SAEVA Proceedings 2015 | Page 29

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