Topics and Briefing Documents

Endectocides and the Environment

Endectocides are used in the treatment and control of internal (endo-) and external (ecto-) parasites in certain animals. Endectocides comprise two groups of closely related compounds, the avermectins and the milbemycins. There are currently three avermectins available for use in UK livestock as endectocides: doramectin, eprinomectin, and ivermectin.

The only milbemycin currently available in the UK is moxidectin.

Endectocides can be used in animals to control internal and external parasites. Due to their broad spectrum of activity, it is possible to treat several species of parasites with one dose of an endectocide.

Individual products have been specifically developed for different uses, and formulated so that animals can be treated effectively. Endectocides can be administered by a variety of routes, orally, by injection or topically. The active ingredient is carried to all parts of the body via the circulation and works by interrupting the nerve transmission of the parasites leading to their paralysis and death.

To keep them healthy. Parasites cause damage to animals leading to loss of condition, suffering and possibly death. Different parasites cause different forms of suffering. For example:

1. Roundworms live in the animals’ gut, causing damage to the gastro-intestinal tract which can result in diarrhoea, loss of condition and reduced nutrient intake and utilisation.
2. Lungworm in cattle cause inflammation and irritation to the lungs, leading to coughing, difficulty in breathing and death in severe cases.
3. Sheep scab mites live in the skin under the wool. Sheep affected with scab experience severe irritation, scratch a lot, stop feeding, lose condition, and in extreme cases animals may die or may have to be destroyed.
4. Horn flies pierce the hide of cattle and feed by sucking the blood. Each fly may feed for up to 40 times each day, causing annoyance and irritation to the host.
5. Lice can cause extreme discomfort to cattle. The lice feed on the dead skin, hair and in some species, blood, causing severe itching. The animal will lose weight and become stressed, and in acute cases the lice can cause anaemia.

Treatment should ‘maintain parasite levels below thresholds of significance to animal health, welfare and productivity,’(1) by a combination of animal husbandry, pasture management, medication and vaccination when available. Farmers need a range of products from which to choose the one most suitable for their animals and farm.

Under UK and EU legislation, all animal medicines must satisfy strict criteria on their efficacy, quality and safety, (including safety to the animal, user, consumer and the environment) before they can gain a marketing authorisation and be allowed to enter the UK market. They are then subject to regular review every few years to ensure that they meet current standards of safety. Increasing emphasis is being placed on environmental safety, with very tough environmental impact assessments required. All the endectocides have been put through this procedure.

After the animal has been treated with an endectocide, the body metabolises the substance to some degree and then it is excreted, predominantly in the faecal matter. These faecal residues retain some of the biological activity of the endectocides and investigation of the consequences of this characteristic forms an important part of the environmental risk assessment.

Results from laboratory studies show that generally there is no effect on adult insects, but some effects on insect larvae. These effects vary amongst species, products and experiments.

Field studies confirm that there are no measurable effects in adult insects and some, variable effects in larvae.

In laboratory experiments, there are very high levels of exposure of the insects being studied to the active ingredients. The insects have no alternative but to live and feed in close association with the compounds. Under field conditions, the majority of insects will never be exposed to the faecal residues and so will not be affected in any way. For those species of insect larvae which may be exposed and affected, their populations appear to readily compensate through immigration or increased breeding performance.

There is no evidence that populations of dung beetles and flies are adversely affected by the use of endectocides. Any specific effects (e.g. on insect larvae) are limited and the total effect on the rest of the population is negligible. Endectocides remain a valuable choice of product for controlling both ecto and endo parasites of domestic animals.

Reviewed October 2007

Reference

(1) Chemical Review Section, (1998) NRA Special Review of Macrocyclic Lactones, National Registration Authority, Australia, May.

Further Information

(2) Wratten, S.D. and Forbes, A.B. (1996) Environmental assessment of veterinary avermectins in temperate pastoral ecosystems, Ann. appl. Biol, 128, 329-348.

(3) Bishop, Y. (ed) (2001) The Veterinary Formulary, (5th edition) The Pharmaceutical Press: London