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 pastu re contaminated with Haemonchus contortus third stage larvae and slugs carrying third stage larvae of Protostrongylus rufescens for an entir e 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 lungwo rm when allowed to graze contaminated pastures for 5 months during the summer and fall in central Maryland. Dorset sheep maintained heavy ab omasal 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 a fter 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 ha d mild respiratory and gastrointestinal signs, and two of five Dorset sheep with both infections had chronic anemia. All lungworm and H. con tortus infected Dorset sheep had decreased numbers of circulating whit e blood eels. There was mastocytosis in the lungs of lungworm infected Dorset and St. Croix sheep when compared to age- and breed-matched co ntrol sheep prevented from acquiring both lungworm and trichostrongyle infections. No difference was noted in the number of mast cells in th e abomasum, duodenum and skin of infected and non-infected Dorset shee p. 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 n ot in the abomasum of infected sheep. Despite the pronounced cellular infiltrates surrounding the adult lungworms, they were viable on recov ery and appeared undamaged when examined histologically.