Introduction Classification Taxonomy Phylum: Nematoda. Superfamily: Filaroidea. Genus: Onchocerca. Species: cervicalis. Distribution Very common where high numbers of the vectors, Culicoidesspp are found. High incidences reported in USA (25-100% horses are infested), Argentina, Australia. Infections only occasionally diagnosed in Northern Europe. Global warming and other changes in climate may increase prevalence. Significance 4th stage larve (microfilariae) cause dermatitis and conjunctivitis/blepharitis Skin: onchocerciasis . Formerly implicated as a cause of 'fistulous withers'. Active Forms Active Form 1 Morphology Typical filarial nematode morphology. Sexual dimorphism, males (2-6 centimeters long) smaller than females (20-30 centimeters long). Color Tolerances Other Survive in nuchal ligament of definitive host (horses). Development Growth Host inflammatory response confines adults within fibrous tissue. Reproduction Females are viviparous and produce microfilariae. Longevity Unknown, but thought to have a long life-span. Active Form 2 Morphology Slender microfilariae 205-240 micrometers in length. Tolerances Other Microfilariae are active and are found in connective tissues of head and neck, and on occasion in the conjunctiva. From these sites microfilariae are ingested by feeding Culicoidesspp. Development Growth Time taken for development of microfilariae in intermediate host to infective stage varies with environmental temperatures. Longevity The lifespan of the adult fly. Resting Forms Clinical Effects Epidemiology Lifecycle Microfilariae in dermis. Ingested by biting flies (Culicoides spp). Develop to infective stage. Deposited by fly on to skin during feeding. Adult worms found in tendons/ligaments, typically nuchal ligament. Transmission Through feeding activity of Culicoidesspp. Pathological effects The host response to dying worms may cause dermatitis with pruritus, alopecia, depigmentation and ulceration. This immunopathologic response (mazzotti reaction) is characteristic of hypersensitivity responses to helminth parasites. Parasite cuticle collagen components have been implicated in inducing such responses viaaggregation of platelets and production of arachidonic acid metabolites. Skin lesions (alopecia, scaling, crusting, inflammatory plaques, erythema, ulceration, lichenification, pruritus) are most usually seen on the forehead, ventral mid-line and pectoral region. Sporadic cases involving the conjunctiva also occur. Adult parasites in nodules are sometimes apparent as painless swellings in the nuchal ligament. Other host effects In general, infections are tolerated well with little pathology. Horses that have clinical lesions have developed hypersensitivity. Control Control via animal Fly avoidance measures - stabling of horses at dawn and dusk. Fly control measures - repellents (eg in USA 2% permethrin products are very effective as repellents), insecticides. Control via chemotherapies Ivermectin Ivermectin is effective against microfilariae 200 micrograms/kg one dose (PO or IM), occasionally second and third dose may be necessary at 1 month interval. Does not kill adults. Repeated treatments advisable in areas where high rates of transmission occur. Treatment may precipitate clinical signs due to intense response to dying parasites In clinical cases non-steroidal anti-inflammatory agents Therapeutics: anti-inflammatory drugs may be beneficial in reducing the pathology associated with the host inflammatory/immune response. Diagnosis Useful samples Isolation Phosphate-buffered saline. Fixation in formalin for preservation. Field diagnosis Clinical signs. Area with large numbers of Culicoidesspp. Seasonality follows biting fly season. Differentiation from cutaneous habronemiasis Habronemiasis , sweet itch Insect hypersensitivity , lesions due directly to biting flies or wounds. Laboratory diagnosis Incubate skin biopsies overnight in blanced salt solution of PBS, centrifuge medium, stain microfilariae in pellet with Giemsa or methylene blue. Further Reading Publications Refereed papers Recent references from PubMed and VetMedResource. Mancebo O A, Verdi J H and Bulman G M (1997) Comparative efficacy of moxidectin 2% equine oral gel and ivermectin 2% equine oral paste against Onchocerca cervicalis (Raillet and Henry, 1910) microfilariae in horses with naturally acquired infections in Formosa (Argentina). Vet Parasitol 73 243-248 PubMed. Klei T R, Torbert B, Chapman M R & Foil L (1983) Prevalence of Onchocerca cervicalis in equids in the Gulf Coast Region. Am J Vet Res 45 (8) 1646-1647 PubMed. Lloyd S and Soulsby E J L (1978) Survey for infection with Onchocerca cervicalis in horses in eastern United States. Am J Vet Res 39, 1962-1963 PubMed.