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What are wound dressings?

Wound dressing is a common medical practice to treat whatever injuries or wounds that can occur on animals as well as humans. Usually they contain disinfectants, and other medicines to ease pain, promote healing, etc.

In the context of parasite control on livestock and pets, there are dressings that contain larvicides to kill maggots of parasitic flies that are specialized in laying their eggs in wounds or injuries. The larvae hatching out of the eggs feed on the host tissues causing extreme damage. Infections by such maggots are called skin or cutaneous myiasis.

Cutaneous myiases affect mainly cattle, sheep and goat, especially those on pasture. But any other animal that gets injured, including pig, poultry, dogs and cats can be affected by such cutaneous myiasis, especially in rural areas of tropical and subtropical regions.

Parasites controlled by larvicidal dressings

Blowfly maggots (Lucilia sericata)

Cutaneous myiases are mostly due to screwworm flies (affect all mammals) and blowflies (affect sheep and goats). Larvicides (also called maggoticides) are insecticides that are effective against the larvae (i.e. the maggots) of insects.

The maggots of other parasitic flies (e.g. bot flies, warble flies) do not remain on the body surface, but penetrate inside the host and cannot be controlled with wound dressings. Such internal myiases are treated mainly with macrocyclic lactones (e.g. ivermectin) administered as injectables or pour-ons.

Larvicidal dressings are ready-to-use products that contain one or more larvicides and that are administered directly to the wound.

They are used either to prevent development of the larvae that hatch out of the fly eggs, or to cure infections with maggots that already infest the wound. Preventive treatment of whole herds is often done after management practices that cause wounds on livestock such as dehorning, tail docking, castration, mulesing, etc. Curative treatment is usually done on individual animals that have been accidentally injured and have wounds already infested with maggots.

Active ingredients and formulations of larvicidal dressings

There are basically three types of formulations for larvicidal dressings: liquids, powders and ointments.

All of them contain one or more generic active ingredients belonging to the organophosphates (e.g. chlorpyrifos, coumaphos, diazinon, dichlorvos), carbamates (e.g. carbaryl, propoxur), synthetic pyrethroids (e.g. cypermethrin, permethrin), neonicotinoids (e.g. imidacloprid) or phenylpyrazoles (e.g. fipronil). 

Most larvicidal dressings kill the maggots by direct contact or ingestion, i.e. they have no systemic mode of action. Their residual effect is usually sufficiently long to allow the injury to heal and stop attracting flies. But sometimes re-treatments may be required.

Larvicidal dressings are usually administered in rather low quantities, just enough to cover the wound. As a general rule such quantities do not create residue problems in milk or meat. Such treatments are also well tolerated by the animals and side effects are infrequent.

There are no reports on serious resistance problems of the concerned fly species to larvicidal dressings.

Resistance of blowflies to insecticides is a serious problem in Australia and New Zealand, but affects herd treatments using dips, sprays, pour-ons, etc. to prevent infestations, not wound dressings. The main difference between dips, sprays, pour-ons and dressings is that wound dressings contain larvicides that are administered in much higher concentrations than dips, sprays or pour-ons.

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