Ectoparasiticides

Introduction

These are chemicals used to treat and control parasites that reside on, or in, the skin and coat of animals. They are a diverse set of products that can be applied in various ways. Some important terms must be discussed first before mentioning how each drug works.

Insecticides kill insects, whilst acaricides kill mites and ticks.

Endectocides are active against both endo and ectoparasites. This are discussed on the anthelmintics pages.

Insect Growth Regulators don't kill adult stages but are able to prevent hatching, larval development or metamorphosis.

Modes of Action

These drugs mainly act on the parasites nervous system and neuromuscular junction. They are selectively toxic by having a greater affinity for the parasite receptor than the animal receptor. They can either by contact posions - arbsorbed through the parasites exoskeleton, or are stomach poisons - they are absorbed through the parasites stomach and so the parasite must bite to be affected. Thus the stomach poisons are usually given systemically to the host animal.

Insecticides

Synthetic Pyrethroids - Cypermethrin, Deltamethrin

• Many products and are in widespread use.
• They increase the permeability of neuronal membranes to sodium ions. This makes the membrane hyperxcitable leading to convulsions and death.
• In high concentrations these products can cause adverse reactions in cats and dogs.
• Piperonyl butoxide is a syngeristic product that is often put in formulations to potentiate the action of the pyrethroids.

Semicarbazones - Metaflumizone

• A new group of insecticides
• They work by blocking sodium ion channels in the axonal membrane of nerves, this prevents the propagation of nerve impulses leading to paralysis of the insect and hence death.

Organophosphates and Carbamates - Diazinon, Dichlorvos

• Their use has been limited of late as they are potentially neurotoxic to animals and man, and can cause environmental damage.
• They are anticholinesterases and so resukt in the build up of acetylcholine in the parasites's synapses, leading to paralysis and death.

Neonicotinoids - Imidacloprid, Nitenpyram

• These are mainly used for pest control in crop protection schemes. The above two drugs though are both currently used for flea control.
• They work by stimulating and then blocking post-synaptic nicotinic acetylcholine receptors which leads to paralysis and death.

Phenylpyrazoles - Fipronil, Pyripole

• These are used for flea control and work by blocking GABA transmitted impulses, thus blocking the influx of chlorine ione into nerve cells.

Macrocyclic Lactones - avermectins and milbemycins

• These are endectocides are will be discussed in full on the anti-helminth pages.

Amidines - Amitraz

• This has limited action in veterinary medicine.
• It works by acting on octoparamine receptor sites, these are within parasite muscles, resulting in increased nervous activity, spasming and death.

Inert Inorganics

These are inert substances, often powder, that are used to treat the environment in a properly managed flea control scheme. They work by dessication of the flea eggs or by clogging their spiracles. An example is Sodium polyborate.

Insect Growth Regulators

Insect juvenile hormone analogues - methoprene, pyriproxyfen These work by blocking receptor sites for insect juvenile hormone. This stops the juvenile insect from continuing along it's natural course of development so it never reaches adult status.

Chitin Inhibitors - Cyromazine, dicyclanil, lufenuron These likewise block insect development by preventing the insect access to chitin, one of the key components to an adult insect's exoskeleton.

Points to Consider When Choosing an Ectoparasiticide

When choosing an ectoparasiticide treatment for a particular animal it is always key to have in mind what animal you are dealing with, what is the main parasite problem, what sort of environment does the animal reside in and how much owner compliance do you think there will be. It is also important to understand the following properties of your drug options:

• Knock-down - the immediate effect on the parasites at the initial treatment. It is essentially the curative effect providing immediate relief.
• Persistent Efficacy - if the product has any residual efficacy and how long it is capable of protecting the animal from re-infestation. This may result from binding to the skin or hair shaft, or from residing in a body reservoir, or form a special formulation of the drug.
• Speed of Kill
• Repellenct - how readily the product repells parasites from landing or feeding on the host. This can be provided by a either a repellent vapour phase of the drug, or from a contact repellency.
• Antifeeding effects - the combined or singular effects of repellency, quick knock-down and behavioural changes can reduce the opportunity for a parasite to bite and feed.
• Spectrum of Activity - how many different species and stages of the life-cycles does the product act upon.
• Ovicidal Activity - The ability of some products to affect the fertility of parasite eggs.
• Larvicidal Activity - is the product active on the animal or in the environment. Which one you desire will depend on the parasites involved and also the host species.

Application

There are numerous ways in which ectoparasiticides are formulated. They can be in solutions, suspensions, emulsions, powders and shampoos. A common formulation seen in practice is when they become incorporated into a plastic matrix, which the product can be released slowly. This use is often seen in flea collars or in cattle ear tags. They can also be given orally or systemically.

For small animals though the most common method is via a spot-on treatment. These can work on the surface by spreading over the skin in a lipid layer, or systemically by being absorbed through the skin and then distributed through the blood.

For large animals sprays, pour ons or dips are the most common. These all work in a similar way to the spot-on formulations.

Dipping

The dipping of sheep is a requisite to control ectoparasites in the sheep population. It is though quite a tricky process to get right and a failure at any one of the points could mean that the sheep aren't covered against parasites or that resistance may build up within the parasite population. On the other hand if the concentration of the dip is too high there is significant danger of toxicity to both the sheep and the handler.

A dip should comprise of:

• A holding pen, so that the sheep aren't stressed before the dip.
• A race so that animals can be lead single file into the dip tank.
• The dip tank should have a ledge so that the sheep must jump in and swim to the far end to aid in coverage of the ectoparisiticide. For sheep scan control it is important the every animal has their head submerged.
• A final draining pen, where the sheep can let the excess ectoparisticide drain off back into the tank prior to turn out.

Other important things for the farmer to consider are:

• The volume of the dip tank; this must be known to prepare the correct concentration of product.
• Some product are absorbed to the skin or wool of the sheep and so the tank concentration will become weaker, exhaustion. Replenishment must occur at regular intervals to thus maintain a constant concentration.
• Products of the same active ingredient shouldn't be used together as they may break and form a toxic layer of compound.
• The life time of a product can be limited by any bacterial contamination.

As the process of dipping is both dangerous to the sheep and handler a set of legal requirements have been put in place, these are:

• A certificate of competence in the UK is needed to purchase and handle sheep-dip and a COSSH risk assessment must be performed before dipping.
• Protective clothing must be worn.
• There are specific ways od disposing of sheep dip, please see the Defra website for more information; http://www.defra.gov.uk/environment/water/ground/sheepdip/index.htm

Problems with Ectoparasiticides

The main problem with ectoparasiticides are their potential toxic nature. This is an increased risk compared to other pharmaceutical products because of the large volumes and high concentrations that are used. This is not only a risk to animals, but to man and to the environment. Man and animals can experience acute toxicty if concentrations are calculated incorrectly. It has been suggested that chronic neuropathic toxicity can occur with repeated exposure to organophosphate dips, this is highly controversial at the moment and currently there has been no absolute proof of this. Due to their potential ability to reach waterways specific regulations have been put in place concerning their disposal. Please look at the above defra link if you want more information.

Resistance

Like all anti-infective agents resistance is starting to develop as a very serious problem within this set of drugs. This has been because the parasites experience high selection pressure, with the vast majority of the parasite population coming in contact with the toxicant. This leads to a higher prevelance of the resistant gene amongst the subsequent surviving generations.

It must be remembered that most reports of resistance aren't in fact this but are a failure of treatment. This can be because of a poor choice of product, poor application or because of an overwhelming environmental challenge.