Effects of anesthesia and surgery on serologic responses to vaccination in kittens

OBJECTIVE: To determine the effects of anesthesia and surgery on serologic responses to vaccination in kittens. DESIGN: Prospective controlled trial. ANIMALS: 32 specific-pathogen-free kittens. PROCEDURES: Kittens were assigned to 1 of 4 treatment groups: neutering at 7, 8, or 9 weeks of age or no neutering. All kittens were inoculated with modified-live virus vaccines against feline panleukopenia virus (FPV), feline herpesvirus (FHV), and feline calicivirus (FCV) at 8, 11, and 14 weeks of age and inactivated rabies virus (RV) at 14 weeks of age. Serum antibody titers against FPV, FHV, and FCV were determined at 8, 9, 11, 14, and 17 weeks of age; RV titers were determined at 14 and 17 weeks of age. RESULTS: Serologic responses of kittens neutered at the time of first vaccination (8 weeks) were not different from those of kittens neutered 1 week before (7 weeks) or 1 week after (9 weeks) first vaccination or from those of kittens that were not neutered. In total, 31%, 0%, 69%, and 9% of kittens failed to develop adequate titers against FPV, FCV, FHV, and RV, respectively, by 17 weeks of age. CONCLUSIONS AND CLINICAL RELEVANCE: Neutering at or near the time of first vaccination with a modified-live virus vaccine did not impair antibody responses in kittens. Many kittens that were last vaccinated at 14 weeks of age had inadequate antibody titers at 17 weeks of age. Kittens may be vaccinated in the perioperative period when necessary, and the primary vaccination series should be extended through at least 16 weeks of age.     

Response of feral cats to vaccination at the time of neutering

OBJECTIVE: To determine whether administration of inactivated virus or modified-live virus (MLV) vaccines to feral cats at the time of neutering induces protective serum antiviral antibody titers. DESIGN: Prospective study. ANIMALS: 61 feral cats included in a trap-neuter-return program in Florida. PROCEDURES: Each cat received vaccines against feline panleukopenia virus (FPV), feline herpes virus (FHV), feline calicivirus (FCV), FeLV, and rabies virus (RV). Immediately on completion of surgery, vaccines that contained inactivated RV and FeLV antigens and either MLV or inactivated FPV, FHV, and FCV antigens were administered. Titers of antiviral antibodies (except those against FeLV) were assessed in serum samples obtained immediately prior to surgery and approximately 10 weeks later. RESULTS: Prior to vaccination, some of the cats had protective serum antibody titers against FPV (33%), FHV (21%), FCV (64%), and RV (3%). Following vaccination, the overall proportion of cats with protective serum antiviral antibody titers increased (FPV [90%], FHV [56%], FCV [93%], and RV [98%]). With the exception of the FHV vaccine, there were no differences in the proportions of cats protected with inactivated virus versus MLV vaccines. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that exposure to FPV, FHV, and FCV is common among feral cats and that a high proportion of cats are susceptible to RV infection. Feral cats appeared to have an excellent immune response following vaccination at the time of neutering. Incorporation of vaccination into trap-neuter-return programs is likely to protect the health of individual cats and possibly reduce the disease burden in the community.     

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.     

Onset of immunity in kittens after vaccination with a non-adjuvanted vaccine against feline panleucopenia, feline calicivirus and feline herpesvirus

The induction of a quick onset of immunity against feline parvovirus (FPV), feline herpesvirus (FHV) and feline calicivirus (FCV) is critical both in young kittens after the decline of maternal antibodies and in cats at high risk of exposure. The onset of immunity for the core components was evaluated in 8–9 week old specific pathogen free kittens by challenge 1 week after vaccination with a combined modified live (FPV, FHV) and inactivated (FCV) vaccine. The protection obtained 1 week after vaccination was compared to that obtained when the challenge was performed 3–4 weeks after vaccination. The protocol consisted of a single injection for vaccination against FPV and two injections 4 weeks apart for FHV and FCV.At 1 week after vaccination, the kittens showed no FPV-induced clinical signs or leukopenia following challenge, and after FCV and FHV challenges the clinical score was significantly lower in vaccinated animals than in controls. Interestingly, the relative efficacy of the vaccination was comparable whether the animals were challenged 1 week or 3–4 weeks after vaccination, indicating that the onset of protection occurred within 7 days of vaccination. Following the 1-week challenge, excretion of FPV, FHV and FCV was significantly reduced in vaccinated cats compared to control kittens, confirming the onset of immunity within 7 days of vaccination.     

Detection of protective antibody titers against feline panleukopenia virus, feline herpesvirus-1, and feline calicivirus in shelter cats using a point-of-care ELISA

Serum antibody titers are a useful measurement of protection against infection (feline panleukopenia virus [FPV]) or clinical disease (feline herpesvirus-1 [FHV] and feline calicivirus [FCV]), and their determination has been recommended as part of disease outbreak management in animal shelters. The objective of this study was to determine the sensitivity, specificity, and inter-observer and inter-assay agreement of two semi-quantitative point-of-care assays for the detection of protective antibody titers (PAT) against FPV, FHV and FCV in shelter cats. Low sensitivity for FPV antibodies (28%) rendered a canine point-of-care assay inappropriate for use in cats. The feline point-of-care assay also had low sensitivity (49%) and low negative predictive value (74%) for FPV PAT detection, but was highly accurate in the assessment of FHV and FCV PAT. Improvements in accuracy and repeatability of FPV PAT determination could make this tool a valuable component of a disease outbreak response in animal shelters.     

Duration of serologic response to three viral antigens in cats

OBJECTIVE: To determine whether vaccinated cats either remained seropositive or responded serologically to revaccination against 3 key viral antigens after extended periods since their last vaccination. DESIGN: Serologic survey. ANIMALS: 272 healthy client-owned cats. PROCEDURE: Cats were > or = 2 years old and vaccinated for feline panleukopenia virus (FPV), feline calicivirus (FCV), and feline herpesvirus (FHV). On day 0, cats were revaccinated with a vaccine from the same line of vaccines as they had historically received. Antibody titers were measured in sera collected on day 0 (prevaccination titer) and 5 to 7 days later (postvaccination titer). Cats were considered to have responded serologically if they had a day-0 hemagglutination inhibition titer to FPV > or = 1:40, serum neutralization (SN) titer to FCV > or = 1:32, SN titer to FHV > or = 1:16, or > or = 4-fold increase in antibody titer after revaccination. RESULTS: The percentage of cats that had titers at or above the threshold values or responded to revaccination with a > or = 4-fold increase in titer was 96.7% for FPV, 97.8% for FCV, and 88.2% for FHV. CONCLUSIONS AND CLINICAL RELEVANCE: In most cats, vaccination induced a response that lasted up to and beyond 48 months for all 3 antigens. Although not equivalent to challenge-of-immunity studies as a demonstration of efficacy, results suggest that revaccination with the vaccine used in our study provides adequate protection even when given less frequently than the traditional 1-year interval. The study provides valuable information for clinicians to determine appropriate revaccination intervals.     

Effect of vaccination on parvovirus antigen testing in kittens

OBJECTIVE: To determine the frequency and duration of feline panleukopenia virus (FPV) vaccine-induced interference with fecal parvovirus diagnostic testing in cats. DESIGN: Prospective controlled study. ANIMALS: Sixty-four 8- to 10-week-old specific-pathogen-free kittens. PROCEDURES: Kittens were inoculated once with 1 of 8 commercial multivalent vaccines containing modified-live virus (MLV) or inactivated FPV by the SC or intranasal routes. Feces were tested for parvovirus antigen immediately prior to vaccination, then daily for 14 days with 3 tests designed for detection of canine parvovirus. Serum anti-FPV antibody titers were determined by use of hemagglutination inhibition prior to vaccination and 14 days later. RESULTS: All fecal parvovirus test results were negative prior to vaccination. After vaccination, 1 kitten had positive test results with test 1, 4 kittens had positive results with test 2, and 13 kittens had positive results with test 3. Only 1 kitten had positive results with all 3 tests, and only 2 of those tests were subjectively considered to have strongly positive results. At 14 days after vaccination, 31% of kittens receiving inactivated vaccines had protective FPV titers, whereas 85% of kittens receiving MLV vaccines had protective titers. CONCLUSIONS AND CLINICAL RELEVANCE: Animal shelter veterinarians should select fecal tests for parvovirus detection that have high sensitivity for FPV and low frequency of vaccine-related test interference. Positive parvovirus test results should be interpreted in light of clinical signs, vaccination history, and results of confirmatory testing. Despite the possibility of test interference, the benefit provided by universal MLV FPV vaccination of cats in high-risk environments such as shelters outweighs the impact on diagnostic test accuracy.     

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.     

Feline panleukopenia virus, feline herpesvirus-1, and feline calicivirus antibody responses in seronegative specific pathogen-free cats after a single administration of two different modified live FVRCP vaccines

Two groups of feline panleukopenia virus (FPV), feline calicivirus (FCV), and feline herpesvirus-1 (FHV-1) seronegative cats (five cats per group) were administered one of two modified live feline viral rhinotracheitis, calicivirus, and panleukopenia virus (FVRCP) vaccines and the serological responses to each agent were followed over 28 days. While all cats developed detectable FPV and FCV antibody titers; only two cats developed detectable FHV-1 antibody titers using the criteria described by the testing laboratory. For FPV and FHV-1, there were no differences in seroconversion rates between the cats that were administered the intranasal (IN) FVRCP vaccine and the cats that were administered the parenteral FVRCP vaccine on any day post-inoculation. For FCV, the cats that were administered the IN FVRCP vaccine were more likely to seroconvert on days 10 and 14 when compared to cats that were administered the parenteral FVRCP vaccine.     

Serologic evaluation of vaccinated American river otters

The Oklahoma Department of Wildlife Conservation acquired 20 American river otters (Lutra canadensis) between 1984 and 1985 for reintroduction into Oklahoma waterways. In 1985, 10 otters were evaluated for serum antibody titers after vaccination with canine distemper virus, canine adenovirus type 2, canine parvovirus (CPV), feline panleukopenia virus (FPV), feline rhinotracheitis virus (FRV), and feline calicivirus. Prevaccination serum-virus neutralization (SVN) antibody to feline rhinotracheitis virus was found in 2 otters and to feline calicivirus in 1 otter. Using an indirect fluorescent antibody (IFA) assay, prevaccination antibody to CPV and FPV was found in 2 otters. A significant increase in SVN antibody titers was found after vaccination of otters with canine adenovirus type 2 (6 of 8 animals) and feline calicivirus (1 of 8 animals). One of 8 otters developed significant antibody titers to CPV and FPV, as measured by IFA assay. Otters did not develop SVN antibody titers to canine distemper virus after vaccination. Antigens of feline leukemia virus, using ELISA, or antibodies to feline infectious peritonitis, using IFA assay, were not found in the 20 otters.     

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.     

Comparison of prevalence of feline herpesvirus type 1, calicivirus and parvovirus infections in domestic and leopard cats in Vietnam

A serosurvey of feline herpesvirus type 1 (FHV-1), feline calicivirus (FCV), and feline parvovirus (FPV) in cats from Ho Chi Minh City area in southern Vietnam was conducted in December 1998, and we compared the results with our previous results in northern Vietnam (Hanoi area). The positive rate of FHV and FCV in domestic cats were 44% and 74%, respectively. They were rather higher than those in Hanoi area, while the seropositivity of FPV (44%) was similar to that in Hanoi area. In leopard cats, the positive rate of FPV was high (3/4) and it indicated that FPV was prevailing in leopard cats in Vietnam.     

Common virus infections in cats, before and after being placed in shelters, with emphasis on feline enteric coronavirus

The purpose of this study was to determine the origin and subsequent spread of feline calicivirus (FCV), feline herpesvirus (FHV), and feline enteric coronavirus (FECV) in cats relinquished to shelters. FCV was isolated from the oral fauces of 11% of healthy cats upon entry, and isolation rates were highest for kittens (33%). FHV shedding was very low (4%) at the time of entry and occurred mainly in juveniles. FECV shedding was also common among newly relinquished cats (33%), especially older kittens and juveniles (90%). The subsequent spread of all three viruses was rapid and efficient in the shelter environment. Fifteen percent of cats were shedding FCV, 52% FHV, and 60% FECV after 1 week. More detailed studies were done with FECV shedding, which could be accurately quantitated. The amounts of FECV shed by infected cats ranged from 10(2)to 10(16)particles/swab of feces. FECV shedding was several logs higher in young kittens with primary infection than adult cats with primary infections. The mean levels of FECV shedding among adults were the same for primary and chronic infections. Although shelters were not the primary source of these viruses for many relinquished cats, factors intrinsic to the shelter environment were critical in amplifying shedding and spread to susceptible individuals. Extrinsic factors were especially important for the spread of FHV and FECV. FHV shedding rates increased from 4% to 50% in 1 week's time. The speed and magnitude of the increase in FHV shedding suggested that there was reactivation of latent infections as well as acquisition of new infections. FECV shedding increased 10 to 1,000,000 fold in 1 week among cats that were already infected at entry, and more than one-half of initially negative cats were shedding FECV a week later. Feline calicivirus infection was the least likely to spread in the shelter. The infection rate only increased from 11 to 15% in 1 week.     

Feline panleukopenia virus, feline herpesvirus-1 and feline calicivirus antibody responses in seronegative specific pathogen-free kittens after parenteral administration of an inactivated FVRCP vaccine or a modified live FVRCP vaccine

Two groups of feline panleukopenia (FPV), feline calicivirus (FCV) and feline herpesvirus 1 (FHV-1) seronegative kittens (six cats per group) were administered one of two feline viral rhinotracheitis, calcivirus and panleukopenia (FVRCP) vaccines subcutaneously (one inactivated and one modified live) and the serological responses to each agent were followed over 49 days (days 0, 2, 5, 7, 10, 14, 21, 28, 35, 42, 49). While the kittens administered the modified live FPV vaccine were more likely to seroconvert on day 7 after the first inoculation than kittens administered the inactivated vaccine, all kittens had seroconverted by day 14. In contrast, FHV-1 serological responses were more rapid following administration of the inactivated FVRCP vaccine when compared with the modified live FVRCP vaccine. There were no statistical differences between the serological response rates between the two FVRCP vaccines in regard to FCV.     

When cats' ways of life interact with their viruses: a study in 15 natural populations of owned and unowned cats (Felis silvestris catus)

In natural populations, virus circulation is influenced by host behavior and physiological characteristics. Cat populations exhibit a great variability in social and spatial structure, the existence of different ways of life within a same population may also result in different epidemiological patterns. To test this hypothesis, we used a logistic regression to analyze the risk factors of Feline immunodeficiency virus (FIV), feline herpes virus (FHV), feline calicivirus (FCV), and feline parvovirus (FPV) infection in owned (fed and sheltered) and unowned (neither fed nor sheltered, unsocialized) cats living in a rural environment in the North Eastern part of France. A serological survey was carried out in 492 non-vaccinated and non-sterilized individuals from 15 populations living in the same area. The prevalence of feline leukemia virus (FeLV) was also studied, but too few were infected to analyze the risk factors of this virus. For each virus, the epidemiological pattern was different in owned and unowned cats. Unowned cats were more frequently infected by directly transmitted viruses like FIV, FHV and FCV (21.22%, 67.66%, 86.52% in unowned cats vs 9.55%, 53.88%, 77.18% in owned cats, respectively), a difference that may be explained by a more solitary and more aggressive behavior in unowned adults, and/or possibly by a higher sensitivity related to a more stressful life. On the contrary, owned cats were more frequently infected with FPV (36.41% in owned cats vs 15.61% in unowned cats), possibly as a result of their concentration around human settlements. The present study showed that owned and unowned cats living in a same area have behavioral and physiological characteristics sufficiently different to influence virus circulation. Pooling different types of cats in a single sample without taking it into account could give a wrong picture of the epidemiology of their viruses. The conclusion of this work can be extended to any epidemiological studies led in wildlife species with flexible behavior as any variations in social or spatial structure, between or within populations, could result in different virus circulation.     

Duration of immunity for canine and feline vaccines: a review

In our studies aimed at assessing the minimum duration of vaccinal immunity (DOI), approximately 1000 dogs have been vaccinated with products from all the major US veterinary biological companies. The DOI for the various products is determined by antibody titers for all dogs and, by challenge studies in selected groups of dogs. Recently, all major companies that make canine vaccines for the U.S. market have completed their own studies; published data show a 3 years or longer minimum DOI for the canine core products, canine distemper virus (CDV), canine parvovirus type 2 (CPV-2), and canine adenovirus-2 (CAV-2). Studies with feline core vaccines - feline parvovirus (FPV), calicivirus (FCV) and herpes virus type I (FHV-1) have shown a minimum DOI of greater than 3 years. Based on these results, the current canine and feline guidelines (which recommend that the last dose of core vaccines be given to puppies and kittens > or =12 weeks of age or older, then revaccination again at 1 year, then not more often than every 3 years) should provide a level of protection equal to that achieved by annual revaccination. In contrast, the non-core canine and feline vaccines, perhaps with the exception of feline leukaemia vaccines, provide immunity for < or =1 year. In general the effectiveness of the non-core products is less than the core products. Thus, when required, non-core vaccines should be administered yearly, or even more frequently.     

Relevance of feline calicivirus, feline immunodeficiency virus, feline leukemia virus, feline herpesvirus and Bartonella henselae in cats with chronic gingivostomatitis

Despite its common occurrence, the aetiology of chronic gingivostomatitis in cats remains uncertain. Aetiology is likely multifactorial, and several infectious agents may be associated with chronic gingivostomatitis. The purpose of this study was to investigate the prevalence of feline calicivirus (FCV), feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), feline herpesvirus (FHV), and Bartonella henselae (B. henselae) in cats with chronic gingivostomatitis and in an age-matched control group. In addition, other factors, e. g., environmental conditions were investigated. In 52 cats with chronic gingivostomatitis and 50 healthy age-matched control cats, the presence of FCV ribonucleic acid (RNA), and FHV deoxyribonucleic acid (DNA) (polymerase chain reaction [PCR] from oropharyngeal swabs), and B. henselae DNA (PCR from oropharyngeal swabs and blood), as well as FeLV antigen (serum), and antibodies against FCV, B. henselae, and FIV (serum) were examined. FCV RNA was significantly more common in cats with chronic gingivostomatitis (53.8%, p < 0.001) than in controls (14.0%); a significant difference was also found in the prevalence of antibodies to FCV between the cats with chronic gingivostomatitis (78.8%, p = 0.023) and controls (58.0%). Of the other infectious agents investigated, there was no significant difference in the prevalence between the cats with chronic gingivostomatitis and the controls. The results of this study allow the conclusion that FCV, but no other infectious agents, is commonly associated with chronic gingivostomatitis in cats.     

Dog response to inactivated canine parvovirus and feline panleukopenia virus vaccines

Inactivated canine parvovirus (CPV) and inactivated feline panleukopenia virus (FPV) vaccines were evaluated in dogs. Maximal serologic response occurred within 1-2 weeks after vaccination. Antibody titers then declined rapidly to low levels that persisted at least 20 weeks. Immunity to CPV, defined as complete resistance to infection, was correlated with serum antibody titer and did not persist longer than 6 weeks after vaccination with inactivated virus. However, protection against generalized infection was demonstrated 20 weeks after vaccination. In unvaccinated dogs, viremia and generalized infection occurred after oronasal challenge with virulent CPV. In contrast, viral replication was restricted to the intestinal tract and gut-associated lymphoid tissue of vaccinated dogs. Canine parvovirus was inactivated by formalin, beta-propiolactone (BPL), and binary ethylenimine (BEI) in serum-free media; inactivation kinetics were determined. Formalin resulted in a greater loss of viral HA than either BEI of BPL, and antigenicity was correspondingly reduced.     

Efficacy of a Feline Leukemia Virus Vaccine in a Natural Exposure Challenge

A commercial feline leukemia virus (FeLV) vaccine was evaluated in a natural exposure system. All kittens were negative for FeLV antigen on two enzyme-linked immunosorbent assay (ELISA) tests and one indirect immunofluorescence antibody (IFA) test before vaccination or exposure. Twenty-three kittens were vaccinated subcutaneously at nine and 12 weeks of age. The vaccinated kittens and 14 unvaccinated littermates were housed in an infected environment starting at 14 weeks. The kittens were exposed for 24 weeks by living in a large room with one feline leukemia virus-positive, asymptomatic adult cat for each five kittens. Sixty-four percent of the unvaccinated kittens and 70% of the vaccinated kittens became infected as determined by ELISA. Forty-three percent of unvaccinated kittens and 39% of vaccinated kittens died. There was no difference between the infection and mortality of vaccinated kittens that developed antibodies to anti-FeLV glycoprotein 70-envelope antigen and those that did not. Consideration should be given to evaluation of feline leukemia virus vaccines using "street" virus in a natural exposure system.