Alveolar mastocytosis and eosinophilia in lambs with naturally acquired nematode infections of Protostrongylus rufescens and Haemonchus contortus.

Specific-pathogen-free Dorset and St. Croix lambs were placed on pasture contaminated with Haemonchus contortus third stage larvae and slugs carrying third stage larvae of Protostrongylus rufescens for an entire grazing season to evaluate breed differences in acquired resistance to these nematodes. Lambs were evaluated for clinical signs, clinical pathology and histopathologic lesions associated with these infections. Both breeds acquired natural infections with H. contortus and lungworm when allowed to graze contaminated pastures for 5 months during the summer and fall in central Maryland. Dorset sheep maintained heavy abomasal worm burdens of H. contortus throughout the grazing period when compared to St. Croix breed sheep. Seven of 12 Dorset sheep and three of 12 St. Croix sheep on pasture acquired heavy lungworm infections after at least 15 weeks of exposure, as evidenced by shedding of first stage larvae in feces and numerous subpleural lung lesions containing adult P. rufescens found at necropsy. All lungworm infected animals had mild respiratory and gastrointestinal signs, and two of five Dorset sheep with both infections had chronic anemia. All lungworm and H. contortus infected Dorset sheep had decreased numbers of circulating white blood cells. There was mastocytosis in the lungs of lungworm infected Dorset and St. Croix sheep when compared to age- and breed-matched control sheep prevented from acquiring both lungworm and trichostrongyle infections. No difference was noted in the number of mast cells in the abomasum, duodenum and skin of infected and non-infected Dorset sheep. A morphologic range of mast cell forms was observed in the lungs of infected sheep including transitional cells and globular leukocytes. The number of eosinophils was significantly greater in the lungs but not in the abomasum of infected sheep. Despite the pronounced cellular infiltrates surrounding the adult lungworms, they were viable on recovery and appeared undamaged when examined histologically.