Comparison Of Pathogenicity And Immunogenicity Of Five Feline Calicivirus Isolates

Feline calicivirus(FCV), is a member of the genus Vesivirus belonging to Caliciviridae. The virus can cause upper respiratory tract disease(URTD)of cats with clinical symptoms like rhinitis, conjunctivitis, acute oral ulcerations, chronic gastritis, lameness and pneumonia. FCV with the only serotype is characterized by the high variation of virulence, genetic and antigenic diversity, which poses challenge to the prevention of FCV infection. Since isolated and identified by Fastier(1957), FCV was also found in Europe, America and Asia, with the tendency of worldwide spread. Cat is as the natural host is susceptible to FCV. A dog was also reported to be infected by FCV. The high virulent strain that currently appeared can cause virulent systemic disease(VSD), posing a serious threat to the health of cat colony.FCV infection can be inhibited by immune response of which clinical symptoms may be reduced or eliminated by artificial immunization and maternal antibody. Therefore, vaccination is still the major measure taken in FCV prevention. Vaccines against FCV have been used for many years mainly containing inactivated and modified live vaccines which are commonly presented in polyvalent and combined vaccines. The use of conventional vaccines has effectively reduced the morbidity and delivery of the virus. Due to the diversity of antigenic and genetic variation of FCV, current commercial vaccines can not provide enough protection. There is much progress in the study of research on the etiology, morphology and molecular biology of FCV but further research on its pathogenicity and immunogenicity is needed, which may contribute to virus selection and development of new type vaccine and laying a foundation for the FCV prevention.Approximately 6-11 week-old healthy unvaccinated kittens were randomly allocated to test group and control group. Kittens in test group were artificially challenged with five isolates from different regions, FCV CH-SH, CH-JL-1, CH-JL-2, CH-JL-3, CH-JL-4 respectively, while group C received the equal amount of F81 cell culture. The body weight, body temperature, morbidity and clinical signs of each group were monitored in the prechallenge and postchallenge phase and scored in accordance with the European Pharmacopoeia. All cats were sacrificed by anesthesia to observe pathological changes of tissues, organs and histopathology 14 days after challenge. The results showed that, in the postchallenge phase, the total morbidity rate was 100%; the mean clinical scores of group SH, JL-1, JL-2; JL-2, JL-3, JL-4 and the control were 8, 4, 9; 10, 5, 8 and 1 respectively. The pathological change indicated that cats were characterized with oral ulcers and an increase in nasal and ocular discharge, after the infection of FCV. The lesions in digestive tract were mild, while there were different degeneration, hemorrhage and obviously pathological “meat-like” changes, with the varying amounts of desquamated alveolar epithelial and macrophages, and small amounts of fibrin exudation and diffuse alveolar damage in lung. The results revealed that all isolates had certain degrees of pathogenicity, among which, JL-2 was the most pathogenic one and JL-4 came second.All isolates inactivated by formalin were mixed with freund’s adjuvant to prepare oil emulsion vaccines. Healthy unvaccinated kittens were randomly allocated to vaccinated(V)and control(C) groups. Kittens in group V were vaccinated with 1.5mL five isolates respectively via the subcutaneous route, while group C received the equal amount of F81 cell culture. The sera specific of each isolate were collected at different time after vaccination. The content of IgG, IL-2, IL-4 and IFN-γ measured by ELISA to evaluate the humoral immunity level, and peripheral lymphocyte proliferation by MTT to evaluate the cell immunity level induced by five isolates. The results showed that all isolates could induce immune response of vaccinated kittens with the order of humoral immunity level: CH-JL-2>CH-JL-1>CH-JL-4>CH-JL-3>CH-SH and that of cell immunity level CH-JL-4>CH-JL-2>CH-JL-1>CH-SH>CH-JL-3. These data demonstrated preliminarily that the immunogenicity order of five isolates was: CH-JL-2 >CH-JL-4>CH-JL-1>CH-JL-3>CH-SH.