Bulgarian Journal of Veterinary Medicine, 2016, 19, No 4, 334–339 ISSN 1311-1477; DOI: 10.15547/bjvm.924 Case report CLINICAL AND MORPHOLOGICAL INVESTIGATIONS IN A SPONTANEOUS CRYPTOSPORIDIUM ENTERITIS OUTBREAK IN CALVES I. KALKANOV 1 , I. DINEV 1 , K. DIMITROV 1 & P. ILIEV 2 1 Department of General and Clinical Pathology, 2 Department of Veterinary Micro- biology, Infectious and Parasitic Diseases, Faculty of Veterinary Medicine, Trakia University, Stara Zagora, Bulgaria Summary Kalkanov, I., I. Dinev, K. Dimitrov & P. Iliev, 2016. Clinical and morphological investiga- tions in a spontaneous Cryptosporidium enteritis outbreak in calves. Bulg. J. Vet. Med., 19, No 4, 334–339. The purpose of this report was to present the results of histopathological examination during the course of a natural Cryptosporidium parvum outbreak, as well as the morphology of cells infiltrating affected gastrointestinal tract areas in newborn calves. The study included 18 calves exhibiting a marked diarrhoeic syndrome, between 1 and 8 days of age. Sporulated C. parvum oocysts were de- monstrated in faecal smears stained by the method of Henriksen. Coproantigens of C. parvum were detected by the rapid Rainbow calf scour 5 BIO K 306 test. Gross lesions were mainly present in the gastrointestinal tract, together with inflammation in regional mesenteric lymph nodes. Microscopic histopathological lesions consisted mainly in desquamative catarrh of intestinal mucosa and intestinal villous atrophy. The affected intestinal epithelium contained multiple Cryptosporidium spp. forms at different stage of the life cycle. The results from histopathological studies of the outbreak allowed confirming some main morphogenetic features of the disease caused by C. parvum in newborn and growing calves. Key words: calves, Cryptosporidium parvum, cryptosporidiosis, pathology Gastrointestinal diseases in newborn and juvenile calves are the commonest prob- lems in this livestock category. The losses are due to high mortality and morbidity rates, reduced weight gain after remission from the disease and disease treatment costs. Neonatal diarrhoea in calves is a multifactorial disease influenced by the breed, feeding regimen and farming con- ditions. The immunological status of the dams from the herd, farm management and the occurrence of different infectious agents are also important (Bendali et al., 1999; Scott et al., 2004). The risk for oc- currence of the disease is the highest du- ring the first month, then decreases as age