Natural pyrethrins are produced by some plants (e.g. chrysanths and marigolds) that were known for their insecticidal properties already in Chinese medicine since 1000 BC and in the Roman Empire. They are extracted from industrially grown plants and are often marketed as pyrethrum, a mixture of these natural compounds.

After the discovery and introduction of synthetic pyrethroids in the 1970s-1980s, resistance to both natural pyrethrins and synthetic pyrethroids developed rather quickly worldwide.

Natural pyrethrins show cross-resistance with synthetic pyrethroids because they share the mode of action. Resistance to synthetic pyrethroids and consequently to natural pyrethrins is found worldwide, is very frequent, develops and spreads very quickly, and can be extremely high (Resistance Factors often >1000), which makes many products affected completely useless.

The most critical cases of veterinary parasites resistant to natural pyrethrins and synthetic pyrethroids are:

There are numerous reports on many other external parasites resistant to natural pyrethrins and synthetic pyrethroids, but so far, such cases remain restricted to limited regions and/or do not represent a global threat for domestic animals yet, and thus are not particularly analyzed in this article. Recommended measures to handle these cases are more or less the same as for the most critical ones: Rotation to chemical classes with different modes of action that remain effective and/or Integrated Pest Management. The following cases can be mentioned:

OUTLOOK. Problems will worsen everywhere. Resistance will continue to spread and to strengthen, because both natural pyrethrins and synthetic pyrethroids are still being used, alone or in mixtures with other chemical classes , which will maintain and strengthen resistance everywhere.

RECOMMENDED MEASURES. The most reasonable measure is to switch to Integrated Pest Management (IPM) and/or to implement whatever preventative measures that reduce the use of any chemicals. Where alternative chemicals of other chemical classes are still working against a given pest, rotation is usually a good option, i.e. to stop using natural pyrethrins or synthetic pyrethroids and to use other products with active ingredients of those still effective chemical classes during several years.

Natural pyrethrins are effective against most insects (flies, fleas, mosquitoes, etc.), ticks and mites of veterinary importance. They have no efficacy whatsoever against internal parasites (roundworms, tapeworms, flukes). They are only available for external use. There are thousands of generic products worldwide, mainly for pets. A key difference with most synthetic pyrethroids is that natural pyrethrins are quickly broken down by sunlight and consequently their residual effect is very short.

For dogs and cats natural pyrethrins are available in numberless low-cost aerosols, baths, dusts, soaps, sprays, and the like. Use in livestock and horses is irrelevant due to their poor residual effect when exposed to sunlight.

Multiresistance is already common and frequent in some species, particularly in the cattle tick Rhipicephalus (Boophilus) microplus, in the housefly Musca domestica, and in mosquitoes.

It is a general rule that compounds that belong to the same chemical class show so-called cross-resistance among them, i.e., if a parasite develops resistance to one compound, it will be more or less resistant to other compounds of the same chemical class.

Where available, follow national or regional recommendations for delaying resistance development or for handling already confirmed cases.

Use of synergists . Quite frequently, products with natural pyrethrins and synthetic pyrethroids contain also synergists (mainly PBO) that are supposed to neutralize resistance. However, PBO works only against the so-called metabolic resistance (enhanced detoxification) caused by mixed function oxidases (= MFO), which is one among several mechanisms by which parasites can become resistant to synthetic pyrethroids, organophosphates and pesticides of other chemical classes. PBO specifically inhibits the activity of MFOs. If metabolic resistance is caused by other enzymes than MFOs, or if resistance is (also) due to other mechanisms such as target site insensitivity, reduced penetration or behavioral modifications, it won't be neutralized by PBO. In the vast majority of cases, producers affected by parasites resistant to synthetic pyrethroids or organophosphates do not know which mechanism makes the parasites resistant, and it is mostly not possible to find it out. Consequently, whether the synergist PBO helps to overcome resistance or not is in fact a lottery.

To evaluate resistance problems it must also be considered that innovation in the field of livestock parasiticides has strongly decreased in the last decades.

This means that the likelihood that new chemical classes with new modes of action against resistant parasites become available is quite slim. The reason is that, in the last decades, almost all animal health companies have focused their R&D investments in the much more profitable business of pet parasiticides. As a consequence, regarding resistance management in livestock and horses, almost nothing really new (i.e. with a new mode of action) has been introduced in the last decades: all new products (mostly new formulations or mixtures) have been basically "more of the same".

If you want to learn more about resistance, read one of the following articles in this site:

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