Pyrethrins are natural active ingredients of plant origin used in veterinary medicine in dogs, cats and livestock against some external parasites (lice, fleas, flies, etc). They are also used against agricultural and household pests.
Common name: PYRETHRINS
Other names: Pyrethrum
Type: INSECTICIDE, ACARICIDE, REPELLENT
Some plants where they are found
- Chrysanthemum cinerariaefolium = Tanacetum cinerariaefolium (Dalmatian chrysanthemum)
- Chrysanthemum coccineum (Persian chrysanthemum)
- Tagetes minuta (southern marigold, wild marigold, black mint)
- Tagetes erecta (Mexican marigold, Aztec marigold)
EFFICACY AGAINST PARASITES
Type of action: Broad-spectrum, contact, non systemic ectoparasiticide: insecticide, acaricide, tickicide, louisicide, larvicide
Main veterinary parasites controlled: flies, ticks, mites, lice, fleas, mosquitoes, etc.
Mechanism of action: Pyrethrins have the same mechanism of action as synthetic pyrethroids. They act on the membrane of nerve cells blocking the closure of the ion gates of the sodium channel during re-polarization. This strongly disrupts the transmission of nervous impulses. At low concentrations insects suffer from hyperactivity. At high concentrations they are paralyzed and die.
However, resistance to pyrethrins and all synthetic pyrethroids is already very frequent worldwide and can be extremely high, particularly in cattle ticks, horn & buffalo flies, houseflies, red fowl mites, mosquitoes, fleas, etc.
A "crushing" efficacy should not be expected from natural products, comparable to that of modern synthetic insecticides, which are more potent and persistent than any natural compound. Simplifying, it can be said that natural active ingredients can be useful in places or seasons with low parasite challenge, but may be insufficient for controlling well established parasite populations (fleas, ticks, mites, etc.) in pets, let alone in livestock.
Efficacy against a specific parasite depends on the delivery form and on the dose administered. Check the labels of the products available in your country.
Click here for general information on features and characteristics of PARASITICIDES.
Oral LD50, rat, acute*: 1200-1500 mg/kg
Dermal LD50, rat, acute*: >1800 mg/kg
* These values refer to the active ingredient. Toxicity has to be determined for each formulation as well. Formulations are usually significantly less toxic than the active ingredients.
Pyrethrins are considered quite safe for humans and domestic animals. But they can be irritant for the skin and the mucosal epithelia (eyes, nose, respiratory system, etc.), also for domestic animals, but much less than for synthetic pyrethroids.
Pyrethrins are very toxic to fish and to numerous beneficial insects.
It must be remembered that the natural origin does not guarantee that such compounds are less toxic than the synthetic parasiticides. They are as "chemical" as the synthetic ones. Toxicity is a matter of dose! General information on the safety of veterinary antiparasitics is available in specific articles in this site (click to visit):
- General safety of antiparasitics for domestic animals
- General safety of antiparasitics for humans
- General safety of antiparasitics for the environment
MARKETING & USAGE
The insecticidal effect of pyrethrum and its extracts was already known in ancient times and such products have been used for centuries for this purpose. It is believed that powders of pyrethrum plants were already used as insecticides in China in the Xth century B.C. But the main chemical molecules (pyrethrins I and II) responsible for the insecticidal effect in the extracts were identified only in the fist half of the XXth century. Soon later both were synthesized in the laboratory. Most pyrethrins marketed today are not manufactures through chemical synthesis but are extracted from harvested plants grown for this purpose in several countries (e.g. Kenya, Croatia, Australia, etc.).
There are thousands of products with natural pyrethrins, mainly for agriculture, horticulture, public and domestic pest control, but also for pets, mainly in low-cost brands. Usage in livestock is more modest.
Use in LIVESTOCK: Yes, rather modest
Use in HORSES: Yes
Use in DOGS & CATS: Yes, abundant. Less in cats because cats may not tolerate pyrethrins and/or synthetic pyrethroids.
Main delivery forms:
- Collars impregnated with insecticides
- Premise and environmental treatment
- Shampoos, soaps, powder, sprays, etc
- Spot-ons, pipettes, squeeze-ons
Use in human medicine: Yes
Use in public/domestic hygiene: Yes
Use in agriculture: Yes
Reported in livestock & horses: Yes, as for all synthetic pyrethroids: very frequent worldwide in such species as cattle ticks (Boophilus spp), horn flies (Haematobia irritans), sheep lice (Damalinia ovis), poultry mites (Dermanyssus gallinae), houseflies (Musca domestica), mosquitoes.
Reported in pets: Yes, quite frequent worldwide in dog and cat fleas (Ctenocephalides spp).
For this reason, many veterinary antiparasitics with natural pyrethrins contain other insecticides and/or a synergist (often PBO = Piperonyl butoxide). The synergist in the formulation is supposed to neutralize resistance of parasites to insecticides.
However, PBO works only against 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 insecticides. 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 or pet owners facing parasites resistant to pyrethrins and synthetic pyrethroids do not know which mechanisms make 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.
Obviously, if a particular brand contains a synergist such as PBO or other synthetic insecticides it cannot be labeled "biological", "ecological" or anything similar suggesting an exclusive natural origin.
In addition to their insecticidal properties, pyrethrins also have a repellent effect against mosquitoes and to a lesser extent against other flying insects. But it lasts only a few houres after treatment, if at all.
Pyrethrins are certainly the most used natural insecticides worldwide. However, resistance is already very frequent. They share two features with many other natural insecticides: they are rather volatile and are unstable when exposed to sunlight. Consequently their effect in animals exposed to sunlight is very short: a few days or even only a few hours. This means that protection of the treated animals against re-infestations (residual effect) is virtually inexistent. This is particularly unfavorable for livestock that would need to be treated very frequently. However, for the same reason they do not leave chemical residues in food commodities (meat, milk, eggs, etc.), which is a benefit sought by organic producers.
The lack of residual effect was the reason why synthetic pyrethroids (permethrin, cypermethrin, deltamethrin, etc) were developed in the 1970-1980s modifying the pyrethrin molecules in the laboratory. Most synthetic pyrethroids are less volatile, resistant to sunlight, and can protect treated animals against re-infestation during several weeks or even months. Synthetic pyrethroids have been massively used in livestock and pets in the 1990s-2000s. They are still used a lot, but usage is declining due to resistance and the introduction of newer chemicals without resistance.
Another disadvantage of pyrethrins and other "natural products" extracted from plants is that quality (and thus efficacy) may vary, even if used following the label instructions, because producing extracts from heterogeneous plant materials is often less reliable than chemical synthesis.
Finally it must be said that in many countries registration of so-called "natural products" does not need a (serious) proof of efficacy, quality and safety as pesticides or veterinary medicines. This means that many such products have not been seriously tested in clinical trials in the field and consequently their efficacy and/or safety may not be granted. For such products, getting a market authorization does not need a substantial investment, which explains why there are so many brands in most countries.
- Click here to view the list of all technical summaries of natural antiparasitic active ingredients in this site.
- Click here to view the list of all technical summaries of antiparasitic active ingredients in this site.
- Click here to visit the article on medicinal plants with antiparasitic properties in this site.