Duration of Immunity in Cats Vaccinated With an Inactivated Feline Panleukopenia, Herpesvirus, and Calicivirus Vaccine

<<Feline Pract 25[4]:12-19 Jul/Aug'97 Clinical Study 19 Refs

F.W. Scott, DVM, PhD (ACVM), and C. Geissinger, BS

Cornell Feline Health Center, Department of Microbiology and Immunology, College of Veterinary Medicine Cornell University, Ithaca, New York 14853

INTRODUCTION

Except for rabies vaccines, the duration of immunity after vaccination has not been evaluated for most veterinary biologics. At this time the minimal duration of immunity must be determined for new veterinary biologics but determination of the actual or maximal duration of immunity is not required. This study reports the virus neutralizing (VN) antibody titers in SPF cats vaccinated as kittens with an inactivated triple vaccine (feline panleukopenia, feline herpesvirus, and feline calicivirus) at 8 and 12 weeks of age over a 6 year period during which they were not given any booster vaccinations.

EXPERIMENTAL DESIGN

An inactivated commercial triple vaccine (Fel-O-Vax PCT:Fort Dodge Laboratories) containing feline panleukopenia virus (FPV), feline herpesvirus (FHV), and feline calicivirus (FCV) was given subcutaneously to 15 SPF kittens at 8 and again at 12 weeks of age. No additional booster vaccines were given during the 6 year study.

These 15 SPF kittens were used to start a breeding colony. Twenty additional female kittens reared from the original queens were not vaccinated and served as the unvaccinated controls from year 1 to year 6 of the study. All of the cats in the study were maintained in isolation rooms with the unvaccinated controls in contact with the vaccinated cats from year 1 until the end of the study.

Blood samples were taken from vaccinates 2 months after the second vaccination and then from vaccinates and controls at 3, 4, 5, and 6 years postvaccination. Serum was separated from the blood samples and stored at -20C until VN assays were done. VN antibody titers against FPV, FHV, and FCV were determined each year during the study shortly after the samples were collected and were retitrated and run again after the year 5 samples were collected so all 4 serum samples from each cat could be assayed on the same day.

RESULTS & DISCUSSION

FELINE PANLEUKOPENIA VIRUS (FPV). FPV titers for all vaccinated cats remained high (1:500 or greater) throughout the 6 year study. The mean FPV antibody titer was 1:6,923 (n=13) at 2 months after vaccination, was 1:4769 (n=13) at 5 years postvaccination, and 1:5,312 (n=8) at 6 years postvaccination. The FPV antibody titers in the unvaccinated control cats remained negative (<1:10) throughout the 6 year study.

Previous studies have confirmed a strong correlation between a positive FPV antibody titer and solid immunity to FPV. A FPV antibody titer of 1:8 or higher are considered completely protective against virulent FPV exposure.

FELINE HERPESVIRUS (FHV): Vaccination resulted in low FHV antibody titers which persisted for several years. Positive FHV antibody titers were present in all 13 vaccinated cats at 2 months postvaccination with a mean titer of 1:18. The mean titers declined to 1:12 (n=15) at 3 years, 1:9 (n=14) at 4 years, 1:4 (n=13) at 5 years, and 1:2 (n=8) at 6 years. FHV antibody titers were negative (<1:2) in 1/15 cats at 3 years, 4/14 at 4 years, 6/13 at 5 years, and 4/8 at 6 years postvaccination. Low titers disappeared and reappeard over time in several vaccinated cats.

The correlation between FHV antibody titers and immunity is not clear. Vaccination, even with MLV FHV vaccines, produces only low FHV antibody titers with some cats failing to show any detectable antibody response to vaccination. A booster vaccine given 3 weeks after the initial vaccination generally produces a mild anamnestic response. Exposure of vaccinated cats to FHV results in local infection with viral replication and shedding with about 10% of the cats showing any clinical signs of infection - usually mild signs of URI for 1-3 days. Previous FHV vaccine studies found that any detectable antibody titer ie., 1:2 or greater, provided "substantial" protection.

FELINE CALICIVIRUS (FCV). All vaccinated cats had positive FCV antibody titers at 2 months, 3 years, and 4 years postvaccination although one cat had a low titer of 1:3 at 4 years postvaccination. At 5 years 1 cat had a negative titer and 2 cats had titers of 1:3. At 6 years 1 cat (different from the negative cat at 5 years) was negative and 3 had titers of 1:2 in the 8 cats tested. The mean titers decreased from 1:145 at 2 months to 1:81 at 3 years, and 1:77 at 4 years, 1:45 at 5 years, and 1:8 at 6 years. Titers of 1:2 or 1:3 may or may not be specific for FCV.

"The correlation between antibody titers and immunity to FCV falls between that for FPV and FHV. FCV antibody titers produced by vaccines are higher than for FHV, but still much less than the FPV titers. The results of this study clearly show that good FCV antibody titers persist for at least 4 years after vaccination. From previous studies, we and others have observed that positive FCV antibody titers of 1:4 or higher are consistent with protection against virus exposure. As with FHV, immunity is not 100%, with infection and viral shed occurring in vaccinated cats exposed to virulent virus."

UNVACCINATED CONTROL CATS. Except for 1 cat with a 1:2 FCV titer at year 3, all 17 control cats had negative FPV, FHV, and FCV antibody titers on samples from years 3 and 4 of the study. At year 5 the 16 remaining controls had negative FPV and FHV antibody titers but 2 cats had low (1:3 and 1:2) FCV antibody titers which were negative at year 6. At year 6 the 9 remaining controls had negative FPV and FHV titers but 1 cat had a FCV titer of 1:2.

CONCLUSION

The authors state that "The present study clearly shows that neutralizing antibodies against FPV, FHV, and FCV persist for at least 3 years in all vaccinated cats. FPV antibodies probably persist for the life of the cat. Antibody titers against FHV and FCV gradually decline over the years, and the exact length of protection is not known. However, based on the results of this study, the results of rabies studies, and information from human medicine, recommendations for routine vaccination of cats in moderate to low-risk populations probably should be modified. In our opinion, cats that are properly vaccinated as kittens should be revaccinated with a single dose of triple vacine at 1 year of age, then revaccinated every 3 years with either a FHV/FCV vaccine or triple vaccine. Manufacturers of vaccines need to test their vaccine for longer durations of immunity, and thus adjust their label recommendations to a more realistic DOI."

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vinid = JA010743, collectionid = J0139704, date0997

Journal info: ISSN 1057-6614; ID=J013 FP

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