Evaluation of a feline viral rhinotracheitis-feline calicivirus disease vaccine.

An attenuated respiratory disease vaccine against feline viral rhinotracheitis (FVR) and feline calicivirus (FCV) disease was evaluated for safety and efficacy in specific-pathogen-free cats. Twenty cats were vaccinated twice intramuscularly, with 28 days between vaccinations. Ten unvaccinated cats were used as contact controls. Adverse effects were not noticed after vaccination, and the vaccinal virus did not spread to contact controls. Arithmetical mean serum-neutralizing titers against vaccinal FCV strain F9 and challenge FCV strain 255 were 1:13 and 1:15 at 28 days after the 1st inoculation. These titers increased to 1:45 and 1:196 after the 2nd inoculation. After challenge exposure of vaccinated cats to virulent FCV 255 virus, mean titers increased to 1:129 and 1:865, respectively for F9 and 255 viruses. The F9 postchallenge mean titer for vaccinated cats was 21.5 times higher than that for the 8 contact controls that survived challenge exposure. The arithmetical mean serum neutralizing titer for FVR was low (1:2) after the 1st vaccination, but increased to 1:35 after the 2nd vaccination. Challenge exposure to virulent FVR virus resulted in a marked anamnestic immune response (mean titer of 1:207, compared with 1:12 for contact controls). In general, vaccinated cats remained alert and healthy after challenge exposure with FCV-255, whereas unvaccinated contact control cats developed definite signs of FCV disease, including central nervous system (CNS) depression (6 of 10) and dyspnea indicative of pneumonia (5 of 10). Two controls died of severe pneumonia. A mild fibrile response was detected in 28% of vaccinated cats, compared with a more severe febrile response in 78% of control cats. Some vaccinated cats developed minute lingual ulcers that did not appear to be detrimental to the health of the cat. After FVR challenge exposure, vaccinated cats were free of serious clinical signs. Five of 18 vaccinated cats had mild signs of FVR, including an occasional sneeze, low temperature, and mild serous lacrimation for 1 or 2 days. Contact controls developed definite clinical signs of FVR. The combined FVR-FCV vaccine appears to be safe and reasonably efficacious. Vaccination against FCV disease and FVR should be part of the routine feline immunization program.     

Interaction of an intranasal combined feline viral rhinotracheitis, feline calicivirus vaccine and the FVR carrier state

Eight cats were vaccinated intranasally with a combined feline calicivirus/feline viral rhinotracheitis (FVR) virus commercial vaccine. Following intranasal challenge with a field strain of FVR virus and subsequent treatment with corticosteroid, no virus was recovered from any of the eight cats, while FVR virus was recovered following corticosteroid treatment from two of four unvaccinated and challenged controls. No evidence was found for the development of an FVR virus carrier state with the intranasal vaccine virus.     

Long-term immunity in cats vaccinated with an inactivated trivalent vaccine.

OBJECTIVE: To evaluate duration of immunity in cats vaccinated with an inactivated vaccine of feline panleukopenia virus (FPV), feline herpesvirus (FHV), and feline calicivirus (FCV). ANIMALS: 17 cats. PROCEDURE: Immunity of 9 vaccinated and 8 unvaccinated cats (of an original 15 vaccinated and 17 unvaccinated cats) was challenged 7.5 years after vaccination. Specific-pathogen-free (SPF) cats were vaccinated at 8 and 12 weeks old and housed in isolation facilities. Offspring of vaccinated cats served as unvaccinated contact control cats. Virus neutralization tests were used to determine antibody titers yearly. Clinical responses were recorded, and titers were determined weekly after viral challenge. RESULTS: Control cats remained free of antibodies against FPV, FHV, and FCV and did not have infection before viral challenge. Vaccinated cats had high FPV titers throughout the study and solid protection against virulent FPV 7.5 years after vaccination. Vaccinated cats were seropositive against FHV and FCV for 3 to 4 years after vaccination, with gradually declining titers. Vaccinated cats were protected partially against viral challenge with virulent FHV. Relative efficacy of the vaccine, on the basis of reduction of clinical signs of disease, was 52%. Results were similar after FCV challenge, with relative efficacy of 63%. Vaccination did not prevent local mild infection or shedding of FHV or FCV. CONCLUSIONS: Duration of immunity after vaccination with an inactivated, adjuvanted vaccine was > 7 years. Protection against FPV was better than for FHV and FCV. CLINICAL IMPLICATIONS: Persistence of antibody titers against all 3 viruses for > 3 years supports recommendations that cats may be revaccinated against FPV-FHV-FCV at 3-year intervals.     

Efficacy of an inactivated feline calicivirus (FCV) vaccine against challenge with United Kingdom field strains and its interaction with the FCV carrier state

The efficacy of an inactivated vaccine derived from feline calicivirus (FCV) strain FS2 was assessed against challenge with three UK field strains of FCV. The mean clinical score, calculated on the number of signs recorded per day over 21 days after challenge, was lower for vaccinated cats when compared to unvaccinated animals though the difference was not statistically significant. All cats excreted FCV throughout the three weeks following challenge and there was no difference in the number of days of virus shedding during this period between vaccinated and unvaccinated animals. The development of FCV serum neutralising antibody titres following vaccination and challenge was recorded. In the second part of the study the ability of vaccinated and challenged cats to become FCV carriers and then infect susceptible in-contact animals was demonstrated.     

High-cell-passage canine coronavirus vaccine providing sterilising immunity

OBJECTIVES: To evaluate the ability of a high-cell-passage canine coronavirus vaccine to immunise dogs against challenge with a field isolate of the virus. METHODS: Three dogs that had previously tested seronegative and virus-negative for canine coronavirus were inoculated twice, at 21-day intervals, with the vaccine and kept under observation. Two seronegative and virus-negative dogs served as unvaccinated controls. For safety tests, two additional dogs were inoculated oronasally with 10 times the vaccinal dose and no reactions were observed. Faecal samples were collected daily from the vaccinated dogs after the first and second inoculations. Both vaccinated and control dogs were challenged two weeks after the second vaccination with a field canine coronavirus strain. Blood samples were collected for serological tests before vaccination and at weekly intervals after vaccinations and challenge. RESULTS: Virus was not detected in faecal samples after the first or second vaccinations by virus isolation assays and PCR. Significantly, the vaccinated dogs did not have clinical signs after challenge and no virus shedding was observed. The two unvaccinated control dogs had moderate enteritis, and virus was detected in cell cultures starting from three days postchallenge (dog 1) and two days postchallenge (dog 2), and by PCR for 23 median days. CLINICAL SIGNIFICANCE: This study showed the efficacy of a high-cell-passage canine coronavirus vaccine in preventing infection of dogs by virulent virus and, specifically, its ability to induce sterilising immunity.     

Vaccine genotype and route of administration affect pseudorabies field virus latency load after challenge

The influence of vaccine genotype and route of administration on the efficacy of pseudorabies virus (PRV) vaccines against virulent PRV challenge was evaluated in a controlled experiment using five genotypically distinct modified live vaccines (MLVs) for PRV. Several of these MLVs share deletions in specific genes, however, each has its deletion in a different locus within that gene. Pigs were vaccinated with each vaccine, either via the intramuscular or intranasal route, and subsequently challenged with a highly virulent PRV field strain. During a 2-week period following challenge with virulent PRV, each of the vaccine strains used in this study was evaluated for its effectiveness in the reduction of clinical signs, prevention of growth retardation and virulent virus shedding. One month after challenge, tissues were collected and analyzed for virulent PRV latency load by a recently developed method for the electrochemiluminescent quantitation of latent herpesvirus DNA in animal tissues after PCR amplification. It was determined that all vaccination protocols provided protection against clinical signs resulting from field virus challenge and reduced both field virus shedding and latency load after field virus challenge. Our results indicated that vaccine efficacy was significantly influenced by the modified live vaccine strain and route of administration. Compared to unvaccinated pigs, vaccination reduced field virus latency load in trigeminal ganglia, but significant differences were found between vaccines and routes of administration. We conclude that vaccine genotype plays a role in the effectiveness of PRV MLVs.     

Attempted immunisation of cats against feline infectious peritonitis using canine coronavirus

Specific pathogen free kittens were vaccinated with an unattenuated field isolate of canine coronavirus (CCV) either by aerosol or subcutaneously, and received boosting vaccinations four weeks later. Aerosolisation elicited a homologous virus-neutralising (VN) antibody response that increased steadily over a four-week period and levelled off one to two weeks after revaccination. The initial aerosolised dose produced an asymptomatic infection with excretion of CCV from the oropharynx up to eight days after vaccination; virus shedding was not detected, however, after the second inoculation. Cats vaccinated subcutaneously developed low VN antibody titres after the first CCV dose and experienced a strong anamnestic response after the second dose. Neutralising antibody titres then levelled off one to two weeks after revaccination at mean values somewhat lower than in cats vaccinated by aerosol. CCV was not isolated from the oropharynx after either subcutaneous dose. Four weeks after CCV boosting inoculations, vaccinated cats and sham-vaccinated control cats were divided into three subgroups and challenged by aerosol with the virulent UCD1 strain of feline infectious peritonitis virus (FIPV UCD1) at three different dosage levels. Five of six cats (including sham-vaccinated controls) given the lowest challenge dose showed no signs of disease, while all other cats developed lesions typical of feline infectious peritonitis (FIP). The five surviving cats developed FIP after subsequent challenge with a fivefold higher dose of FIPV. Thus heterotypic vaccination of cats with CCV did not provide effective protection against FIPV challenge.     

Use of serologic tests to predict resistance to feline herpesvirus 1, feline calicivirus,and feline parvovirus infection in cats

OBJECTIVE: To determine whether detection of virus-specific serum antibodies correlates with resistance to challenge with virulent feline herpesvirus 1 (FHV-1), feline calicivirus (FCV), and feline parvovirus (FPV) in cats and to determine percentages of client-owned cats with serum antibodies to FHV-1, FCV, and FPV. DESIGN: Prospective experimental study. ANIMALS: 72 laboratory-reared cats and 276 client-owned cats. PROCEDURES: Laboratory-reared cats were vaccinated against FHV-1, FCV, and FPV, using 1 of 3 commercial vaccines, or maintained as unvaccinated controls. Between 9 and 36 months after vaccination, cats were challenged with virulent virus. Recombinant-antigen ELISA for detection of FHV-1-, FCV-, and FPV-specific antibodies were developed, and results were compared with results of hemagglutination inhibition (FPV) and virus neutralization (FHV-1 and FCV) assays and with resistance to viral challenge. RESULTS: For vaccinated laboratory-reared cats, predictive values of positive results were 100% for the FPV and FCV ELISA and 90% for the FHV-1 ELISA. Results of the FHV-1, FCV, and FPV ELISA were positive for 195 (70.7%), 255 (92.4%), and 189 (68.5%), respectively, of the 276 client-owned cats. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that for cats that have been vaccinated, detection of FHV-1-, FCV-, and FPV-specific antibodies is predictive of whether cats are susceptible to disease, regardless of vaccine type or vaccination interval. Because most client-owned cats had detectable serum antibodies suggestive of resistance to infection, use of arbitrary booster vaccination intervals is likely to lead to unnecessary vaccination of some cats.     

Three-year duration of immunity in cats following vaccination against feline rhinotracheitis virus, feline calicivirus, and feline panleukopenia virus

Forty-two seronegative cats received an initial vaccination at 8 weeks of age and a booster vaccination at 12 weeks. All cats were kept in strict isolation for 3 years after the second vaccination and then were challenged with feline calicivirus (FCV) or sequentially challenged with feline rhinotracheitis virus (FRV) followed by feline panleukopenia virus (FPV). For each viral challenge, a separate group of 10 age-matched, nonvaccinated control cats was also challenged. Vaccinated cats showed a statistically significant reduction in virulent FRV-associated clinical signs (P = .015), 100% protection against oral ulcerations associated with FCV infection (P < .001), and 100% protection against disease associated with virulent FPV challenge (P < .005). These results demonstrated that the vaccine provided protection against virulent FRV, FCV, and FPV challenge in cats 8 weeks of age or older for a minimum of 3 years following second vaccination.     

Intranasal vaccination of pigs against Aujeszky's disease: protective immunity at 2 weeks, 2 months and 4 months after vaccination in pigs with maternal antibodies.

The purpose of the study was to evaluate the short- and long-term immunity after intranasal vaccination in pigs with maternally derived antibodies (MDA). In two experiments, 10-week-old pigs with moderate MDA titres against Aujeszky's disease virus (ADV) were vaccinated intranasally with the Bartha strain of ADV to evaluate the protective immunity conferred at 2 weeks, 2 months and 4 months after vaccination. Protection was evaluated on the basis of severity of clinical signs, periods of fever and growth arrest, and duration and amount of virus excreted after challenge with a virulent ADV. During the first 2-3 weeks after vaccination, antibodies to ADV continued to decline as in unvaccinated control pigs. After that, antibody titres stabilized or gradually increased. At 2 weeks, 2 months and 4 months after vaccination, vaccinated pigs were significantly better protected than unvaccinated controls. The vaccinated pigs challenged 2 weeks after vaccination hardly developed any sign of disease. Mild signs of Aujeszky's disease and a growth arrest period of 5 days were observed in vaccinated pigs challenged 2 months after vaccination, whereas vaccinated pigs challenged 4 months after vaccination developed severe signs of disease and a growth arrest period of 13 days. Vaccinated pigs challenged 2 weeks after vaccination did not excrete challenge virus, and pigs challenged 2 or 4 months after vaccination excreted far less virus than unvaccinated controls. The results demonstrate that intranasal ADV vaccination of pigs with moderate MDA titres protected them from 2 weeks to at least 4 months after vaccination. Immunity steadily declined, however, after vaccination.     

A comparison of the efficacy of two commercial Aujeszky's disease vaccines with glycoprotein-I deletion in pigs

Two commercial Aujeszky's disease vaccines, a modified killed vaccine and a sub-unit vaccine, both carrying a deletion of glycoprotein-I, were evaluated in pigs. Each vaccine was administered to two groups of four pigs, twice at 4-week intervals, with two pigs held as unvaccinated controls. All pigs were challenged with a New Zealand field isolate of Aujeszky's disease virus 3 weeks after the second vaccination. The results indicate that the sub-unit vaccine was able to protect pigs against clinical Aujeszky's disease much better than the pigs vaccinated with the modified killed vaccine when challenged with a virulent virus. However, the amount and the duration of virulent virus excretion following challenge was greater with the sub-unit vaccine than the modified killed vaccine. Pigs vaccinated with the sub-unit vaccine were shown to be latently infected following challenge. Latent infection was demonstrated by excretion of Aujeszky's disease virus from the nasal cavity after dexamethasone treatment and seroconversion of a sentinel in contact pigs to Aujeszky's disease virus.     

Efficacy of a modified live intranasal bovine respiratory syncytial virus vaccine in 3-week-old calves experimentally challenged with BRSV

Two experimental bovine respiratory syncytial virus (BRSV) challenge studies were undertaken to evaluate the efficacy of a single intranasal dose of a bivalent modified live vaccine containing BRSV in 3-week-old calves. In the first study, vaccine efficacy was evaluated in colostrum deprived (maternal antibody negative) calves 5, 10 and 21 days after vaccination. Nasal shedding of BRSV was significantly reduced in vaccinated calves challenged 10 or 21 days after vaccination. Virus excretion titres were also reduced in vaccinates challenged 5 days after vaccination but reduction in duration of shedding and total amount of virus shed were not statistically significant. Clinical disease after challenge in this study was mild. In the second study, vaccine efficacy was assessed in calves with maternal antibodies against BRSV by challenge 66 days post-vaccination. Vaccination significantly reduced nasal shedding after challenge and the severity of clinical disease was also reduced.     

Efficacy and safety of a feline immunodeficiency virus vaccine

Fel-O-Vax FIV is an inactivated virus vaccine designed as an aid in the prevention of infection of cats, 8 weeks or older, by feline immunodeficiency virus (FIV). It contains two genetically distinct FIV strains. The efficacy of this vaccine was demonstrated in a vaccination-challenge study designed to meet various regulatory requirements for registering the vaccine. Eight-week-old kittens were vaccinated with an immunogenicity vaccine which contained minimal release levels of FIV antigens formulated with a proprietary adjuvant system. Twelve months later, all vaccinates and controls were challenged with a heterologous FIV strain. Following the vigorous challenge exposure, cats were monitored for FIV viremia. It was found that 16% of the vaccinated cats developed viremia while 90% of the controls became persistently infected with FIV, which demonstrated that the vaccine was efficacious and the protective immunity lasted for at least 12 months. The safety of the vaccine was demonstrated by a field safety trial in which only 22 mild reactions of short duration were observed following administering 2051 doses of two pre-licensing serials of Fel-O-Vax FIV to cats of various breeds, ages and vaccination histories. Thus, Fel-O-Vax FIV is safe and efficacious for the prevention of FIV infection in cats.     

Immunogenicity and Efficacy of a Commercial Feline Leukemia Virus Vaccine

Twenty young adult specific pathogen-free cats were randomly divided into two groups of 10 animals each. One group was vaccinated with two doses of feline leukemia virus vaccine according to the manufacturer's recommendations. All 20 cats were challenge exposed oronasally (4 times over a 1-week period), beginning 3 weeks after immunization, with a virulent subgroup A strain of FeLV (CT600-FeLV). The severity of the FeLV infection was enhanced by treating the cats with methylprednisolone acetate at the time of the last FeLV exposure. Ten of 10nonvaccinated cats became persistently viremic compared with 0/10 of the vaccinates. ELISA antibodies to whole FeLV were present at high concentrations after immunization in all of the vaccinated cats, and there was no observable anamnestic antibody response after challenge exposure. ELISA antibodies to whole FeLV appeared at low concentrations in the serum of nonvaccinated cats after infection but disappeared as the viremia became permanently established. Virus neutralizing antibodies were detected in 3/10 vaccinates and 0/10 nonvaccinates immediately before FeLV challenge exposure, and in 8/10 vaccinates and 1/10 nonvaccinates 5 weeks later. Although vaccination did not consistently evoke virus neutralizing antibodies, it appeared to immunologically prime cats for a virus-neutralizing antibody response after infection. Active FeLV infection was detected in bone marrow cells taken 14 weeks after infection from 10/10 nonvaccinates and 0/10 vaccinates. Latent FeLV infection was not detected in bone marrow cells from any of the vaccinated cats 14 weeks after challenge exposure.     

Evaluation of the efficacy provided by a Recombinant Canarypox-Vectored Equine West Nile Virus vaccine against an experimental West Nile Virus intrathecal challenge in horses

Efficacy of the Recombitek Equine West Nile Virus (WNV) vaccine was evaluated against a WNV intrathecal challenge model that results in WNV-induced clinical disease. Ten vaccinated (twice at days 0 and 35) and 10 control horses were challenged 2 weeks after administration of the second vaccine with a virulent WNV by intrathecal administration. After the challenge, eight of 10 controls developed clinical signs of encephalomyelitis whereas one vaccinate exhibited muscle fasciculation only once. Nine controls and one vaccinate developed a fever. Histopathology revealed mild to moderate nonsuppurative encephalitis in eight controls and one vaccinate. None of the vaccinates and all of the controls developed WNV viremia after challenge. All vaccinated horses developed antibodies to WNV after vaccination. These and results of previous studies demonstrate efficacy of the Recombitek WNV vaccine against WNV-induced clinical disease and natural challenge with WNV-infected mosquitoes.     

Longevity of protective immunity to experimental bovine herpesvirus-1 infection following inoculation with a combination modified-live virus vaccine in beef calves

OBJECTIVE: To determine whether a combination viral vaccine containing modified-live bovine herpesvirus-1 (BHV-1) would protect calves from infection with a recent field isolate of BHV-1. DESIGN: Randomized controlled trial. ANIMALS: Sixty 4- to 6-month-old beef calves. PROCEDURE: Calves were inoculated with a placebo 42 and 20 days prior to challenge (group 1; n = 10) or with the combination vaccine 42 and 20 days prior to challenge (group 2; 10), 146 and 126 days prior to challenge (group 3; 10), 117 and 96 days prior to challenge (group 4; 10), 86 and 65 days prior to challenge (group 5; 10), or 126 days prior to challenge (group 6; 10). All calves were challenged with BHV-1 via aerosol. Clinical signs, immune responses, and nasal shedding of virus were monitored for 14 days after challenge. RESULTS: Vaccination elicited increases in BHV-1-specific IgG antibody titers. Challenge with BHV-1 resulted in mild respiratory tract disease in all groups, but vaccinated calves had less severe signs of clinical disease. Extent and duration of nasal BHV-1 shedding following challenge was significantly lower in vaccinated calves than in control calves. In calves that received 2 doses of the vaccine, the degree of protection varied with the interval between the last vaccination and challenge, as evidenced by increases in risk of clinical signs and extent and duration of viral shedding. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that this combination vaccine provided protection from infection with virulent BHV-1 and significantly reduced nasal shedding of the virus for at least 126 days after vaccination.     

Development of a classical swine fever subunit marker vaccine and companion diagnostic test

The development of a classical swine fever (CSF) subunit marker vaccine, based on viral envelope glycoprotein E2, and a companion diagnostic test, based on a second viral envelope glycoprotein E(RNS), will be described. Important properties of the vaccine, such as onset and duration of immunity, and prevention of horizontal and vertical transmission of virus were evaluated. A single dose of the vaccine protected pigs against clinical signs of CSF, following intranasal challenge with 100LD(50) of virulent classical swine fever virus (CSFV) at 2 weeks after vaccination. However, challenge virus transmission to unvaccinated sentinels was not always completely inhibited at this time point. From 3 weeks up to 6 months after vaccination, pigs were protected against clinical signs of CSF, and no longer transmitted challenge virus to unvaccinated sentinels. In contrast, unvaccinated control pigs died within 2 weeks after challenge. We also evaluated transmission of challenge virus in a setup enabling determination of the reproduction ratio (R value) of the virus. In such an experiment, transmission of challenge virus is determined in a fully vaccinated population at different time points after vaccination. Pigs challenged at 1 week after immunization died of CSF, whereas the vaccinated sentinels became infected, seroconverted for E(RNS) antibodies, but survived. At 2 weeks after vaccination, the challenged pigs seroconverted for E(RNS) antibodies, but none of the vaccinated sentinels did. Thus, at 1 week after vaccination, R1, and at 2 weeks, R=0, implying no control or control of an outbreak, respectively. Vertical transmission of CSFV to the immune-incompetent fetus may lead to the birth of highly viraemic, persistently infected piglets which are one of the major sources of virus spread. Protection against transplacental transmission of CSFV in vaccinated sows was, therefore, tested in once and twice vaccinated sows. Only one out of nine once-vaccinated sows transmitted challenge virus to the fetus, whereas none of the nine twice-vaccinated sows did. Finally, our data show that the E(RNS) test detects CSFV-specific antibodies in vaccinated or unvaccinated pigs as early as 14 days after infection with a virulent CSF strain. This indicates that the E2 vaccine and companion test fully comply with the marker vaccine concept. This concept implies the possibility of detecting infected animals within a vaccinated population.     

Experimental infections with fowl pox in chickens

Seven groups of chickens were challenged with a field isolate of fowl pox virus at 18 weeks old. The birds in the groups that had been vaccinated 3 weeks previously with fowl pox vaccinates showed no signs of disease. Birds which had not been vaccinated against fowl pox developed upper respiratory disease after challenge, and some birds had diphtheritic tracheitis and laryngitis which appeared identical to that commonly seen under field conditions. Seven days after challenge, fowl pox virus was recovered from the tracheas of unvaccinated birds, but not from the vaccinated ones.

Intercurrent Mycoplasma gallisepticum infection appeared to extend slightly the period of respiratory disease but was not essential for development of the diphtheritic lesion.     

Canine parvovirus type 2 vaccine protects against virulent challenge with type 2c virus

The ability of dogs vaccinated with a live attenuated CPV type 2 (Nobivac Intervet) vaccine to resist challenge with a current CPV2c isolate was investigated. Six SPF beagle dogs were given the minimum recommended course of vaccination, comprising a single inoculation of vaccine (Nobivac Lepto+Nobivac Pi) at 8-10 weeks of age followed 3 weeks later with a parvovirus vaccine in combination with distemper, adenovirus and parainfluenza virus (Nobivac DHPPi) and a repeat leptospirosis vaccine. Six control dogs were kept unvaccinated. All animals were challenged orally with a type 2c isolate of CPV and monitored for clinical signs, virus shedding, white blood cell fluctuations and serological responses. All vaccinated dogs were fully protected; showing no clinical signs nor shedding challenge virus in the faeces, in contrast to control animals, which displayed all the typical signs of infection with pathogenic CPV and shed challenge virus in the faeces.     

Enhancement after feline immunodeficiency virus vaccination.

Cats were vaccinated with one of the three preparations: purified feline immunodeficiency virus (FIV) incorporated into immune stimulating complexes (ISCOMs), recombinant FIV p24 ISCOMs, or a fixed, inactivated cell vaccine in quil A. Cats inoculated with the FIV ISCOMs or the recombinant p24 ISCOMs developed high titres of antibodies against the core protein p24 but had no detectable antibodies against the env protein gp120 or virus neutralising antibodies. In contrast, all of the cats inoculated with the fixed, inactivated cell vaccine developed anti-env antibodies and four of five had detectable levels of neutralising antibody. However, none of the vaccinated cats were protected from infection after intraperitoneal challenge with 20 infectious units of FIV. Indeed there appeared to be enhancement of infection after vaccination as the vaccinated cats become viraemic sooner than the unvaccinated controls, and 100% of the vaccinated cats became viraemic compared with 78% of the controls. The mechanism responsible for this enhancement remains unknown.