- For both viruses this has generally been measured by serum virus neutralising (VN) antibody titres, though for FHV particularly, other immune mechanisms, especially cellular immunity, may be a truer reflection of immune status. Even for FCV, where VN antibody was traditionally considered to be the hallmark of immunity, some cats with no detectable VN antibody may show immunity to rechallenge with a heterologous FCV strain. The ultimate test of immunity is, of course, response to challenge.
- In FHV, just after initial infection, cats are generally resistant to challenge, although VN antibody titres are generally low and in some cases undetectable.
- After six months or more, however, protection may only be partial and indeed cats may reinfect themselves at any time. Following exogenous or genous reinfection, VN antibody titres rise to more moderate levels and thereafter, independent of virus shedding episodes, remain relatively stable.
- In FCV, VN antibody titres are higher than those in FHV, and immunity following natural infection is thought to be longer lived. However, there is some variation depending on the virus strain involved and whether or not homotypic or heterotypic responses are being considered.
- The duration of immunity following vaccination is not known. Most challenge studies have been done within 3 months of vaccination, but equivalent protection has been reported for FHV after a year and for FCV 10-12 months. Most manufacturers recommend yearly revaccination or 6-monthly in some circumstances
- Maternally derived antibody (essentially colostral) in kittens may persist for 2-10 weeks for FHV (mean levels falling below detectable (less than 1 in 2) levels by 9 weeks of age). For FCV, titres may be more persistent, antibody declining to undetectable levels by 10-14 weeks of age. However, for FCV, low levels of maternally derived antibody do not necessarily protect against infection or disease. In contrast, some kittens with no detectable FHV antibody appeared to be protected against disease, though not infection.
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