CHEMICAL CLASSES of VETERINARY PARASITICIDES for use on LIVESTOCK, HORSES, DOGS and CATS



From a purely chemical point of view almost all active ingredients with parasiticidal activity (either external or internal parasiticides) discovered so far are synthetic organic molecules, i.e. they do not occur in nature but have been synthesized in the laboratory. Very few such active ingredients occur naturally in plants or other organisms. And even fewer are of mineral (i.e. inorganic) origin.
Most active ingredients can be grouped into chemical classes or families with similar functional groups, i.e. they share a specific molecular structure. E.g. organophosphates are all derivatives of phosphoric acid.
Active ingredients of the same chemical groups have usually the same mechanism of action at the molecular level. What differs considerably for active ingredients of the same chemical class is often the spectrum of activity, the toxicity to both parasites and non-target organisms, the behavior in the environment, etc.
The most relevant chemical classes of parasiticides discovered so far are the following ones, ordered by spectrum of activity and by the decade the first compounds were introduced, regardless of whether they are still marketed today or not:
Endectocides: active against both external and internal parasites
  • Macrocyclic Lactones (1980s) or endectocides: broad-spectrum systemic ecto and endoparasiticides,
Ectoparasiticides: active against external parasites (mainly insects, ticks and mites)
  • Organochlorines (1940s): broad-spectrum insecticides and acaricides, nowadays prohibited in most countries.
  • Organophosphates (1950s): broad-spectrum insecticides and acaricides. Are being phased out in some countries.
  • Carbamates (1950s): broad-spectrum insecticides and acaricides. Are being phased out in some countries.
  • Amidines (1960s): mainly acaricides and tickicides.
  • Synthetic Pyrethroids (1970s): broad-spectrum insecticides and acaricides.
  • Benzoylureas (1970s): development inhibitors (= growth regulators).
  • Juvenile Hormone Analogues (1970s): development inhibitors (= growth regulators).
  • Neonicotinoids (1990s): broad-spectrum insecticides and acaricides.
  • Phenylpyrazoles (1990s): broad-spectrum insecticides and acaricides.
  • Spinosyns (1990s): broad-spectrum insecticides and acaricides, partly systemic.
  • Isoxazolines (2010s): broad spectrum, systemic insecticides and acaricides.
A few relevant ectoparasiticides do not belong to these chemical classes, e.g. cyromazine, dicyclanil, metaflumizone.
Other chemicals frequently used on livestock and pets against external parasites are not properly ectoparasiticides, nor build an own chemical class, but they share a common funtionality regarding their use against external parasites:
  • Repellents: do not kill the parasites, but keep them away from the treated animals. They are not properly a chemical class, but a functional class.
  • Synergists: enhance the parasiticidal activity of certain active ingredients or help to overcome resistance.
Endoparasiticides = anthelmintics: active against internal parasites mostly parasitic worms (roundworms, tapeworms, flukes)
  • Benzimidazoles (1960s): broad-spectrum anthelmintics: nematicides, taenicides, flukicides
  • Imidazothiazoles (1960s): broad-spectrum nematicides (only against roundworms)
  • Tetrahydropyrimidines (1960s): narrow-spectrum nematicides
  • Isoquinolines (1970s): taenicides
  • Salicylanilides (1970s): narrow-spectrum nematicides, taenicides, flukicides
A few relevant endoparasiticides do not belong to these chemical classes, e.g. clorsulon, monepantel, nitroscanate, nitroxinil, piperazine derivatives.
Other parasiticides: a few parasiticides are of mineral (i.e. non synthetic) origin, or are directly obtained from plants:
  • Natural insecticides: mainly pyrethrins and rotenone
  • Copper: used mainly as an anthelmintic against a few parasitic roundworms in the stomach of ruminants.

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