INTERNAL PARASITICIDES: ENDOPARASITICIDES or ANTHELMINTICS for veterinary use on LIVESTOCK - cattle, sheep, goats, pig, poultry - DOGS, CATS & HORSES

What is an internal parasiticide or an endoparasiticide?

Internal parasiticides (= antiparasitics) are products used to control parasites that attack livestock, pets or other animals internally, mainly roundworms, tapeworms and flukes. They are also called endoparasiticides because internal parasites are also known as endoparasites. This is in contrast with the ectoparasiticides used against external parasites known as ectoparasites. There are also parasiticides that are effective against both endo and ectoparasites and are called endectocides (e.g. ivermectin).

Within the endoparasiticides, the so-called anthelmintics are active against parasitic helminths, i.e. parasitic worms. Other endoparasiticides are active against parasitic organisms belonging to the protozoans, i.e. unicellular organisms such as haemoparasites (=blood parasites) or coccidia. The products for controlling these unicellular parasites are different from those against helminths and are not described here.

Most anthelmintics for livestock and pets are considered as veterinary medicines, whereas most external parasiticides are considered as pesticides. As a general rule, parasiticides for external use (e.g. dipping, spraying, pour-ons, spot-on, etc.) that contain active ingredients also used in agriculture or hygiene are considered as pesticides. Whereas parasiticides for internal use (e.g. injectables, drenches, tablets, pills, etc.) that contain active ingredients not widely used in agriculture or hygiene tend to be considered as veterinary medicines. However, there are numerous exceptions.

A curious practice is to use two different names for exactly the same active ingredient, one for its use in agricultural products, the other one for its use in veterinary products, e.g. diazinon (agricultural use) = dimpylate (veterinary use); or trichlorfon (agricultural use) = metrifonate (veterinary use). To keep it complicated, not all countries follow this distinction. Or both names may be used in the same country for veterinary use, etc.

Classifications of anthelmintics

Antelmintics can be classified by various criteria:

Developmental stage affected by the active ingredient:

• Adulticides kill the adult worms
• Larvicides kill the immature worms, i.e. the larvae
• Ovicides kill the eggs

A single active ingredient can be adulticide and larvicide, or only larvicide, or only adulticide, etc.

Type of parasites controlled:

• Nematicides: effective against roundworms  (= nematodes)
• Taenicides or cestodicides: effectice against tapeworms (= cestodes)
• Flukicides or trematodicide: effective against flukes (= trematodes)

A single active ingredient can be only nematicide, or both nematicide and taenicide, etc.

Chemical classes of anthelmintics

From a purely chemical point of view almost all active ingredients with parasiticidal activity (either ecto or endoparasiticides) 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 (e.g. copper).

Many of them can be grouped into chemical classes or families with similar functional groups, i.e. they share a specific molecular structure.

Active ingredients of the same chemical groups have usually the same mechanism of action at the molecular level. What differs considerably is the spectrum of activity, the toxicity to both parasites and non-target organisms, their behavior in the environment, etc.

The most relevant chemical classes of anthelmintics discovered so far are the following, ordered roughly by the time the first compounds were introduced, and regardless of whether they are still marketed today or not:

• 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

• Macrocyclic lactones (1980s) are a special chemical class that have both broad-spectrum ectoparasiticidal and endoparasiticidal activity, the reason for beeing called endectocides.

A few active ingredients do not belong to these chemical classes, e.g. clorsulon, monepantel, nitroxinil, piperazine derivatives.

Delivery forms of anthelmintics

Most anthelmintics are administered to livestock and pets in one of the following delivery forms:

For internal use (oral or injectable administration)

• Additives for feed or drinking water: used mainly on pig and poultry, less on cattle and sheep, seldom on pets.
• Injectables: used mainly on cattle, sheep, goats and pigs; scarcely on poultry and pets.
• Drenches = Oral liquids: used both on livestock (drenches) and pets.
• Tablets, pills, etc. = Oral solids, etc.: used mainly on pets and poultry, scarcely on ruminants and pigs.
• Slow-release boluses: used mainly on ruminants, i.e. on cattle, sheep and goats.

For external use (i.e. topical administration)

• Pour-ons: used mainly on cattle, sheep, goats and pigs.
• Spot-ons: used on dogs and cats; not used on livestock for anthelmintics