Their parasiticidal properties were discovered in the 1960's.
The most relevant active ingredient of this chemical class is amitraz, which is still extensively used in livestock, especially in cattle, but also in dogs, mainly against ticks and mites. Another amidine still used in livestock in a few countries is cymiazole.
Click here for a general introduction to ectoparasiticides and their most important features.
Mode of action and characteristics of amidines
The acaricidal activity of amidines is due to their antagonistic effect on octopamine receptors of the nerve cells in the brain: the parasites become hyperexcited, paralyzed and eventually die. Hyperexcitation also has a detachment effect on ticks: they leave the host before completing or even initiating their blood meal. They also have a repellent effect that keeps many ticks away from treated animals.
Amidines have a tarsal effect, i.e. they act by contact. They are effective against several lice species too, but neither against most other insects such as flies, mosquitoes, fleas, etc, nor against fly maggots (myiasis). For livestock operations that require a simultaneous control of ticks and flies (usually the case in many parts of Latin America, Australia, Africa and Europe) this is a clear weakness of amitraz products.
The only two amidines nowadays used in livestock or pets are:
- Amitraz: acaricide, scabicide, louisicide. Extensive use in livestock (e.g. dips, sprays; pour-ons only for pig), frequent use in dogs (e.g. collars, spot-ons, etc.)
- Cymiazol: acaricide, scabicide, louisicide. Very scarcely used in cattle (e.g. in South Africa and Mexico). Also used in bees against the Varroa mite. Not used in pets.
Amitraz has the specific inconvenient of being unstable in the dipwash. Either you stabilized the dipwash adding large amounts of calcium hydroxide, or the consumed dipwash has to be fully replenished, which is more product-consuming.
Cymiazol is stable in the dipwash, but has almost no residual effect, i.e. the animals have to be dipped more frequently, which is also a disadvantage.
Amitraz is also used in agriculture, cymiazol not.
Delivery forms of amidines
Most common formulations with amidines for livestock are liquid concentrates (EC) for dipping and spraying. Often in mixtures with synthetic pyrethroids. Some of these formulations may also be approved for dogs in some countries.
There are a few pour-on formulations with amitraz, mainly for pigs. In the 1970's many efforts were made to develop amitraz or cymiazol pour-ons for cattle, but they were not successful. The main reason was that cattle, especially calves, did not tolerate such products. The sedative effect that amidines show on livestock is stronger with pour-ons than with dips and sprays.
For dogs, amitraz has been traditionally used in collars and baths, and in the last years several companies have added amitraz to their flea control spot-ons in order to make them work against ticks as well.
Safety of amitraz
Amidines, including amitraz shall never be administered to horses, because they do not tolerate them. Amitraz is also toxic to cats and rodents.
At a dosage close to the therapeutic one, amidines can show a sedative effect in cattle. After dipping or spraying cattle with amitraz or cymiazol, it is typical that a few animals lay down or fall to the ground and remain sleepy for a certain time, especially young or weaker ones. Normally they recover spontaneously or they can be sprayed with abundant water (which washes away part of the product that should protect them...) to accelerate recovery.
This sedative effe can also affect dogs, wherby puppies, small breeds as well as weak, sick or old animals may be more susceptible.
In the last years poisoning of cats, puppies and small breed dogs with amitraz has become more frequent and dramatic. It is basically a consequence of undue use in cats of spot-ons that contain amitraz and are only allowed for dogs. Or also a consequence of simple overdosing of puppies or small dogs with such products.
Additional specific information (toxicity, intoxication symptoms, adverse drug reactions, antidote, etc.) on the safety of amidine active ingredients for veterinary use is available in specific articles in this site:
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
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!!!
Parasite resistance to amidines
Amidines have a different mode of action than most other ectoparasiticides used in livestock (e.g. organophosphates, synthetic pyrethroids, macrocyclic lactones, etc.). Consequently they have no cross-resistance with them, i.e. are effective against many parasites that are resistant to these other chemical classes.
But there is reported resistance to amitraz in the cattle tick (Boophilus = Rhipicephalus microplus) in Australia and Latin America, and in the tropical cattle tick (= blue tick, Boophilus = Rhipicephalus decoloratus) in Southern Africa. Interestingly, after their introduction in the early 1970's, both amitraz and cymiazol were not broadly used in many regions because they were soon largely replaced by synthetic pyrethroids that were as effective against ticks but controlled flies and other insects as well. As a consequence, resistance of cattle ticks to amitraz was not an issue in many places during decades.
In the meantime, resistance of ticks (and flies!) to synthetic pyrethroids has reached such levels, that amitraz has experienced a very strong revival in many countries in the last decades. Whereas in the 1990's only a few commercial brands with amitraz for cattle were available in Latin America (basically the original ones: TAKTIC, TRIATIX, etc.), today there are dozens of brands in the market: usage has rocketed, probably because it is the only reliable tickicide left for dipping and spraying cattle after the failure of synthetic pyrethroids and the general rejection of organophosphates. As a consequence, resistance of the cattle tick to amitraz is now spreading and strengthening rapidly.