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Equine ectoparasite control

by
01 November 2015, at 12:00a.m.

NICOLA RATCLIFFE of MSD Animal Health looks at the various sensitivities of horses and types of pruritus caused by flying and biting parasites and what vets can do about them

WHILE there is no way to completely eliminate flying and biting parasites, a co-ordinated approach can significantly reduce their numbers and will help horse owners to keep their horses more comfortable.

Culicoides hypersensitivity Insect bite hypersensitivity, more commonly referred to by owners as sweet itch, is a seasonal pruritus related to the presence of Culicoides spp., Simulium spp., Stomoxys calictrans and/or Haematobia irritans.

The disorder is attributed to a Type II (cytotoxic) and a Type IV (cell mediated or delayed) hypersensitivity reaction to the salivary antigens of these flies. Individual equines may be more sensitive to one species of fly than the other and certain breeds, for example Welsh ponies, are more likely to develop the condition, supporting the hypothesis that sensitivity may be genetic in origin.

There are three patterns of skin disease associated with the condition:

  1. Dorsal from the ears to the tail (most common)
  2. Ventral midline
  3. Varying combinations of both

With all patterns of skin disease, the initial lesions are papules and nodules which are severely pruritic. The severe pruritus encourages self-mutilation resulting in broken hairs, alopecia and fresh excoriations and scabs.

With further attacks and selfmutilation the skin damage becomes more severe and over a period of time skin changes become permanent with chronic thick lichenified folds and rugae. Lesions heal and the hair grows back in the winter only to return again the following spring.

Treatment and control

Practical treatment revolves around anti-inflammatory therapy, which is often unsatisfactory and can have serious side-effects if used longterm. Preventive control measures are therefore crucial to avoid the condition and limit the suffering which can arise from the intense and unrelenting itching.

Control is aimed at minimising exposure to midge bites and often includes some of the following:

  • Starting control before the midge season – owners must be advised not to wait for their horse to start itching
  • Stabling a horse during dawn and dusk when midge activity is greatest
  • Turning horses out in fields with lower midge burdens such as breezy pastures, higher ground and away from woodland
  • Preventing horses from grazing areas that have ponds nearby as these naturally attract the midges
  • Using an effective fly rug to prevent midge contact with a horse’s skin
  • Using an effective long-acting product containing permethrin and citronellol that is licensed to both kill and repel flies

Lice (pediculosis)

Both biting and sucking lice affect horses and donkeys and are a common ectoparasitic skin disease of horses worldwide. Just a small number of the biting louse, Damalinia equi, will cause severe pruritus, scurf and alopecia to the head, neck and dorso-lateral trunk. Careful examination is required so as not to miss the lice and in some cases skin scrapings may be required.

The sucking louse, Haematopinus asini are usually found in greater numbers and can be easier to see by the naked eye particularly over the base of the mane, tail and croup. They are more common during the winter and early spring when horses have thick winter coats.

Affected horses rub and bite at their head, neck, mane, flanks and tail, often resulting in rough coats with varying degrees of alopecia and self-inflicted lesions.

Horses with large burdens will show a deterioration in bodyweight and health status if left untreated because of the physical demands of the intense pruritus and in the case of sucking lice, blood loss and the impending anaemia.

Both types of lice are transmitted by close contact and therefore horses housed in close proximity, and as such living in a prominent breeding ground, are more likely to develop infestation.

Equally, horses which are unhealthy, such as those suffering from immune suppression disorders such as Pars Pituitary Intermedia Dysfunction (PPID, Equine Cushings Disease) are particularly susceptible and liable to harbour enormous populations. Lice can also be transmitted via indirect contact through fomites such as grooming equipment and rugs.

The differential diagnosis for tail rubbing, which can occur with louse infestations, is Oxyuris equi infestation, Culicoides hypersensitivity, or tail mange due to Psoroptes cuniculi or Chorioptes equi parasites.

Control involves treating all affected horses and in contacts two weeks apart as well as fomites such as tack, grooming equipment and rugs.

Topical application

Relaunched in 2015, Coopers Fly Repellent Plus is the only product licensed to kill and repel flies on horses. It contains permethrin (cis:trans 25:75) 1.05%w/v and citronellol in a long-lasting, ready-to-use formulation. It helps to rid horses of fly annoyance, thus improving welfare and comfort while also being effective in the control of biting lice and as an aid in the control of sweet itch.

The active ingredients are tried, trusted and proven to be effective and the product does not contain any ingredients on the 2015 FEI list of banned substances.

Permethrin is a residual insecticide which not only kills flies but actively discourages them from resting on a treated area through a contact repellent effect. It is a highly effective insecticide of extremely low mammalian toxicity, with the 25:75 CIS:TRANS isomer ratio being the least toxic form of permethrin available. This means that as well as offering powerful protection against ectoparasites, it is gentle on the horse’s coat and skin. The citronellol component is well recognised for its pleasant fragrance and also possesses good fly repellent properties.

  • Coopers Fly Repellent Plus contains permethrin and citronellol. It is a ready-touse topical application licensed to repel and kill biting and other flies on horses; as an aid in the control of sweet itch and for the treatment and control of infestations of the biting louse Damanlinia equi. Category AVM-GSL.