South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Histopathologic findings in biopsy samples collected from affected skin are usually non-specific and
include superficial to deep perivascular dermatitis with infiltration of eosinophils and lymphocytes into the
dermis which can also be observed with other pruritic skin problems including atopic dermatitis, food
allergy, and ectoparasites. Skin biopsies are usually not necessary to pursue, however, when the clinical
diagnosis remains uncertain, collection of multiple skin biopsies can help either rule out or identify other
disease processes other than allergic skin diseases.
Treatment of equine insect hypersensitivity includes decreasing exposure to insects as well as management
practices and medications to limit pruritus. Twice daily application of a 2% permethrin fly spray is fairly
effective at repelling Culicoides spp., flies, and mosquitoes spraying at dawn and dusk. Because fly
repellant sprays that can be purchased at feed and tack stores can vary in both active ingredient and
concentration, it is important to have clients read the product label and select sprays with greatest
concentration of pyrethroid/ permethrin. Pyrethrum is a botanical insecticide produced by grinding up
flower heads of certain Chrysanthemum spp. grown mostly in Kenya. Species grown in the United States do
not produce the insecticidal compounds. Pyrethrins are the actual insecticidal compounds in pyrethrum and
work as neurotoxins to provide a quick “knockdown” effect. However, pyrethrins degrade quickly when
exposed to sunlight and therefore provide a short period of activity after application. Pyrethroids are
“pyrethrin-like” synthetic compounds that provide greater insect control and longer residual activity.
Permethrin is one of several synthetic pyrethroid insecticides currently available. There are also anecdotal
reports of weekly application of fipronil spray (Frontline spray by Merial) as an insect repellent but this is
an off-label use of this product marketed for flea and tick control for dogs and cats. Also the use of cattle
ear tags impregnated with pyrethroids or organophosphates attached to halters or braided into mane and tail
hair may be effective. Use of topical insect repellents marketed for human use that contain varying
concentrations of N, N-diethyl-meta-toluamide (DEET) is not recommended in horses as profuse sweating,
irritation, and exfoliation has been reported following repeated application of DEET at concentrations of
50% or greater to horses. Repeated application to the skin of a 15% DEET solution also produced
hypersteatosis, an excessive activity of the sebaceous glands in horses.
Because all topically applied insect repellents have only limited efficacy, additional management practices
can often be helpful in decreasing both exposure and overall insect burden in the stable environment.
Stabling horses at dawn and dusk hours, peak feeding times for Culicoides spp. and mosquitoes, can limit
exposure. Culicoides spp. are able to pass through regular screens and mosquito netting and extra fine mesh
screening can be challenging to keep clean in a barn environment. Use of powerful hallway and stall front
fans may be a more practical approach because Culicoides spp. and mosquitoes are rather weak fliers. In
addition to these management practices, use of fly masks and body sheets can also reduce exposure and
these can be used both in the stall and at pasture.
Insect burden in the environment can be reduced in several ways:
1.
Minimizing sites where water can accumulate and draining marshy areas adjacent to stables.
2.
Frequent dumping and cleaning of water buckets and troughs.
3.
Products containing the bacterium Bacillus thuringiensis subspecies israeliensis (mosquito dunks)
kill mosquito and black fly larvae in water sources for 30 days or longer.
4.
Because many fly larvae develop in manure piles and other accumulations of organic debris,
frequent manure removal or dragging pastures to break up manure piles are also recommended for both
insect and internal parasite control.
5.
To decrease insect burden in the stable, overhead spray or mist insecticide systems can be installed
and programmed to discharge repellent multiple times a day.
6.
To further reduce fly burden, feed-through fly control products can be used. These products are top
dressed on grain an d are passed unchanged in the faeces. The feed-through chemicals work by
preventing development of fly larvae into adult flies within manure piles. Newer feed-through products
are organophosphate-free: Methoprene is an insect growth regulator that interferes with function of a
fly growth hormone that regulates progression through the different life stages. Two other compounds,
diflubenzuron and cryomazine, are chitin synthesis inhibitors that interfere with production of chitin,
the major component of the cuticle. Because mammals do not have insect growth hormone or chitin,
these compounds are very safe to use. Although limiting insect exposure is critical for managing horses,
management practices alone may not be effective in controlling pruritus. Thus, concurrent use of anti29