Eosinophilic Mediastinitis, Myositis, Pleuritis, and Pneumonia of Cattle Associated with Migration of First-Instar Larvae of Hypoderma Zineatum

Abstract

Abstract. Migrating first-instar larvae of Hypoderma Zineatum are a frequent cause of focal inflammatory lesions in connective tissues of the mediastinum, parietal and visceral pleura, peritoneum, lungs, diaphragm, and other loci. The lesions are characterized grossly by foci of yellowish or greenish gelatinous edema and microscopically by infiltration of the edematous tissue by a dense array of eosinophils. Lesions were recognized during a period of several weeks in late spring; the timing was attributable to events in the life cycle of the fly. The larvae, which were small (≈ 1 x 4.5 mm), transparent, and unobtrusive, were recovered from lesions in 12 of 20 cattle in which careful parasitologic examination was made.The life history, pathologic effect and myiasis control programs for Hypoderma lineatum and Hypoderma bovis (Diptera: Oestridae) have recently been reviewed 3,4,13 General aspects of the life cycle of H. lineatui are well known. In early spring, adults emerge from pupae that developed from third-instar larvae. These larvae entered the soil during late winter, following emergence from subcutaneous warble cavities. The adult life span is 3-8 days, during which fertilization and egg production occurs. Eggs are deposited on hairs of legs and ventrum of cattle and hatch in a few days, and first-instar larvae penetrate the skin and enter the subcutis within hours. Exact routes of the extensive migration that occurs in the host are not clear, but the esophageal submucosa is generally involved. First-instar larvae remain in the esophageal submucosa for 5-6 months and ultimately reach the subcutis of the back, where they induce a warble cavity, molt twice, and remain for 5-8 weeks. Larvae emerge from the warble, fall to the ground, pupate, and initiate another cycle.Migration is dependent upon elaboration of a collagenase by larvae, and the migratory route is largely or entirely through the collagenous stroma of various organs.19 Although veterinarians are quite familiar with the esophageal and cutaneous lesions of H. lineatum infections, the pathologic changes associated with other segments of their larval migration are less well known. In addition to their usual presence in the esophageal submucosa, first-instar larvae have been observed in various loci, including the subcutis of the limbs and trunk; intermuscular fascia; fascia of epineurium of nerve trunks; peritracheal, periesophageal, and parietal pleural and peritoneal connective tissues; pericardium; visceral peritoneum of spleen, liver, rumen, and intestine; diaphragm; intercostal muscles; and the perirenal connective tissue. 9,19 The histopathology of esophageal lesions induced by first-instar larval migration has been described. 18,19 Lesions in periesophageal, peritracheal, and pleural connective tissues have also been reported. 19Economic loss resulting from Hypoderma lineatum infection includes damage to the hide and carcass ("licked" beef, butcher's jelly), rare anaphylactic reactions, the cost of treatment, treatment accidents (esophagitis an...