Pathological and epidemiological investigations into listerial encephalitis in sheep

Three outbreaks of encephalitis caused by Listeria monocytogenes were investigated in the migratory flocks of sheep in Punjab, India. Sixty-nine sheep were affected out of 930 animals (875 sheep and 55 goats) during these outbreaks, with cumulative morbidity, mortality and case fatality rate of 7.89, 7.08 and 89.85%, respectively. Incidence of disease was low in sheep below 6 months of age with cumulative morbidity and mortality rate of 3.75% as compared to 9.5 and 8.3%, respectively in sheep above 6 months of age. Case fatality was highest (100%) in young sheep below 6 months of age when compared to 88.3% in adult sheep above 6 months of age. Clinical signs included circling movements, unilateral facial paralysis, twisting of neck, stiffening and opisthotonus, blindness, corneal opacity and retention of food in mouth. Histopathologically, extensive perivascular cuffing, microabscesses and meningitis were observed in medulla oblongata, pons and anterior part of spinal cord. In few cases, microabscesses were also present in liver. Cerebrospinal fluid revealed higher concentration of total proteins with increase in globulins. Isolation of L. monocytogenes was made from hind brain and anterior part of spinal cord using cold enrichment method. There was no symptom of disease in the goats.