Cutaneous Onchocerciasis

Cutaneous onchocerciasis, which is one of the differential diagnoses for ventral midline dermatitis, occurs worldwide and is caused by the microfilariae of Onchocerca spp. The Onchocerca species that can cause cutaneous disease in horses are O cervicalis, O gutturosa, and O reticulata. In the United States, O cervicalis and O gutturosa are the most common species. The adult resides in the ligamentum nuchae in horses, whereas the microfilariae are found in the dermis. Body sites most commonly infested with the microfilariae include the face, eyelids, neck, and ventral midline, particularly the umbilical area. Biting insects such as Culicoides and Simulium spp function as vectors. The number of microfilariae is particularly high in warmer months, coinciding with the time of the year when the vectors are most active. Cutaneous onchocerciasis was very common in the United States in the late 1970s, particularly in the southern states, where more than 80% of horses that underwent biopsy had positive findings for microfilariae. At present, although many horses are still infested with the adult parasite, the cutaneous disease has become rare because of the common use of avermectins for routine deworming, which eliminates microfilariae in the skin. In other countries, onchocerciasis is not rare. In an epidemiologic study in Brazil in 2004, O cervicalis was detected in midventral skin biopsy specimens in 17.9% of 1200 horses, and adult worms were recovered from the ligamentum nuchae in 16.6% of the same animals.

The cutaneous disease associated with O cervicalis develops as a hypersensitivity reaction to the microfilariae, which is why only some horses develop cutaneous disease although most are infested. Cutaneous signs include alopecia, scaling, depigmentation, plaques, and crusting. Annular lesions on the center of the forehead are believed to be suggestive of cutaneous onchocerciasis. Onchocerca cervicalis microfilariae can also affect the eye and be responsible for uveitis, keratitis, and vitiligo of the bulbar conjunctiva. Horses infested with O gutturosa and O reticulata may also develop nodules associated with the adult worms. Pruritus is variable and can be severe in some horses. Secondary skin infections may develop and contribute to the pruritus.

Differential diagnoses for cutaneous onchocerciasis include Culicoides hypersensitivity, staphylococcal folliculitis, dermatophytosis, fly bite dermatosis, environmental allergies, and food allergy. Clinical suspicion of cutaneous onchocerciasis is raised when a history of inconsistent or minimal deworming accompanies the typical cutaneous signs. Diagnosis can be challenging in the sense that the simple demonstration of microfilariae on skin scrapings does not prove a causal effect for the skin disease and may be an incidental finding. Skin biopsy shows a superficial perivascular eosinophilic dermatitis. Microfilariae are targeted by the eosinophilic inflammation, and that can lead to the formation of eosinophilic granulomas. The presence of the microfilariae and the inflammatory response around them are highly suggestive of cutaneous onchocerciasis.

Treatment of cutaneous onchocerciasis is aimed at both killing the microfilariae and decreasing the inflammatory response associated with the hypersensitivity. Both ivermectin and moxidectin are effective in killing the microfilariae but not the adult parasite, so deworming should be repeated on a regular basis. One study evaluated the efficacy of a single dose of ivermectin (0.2 mg/kg), in injectable or paste formulations, against microfilariae of O cervicalis and associated skin lesions in 20 naturally infected horses that were monitored 21, 42, and 63 days after treatment. Microfilariae were absent by day 21 after treatment in all but 1 horse. The authors reported that active lesions improved or were resolved completely by 63 days. In biopsy specimens, the severity of inflammation was reduced in all horses by 63 days after treatment, but there was a residual population of inflammatory cells. In that study, no adverse reactions of treatment were observed in any of the horses. Worsening of lesions can occur, however, after treatment with ivermectin, likely as a consequence of death of the microfilariae. This was demonstrated in a study in which 8 of 12 horses with cutaneous onchocerciasis that were treated with a single oral dose of ivermectin developed transient skin reactions, such as hives and pitting edema of the ventral midline and intermandibular space, 4 to 24 hours after treatment. It is thus important to combine the deworming with glucocorticoid therapy in severely affected horses to minimize the allergic reactions secondary to killing of the microfilariae. It is also recommended to implement regular deworming to minimize future relapses in hypersensitive horses.

Culicoides Hypersensitivity

Culicoides spp bites and the resulting hypersensitivity are another common cause of ventral midline dermatitis in horses. Many different species of Culicoides have been described, and some have a distinct predilection for feeding on the ventral abdomen and causing ventral dermatitis.

Culicoides are very small biting flies that are particularly active from dusk to dawn and breed in standing water such as ponds and lakes. They are poor fliers, flying only for short distances and not against the wind. It is common to have more than one species of Culicoides feeding on one horse and, depending on the species involved, the distribution of the lesions can be primarily ventral or can be more generalized to include the lower limbs, dorsal areas, ears, face, neck, and rump.

Culicoides hypersensitivity is considered a mix of both type I and type IV hypersensitivity reactions against several antigens in Culicoides saliva. The lesions consist of papules that crust over and can induce severe pruritus and frequently lead to secondary bacterial infections. Culicoides hypersensitivity is considered one of the most common causes of severe pruritus in horses. Besides inducing hypersensitivity, Culicoides spp transmit many diseases, including but not limited to Onchocerca, bluetongue virus, and African horse sickness.

Diagnosis of Culicoides hypersensitivity is made from clinical signs, history (in most regions this is a seasonal dermatitis seen only in the warmer months), lifestyle (horse out on pasture at peak feeding times in paddocks close to standing water), and the lack of consistent use of fly repellents. Allergy testing can be considered to confirm a clinical suspicion, but it is important to note that normal horses may also show positive results on both intradermal and serology testing. Thus the detection of allergen-specific immunoglobulin E indicates exposure and development of immunoglobulin E but does not necessarily confirm causation by Culicoides. Conversely, some allergic horses may have a negative immediate reaction to intradermal injection of Culicoides allergen. Such horses may have type IV hypersensitivity, which will only be evident 24 to 48 hours after the test. For these reasons, the results of allergy testing must be interpreted in conjunction with the history and the clinical signs. The ultimate diagnosis relies on resolution or decrease of clinical signs in response to aggressive insect control.

Treatment of the ventral dermatitis caused by Culicoides involves use of fly repellents to prevent additional bites and reduction of inflammation by use of either topical or systemic glucocorticoids, depending on the severity of the inflammation. Although many products on the market are labeled as fly repellents, most are insecticides and not true repellents. True repellent activity against biting insects requires high concentrations of permethrin, which is crucial to provide relief to hypersensitive horses. Many spot-on formulations containing 44% to 64% permethrin, which provides good repellent activity, are available specifically for use in horses. These products can be used on problem areas once weekly, and sprays with lower concentrations (2% permethrin) may be used to cover the rest of the body. Sprays should be used daily for maximal protection, particularly in hot and humid climates, because the efficacy is decreased by exposure to rain and heavy sweating. Other synthetic pyrethroids, such as cypermethrin-containing products,¹ can be effective repellents provided that they are applied daily. To minimize bites, it is helpful to move horses to paddocks further away from standing water and to keep horses in the barn in front of fans during peak insect feeding times. These measures help minimize exposure to Culicoides because Culicoides fly only for short distances and cannot fly against the wind. Fly masks and fly sheets may be used as long as they are changed frequently and kept clean and dry. Incorrect use and maintenance of these items that can trap moisture in the heat of the summer may predispose horses to secondary infections.

Because many Culicoides-hypersensitive horses develop a secondary bacterial infection that significantly adds to pruritus severity, antimicrobial treatment is needed in most cases. In mild cases, this can be accomplished by use of topical therapy, such as benzoyl peroxide or chlorhexidine shampoo (weekly), or topical application of oxychlorine-based sprays (daily). In more severe cases, oral antimicrobial therapy may be needed. A good choice is use of an oral potentiated sulfonamide for a minimum of 2 weeks. Because antimicrobial resistance is a growing concern in medicine, topical therapy should be tried first rather than administering systemic antimicrobials in all cases.