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What are organochlorines?

Organochlorines, also called chlorinated hydrocarbons are the first chemical classes of synthetic pesticides discovered in the 20th century and subsequently manufactured and used massively worldwide in agriculture and hygiene, as well as on livestock, horses and pets.

Molecular structure of DDTThe ultimate organochlorine representative is DDT (dichlorodiphenyltrichloroethane), the first synthetic organic insecticide with broad spectrum of activity. It had already been synthesized and described in the late 19th century. But it was the Swiss chemist Paul Hermann Müller who first discovered its insecticidal efficacy against external parasites of humans, animals and plants. He was awarded the Nobel Prize in 1948 for this discovery. DDT played an essential role in 2nd World War to protect the allied troops from typhus and malaria in the Pacific War.

In the agricultural and livestock industry DDT helped to replace much more toxic pesticides like arsenic derivatives. For decades DDT was the most used insecticide worldwide. In 1960 more than 74'000 tons were produced and consumed only in the USA.

But in the 1950's it was already discovered that DDT is quite toxic to many bird species and that it bio-accumulates in the chain food. This means that what gets into a living organism, including humans (e.g. as residues in food) is only very slowly excreted or metabolized, and if the input continues, the amount in the organism increases. It is now 50 years from the publication of the famous book "The Silent Spring" by Rachel Carson in 1962, which pointed to this problems, initiated the decline of organochlorines, and started raising the public awareness on the environment pollution with agrochemicals. Slowly but steadily, DDT was phased out in many countries, and withdrawal of other organochlorines followed. Today, the WHO still recommends DDT for mosquito control in certain areas with endemic malaria.

There is also additional information in this site on the general features of parasiticides and ectoparasiticides, as well as on parasiticidal chemical classes and active ingredients.

Active ingredients

Other parasiticidal organochlorines introduced after DDT and broadly used on livestock for many years are:

Dieldrin usage in livestock horses or pets was stopped long ago, but lindane (= BHC, = gamma-hexachlorocyclohexane, = gamma HCH, = gammaxene) was used in livestock and pets in several in European countries and elsewhere until the end of the 1990's. It was particularly effective against scab and mange mites. It is still used in pets in a few countries, e.g. in Latin America.

A few organochlorines were used as flukicides (e.g. hexachloroethane, hexachlorophene, hexachlorparaxylene) in livestock, but they were replaced decades ago by newer, more efficient and less toxic anthelmintics (e.g. benzimidazoles, closantel, etc).

Mode of action and characteristics of organochlorines

Molecular structure of LindaneOrganochlorines 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. This toxic effect occurs not only on insects, but on many vertebrates as well, since the ion gates of the sodium channel in the cellular membranes of nerve cells work similarly in many living organisms.

Most organochlorine insecticides have a broad spectrum of activity against insects, mites, ticks, etc. They can have both adulticidal and larvicidal activity, by contact as well as after oral ingestion. They also have a long residual activity: being very lipophylic they are stored in fat, both inside the body as well as outside (e.g. wool grease).

Safety of organochlorines

Acute mammal toxicity of most organochlorines is comparable to that of organophosphates. Most organochlorines are similar to DDT regarding their negative environmental impact and their bio-accumulation in the food chain. Nowadays, they have been banned almost worldwide for use in livestock and horses and only a few products for use in pets remain available in a few countries. 

General information on the safety of veterinary antiparasitics is available in specific articles in this site (click to visit):


Never use livestock, horse or poultry products on dogs and/or cats, unless explicitly approved for dogs and/or cats too. Without reliable use instructions they can be easily overdosed, and pets may not tolerate formulations developed for use on livestock, horses and/or poultry.  Some active ingredients may be toxic to particular animals.

Never use agricultural or hygiene products on livestock, horses, poultry or pets, unless explicitly approved for veterinary use, which is quite unusual. Even if the specific active ingredient is approved for some veterinary use. The formulations for agricultural and/or hygiene use are mostly different than those for veterinary use and  may be toxic to or not be tolerated by animals.

It is obvious that veterinary medicines are not intended for and should never be used on humans!!!

Resistance of ectoparasites against organochlorines

Housefly resistance to DDT was already reported in 1947, and resistance to DDT and other organochlorines appeared subsequently in numerous parasites (e.g. ticks, mosquitoes, fleas, etc.). Although organochlorines have not been used in many countries for more than 20 years, there are still parasite populations in the field that remain resistant to them, particularly houseflies, Boophilus cattle ticks, and mosquitoes.

It is known that resistance to some organochlorines (especially to dieldrin) shows cross-resistance with more modern insecticides such as synthetic pyrethroids (e.g. cypermethrin, permethrin) and phenylpyrazoles (e.g. fipronil), to chemical classes that also act on the transmission of nervous signals through the ion gated sodium channel.

Visit also the articles in this site about parasite resistance to antiparasitics and  how it develops.

Other articles in this site

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