An overview of successful TGEV vaccination strategies and discussion on the interrelationship between TGEV and PRCV

Porcine respiratory coronavirus (PRCV) is a new variant of TGE with an altered pathogenesis. PRCV multiplies mainly in tonsilar tissues and the respiratory tract. There are no enteric symptoms and in experimentally infected pigs, even the respiratory tract infection is usually asymptomatic. PRCV is spread aerogenically through herds and the significance of PRCV as a pathogen in swine has yet to be determined. Despite the differences in pathogenesis and tissue tropism, the behavior of TGEV and PRCV are closely related antigenically. PRCV induces an antibody response in pigs that cannot be distinguished from TGEV-infected pigs by conventional serological assays. PRCV sensitized animals are not protected from TGEV challenge nor is the milk antibody provided to nursing piglets completely effective in prevention of TGEV infections; thus PRCV is not a good vaccine candidate for TGEV infections. PRCV subclinical infections have led to several reported cases of enzootic TGEV in herds that had been diagnosed as TGEV immune strictly on the basis of serum neutralizing titers which were later found to be due to exposure to PRCV. Vaccination studies conducted with the Ambico, oral modified live TGEV vaccine have led to some startling new results: (1) Use of Ambico TGEV modified live vaccine has been shown to provide complete protection against subsequent PRCV challenge and (2) the effectiveness of TGEV vaccination is actually enhanced by previous exposure to PRCV (3) Weanling pigs which have passively acquired circulating TGEV neutralizing antibodies are protected from subsequent PRCV infections.