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.
- One-host Cattle ticks: Rhipicephalus (Boophilus) spp in CATTLE, worldwide in tropical and subtropical regions.
- Horn and Buffalo flies: Haematobia irritans in CATTLE, mainly in the Americas and Australia.
- Sheep body lice: Damalinia (=Bovicola) ovis in SHEEP, only in Australia and New Zealand.
- Red fowl mites: Dermanyssus gallinae in POULTRY, worldwide.
- Houseflies: Musca domestica, in LIVESTOCK OPERATIONS, worldwide.
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:
- Fleas: cat fleas Ctenocephalides felis in DOGS and CATS. Flea resistance to synthetic pyrethroids was common in the 1990s worldwide. However, after the avalanche of new products with many different modes of action in the last decades, flea resistance to insecticides is nowadays not a real issue.
- Multi-host ticks: Cayenne ticks Amblyomma cayennense in CATTLE in Brazil and Mexico LIKELY TO INCREASE in Latin America; Brown dog ticks Rhipicephalus sanguineus in DOGS in several countries; Tropical bont tick Amblyomma variegatum in CATTLE in Ghana; Brown ear ticks Rhipicephalus appendiculatus in Cattle in South Africa; Rhipicephalus bursa in CATTLE in Iran. Hyalomma anatolicum in CATTLE in India.
- Soft ticks: Argas persicus and Ornithodoros lahorensis in Iran.
- Mites: Northern fowl mites: Ornithonyssus sylviarum in POULTRY in the US; Sheep mange mites Psoroptes ovis in SHEEP in the UK.
- Flies: Stable flies Stomoxys calcitrans in all kinds of LIVESTOCK in the US, France, Germany and several other countries: LIKELY TO INCREASE; lesser housefly Fannia canicularis in the USA.
- Lice: Goat lice Bovicola spp and horse chewing lice Bovicola (Werneckiella) ocellatus, both in the UK; cattle chewing louse Bovicola bovis in UK and IRL in the 2010s.
- Bed bugs: common bed bugs Cimex lectularius in the US, Denmark and several other countries: LIKELY TO INCREASE; tropical bed bug Cimex hemipterus in Sri Lanka.
- Black flies: Simulium spp in Argentina.
- Mosquitoes: Culex spp, Aedes spp, Anopheles spp, etc. in LIVESTOCK and PETS, worldwide. Resistance of mosquitoes to synthetic pyrethroids is very frequent everywhere and can be rather strong. But their control is usually more a task for health authorities than for producers or pet owners. Mosquitoes are seldom a problem for the livestock industry. For pet owners they are more relevant because mosquitoes are vectors of various diseases (e.g. heartworms), but there is not a lot they can do in terms of chemical protection against mosquitoes.
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.
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.
- The last "new" chemical class of nematicides for cattle and horses (macrocyclic lactones) was introduced in the 1980s, for sheep and goats in the early 2000s (monepantel, derquantel).
- The last "new" tickicide for cattle (fluazuron belonging to the benzoylphenyl ureas) was introduced in the 1990s.
- The last "new" ectoparasiticides for sheep (dicyclanil, spinosad) were introduced in the 1990s.
- The last "new" flukicide for cattle and sheep (triclabendazole) was introduced in the 1970s.
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:
- Resistance Basics: what is resistance, types of resistance, etc.
- Resistance Development: how does resistance develop and what drives it.
- Resistance Diagnosis: how to find out whether a product failure is due to resistance or not.
- Resistance Prevention and Management: how to prevent, delay or manage resistance.
- Integrated Pest Management (IPM): A global approach to parasite control without relying only on chemicals.
Cick here to get to the section on RESISTANCE in this site.