Abstract
In the late 1990s and early 2000s, we received reports of significant increases in the number of cats developing panleukopenia in certain shelters around the US. These reports were of great concern because feline panleukopenia caused by feline parvovirus (FPV) had been well controlled through vaccination and few or no major problems with FPV that had been reported, even in shelters, for many years. When the shelters with outbreaks were questioned, it was frequently found that they had started using a three-way intranasal vaccine instead of the three-way parenteral vaccine containing FPV, feline calicivirus (FCV), and feline herpesvirus type-1 (FHV-1). The parenteral vaccine had been used for years. However, the reason the shelters had changed to the intranasal vaccine was to avoid the possible development of vaccine associated sarcomas (VAS) from the injectable vaccines. What many people failed to understand was the three-way parenteral product was not associated with the increase in VAS; it was adjuvanted rabies and leukemia vaccines that were primarily responsible for the increase in VASs. Also, what everyone making the change to intranasal failed to understand was the intranasal route is inferior to the parenteral route for immunization against FPV. Thus, in an attempt to prevent the development of a VAS that occurs at a prevalence of 1 per 1000 to 10,000 cats, they had created an opportunity for 1 out of every 2–4 cats to get infected with FPV, become severely ill, and die. Although the intranasal route is excellent in providing immunity to the respiratory viruses like FCV and FHV-1, it is not as effective as parenteral administration in preventing FPV. FCV and FHV-1 replicate locally in the respiratory epithelial and endothelial cells, whereas FPV must replicate systemically in lymphoid tissue. Virulent FPV, as well as modified live (ML) FPV requires actively dividing lymphocytes to first replicate before this virus can eventually move to organs like the gastrointestinal tract of the adult or the brain of a fetus. The lymphoid cells must be actively dividing for FPV to replicate, therefore, without these actively dividing lymphocytes, replication of FPV does not occur and thus there will be no immune response to FPV. Parenteral immunization with ML FPV ensures that enough virus is present to infect lymphoid cells in the draining lymph nodes and/or spleen after parenteral vaccination. The replication of virus and production of new virus are required for the development of a rapid and effective immune response. Protection from FPV is dependant entirely on the development of IgG neutralizing antibodies, with local secretory antibody and CMI playing no major role in protection.
Intranasal administration is less likely to achieve effective immunization with FPV, especially in kittens with maternally derived antibody (MDA) as the small amount of virus reaching the lymphoid cells is effectively blocked by the MDA. Also, when a vaccine is given intranasally, as anyone who has administered an intranasal vaccine knows, much of the vaccine ends up on the person vaccinating or in the cat's hair or on the table, reducing the likelihood that enough of the FPV will ever reach the lymphoid cells that need to get infected. Also even when most, if not all, of the vaccine gets into the nasal passages, much of it then ends up in the oral cavity and intestinal tract, where it is unable to infect cells, thus it does not immunize the cat, as we have reported previously. 1 Because of the problems associated with effective immunization to protect cats from FPV when the intranasal product is used, the AAFP Shelter Vaccine Guidelines recommend that when a three-way intranasal product containing FPV virus is used, MLV-FPV vaccine should also be administered to ensure the cat develops FPV immunity. FPV, without exception, is the most important feline vaccine a cat receives, as FPV infection of susceptible cats under the age of 1 year will likely lead to death in 50% or more of the diseased animals. No other disease of the cat will cause this high level of mortality that FPV causes in susceptible kittens in the shelter environment. Virulent FPV is almost always present in most shelter environments, due to the exceptional stability of the virus in that environment. In contrast to the high risk for FPV infection and disease in a shelter, the pet cat is at relatively low risk of infection because FPV is unlikely to be present in its environment. Therefore, the intranasal three-way vaccines would be much more acceptable for use in pet cats, due not only to the lower risk of infection but also because the pet cat will receive multiple doses of the vaccine, making it more likely that FPV immunization with an intranasal vaccine will occur. Also, most pet cats will be more receptive to intranasal administration of a vaccine than the shelter cat. The article by Lappin et al 2 appearing in this issue shows that under ideal conditions, intranasal administration of the three-way product to 10-month-old antibody negative cats is as likely to provide immunity for FPV as the parenteral three-way product. There is no concern about intranasal FCV and FHV-1 vaccines, therefore I, as well as many others, highly recommend the intranasal MLV vaccines for protection of pet and shelter cats against FCV and FHV-1. The results of this article should not be used by anyone to start using intranasal only vaccine in the shelter environment to prevent FPV. Instead, the AAFP Shelter Guidelines should be followed that recommend a parenteral MLV-FPV vaccine be given to all cats at entry or as soon after entering the shelter as possible. Cats receiving the parenteral FPV vaccine should develop immunity if the cats have no MDA to block immunity to FPV, within 3 days after vaccination.