of treatments needed to keep the animals clean of parasites is a characteristic symptom of resistance development.

However this can also happen without a resistance problem. A typical situation is when parasite populations unexpectedly explode due to extremely favorable weather and/or ecological conditions, i.e they appear quite suddenly in huge numbers, and their control requires more frequent treatments. In this case you must be aware that, with very few exceptions (e.g. some lousicides), parasiticides do not control 100% of the parasites. Good parasiticides may achieve 95% or even 99%, whereas many products may achieve only 80% control. This means that they allow survival of 1 to 20% of the parasites. If e.g. an animal is infested with 100 ticks and you kill 99%, only 1 will survive that you won't probably find. But if it is infected with 10000 ticks and you kill 99%, 100 will survive, and you will certainly find a few dozens.

There are also single or chronic errors in product delivery that can progressively reduce product efficacy and suggest that efficacy is decreasing due to resistance. If a plunge dip for cattle becomes increasingly filled with dirt and mud, even if it is replenished correctly, the concentration of the active ingredient in the wash will diminish because many active ingredients adhere to the mud particles and will not be available to impregnate the animal's hair. Along a treatment season, product efficacy seems to diminish because more and more parasites survive dipping. If you add the effect of the progressive tick infestation level of the pastures along the tick season, the global impression is that the product (which is being used at the same strength against an increasing tick challenge) seems to lose efficacy and one may be tempted to conclude that there is a resistance problem.

A similar effect can happen if a plunge dip was not correctly calibrated or if it gets regularly flooded. Or if in a spray race the noozles get increasingly clogged, or the pressure drops due to dirt or leaks. Or if the same dose of pour-ons, drenches or injectables is being administered to fast growing animals.

5. For how long has the same "chemical class" been used in the property?

As already said, resistance to a parasiticidal active ingredient does not appear suddenly, but takes several years of rather intense usage of the same active ingredient or other active ingredients with the same mode of action. The crucial question is not which product (or brand) has been used in the past, but which parasiticidal chemical class has been used in the property and for how long.

It is rather irrelevant which brands, formulations or administration methods were used: the crucial information is which active ingredient(s) has been used and to which chemical class it belongs. Unfortunately it happens that, for whatever reasons, some laboratories may change the active ingredient in a given brand, or may change the brand without changing the active ingredient.

If for controlling one of the six key parasites mentioned before the same chemical class has been used without interruption during 4 or more years, the likelihood of resistance development is real, particularly to synthetic pyrethroids, organophosphates, amitraz, fipronil and macrocyclic lactones.

The same applies to anthelmintic benzimidazoles, levamisole, macrocyclic lactones and monepantel used against gastrointestinal roundworms of livestock and horses.

6. Are there reports on confirmed resistance in the neighborhood?

If resistance to a particular parasite has been confirmed in a neighbouring property, the probability that a product failure dealing with the same parasite in its own property may be due to resistance increases considerably.

This is particularly true for horn and buffalo flies, houseflies and blowflies due to their ability to fly for long distances, and to the effect of strong winds and livestock movements that can cause resistant individual parasites to be transported from one property to another one.

For cattle ticks the risk is smaller, because free-living larval stages remain where they hatched out of the eggs. But livestock movements or floodings may dislocate resistant individuals from one property to another one.

Obviously, the absence of confirmed resistance cases in the neighborhood doesn't mean anything: perhaps it has not been confirmed yet, or it is still unknown, etc. There is always someone that has to be the first.

7. Make a control trial under strict supervision

If product failure, repeated or not, cannot be explained by incorrect treatment, some climatic or ecological peculiarity, a particular livestock handling event, product deficiency (expired, spoiled, etc.) or an administration error, you can try a strictly controlled treatment with fresh product run under close supervision to prevent errors. This may be difficult to do with preventative products (e.g. against blowfly strike).

Strictly follow all the label instructions. You may run this trial with a few animals (10 to 20) to make it easier to check the infestation levels before and after the treatment. If possible use two groups of animals: one that will be treated, and another one that won't be treated. Examine the animals to estimate the infestation level before treatment. Don't run the trial if the infestation level is too low: results could be misleading or uncertain. Compare the infestation level after treatment in both groups (treated and untreated).

If you are dealing with a spray race (or shower dip or similar), ensure that the noozles are not clogged, that there are no leaks in the tubing, that the water pressure is constant, etc. If you are using a pour-on, a drench or an injectable weight each individual animal before treatment, ensure that the guns deliver the correct volume, etc.

If you are dealing with a plunge dip, empty the vat, clean it, charge it with fresh product and run the trial as correctly as possible (e.g. no rain before or after the trial).

The following real case illustrates that almost anything can go wrong. In the 1980s, a customer complained that his plunge dip for cattle was not working proprerly. After thorough interrogation of all people involved, it was decided to empty the vat and to run a trial with fresh product. After emptying the vat, in addition to a considerable amount of dirt, dozens of unopened product containers were found in the bottom...

For running such a trial it is highly recommended to ask for support from a veterinary doctor, an extension officer or a representative of the manufacturer.

8. Conclusion

If the problem:

  • affects one of the species mentioned above (see chapter 2)
  • has been occurring for a certain time and is worsening
  • usually affects most animals in the property

and

  • the product (active ingredient) concerned has been uninterruptedly used during more than 3-5 years
  • incorrect use has been reasonably excluded
  • a control trial has resulted in insufficient control

you can conclude that product failure is likely due to resistance.

If you can send samples of the suspected parasites to a lab in order to get a reliable diagnosis, do it. Even if it takes months to get a result it will determine what you have to do to handle the problem in the future. If you can't, the articles in this site on resistance management and resistance prevention may help you to deal with the problem.