Noncore vaccines: which ones and how often? Part 2 – cats (Proceedings)

Classification of feline vaccines

As with canine vaccines, feline products are now categorized as core, noncore, and not generally recommended. As the names imply, vaccines that all cats should receive are core, while noncore are optional based on lifestyle, local disease prevalence, and risk/benefit ratios. These categories are from the 2006 AAFP guidelines (see Proceedings on core vaccines).

Core vaccines include panleukopenia, herpes, calici, and rabies. Vaccines classified as noncore and not generally recommended are discussed below.

Noncore vaccines

Leukemia virus (FeLV)

There is ongoing debate as to which cats should be vaccinated for FeLV, and if so when and how often. It can be difficult to assess an individual cat's risk of being exposed. Certainly cats that spend time outdoors are more likely to be infected than indoor cats, but lifestyles and confinement can change through a cat's lifetime. Owners may believe they will keep new them indoors for life, but cats may have other ideas.

Kittens are much more susceptible to FeLV infection than adults. Cats older than 1 year of age can be exposed but are less likely to become persistently infected. The minimum age that kittens can be vaccinated (according to label recommendations) is 8, 9, or 10 weeks, depending on the brand, so FeLV vaccine is typically not given to kittens presented at 6-8 weeks of age.

After two vaccinations 3-4 weeks apart (for either kittens or adults), a booster is given 1 year later. Afterward, annual revaccination is recommended as extended DOI studies have not been performed. A 1-year challenge study is available for one vaccine (Leukocell 2, Pfizer). Kittens were vaccinated at approximately 9 and 12 weeks of age and challenged oronasally one year later. 14/18 vaccinates (78%) were protected. However, only 9/15 (60%) of controls (nonvaccinates) became infected, so the preventable fraction was 63%. The likely explanation for the 6/15 young adult cats that were not infected by the challenge is age-related resistance. The 4/18 cats not protected despite vaccination indicates that FeLV does not always induce immunity. A relative lack of efficacy was observed in an outdoor colony where 3 of 5 FIV+ cats became infected with FeLV even after annual vaccination (Leukocell 2). All 19 FIV- cats were protected from FeLV up to 5 years with annual vaccination.

Efficacy studies of FeLV vaccines are available but difficult to compare, as different models of vaccination, challenges, and outcomes were used. An older study of one product showed that immunity lasted for 3 years, and based on knowledge of immune system memory, it is reasonable to expect that annual revaccination is not absolutely necessary. Another study found that cats vaccinated and then challenged with FeLV did not have viremia (ELISA negative) but still tested positive for proviral RNA by quantitative and real-time RT-PCR testing. Therefore, FeLV vaccination is unlikely to provide "sterilizing" immunity but does seem to protect against viremia, shedding, and clinical disease.

The safety of FeLV vaccines has been a concern ever since the recognition of injection-site (or vaccine-associated) sarcomas. Some studies suggest that both rabies vaccine (RV) and FeLV are associated with an increased risk of post-vaccination chronic inflammation, most likely due to adjuvants. Some practitioners now limit FeLV vaccines to cats at risk of exposure, while others recommend routine vaccination to most or all cats. The current AAFP recommendation is to vaccinate all kittens due to their higher risk of infection and unknown indoor/outdoor status later in life. For adult cats, more selective use is appropriate, considering that inflammation and neoplasia (while rare) can be a significant problem.

Another strategy to minimize the risk of FeLV vaccine-associated sarcomas is to use a nonadjuvanted product. Currently, only one type is available in the U.S. that is produced with recombinant technology and delivered transdermally with a needle-free injection device. Studies and clinical experience to date suggest that post-vaccination inflammation does not occur, which indicates that neoplastic transformation is unlikely. This vaccine stimulates both humoral and cell-mediated immunity because of delivery directly to dendritic (antigen-presenting cells) in the skin. This is in contrast with other vaccines that are injected into the subcutaneous space and are not in direct contact with the immune system.

Immunodeficiency virus (FIV)

Vaccines to protect against FIV have been difficult to produce due to variation in strains (also called clades) and possible lack of cross-protection. Currently, one type is available (Fel-O-Vax FIV, Fort Dodge) that contains clades A and D. In most areas of the U.S., clade B is the most common field strain. The prelicensing study by the manufacturer indicated a preventable fraction of 82% after a one-year challenge. Subsequent studies using clades A or B as challenge have yielded conflicting results. Space limitations prevent a full discussion of the published research to date.

The vaccine is given at 8 weeks or older with 2 additional doses needed at 2-3 week intervals. The product is a killed, adjuvanted vaccine and recommended for annual revaccination. Reports of FIV vaccine-associated inflammation or neoplasia have not been published, but there is a theoretical risk. The vaccine consistently induces anti-FIV antibody formation, which leads to positive results in all diagnostic tests (ELISA, Western blot, IFA, immunochromatography). PCR assays have been developed to detect FIV but to date have not been successful or accurate in differentiating vaccinated from infected cats. A discriminant ELISA test has been developed to identify vaccinates from exposed cats, but is not commercially available.

The most susceptible population is outdoor fighting cats. If FIV vaccines are used, cats should first be tested FIV- and then ideally identified as an FIV vaccinate to prevent diagnostic confusion in the future. Microchips, tattoos, or other means of permanent identification have been suggested.

Chlamydophilosis

The syndrome caused by Chlamydophila felis (formerly Chlamydia psittaci var felis) is typically conjunctivitis with or without mild sneezing. As the infection is mild, easily diagnosed, and responds well to treatment, vaccination is considered noncore. Both adjuvanted and nonadjuvanted killed and MLV vaccines are available in combination with other antigens. As with FHV-1 and FCV vaccines, there is no protection against infection. Instead, the severity of disease is lessened in vaccinates but the organism may continue to be shed.

The schedule is the same as the other respiratory vaccines. No duration of immunity studies have been published, so annual revaccination is recommended. Adverse reactions to this vaccine are rare but transient fever, anorexia, lethargy, and lameness have been reported 1-3 weeks post-vaccination.

Bordetellosis

Until recently Bordetella bronchiseptica (Bb was not considered an important pathogen in cats. Research and field cases have now demonstrated upper respiratory disease in cats infected with Bb. Currently, only one vaccine is available, a topical (IN) MLV product (Protex-Bb, Intervet/ Schering-Plough). The manufacturer recommends one dose for kittens 4 weeks of age or older with no revaccination interval specified. A challenge study demonstrated at least one year duration of immunity. It is difficult to recommend this vaccine for routine use, as respiratory disease caused by Bb is uncommon and is treatable. In shelter situations, anecdotal reports are mixed.

Not generally recommended vaccines

Feline infectious peritonitis (FIP)

One vaccine has been marketed to prevent FIP, a disease caused by a mutation of feline enteric coronavirus (FCoV). While exposure to coronavirus is common (approximately 40-50% of household cats and up to 90% in catteries), only a small percentage of cats develop FIP. A genetic predisposition to disease is possible, and kittens are at higher risk than adult cats.

The manufacturer's recommendation (Primucell FIP, Pfizer) is to vaccinate kittens 16 weeks of age or older followed by a booster 3-4 weeks later and annual revaccination. Early studies by the manufacturer indicated good protection against FIP. Subsequent research has indicated no benefit to vaccination in cats already exposed to FCoV (Ab positive). One study of natural exposure found that vaccinated cats were more likely to die of FIP than nonvaccinates (preventable fraction of -20%). If FIP vaccination is considered, kittens or cats should first be tested for anti-FCoV antibodies. If positive, the vaccine will not be protective.

Giardia lamblia

This killed, adjuvanted vaccine may reduce shedding of cysts in infected cats but does not prevent infection. The manufacturer recommends vaccinating kittens 8 weeks or older with a booster 2-4 weeks later, then annual revaccination. A prelicensing challenge study one year after vaccination demonstrated a reduction in diarrhea, cyst shedding, and infection. Another study of young cats followed for 28 weeks showed no difference in cyst shedding between vaccinates and controls, and no reduction in clinical signs (although diarrhea was very mild in both groups). Variations in field strains and the fact that disease is mild or asymptomatic in some cats makes efficacy and DOI studies difficult.