It is always preferred to prevent rather than treat infections. Avoiding exposure is the most effective way to prevent infections. Most infectious agents of cats are transmitted in fecal material, respiratory secretions, reproductive tract secretions, urine; by bites or scratches; or by contact with vectors or reservoirs. Many infectious agents are environmentally resistant and can be transmitted by contact with a contaminated environment. Recognition of risk factors associated with infectious agents is the initial step to prevention of infectious diseases. Veterinarians should strive to understand the biology of each infectious agent so that they can counsel clients and staff on the best strategies for prevention. Vaccines available for some infectious agents can prevent infection or lessen clinical illness when infection occurs. However, vaccines are not uniformly effective and are not available for all pathogens; thus, it is paramount to develop sound biosecurity procedures to avoid exposure to infectious agents when developing a preventative medicine program.

The American Association of Feline Practitioners and the Council on Biological and Therapeutic Agents have published information concerning cat vaccination guidelines in the last several years. The AAFP guidelines are endorsed by AAHA and the American College of Veterinary Internal Medicine (ACVIM).

There are many vaccine antigens available for administration to cats. For some of the antigens, there is strong consensus in the United States that all cats should be immunized ("core" vaccines). For other vaccine antigens, there are differences in regional prevalence of the disease in question or other reasons that make some antigens optional for some pets. All kittens and all adult cats with unknown vaccination history should be optimally immunized with core vaccine antigens. Optional antigens and administration intervals should be individualized to each patient upon consultation with the owner and a discussion of benefits, risks, and costs. After the kitten vaccine series, each cat should be presented to the veterinary clinic for a general health examination and a vaccine needs risk assessment yearly. The following is a brief discussion of the feline vaccine antigens currently available.

Vaccination protocols for cats

All healthy kittens and adult cats without a known vaccination history should be routinely vaccinated SQ or IM for panleukopenia, rhinotracheitis, and calicivirus (FVRCP); intranasal products can also be used. Most vaccine-associated soft tissue sarcomas have been associated with adjuvanted feline leukemia virus and rabies virus vaccines. However, tumors at injection sites have also been documented after both killed and modified live FVRCP vaccines. Recently, parenterally administered FVRCP vaccines have been shown to induce antibodies that recognize renal tissues of cats (Lappin et al, 2002). However, disease causation has not been determined to date. Thus, intranasal FVRCP vaccines may be safer than injectable FVRCP vaccines.

Modified-live products should not be administered to clinically ill, debilitated, or pregnant animals, but are preferred over killed products in healthy cats, since cell-mediated immune responses are superior. Kittens presented at 6 to12 weeks of age should receive a modified live or killed FVRCR with boosters given every 3-4 weeks until 12 weeks of age. Kittens presented at > 12 weeks of age and adult cats with unknown vaccination history should receive 2 killed or 2 modified live FVRCP 3-4 weeks apart. Parenteral products should be given SQ, low on the right forelimb so vaccine reactions can be tracked.

All cats in the United States should be vaccinated against rabies. Rabies vaccine should be administered SQ in the lower right rear limb at 12 or 16 weeks of age depending on local ordinances. Use of IM injection does not lessen the risk of vaccine sarcoma development but may make it harder to feel the mass as it develops. A new canarypox vector rabies vaccine is available that has minimal tissue irritation and so it may be less likely to be associated with soft tissue sarcomas. However, this hypothesis has not been proven.

In the United States, at one year of age or one year after the last vaccination, booster FVRCP and rabies virus vaccines should be administered. After one year of age, risk of infection by herpesvirus 1, calicivirus, and panleukopenia should be assessed yearly. In low risk cats, FVRCP vaccines can be administered every third year. The canarypox vector rabies vaccine is currently only approved for intervals of 1 year. If a rabies product with known duration of immunity of 3 years is used, it should then be administered every 3 years; more frequent vaccination is not required for immunity and only increases the risk for vaccine reactions. However, county rabies vaccination guidelines should be followed.

Serology can be used in lieu of arbitrary vaccination with FVRCP (Lappin et al, 2002). In a study of 72 vaccinated and control cats that assessed 2 different parenteral vaccines, the positive predictive value of antibody titers against panleukopenia, calicivirus, and herpesvirus 1 were all 100% in appropriately vaccinated cats. Results were similar using virus neutralization (New York State Veterinary Diagnostic Laboratory, Ithaca, NY) or ELISA (Heska Corporation, Fort Collins, CO). In that study, 70.7%, 92.4%, and 68.5% of randomly screened, client-owned cats had titers predictive of protection against herpesvirus 1, calicivirus, and panleukopenia virus, respectively. These results suggested that use of an arbitrary vaccine interval leads to unneeded vaccination of the majority of cats.

Optional vaccines currently available for use in cats include Chlamydophyla felis (previously Chlamydia), feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), feline infectious peritonitis virus, Bordetella bronchiseptica, Giardia, and ringworm.

In the United States, Chlamydophila s infection in cats generally only results in mild conjunctivitis, and so whether vaccination is ever required is controversial. The use of this vaccine should be reserved for cats with a high risk of exposure to other cats and in catteries with endemic disease. Duration of immunity for Chlamydophila vaccines may be short-lived, so high-risk cats should be immunized prior to a potential exposure.

Many cats have antibodies against Bordetella bronchiseptica and there are sporadic reports of severe lower respiratory disease due to bordetellosis in young kittens. However, since significance of the problem for pet cats is undefined in the United States, Bordetella vaccination should be considered primarily for use in cats at high risk for exposure. In an 11 year period at the Diagnostic Laboratory at Colorado State University, B. bronchiseptica was isolated from < 3% of the lower airway cultures and nasal cultures from clinically ill, client-owned cats (Stein and Lappin, unpublished data, 2004). Since the disease is apparently not life-threatening in adult cats, is uncommon in pet cats, and responds to a variety of antibiotics, routine use of this vaccine in client-owned cats seems unnecessary.

Several FeLV vaccines are currently available. Due to difficulties in assessment of efficacy studies it is unclear which vaccine is optimal. FeLV vaccines are potentially indicated in cats allowed to go outdoors or that have other exposure to cats of unknown FeLV status. The vaccines are likely to be most helpful in kittens because as cats age, there is an acquired resistance to FeLV infection that limits usefulness of vaccination. Vaccinated cats should receive 2 vaccinations initially. Adjuvanted products should be administered SQ in the distal left rear limb due to the risk for development of soft tissue sarcomas. Duration of immunity is unknown, so annual or biannual boosters are currently recommended. The vaccines are not effective in persistently viremic cats and so are not indicated. However, administration of the vaccine to viremic or latent cats does not have increased risk of vaccine reaction. FeLV testing should be performed prior to vaccination because the retrovirus serologic status of all cats should be known so appropriate husbandry can be maintained.

A killed vaccine containing immunogens from 2 FIV isolates was recently licensed for use in the United States (Fort Dodge Animal Health, Overland KS). In pre-licensing studies, 689 cats received 2,051 doses of vaccine with side-effects detected in < 1%. In a challenge study performed 375 days after inoculation with 3 doses (3 weeks apart), 84% of the vaccinates did not become FIV-infected and 90% of the controls became FIV-infected giving a preventable fraction of 82%. However, the efficacy and safety of the vaccine has not been assessed under field conditions in large numbers of cats with large multiple FIV strains. Whether the vaccine will induce vaccine sarcomas is currently unknown. One of the primary problems with FIV vaccination at this time is that the vaccine induces antibodies detectable by the currently available antibody test. Thus, after vaccination, the practitioner will be unable to determine whether the cat is infected by FIV. PCR for detection of FIV provirus is available in some laboratories, but standardization and external quality control for laboratories providing PCR testing is not currently performed.

An intranasal coronavirus vaccine that may protect some cats from developing feline infectious peritonitis virus infection is currently available in the United States. The vaccine appears to be relatively safe. In pet cats, the seroprevalence of coronavirus infection is approximately 20% to 70%, but the incidence of disease due to feline infectious peritonitis virus infection is only 1 in 5000 single cat households. Since the incidence of disease is low, cats are commonly exposed to coronaviruses prior to vaccination, the duration of immunity is short, and the efficacy is less than 100%, coronavirus vaccination is currently considered optional for pet cats. The vaccine may be indicated for seronegative cats entering a known coronavirus-infected household or cattery. The efficacy of this vaccine has not been proven in cats with positive coronavirus serology. Many cats that are to be exposed to coronaviruses have done so by 16 weeks of age and so if used, the vaccine may be more effective at 8 and 12 weeks of age.

A Giardia spp. vaccine has been introduced for use in cats in the United States. When given twice, the vaccine lessens numbers of cysts shed and lessens clinical disease on challenge with one heterologous strain. While the no significant side-effects were reported in preliminary studies, the vaccine is adjuvanted and given SQ and so may ultimately be proven to be associated with fibrosarcomas. Since the disease is usually not life-threatening and has response to therapy of at least 90%, routine use in client-owned cats seems unnecessary. Additionally, it is now known that there are multiple Giardia spp., including a feline specific strain. It is unknown whether the vaccine is protective against strains other than the one used in challenge studies. Based on a study in dogs, it has been proposed that the vaccine may have utility as an immunotherapeutic agent in cats with recurrent or persistent infection. However, in one experimental study, the vaccine was ineffective for the treatment of giardiasis.

A killed ringworm vaccine is available for use in cats in the United States. This vaccine is indicated for treatment of disease in some situations but not as a preventative. Since the product is adjuvanted, granuloma formation occurs in some cats.

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