EXTERNAL PARASITICIDES (ectoparasiticides) for veterinary use on LIVESTOCK - cattle, sheep, goats, pig, poultry - DOGS, CATS & HORSES What is an external parasiticide or an ectoparasiticide?

External parasiticides (= antiparasitics) are products used to control parasites that attack livestock, pets, horses or other animals externally: flies, ticks, mites, fleas, etc. They are also called ectoparasiticides because external parasites are also known as ectoparasites. This is in contrast with the endoparasiticides used against internal parasites known as endoparasites. There are also parasiticides that are effective against both ecto and endoparasites and are called endectocides (e.g. ivermectin).

Most ectoparasiticides for livestock, horses and pets are considered as pesticides, a few ones are considered as veterinary medicines. In fact, the terms "pesticide" and "veterinary medicine" are not clearly defined. As a general rule, parasiticides for external use (e.g. dipping, spraying, pour-ons, spot-ons, 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, 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. Thanks God, only a few active ingredients have been privileged with two or more names.

Classifications of ectoparasiticides (of insecticides in general)

Ectoparasiticides can be classified according to various criteria. The most common ones are:

Developmental stage affected by the active ingredient:

• Adulticides kill the adult parasites
• Larvicides kill the larvae
• Ovicides kill the eggs (rather seldom)

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

Type of parasites controlled:

• Insecticides kill insects.
• Acaricides kill mites and/or ticks
• Tickicides kill ticks
• Lousicides kill lice
• Scabicides kill scab
• Etc.

A single active ingredient can be only insecticide, or only tickicide, or both, etc.

An unofficial but pragmatic classification very much used in the Animal Health industry is to distinguish between four major groups:

• Classical ectoparasiticides: refer to synthetic organic active ingredients that kill parasites mainly by contact and are used in dips, sprays, pour-ons, spot-ons, etc. This is the largest group and includes organochlorines, organophosphates, carbamates, amidines, synthetic pyrethroids, neonicotinoids, phenylpyrazoles, etc. Many such active ingredients are also used in agriculture and in public and domestic hygiene.

• Endectocides: refer to active ingredients of a specific chemical class, the macrocyclic lactones, which are effective against numerous external and internal parasites of livestock and pets. The most famous one is ivermectin. They all act systemically, but can also act by contact and orally. Most of them are semi-synthetic derivatives of natural fermentation products. A few are also used in agriculture and hygiene. A few ones (e.g. doramectin, moxidectin, selamectin) are used only against veterinary parasites.

• Development inhibitors: refer to active ingredients that do not directly kill the parasites but interfere with their development. Immature stages cannot molt properly and die, which interrupts their lifecycle. A few compounds are analogues of natural insect hormones. Others are organic compounds belonging to various chemical classes (e.g. benzoylureas). They are also used in agriculture and hygiene.

• Natural parasiticides: refer to active ingredients of biological origin (mostly plants or micro-organism; e.g. pyrethrins, rotenone), either extracted from plats that are grown and harvested for this purpose, or industrially produced as other synthetic compounds; or of mineral origin, i.e. not of organic but of inorganic origin (e.g. borax, silica). They are also used in agricultures and hygiene.

Chemical classes of external parasiticides

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.

Many of them 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 mechanim 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 ectoparasiticides 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:

• Organochlorines (1940s): broad spectrum insecticides and acaricides, nowadays prohibited in most countries
• Organophosphates (1950s): broad spectrum insecticides and acaricides, nowadays increasingly restricted in many countries
• Carbamates  (1950s): broad spectrum insecticides and acaricides, nowadays increasingly restricted in many 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)
• Macrocyclic Lactones (1980s) or Endectocides: broad sepctrum systemic ecto and endoparasiticides,
• 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 insecticides and acaricides, systemic

• 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. cyromazine, dicyclanil, metaflumizone.

There are other chemicals frequently used on livestock and pets that are not properly ectoparasiticides:

• Repellents: do not kill the parasites, but keep them away from the treated animals
• Synergists: enhance the parasiticidal activity of certain active ingredients or help to overcome resistance

Delivery forms of ectoparasiticides

Most ectoparasiticides are administered to livestock and pets in the following delivery forms:

For external use (i.e. topical administration)

• Collars impregnated with insecticides; used only on pets.
• Dipping: used mainly on cattle, sheep and goats.
• Dressing: used on livestock and pets.
• Dusts, back rubbers and other self-treatment devices: used mainly on poultry, pig and cattle.
• Ear-tags impregnated with insecticides: used only on cattle.
• Pour-ons: used mainly on cattle, sheep, horses, goats and pigs.
• Shampoos, soaps, sprays, powders, creams and the like: used mainly on pets.
• Spot-ons: used mainly on dogs and cats, very seldom on livestock.
• Spraying: spray-races, hand spraying, jetting: used mainly on livestock and horses.

For internal use (i.e. oral or injectable administration)

• Additives for feed or drinking water: used mainly on pig and poultry, less on cattle, horses or sheep, seldom on pets.
• Drenches or other liquid formulations for oral administration (used mainly against worms)
• Tablets, pills, capsules, or similar solid forms for oral administration (used mainly against worms)
• Injectables
• Slow-release boluses: for oral administration, used mainly on ruminants, i.e. on cattle, sheep and goats.

Agricultural origin and use of many ectoparasiticides

It is good to know that most ectoparasiticides used in Animal Health on livestock, horses and pets were primarily discovered and introduced as pesticides for agriculture. In fact, most veterinary ectoparasiticides are spin-offs of agricultural pesticides.

There are various reasons for this. First, because agricultural pests and veterinary ectoparasites are mostly insects, ticks, mites etc, and what works against agricultural pests, works often against veterinary ectoparasites too. Second, because the market potential of Animal Health ectoparasiticides is much smaller than the market potential of agricultural pesticides, and most agrochemical companies understandably focus first on agricultural pests. Once they find an active ingredient that is suitable for agriculture, they will investigate its veterinary potential.

A notable exception is ivermectin, which was first introduced for use on animals and later on for agriculture and human medicine. Probably because the company that discovered and introduced it (MS&D) was primarily not an agrochemical but a pharmaceutical company.

Other exceptions are e.g. dicyclanil, doramectin, eprinomectin, fluazuron, moxidectin, and pyriprole. They all were introduced for use on animals and so far they are not used on crop protection.