Insect Hypersensitivities

CHAPTER 148 Insect Hypersensitivities



Annette Petersen


Insect hypersensitivity is the most common cause of pruritus in horses. A hypersensitivity reaction to bites by various Culicoides spp. flies is the best documented cause of the syndrome, but allergic reactions to bites by Simulium spp. (black flies), Haematobia irritans (horn flies), Stomoxys calcitrans (stable flies), Tabanus spp. (horseflies), and mosquitoes are also recognized. A hypersensitivity reaction to the migrating microfilarial stage of Onchocerca cervicalis can cause a similar clinical syndrome, but this organism is not an insect and has largely been eliminated since introduction of ivermectin for parasite control nearly two decades ago. Insect hypersensitivity secondary to Culicoides spp. bites is a problem with a worldwide distribution, as reflected by the many different names for the syndrome: Culicoides hypersensitivity, summer itch, summer “fungus,” sweet itch, lichen tropicus, Queensland itch (Australia), Sommerekzem (Germany), Manochsvanskorv (Sweden), Kasen (Japan), and others.


Nearly 1000 Culicoides spp. are found worldwide, and common names for these insects include gnats, no-see-ums, biting midges, sand flies, and punkies. Culicoides spp. are tiny winged insects, 1 to 2 mm long, that breed in standing water such as ponds, lakes, irrigation canals, marshes, swamps, and watering troughs. They cannot fly more than 1 to 2 km, and only the female fly feeds, usually at dawn and dusk, when wind is minimal. Their numbers are greatest under warm, humid conditions. The allergic reaction is thought to be the combined effect of type 1 (immediate) and type 4 (delayed) hypersensitivity reactions to salivary antigens of the biting insects.



CLINICAL SIGNS


Signs of insect hypersensitivity can be observed in any breed, age, or sex of horse, but several breeds appear to have a hereditary predisposition: Icelandic, German Shire, Arabian, Connemara, Swiss Warmblood, American Quarter Horse, and ponies. Signs may first develop at 2 to 4 years of age and often become more severe as horses age. Not surprisingly, pruritus and papules develop during insect season. Typically pruritus starts in the spring, becomes worse in the summer, and regresses in the fall in temperate regions. Pruritus frequently becomes more severe and of longer duration each subsequent year, especially when concurrent environmental allergies (atopic dermatitis) are present. In more tropical regions, pruritus can be a year-round problem.


With hypersensitivity to Culicoides spp., pruritus is most commonly observed along the dorsum, and skin lesions include small papules and secondary excoriation along the mane, rump, and tail base. As the condition progresses, lesions may extend to the neck and shoulder regions and to the ears and face. Less commonly a more ventral distribution of skin lesions develops, depending on the feeding sites of the offending insects. In these instances lesions are found along the ventral aspect of the thorax and abdomen and in the axillary and inguinal regions, upper limbs, and intermandibular space. With chronicity and continued self-trauma, “buzzed-off manes” and “rat tails” may develop, along with lichenification and hyperpigmentation (melanotrichia, melanoderma) of the skin. Severe cases of insect hypersensitivity can lead to development of skin folds or rugae at the base of the mane and tailhead from chronic self-trauma. Affected horses are frequently observed to scratch and chew themselves as well as to rub against fence posts, trees, and stall walls. In some instances chronic pruritus can also lead to behavior changes (e.g., restlessness, irritability) and weight loss, making the horse unsuitable for riding.


Hypersensitivity to fly bites tends to induce pruritus and multiple papules, starting in the skin over the neck and shoulders. The mane and tail are often unaffected with fly-bite hypersensitivity, but lesions can extend to the head and may become generalized. Papules frequently become ulcerated consequent to self-trauma. More severely affected horses can also develop secondary bacterial infection of the skin (folliculitis). Signs of this complication range from erected hairs over small papules, with or without crusts, to deeper lesions with nodules and draining tracts. Urticaria is an uncommon manifestation of insect hypersensitivity in horses.



DIAGNOSIS


Diagnosis of insect hypersensitivity is established by recognition of a warm-weather seasonal occurrence of mane and tail pruritus and by ruling out other causes, such as atopic dermatitis or infestation by ectoparasites such as ticks and mites. Improvement with insect control provides additional support for a diagnosis of insect hypersensitivity. Results of a complete blood cell count and serum chemistry profile are usually normal, although some horses with insect hypersensitivity may have eosinophilia.


Intradermal testing with whole-insect antigen preparations is available at many referral practices. Interpretation of results of intradermal testing with insect antigens in horses requires experience because clinically normal horses frequently have positive reactions that may increase in severity and number with age. Clinically affected horses generally have a greater number of reactions as well as larger reactions. Reaction to a single insect extract would be an unusual finding; more commonly, affected horses react to multiple insect antigens. A further limitation of intradermal testing is that no commercial Culicoides spp. allergen extracts have been prepared specifically for use in horses. Nearly 1000 Culicoides spp. have been identified, and although it is possible that salivary antigens are highly conserved across the genus, this may not be the case. Limited investigation in the Netherlands has revealed that Culicoides

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Insect Hypersensitivities

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