Inmunidad pasiva contra infecciones virales entéricas: rotavirus bovino / Passive inmunity against enteric infections: bovine rotavirus

RESUMO

Enteric viral infections in calves are widespread and most occur as enzootics in calves under 3 weeks of agne. Both clinical and subclinical infections occur, with the severity of the disease influenced by such factors as the level of passive immunity and the challenge dose and strain of virus. Rotaviruses and coronaviruses are primary enteric viral pathogens commonly associated with the neonatal calf diarrhea syndrome. Rotaviruses and coronaviruses replicate in intestinal villous enterocytes, resulting in villous atrophy and consequently a malabsorptive diarrhea. Two vaccination approaches have been developd to try to prevent rotavirus and coronavirus infections in calves. The first involves administrtion of modified live rotavirus and coronavirus to newborn calves to stimulate active immunity. This procedure was not efficacious under field conditions, presumably due to the widespread presence of maternal antibodies which neutralize the vaccine viruses. The second approach relies on rotavirus and coronavirus vaccination of cows to enhance antibody titers in mammary secretion and thus provide passive immunity. The variables involved in this latter approach using bovine rotavirus as a model are defined in this report. An analyses of passive and active immunity in calves supllemented with colostrum from rotavirus vaccinated cows and challenged by rotavirus is also desribed. Results indicate that a commercial vaccine failed to increase rotavirus antibody titers in mammary secretions, but experimental vaccines developed in this laboratory significantly increased such titers. Furthermore, passive protection against rotavirus was mediated by optimal levels of colostral rotavirus antibodies, primarily of the IgGl class, present at frequent intervals within the calfs intestine (lactogenic immunity). Serum antibodies alone in the newborn calf were less protective, but may moderate the severity of rotavirus infecctions