Current Thoughts on Feline Immunodeficiency Virus Infection

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FELINE INFECTIOUS DISEASES

CURRENT THOUGHTS ON FELINE IMMUNODEFICIENCY VIRUS INFECTION Ellen Elizabeth Sparger, DVM, PhD

The feline immunodeficiency virus (FIV) is a lentivirus initially isolated from cats in a northern California cattery.75 This novel feline pathogen was initially designated the feline T-lymphotrophic virus (FTLV) because of its isolation from peripheral blood lymphocytes (PBL) from infected cats and its apparent tropism for feline T-lymphocytes in vitro. FTLV was renamed FIV after clinical studies confirmed an association between FIV infection and disorders related to an immunodeficient state in domestic cats. 109 Ultrastructural morphology of FIV viral particles, biochemical characteristics of its reverse transcriptase, and nucleotide sequence of its genome have supported the classification of FIV as a member of the lentivirus subfamily of retroviruses.?•· 75• 95 As a lentivirus that produces an immune deficiency in its host, FIV has received significant interest as an animal model for the human immunodeficiency virus (HIV) (causative agent of AIDS) infection in humans. Recent studies indicate FIV is most similar in its biologic activities in the host to the primate lentiviruses including HIV, yet more similar to the lentiviruses of the ungulate species (visna virus) in its genetic organization and gene regulation. 74 • 92• 95 Investigations characterizing FIV as an animal model for human AIDS has yielded significant information concerning FIV infection in naturally infected cats and may also yield FIV vaccines and antiviral therapeutics useful for the pet cat population. IN VITRO GROWTH CHARACTERISTICS

Peripheral blood lymphocytes from healthy feline donors yield cytopathic effects (CPE) consistent with that reported with human PBL infected with HIV, From the Department of Medicine, University of California, Davis, School of Veterinary Medicine, Davis, California

VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 23 • NUMBER 1 • JANUARY 1993

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the causative agent of the acquired immune deficiency syndrome (AIDS) in humans/5 Initial studies suggested that FIV proliferates in primary feline Tcells and feline lymphoid cell lines. Recent studies indicate that FIV replicates in CD4+, CD8+,t 5 and, possibly, B cells30 and reveal a broader tropism for FIV. The prototype isolate of FIV has been found to infect primary feline macrophages,16 feline fibroblast cell lines, including Crandell feline kidney cells (CrFK); 108 feline tongue cells (Fc3Tg); G355-5 cells derived from feline fetal brain tissue; 78 and feline astrocytes. 26 FIV has not been found to replicate in primary human, mouse, or dog PBL or T-lymphoid cell lines.109 Similar to other lentiviruses, cell tropism, as well as the ability to induce CPE in vitro, appears to be FIV-strain-dependent.'8 The correlation of these in vitro growth properties with in vivo pathogenicity has not been well characterized at this time. GENETIC ORGANIZATION AND VIRAL PROTEINS

The proviral form of FIV has been molecularly cloned; the genome contains 9,472 bases with open reading frames (ORF) for the virion precursor polypeptides gag, pol, and env (Fig. 1). 73 • 74 • 95 A major gag protein of 24 kD, smaller gag proteins of 10 kD and 15 kD, a gag polyprotein precursor of 49 kD, a reverse transcriptase (RT) of 62 KD, and an endonuclease of 31 kD are indicated by nucleotide sequence and protein analysis by polyacrylamide gel electrophoresis and immunoblotting. 93 The major envelope glycoprotein is approximately 130 kD, and a glycosylated transmembrane protein is 40 kD. Homologies within the FIV pol gene when compared to conserved regions of pol genes of other lentiviruses substantiate the classification of FIV as a lentivirus. The percentage homology of FIV to the other lentiviruses ranges from 41% to 45% within the RT domain of pol and does not indicate that FIV is more closely related to any other single lentivirus. 95 Interestingly, ORF PrL encoded within pol appears to encode a protein with a dUTPase activity, also observed in equine infectious anemia virus (EIAV) and visna virus, but not in the primate lentiviruses. 28 As found with other lentiviruses, sequence variation in env is characteristic of independent FIV isolates. The env genes of two unique isolates of FIV show about 15% variation in the predicted amino acid sequences within their N-terminal domains.>• A similar variation was observed between Petaluma FIV and a Japanese FIV isolate. 65 The role of env variation in FIV cellular tropism and pathogenicity has yet to be well defined. Variation in FIV env proteins is expected to play a major role in the design of FIV vaccines. Similar to other lentiviruses, the FIV genome is complex and appears to encode regulatory proteins not encoded by viruses of the oncornavirus subfamily of retroviruses (Fig. 1). ORF 1 overlapping the 3' end of pol may be the counterpart of the viral infectivity factor (VIF) of primate lentiviruses. Regulatory viral proteins may be encoded by several short orfs present in the accessory region between pol and env and within env. Although ORF 2 has been postulated to encode a transcriptional transactivator or tat-like protein, an FIV tat has not LTR

LTR

Figure 1. The FIV proviral genome (78) of the PPR strain of FIV.

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yet been identified. Studies evaluating FlY gene regulation do not provide evidence that FlY encodes a tat protein similar to HIY and that FlY gene regulation is more similar to the ovine lentivirus visna virus. 92 Recent functional and structural studies indicate that ORF L located in the 5' terminal of env, and ORF H located in the 3' terminal of env and extending into the 3' long terminal repeat (LTR), encode a protein analogous to the lentivirus posttranscriptional transactivator rev. 54• 77 The FIV LTR has been found to contain enhancer sequences that respond to activation signals in activated T-cells and that may be responsible for increased virus expression in FlY-infected cells that are activated. 92 Functional and structural properties of these regulatory genes (tat and rev) and the LTR of the lentiviruses are under intensive study. These genes and the LTR are thought to play major roles in latency and pathogenesis of lentiviral infection and may serve as targets for antiviral therapeutics. EPIDEMIOLOGY

Current knowledge of the epidemiology of FlY infection is based on serosurveys conducted in the United States, Canada, Great Britain, continental Europe, Australia, New Zealand, China, and Japan. 10• 12• 20• 32• 36• 47 • 53 • 61 • 94 • 106 The findings in each study were similar. Approximately 89% of the Japanese cats evaluated were clinically ill, whereas the remaining were healthy. 53 The overall infection rate of Japanese cats including ill and healthy cats was 28.9%; the infection rate in ill cats was 43.9% versus 12.4% in healthy cats. Approximately 89% of the cats in one large United States study 106 involving 2,765 cats were either ill or in a high-risk group of infection, whereas 11% were healthy and considered to be in a low-risk group. In this study, 14% of the ill cats were FlY seropositive, whereas 1.2% of low risk or healthy cats were seropositive. The greater density of free-roaming unneutered cats in Japan may at least partially account for the differing incidences from the two studies. Neither study involved randomly selected cats, and the cat populations selected in each study differed in description. All epidemiologic studies have indicated that outdoor free-roaming male cats are at greatest risk for FlY infection. The infection rate was two to three times higher in male versus female cats, and the peak incidence of FlY incidence occurred in cats between the ages of 5 and 10 years. In contrast, the peak incidence of FeLY infection occurs in cats between the ages of 1 and 5 years. 47 Domestic cats had a higher incidence of infection than outbred cats. The incidence in breeding catteries is unknown but is thought to be low. By serologic methods, FlY infection has been detected in several species of nondomestic cats, including captive and free-ranging populations. 7• 8• 62 Infected species within captivity have included snow leopards, lions, a jaguar, a white tiger, and a Pallas cat, and, within free-ranging cats, Florida panthers and a bobcat. Portions of the FlY genome of viruses isolated from the Florida panthers (Olmsted R, personal communication, 1991) and from the Pallas cat' have been molecularly cloned and evaluated. Observations from these preliminary investigations indicate that strains of FlY infecting nondomestic cats are related to domestic strains of FlY but exhibit significant nucleic acid dissimilarity. FlY infection and FeLY did not appear to be directly associated in the two large serosurveys, because the incidence of FeLY infection in FlY-infected cats (12-16%) was similar to the incidence of FeLY infection in FlY-negative cats. 53• 106 In other studies, an association between FlY and FeLY infection was observed. 23• 36• 72 Cats infected with both FeL Y and FlY were slightly younger and had a

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shorter lifespan. The incidence of the feline syncytia-forming virus (FeSFY) infection appeared to strongly correlate directly with the incidence of FlY infection.106 FlY infection was found in 74% of FeSFY-infected cats, whereas 37% of non FeSFY-infected cats were FlY-positive. This association of FlY and FeSFY infections in cats may be due to a similar mode of transmission (see later discussion). Coinfection of FeSFY in FlY-positive cats significantly complicates FlY isolation, because both viruses latently infect peripheral blood mononuclear cells in the host. Observations from serosurveys indicate that FlY has infected cats for quite some time. Cat serums positive for FlY antibody have been reported as far back as stored sera are available, which includes 1975 to 1976 in Europe, 37 1972 in Australia, 85 and 1968 in the United States and Japan. 33· 89 TRANSMISSION

Like most lentiviruses and unlike oncornaviruses such as FeLY, FlY is primarily cell-associated. However, virus has been isolated from serum, plasma, saliva, and cerebrospinal fluid . Asymptomatic FeLY-infected cats shed relatively large quantities of virus in saliva, urine, and other body fluids. The ease of recovery from cells and body fluids from FlY-infected cats tends to correlate with the stage of infection.106 FIV may be difficult to isolate from PBL and especially difficult to isolate from body fluids of asymptomatic cats infected with FlY, whereas virus is readily recovered from ill cats in the terminal stages of disease. Inflammation of the oral cavity in a FlY-infected cat increases virus levels in the saliva. Based on the epidemiology and experimental inoculation studies, a major mode of transmission of natural infection appears to be inoculation by bite wound. Healthy cats housed continuously for up to 4 years with experimentally infected SPF cats and bred with infected cats have remained seronegative and virus negative for FlY. However, within this group of healthy contact control cats, a subgroup of cats have remained seronegative yet are positive for FlY DNA by polymerase chain reaction (PCR) analysis of bone marrow or PBL and for FlY RNA by in situ hybridization of their PBL. 25 These cats have remained in a pathogen-free environment and have exhibited little or no fighting among themselves. These findings suggest that close physical contact may be a more effective mode of transmission than originally thought and that the size of the population of seronegative cats naturally infected with FlY needs to be determined. In the index California cattery over a period of 3 to 4 years, two cats per year have seroconverted within a pen housing both seropositive and seronegative cats (15 cats total) (Sparger EE, unpublished data). In a British study/7 the prevalence of a FlY-seropositive state in a multiple cat household with FlY-infected cats was 21 %. Epidemiology studies reveal older, outdoor free-roaming male cats to be at highest risk for FlY infection. These findings support fighting and biting as major factors in transmission, but close contact and other unidentified factors also may facilitate transmission of FIV. Typically, experimentally and naturally infected queens have given birth to virus-negative kittens that remained virus-negative despite nursing from the infected queens.109 In more recent studies, 18 • 101 queens infected with FlY during gestation have transmitted virus to their offspring either in utero, by colostrum, or saliva. These findings suggest that in utero and nursing are inefficient modes of transmission unless the queen is acutely infected during gestation. Seroepidemiologic studies indicate that FlY infection occurs most frequently via

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horizontal transmission in the postnatal period. The infection rate in cats younger than 6 months of age is very low and rises progressively thereafter. Venereal transmission of infection has not occurred with the breeding of experimentally infected queens with uninfected male cats or with breeding of infected males with uninfected females. Long-term studies of multiple cat households using PCR analysis of test cat tissues for assessment of viral infection will be necessary to fully understand transmission of FIV infection.

CLINICAL STAGES OF FIV INFECTION

Much of what is known about clinical disease associated with FIV infection has been gleaned from studies of the index cattery, epidemiologic surveys, and experimental inoculation of specific-pathogen-free cats.26• 29• 53• 59• 75• 106• ' 09 Although experimental inoculation studies have provided insight into the acute stages of infection, evaluation of FlY-infected cats in the field has allowed assessment of animals in the terminal stages of infection. These ongoing studies indicate that there are five phases of infection: (1) an acute stage occurring several weeks after infection and lasting approximately 4 to 16 weeks, (2) an asymptomatic carrier (AC) phase lasting months to years, (3) persistent generalized lymphadenopathy (PGL), (4) AIDS-related complex (ARC), and (5) a terminal stage characterized by miscellaneous disorders and opportunistic infections typical of AIDS (Table 1). 52• 88 With the resolution of acute stage clinical disease, cats have remained as asymptomatic carriers for up to 4 years in ongoing experimental inoculation studies. This AC phase of FIV infection has also been Table 1. STAGES OF FIV INFECTION Time Stage

Onset*

Duration

Acute stage

4 weeks

4-16 weeks

Asymptomatic carrier Persistent generalized lymphadenopathy AIDS-related complex

Variable Variable

Months to years 2-4 months

Variable

Months to years

AIDS

Variable

Less than 1 year

'Postinfection

Clinical Abnormalities Persistent generalized lymphadenopathy, neutropenia, fever, depression, diarrhea, superticial skin infections, or none None or low grade neutropenia Persistent generalized lymphadenopathy Weight loss, chronic diarrhea, chronic stomatitis and gingivitis, chronic respiratory disease, chronic skin infections, persistent generalized lymphadenopathy, hematologic abnormalities Opportunistic infections, emaciation, lymphoid depletion, and miscellaneous disorders including neurologic, ocular, renal, immunologic, or neoplastic disease

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observed in FlY-seropositive field cats; however, the duration of this stage has not been well characterized in cats naturally infected with FlY. In one field study/" the median age of healthy FlY-positive cats was 4 years; the median age of clinically ill FlY-infected cats was 10 years. 90 In a recent Japanese study,S1 11 naturally FlY-infected cats in the AC phase of infection were observed for 2 years. Four of the 11 cats showed progression of the disease within the 2-year period. The yearly mortality among the FlY-infected cats in the index Petaluma cattery has been 15 to 20% since the peak of mortality was reached in 1987 (Sparger EE, unpublished data). It should be noted, however, that all FlYinfected cats in this cattery are ill. In a retrospective study of 42 cats naturally infected with FlY without simultaneous infection of FeLY,'2 FIV infection resulted in a mean survival of 24.4 months after diagnosis. Again, 85% of the FlY-infected cats in this study were exhibiting FlY-associated clinical disease at the time of diagnosis. The percentage of FlY-infected AC cats that will experience the final or AIDS stage of infection is unknown. CLINICAL SIGNS

Kittens inoculated by either the intraperitoneal or intravenous route with FlY from tissue culture cells usually seroconvert in 2 to 4 weeks postinoculation, although one cat seroconverted after 14 months. 109 Within the same time, virus can be cultivated from PBL isolated from inoculated kittens. The average period between acute infection and seroconversion for naturally infected cats where inoculation doses may be very small has not been determined. Within 4 to 12 weeks postinoculation, kittens consistently suffer peripheral generalized lymphadenopathy characterized as exuberant follicular hyperplasia, leukopenia, and neutropenia. 63• 109 Normal myeloid activity or mild myeloid hyperplasia in the bone marrow has been reported associated with this neutropenia. 63 Less consistent abnormalities reported are fever, diarrhea, facial pyodermas, and anemia. These clinical and hematologic abnormalities are transient in the majority of experimentally inoculated cats; however, persistent neutropenia has been observed in a small percentage of these cats. 35• 63 The severity of acute stage disease is also dependent on FlY strain. Stage 2, or the AC phase, of FlY infection is characterized by its lack of clinical abnormalities. Asymptomatic cats experimentally infected for 2 years or more have experienced abnormal T4:T8 ratios and depressed lymphocyte blastogenesis responses to T-cell mitogens. 5 Despite these hematologic and immunologic abnormalities, this asymptomatic phase of infection is maintained in cats housed in a specific-pathogen-free environment. Onset of immune incompetence during the AC stage of infection and its role in progression into PGL and ARC has not been well characterized in naturally infected cats. The PGL and ARC phases of FlY infection were observed in the Japanese study of 11 naturally infected cats. 51. 52 The four cats that progressed from AC phase to clinical disease developed PGL, which later progressed into ARC. Findings of this Japanese study and a larger study50 of 700 FlY-infected cats suggest that the duration of the PGL stage of infection is short, lasting months at most, and is quickly followed by the ARC stage of infection. Clinical signs associated with ARC include weight loss, chronic diarrhea, chronic upper respiratory disease, chronic stomatitis/gingivitis, chronic skin infections, and lymphadenopathy. 52 • 53 • 88 • 106 Infections associated with ARC are usually secondary and bacterial in origin, not opportunistic. Inflammation of the oral cavity, including the gingiva, periodontal tissues, cheeks, oral fauces, or tongue, is

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the most common clinical entity associated with ARC and is observed in about 50% of FlY-infected cats. 53• 106 This inflammation may be of a purulent necrotizing nature or of a proliferative nature with plasmacytic infiltrates (Lowenstine L, unpublished data). Thirty five percent to 75% of FlY-infected cats suffer hematologic abnormalities including anemia, leukopenia, neutropenia, neutrophilia, and, less frequently, thrombocytopenia and pancytopenia.88• 106 The most common abnormalities include leukopenia and anemia. Leukopenia may be due to an absolute neutropenia, lymphopenia, or both. Neutropenia may be observed during any stage of infection, whereas lymphopenia is usually associated primarily with the AIDS stage of infection. Greiseofulvin therapy may precipitate severe neutropenia in FlY-infected cats. 87 Anemias generally are nonregenerative in character. A Coomb's positive anemia has been observed in some FlY-infected cats (Pedersen, NC, personal communication, 1991), and hemobartonellosis is commonly associated with such anemias. Bone marrow abnormalities have been observed in cats in AC and, more commonly, in the ARC or AIDS stages of infection. 88 Increased numbers of lymphocytes, plasma cells, and eosinophils in the bone marrow were characteristic of abnormalities observed in FlY-infected AC cats. Hyperplasia of specific cell lineages, neoplasia (myeloproliferative disorders), and dysmorphic features were more consistent bone marrow changes observed with the ARC and AIDS stages of FlY infection. Similar findings in the bone marrows of HIYinfected humans have been reported. 88 Using in situ hybridization techniques to localize viral RNA to specific cells in tissues of FlY-infected cats, FlY RNA has been observed in megakaryocytes and in an unidentified mononuclear cell population thought to be macrophages in the bone marrow. 9 Mechanisms for bone marrow abnormalities or for cytopenias observed in FlY-infected cats have not been elucidated but are under investigation. About 25% of ill FlY-infected cats suffer chronic upper respiratory disease usually manifested as recurrent conjunctivitis and rhinitis, which may or may not be associated with stomatitis. 53• 106 Disease of the lower respiratory tract also has been reported and may include chronic bronchitis, bronchiolitis, and pneumonitis. Approximately 15% of ill FlY-infected cats experience chronic infections of the skin or external ear canal. 53• 106 Skin infections may include bacterial pyodermas and chronic abscesses. FlY-infected cats with Otodectes cyanotis infestations tend to suffer purulent and proliferative otitis externa, which is not a typical manifestation of immune-competent cats (Sparger EE, unpublished data). Chronic enteritis and emaciation may be a primary manifestation in about 10% of FlY-infected cats or may be associated with other clinical abnormalities observed with ARC. 53 • 106 FlY-infected cats may suffer acute necrotizing typhilitis and colitis associated with a mesenteric lymphadenitis and abdominal pain. 109 This typhilitis and colitis appears to be associated with a vasculitis of the lower bowel and usually is fatal. Chronic bacterial infections of the urinary tract have been observed with ARC. Cats diagnosed with the ARC stage of FlY infection usually die within a year. In a larger study50 assessing 700 FlY-infected cats, only 7% of the cats were experiencing the AIDS stage of infection. Cats suffering the terminal or AIDS stage of FlY infection may experience opportunistic infections in multiple sites of the body, emaciation, lymphoid depletion, or miscellaneous diseases and usually die within 1 to 6 months. Miscellaneous disorders, including neurologic, ocular, renal, immunologic, or neoplastic disease may occur with AIDSassociated disorders or may occur as sole manifestations of FlY infection. 53• 106 Opportunistic agents associated with FlY infection include generalized cowpox

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virus infection, 13 feline calicivirus, 56 generalized demodectic19· 94 and notoedtric mange53 mite infestations, toxoplasmosis,S9· 105 candidiasis and cryptococcoses, 53 atypical mycobacteriosis/'· 94 Streptococcus canis/6 and Haemobartonella felis infection. 10. 36, 44. 53 Neurologic dysfunction has been the presenting complaint in about 5% of FIV-infected cats, and it may also be associated with the AIDS stage. 79· 90• 103· 106 Neurologic signs such as behavioral changes or psychotic behavior, dementia, facial twitching movements, and seizures have been reported in FIV-positive cats. A significant percentage of FIV-infected cats exhibit microscopic lesions in their CNS (Lowenstine L, Rideout B, unpublished data). 26 Abnormally slow motor and sensory nerve conduction velocities have been observed in many naturally infected cats without clinical neurologic deficits. 103 Glial cells and macrophages have been identified as specific cell populations in the central nervous system, which may be infected with FIV. 26· 57 Neurologic disease with FIV infection may less commonly result from secondary infectious agents such as toxoplasmosis and cryptococcosis. Inflammatory disease of the eye, especially the anterior uveal tract, has been seen in cats naturally infected with FIV. 29· 37· 43· 59 Ocular lesions observed with FIV infection include anterior uveitis, glaucoma, pars planitis, and chronic keratitis and may be associated with other coinfecting agents such as Toxoplasma gondii or FeLV. In other cases, FIV is the only obvious agent. Nonspecific renal disease and chronic interstitial nephritis have been associated with FIV infection in some cats (Rideout B, personal communication, 1990). 53·% This finding may simply reflect the tendency of ill FIV-infected cats to be of advanced age, because renal disease is common in old cats. Neoplastic disorders have been associated with FIV infection in the field and the experimental setting. Neoplasias associated with FIV infection have included FeLV-negative lymphomas, 10• 53· 86· 106 myeloproliferative disease, 10• 53 • 109 fibrosarcomas, 86 mammary gland carcinomas, 44 • 53 and squamous cell carcinomas.49 Tre significance of neoplasia in FIV-infected cats is unknown. The role of FIV as a direct or indirect oncogenic agent has yet to be determined. Lymphomas have been the most common tumor associated with FIV infection and usually have been solitary and of the B-cell type (Lowenstine L, unpublished data). One study86 found the relative risks for developing leukemia/ lymphoma were 5.6, 62.1 and 77.3 times greater in cats infected with FIV, FeLV, or FeLV/FIV, respectively. Lymphoid tumors occur in younger FeLVinfected cats with a mean age of 3.8 years and older FIV-infected cats with a mean age of 8.7 years. Lymphomas in FIV-infected cats usually have been associated with the head and neck (nasopharyngeal lymphomas) (Lowenstine L, unpublished data). Lymphomas in the nasal passages appear to arise out of surrounding plasmacytic-lymphocytic inflammation. DIAGNOSIS

In general, diagnosis of lentivirus infections in all species has involved detection of antibody. Consistently, the presence of antibodies to a lentivirus has signified infection, and this same phenomenon appears to be true for FIV infection. The first diagnostic assay to be developed for diagnosis of FIV antibody was an indirect immunofluorescence assay (IFA) that used FlYinfected PBL for the detection of FIV antibody. 75 The next IFA developed used FIV-infected CrFK cells and proved to be much more sensitive than the original IFA. 71 • 106 An ELISA involving microtiter plates coated with purified virus and

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Table 2. FACTORS PREDISPOSING FALSE-POSITIVE FIV ANTIBODY ASSAYS Operator error Maternal FIV antibody Nonspecific reactivity Testing in populations at low risk for infection

the CITE (IDEXX Systems, Portland, ME) test using membrane filter paper coated with virus was also engineered for the detection of antibody and is equivalent in sensitivity to the IFA. 71 A false-positive result may be due to (1) poor technique of the operator; (2) the presence of maternal FIV antibody in FIV-free kittens for as long as 4 months after birth; and (3) nonspecific reactivity resulting from cell culture antibody associated with immunizations with common feline vaccines (Table 2). 6 Specificity of the ELISA and CITE test appears to drop in the face of testing cats at low risk for FIV infection. 6• 41 • 45 There is general agreement that a positive CITE test or ELISA should be rechecked by either an IFA or Western blot test. Western blot is another assay that allows detection of antibodies to specific viral proteins, and it has been used as a backup assay for ELISA and CITE tests. Western blot is as sensitive as the ELISA, CITE test, or IFA, but is more specific. (Western blot is commercially available at this time through the National Veterinary Laboratory, Inc., Franklin Lakes, NJ.) The radioimmunoprecipitation assay (RIPA) yields the most sensitive and specific detection system for FIV antibody but is not feasible for commercial use. Currently, antibody detection by either IFA, ELISA, CITE test, or western blot is used for routine diagnosis of FIV infection (Table 3). A CITElike test or ELISA using recombinant FIV gag proteins, including p17 and p24, has been evaluated recently and found to have greater or equal sensitivity and specificity compared with the currently available assays using purified whole virus for substrate. 82 Diagnostics using FIV recombinant proteins for antibody detection should be commercially available in the near future. Because the level of viral expression is usually depressed within the host, at least until terminal stages, antigen detection assays have not been useful for routine diagnosis in lentivirus infections. 38 • 66 There have been occasions, however, when FIV DNA or virus was detected in the blood in the face of extremely low or nondetectable levels of antibody. 25 · 40• 44• 109 In acute natural infection, the average length of time between infection and seroconversion is not known, but preliminary evidence suggests that seroconversion in some cases may be delayed from months to even years. During this early stage of infection, FIV proviral DNA in either PBL or bone marrow may be detected by PCR, whereas serum antibody is undetectable by conventional serologic assays and virus may or may not be detected by classical virus isolation techniques. 25 Either PCR for FIV DNA, a specific viral antigen assay, or virus isolation would be necessary for the diagnosis of FIV in this situation and currently are not Table 3. DIAGNOSTIC ASSAYS FOR DETECTION OF FIV ANTIBODY Assay ELISA (IDEXX Systems, Portland, ME) CITE (IDEXX Systems, Portland, ME) Indirect immunofluorescence assay Western blot (National Veterinary Laboratory, Inc., Franklin Lakes, NJ)

Substrate

Use

Purified virus Purified virus Virus-infected cells Purified virus

Screening Screening Confirmatory Confirmatory

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available as commercial assays. The frequency of a seronegative state in a FlYinfected cat is unknown but is being evaluated.

PATHOLOGY Because a wide gamut of clinical syndromes may be associated with FlY infection, the pathology associated with FlY also varies greatly. Clinical disease most frequently involves the oral cavity, respiratory tract, intestinal tract, skin, and lymphoid tissues; consequently, the most common pathologic lesions of FlY infection involve these same tissues . Pathology involving the central nervous system, liver, and kidney also has been observed. This description of pathologic findings is derived from a University of California study (Lowenstine L, Rideout B, unpublished data) involving a series of cats naturally and experimentally infected with FlY and from published studies.3• 14• 17• 84• 109 Pathologic changes in the intestinal tract consistently have been observed in FlY-infected cats, with or without the common clinical findings of chronic diarrhea and wasting. The most frequent lesions include small intestinal villous blunting, loss of villi, and crypt dilatation, which are similar to changes reported in feline parvovirus-like enteritis and enteritis associated with FeL Y infection.83 Other frequently observed lesions of the intestinal tract include ulceration and necrotizing pyogranulomatous inflammation of the large intestinal tract with submucosal infiltration of neutrophils, macrophages, and histiocytes. FlYinfected cats showing necrotizing typhlitis and colitis experience an acute and fulminating diarrhea, often with a fatal outcome. Although secondary infectious agents have been presumed to play a role in these intestinal lesions, attempts to identify a secondary invader have been unsuccessful. It may be possible that FlY infection of a subpopulation of cells in the intestinal epithelium may be directly responsible for intestinal disease. Primary hepatic lesions also have been reported and have included serous hepatitis with accompanying peliosis hepatitis and cholangiohepatitis of unknown origin. Pathologic lesions of the oral cavity may include an ulcerative stomatitis with extensive submucosal and epithelial plasmacytic infiltrates. These accumulations of plasma cells may involve underlying muscle, salivary glands, and draining lymph nodes. Plasmacytic infiltrates also may be seen in the spleen within the same cat. Mucosal inflammation characterized by infiltrations with neutrophils, lymphoid cells, and mast cells without this intense plasmacytic infiltration have been observed. Three predominant patterns of lymphoid tissue changes have been observed which are similar to patterns reported in HIY infection in humans. These patterns include follicular hyperplasia, follicular involution, and a mixture of hyperplasia and involution in the same lymph node. Follicular hyperplasia with the presence of secondary follicles and massive plasmacytic infiltration of the cords and paracortex has been a consistent finding of the acute stage of FlY infection and infrequent in the terminal stages of infection. This same pathology is also very similar to histopathologic changes in human ARC patients. 64 Another frequent finding has been a plasmacytic infiltration of lymph nodes draining the oral cavity of FlY-infected cats with plasmacytic stomatitis. Plasmacytic infiltration and follicular hyperplasia have also been observed in spleens from FlY-positive cats who may or may not be suffering a plasmacytic stomatitis. Plasmacytic infiltrates and follicular hyperplasia in the FlY-infected cat suggest altered regulation of B-cell responses, a phenomenon documented in human AIDS patients. 58 Lymphoid changes in the terminal stages of FlY

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infection have included follicular atrophy, paracortical cell depletion, and fibrosis. Lymphoid depletion is also reported in human AIDS patients. With monoclonal antibodies to feline T-cell markers now available, lymphoid pathology in FlY infection may be further characterized and compared to the lymphoid pathology in human AIDS patients. Lesions in the central nervous system have been commonly observed in FlY infection and have not always been associated with clinical neurologic disease. Central nervous system pathology in FlY infection has primarily involved the cerebral cortex thalamus, and midbrain; neurologic abnormalities such as dementia, behavioral changes, and seizure activity also reflect cortical disease. Choroid plexus fibrosis, both gliosis and glial nodules, amphophilic hyaline bodies in the cortex, white matter vacuolation, and perivascular cuffing by macrophages and lymphocytes have been pathologic lesions observed in the brains of infected cats. The significance of these lesions in the pathogenesis of FlY infection and clinical neurologic disease has not been determined.

INFECTION AND IMMUNITY

Induction of humoral immunity has been best evaluated in cats experimentally inoculated with FIV.45 • 71 • 109 In kittens inoculated by either the intraperitoneal or intravenous route with FlY, antibodies to the major gag and transmembrane envelope proteins appear within 2 to 4 weeks postinoculation and are followed by the appearance of antibodies to the smaller gag proteins and polymerase gene products. Antibodies to the external envelope glycoprotein also appear early in infection and remain in high concentration throughout the course of infection. Recent studies indicate that FlY-infected cats harbor a low titer of neutralizing antibody, which is unable to arrest an established infection. 11 Ongoing studies of cellular immunity specific to FlY in infected cats should further characterize the immune response to FlY in FlY-infected cats. In lentivirus infection in other species, the presence of cellular immunity and neutralizing antibody titers have been observed in the host in the face of latent infection.'" Onset of AIDS has also been associated with a loss of neutralizing antibody and cellular immunity in HIY-infected individuals. 81 • 100 There is evidence in HIY infection that virus shedding increases greatly when individuals become symptomatic for AIDS, and this same phenomenon has been reported in FlY-infected cats. 102• 109 Events that lead to immunodeficiency are not well understood in FlY or HIY infection (Table 4). A reversal in the ratio of CD4+:CD8+ lymphocytes in the blood usually marks the onset of AIDS in HIY-infected individuals. 24 • 80 The ratio is altered as a result of an absolute depletion of circulating lymphocytes Table 4. IMMUNOLOGIC ABNORMALITIES ASSOCIATED WITH FIV INFECTION CD4+ T-lymphocyte depletion Depression of lymphocyte blastogenesis responses toT-cell and B-cell mitogens Reduction of ability of lymphocytes to produce and respond to interleukin-2 (IL-2) Reduced ability to mount an antibody response to T cell-dependent immunogens B-cell and plasmacytic dysplasias

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expressing the CD4+ surface antigen. CD4+ lymphocytes function as helper/ inducer cells in cellular immunity and their depletion results in a rapid decrease in cellular immune functions . Monoclonal antibodies to feline CD4-like and CDS+ proteins have recently become available. 1' 55' 97 Assessment of CD4:CDS ratios in FIV infected cats reveal alterations similar to those of humans with AIDS. 2, 5, 39, 60 , 70, 99 Naturally infected cats with signs of illness usually have inverted CD4:CDS ratios as a result of CD4+ lymphocyte depletion, In experimentally infected cats, inversion of this ratio may occur early in infection or 2 or more years after infection, Although FlY infects peripheral CD4+ T-cells in vivo, virus also has been localized to peripheral CDS+ T-cells and to circulating B cells, which appear to carry the greatest virus load. 30 The mechanism for specific CD4 + lymphocyte depletion in FlY-infected cats is not known, although in vitro FIV infection of a feline T-cell line resulted in transient CD4 surface protein expression in conjunction with syncytium formation. 104 The identity of a cellular receptor for FlY has not been confirmed but does not appear to be a CD4 + cell-surface protein.48 Mechanisms for CD4 + cell depletion are under rigorous investigation in HIY infection in humans and may be similar to those causing CD4 + cell depletion in FlY infection in cats. Lymphocytes from FlY-infected cats also exhibit depressed blastogenesis responses to T-cell and B-cell mitogens, a diminished ability to mount an in vivo antibody response to T-dependent immunogens, a reduced ability to produce IL-2, and a reduced proliferative response to IL-2, 5, 39, 60, 99 Although reduction of certain T-cell functions occur early in experimental FlY infection during the AC stage of infection, the progressive deterioration ofT-lymphocyte function appears closely associated with disease progression in FlY infection. 51 Depletion of C04+ lymphocytes and suppression ofT-cell function may at least partially account for the immunosuppressed state and subsequent secondary and opportunistic infections observed in FlY-infected cats. However, the factors responsible for the transition from an asymptomatic latent stage of infection to frank AIDS have not been well characterized. Increased virus expression, loss of viral immunity, and subsequent loss of CD4+ lymphocytes may result from the interaction of multiple cofactors. Agents and/or vaccines that trigger a generalized activation of T-cells98 and specific agents such as herpes viruses may directly enhance HIY replication.34 The emergence of more pathogenic HIV variants in the host over time may also play a role in the induction of AIDS in HIY-infected humans. 21' 31 Similar cofactors in the induction of AIDS in FlY-infected cats are currently being investigated. FlY may serve as a very useful animal model for addressing the issue of the activation of the latent, stage of lentivirus infection. Identification of cofactors and their role will be critical for designing useful antiviral therapeutics. THERAPY AND VACCINES

FlY-infected cats in the AC stage of infection or cats suffering very mild clinical signs with FlY infection should avoid exposure to other infectious disease and stress of any type . This implies isolation from other cats, which is also recommended to avoid transmission to uninfected cats. A high-quality diet, control of parasites, and early, aggressive treatment of incidental infections may delay onset of FlY-associated disease. Studies suggest that the FlY-infected cat may not be immunologically competent to respond to immunizations against common feline infections and that immune activation induced by vaccination may actually enhance virus expression and accelerate CD4 + T-cell depletion

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associated with FlY infection. If vaccinated at all, FIV-infected cats should receive killed vaccines only. Cats in the ARC and earlier stages of FlY infection tend to respond to symptomatic and supportive therapy, including antibiotics, fluid therapy, and transfusions. Corticosteroid therapy may aid in the treatment of certain immune-mediated complications associated with FlY infection in the absence of obvious opportunistic infections. FlY-infected cats suffering pars planitis and anterior uveitis have responded to topical corticosteroid therapy. 29 Corticosteroid therapy also has been successful in the treatment of autoimmune thrombocytopenia and polyarthritis associated with FlY infection (Pedersen NC, personal communication, 1991). Current studies suggest that FlY may serve as a valuable model for the evaluation of antiviral drug therapy. 2• 67• 68• 69 The FlY reverse transcriptase (RT) is similar to the HIY RT in its sensitivity to the active forms of several antiviral agents, including 3' -azido-3' -deoxythymidine (AZT) and phosphonoformate (PFA). The concentration of AZT required to inhibit replication is similar for FlY and HIY. Thus, FlY should be useful for in vitro studies of RT-targeted antiviral agents. Successful use of AZT in clinically ill FlY-infected cats, with tolerable side effects, has been recently reported. 42• 91 The acyclic purine nucleoside analog, 9-(2-phosphonomethoxyethyl)adenine (PMEA), is also an effective inhibitor of the RT of FIY. 27 PMEA appears even more effective than AZT in treatment of the certain clinical disorders associated with FlY infection. 42 However, the severe hematologic disorders associated with PMEA therapy at dosages tested may prohibit its use in cats suffering the later stages of FlY infection. Low-dose oral a-interferon therapy has been reported successful for FeLYinfected cats suffering from nonneoplastic FeLY-related conditions such as immunosuppression and nonregenerative anemia.• With use of recombinant and leukocyte-derived human a-interferon products available by prescription, clinical benefits such as increased appetite and energy, enhanced recovery from secondary infections, and enhanced survival time were observed. Positive results with FlY infection have been observed by the same investigator. Investigations evaluating experimental biological response modifiers for FlY infection as an animal model for HIY infection are currently funded and may yield agents that may be useful for pet cats naturally infected with FlY. FlY will be useful as an animal model for vaccine studies involving conventional approaches (e.g., whole killed virus, attenuated virus) and genetically engineered materials (e.g., recombinant hybrid viruses, virion subunits). Development and investigation of FlY vaccines are in the early stages. One experimental studyJ 07 reported protection from FlY infection in cats vaccinated with either a preparation consisting of inactivated whole virus or a preparation of fixed FlY-infected cells. In a second study testing vaccine preparations consisting of either FlY whole virus incorporated into immune-stimulating complexes (ISCOMs), recombinant FlY p24 ISCOMs, or a fixed, inactivated cell vaccine, vaccinated cats were not protected from FlY infection upon challenge. In addition, FlY infection was enhanced in vaccinated cats compared to FlY infection in unvaccinated cats. 46 Results from these preliminary studies indicate that characterization of a protective immune response to FlY will be critical for the development of an effective vaccine. Important considerations with the use of a FlY vaccine is that vaccinated cats will be positive to FlY diagnostic tests currently in use .

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PUBLIC HEALTH CONSIDERATIONS

There is no current evidence that FIV will infect humans or any other species. 109 FIV will not infect human primary lymphocytes or human lymphocyte celllines75 and is antigenically and genetically distinct from other lentiviruses, including HIV. 71 • 74 • 93• 95 All retroviruses, including lentiviruses, are speciesspecific. When retroviruses do cross species, adaptation to the new host requires millions of years and the same retrovirus becomes specific for that host. In limited studies, humans exposed to infection via either bite wounds from infected cats or virus-infected tissue culture supernatants were all found seronegative for FIV. 22 • 106 Lentiviruses appear to be species-specific in general.

References 1. Ackley CD, Hoover, EA, Cooper MD: Identification of a CD4-Iike homologue in the

cat. Tissue Antigens 35:92-98, 1989 2. Ackley CD, Yamamoto JK, Levy N, et al: Immunologic abnormalities in pathogenfree cats experimentally infected with feline immunodeficiency virus. J Virol64:56525655, 1990 3. Akihiro K, Ishida T, Matsumura S, et al: Pathology of feline immunodeficiency virus (FIV) infection. In Proceedings of the 5th International AIDS Conference, Montreal, 1989 4. Barlough JE: A special report to practitioners: Colloquim on feline leukemia virus and feline immunodeficiency virus. Feline Pract 19:6-14, 1991 5. Barlough JE, Ackley CD, George JW, et al: Acquired immune dysfunction in cats with experimentally induced feline immunodeficiency virus infection: comparison of short-term and long-term infections. J Acquir Immune Defic Syndr 4:219-227, 1991 6. Barr BC, Pough BB, Jacobson RH, et al: Comparison and interpretation of diagnostic tests for feline immunodeficiency virus infection. J Am Vet Med Assoc 199:13771381, 1991 7. Barr M, Williamson AL, Olmsted R, et al: Characterization of two novel feline immunodeficiency virus isolates: A domestic cat isolate from the northeastern United States and a nondomestic cat isolate from the USSR. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 8. Barr MC, Calle PP, Roelke ME, et al: Feline immunodeficiency virus infection in nondomestic felids. J Zoo Wildl Med 20:265-272, 1989 9. Beebe A, Gluckstern T, Barlough J, et al: Effects of infectious and non-infectious cofactors on feline immunodeficiency virus gene replication in vivo. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 10. Belford CJ, Miller RI, Rahaley RS, et al: Evidence of feline immunodeficiency virus in Queensland cats: Preliminary observations. Aust Vet Pract 19:44-46, 1989 11. Bendinelli M, Tozzini F, Matteucci D, et al: FIV neutralization by sera of naturally and experimentally infected cats. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 12. Bennett M, McCracken C, Lutz H, et al: Prevalence of antibody to feline immunodeficiency virus in some cat populations. Vet Rec 124:397-398, 1989 13. Brown A, Bennett M, Gaskell CJ: Fatal poxvirus infection in association with FIV infection. Vet Rec 124:19-20, 1989 14. Brown PJ, Hopper CD, Harbour DA: Pathological features of lymphoid tissues in cats with natural feline immunodeficiency virus infection. J Comp Pathol 104:345355, 1991 15. Brown WC, Bissey L, Logan KS, et al: Feline immunodeficiency virus infects both CD4+ and CDS+ T lymphocytes. J Virol 65:3359-3364, 1991

CURRENT THOUGHTS ON FELINE IMMUNODEFICIENCY VIRUS INFECTION

187

16. Brunner D, Pedersen NC: Infection of peritoneal macrophages in vitro and in vivo with feline immunodeficiency virus. J Virol 63:5483-5488, 1989 17. Callanan J, Thompson H, O'Neil B, et al: Clinical and pathological findings in FIV experimental infection. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 18. Callanan JJ, Hosie MJ, Jarrett 0: Natural transmission of FIV in kittens. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 19. Chalmers S, Schick RO, Jeffers J: Demodicosis in two cats seropositive for feline immunodeficiency virus. JAm Vet Med Assoc 194:256-257, 1989 20. Chan SG: The feline population and the existence of feline immunodeficiency virus and feline leukemia virus in Guangzhou (Canton) China. In Proceedings of the Conference of Feline Immunodeficiency Researchers, Davis, CA, 1990 21. Cheng-Mayer C, Seto D, Tateno M, et al: Biologic features of HIV-1 that correlate with virulence in the host. Science 240:80-82, 1988 22. Childs JE, Witt CJ, Glass GE: Feline immunodeficiency virus: a survey in Baltimore, Feline Pract 18:11-14, 1990 23. Cohen NO, Carter CN, Thomas MA, et al: Epizootiologic association between feline immunodeficiency virus infection and feline leukemia virus seropositivity. JAm Vet Med Assoc 197:220-225, 1990 24. Dalgleish AG, Beverly PCL, Clapham PR, et al: The CD4(T4) antigen is an essential component of the receptor for the AIDS retrovirus. Nature 312:763-767, 1984 25. Dandekar S, Beebe A, Barlogh J, et al: Presence of feline immunodeficiency virus nucleic acids in seronegative cats. In Proceedings of the First international conference of feline immunodeficiency virus researchers, Davis, CA, 1991 26. Dow SW, Poss ML, Hoover EA: Feline immunodeficiency virus: a neurotropic lentivirus. J Acquir Immune Defic Syndr 3:658-668, 1990 27. Egberink H, Borst M, Niphuis H, et al: Suppression of feline immunodeficiency virus infection in vivo by 9-(2-phosphonomethoxyethyl) adenine. Proc Natl Acad Sci USA 87:3087-3091, 1990 28. Elder JH, Phillips TR, Lerner DL, et al: Characterization of the pol gene products of feline immunodeficiency virus. In RNA Tumor Viruses. Cold Spring Harbor, NY, Cold Spring Harbor Laboratory, 1991 29. English RV, Davidson MG, Nasisse MP, et al: Intraocular disease associated with feline immunodeficiency virus infection in cats. JAm Vet Med Assoc 196:1116-1119, 1990 30. English RV, Johnson C, Tompkins MB: The B cell is a major reservoir for virus in cats infected with feline immunodeficiency virus. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 31. Fisher AG, Ensoli B, Looney D, et al: Biologically diverse molecular variants within . a single HIV-1 isolate. Nature 334:444-447, 1988 32. Fleming EJ, McCaw DL, Smith JA, et al: Clinical, hematologic, and survival data from cats infected with feline immunodeficiency virus: 42 cases (1983-1988). J Am Vet Med Assoc 199:913-916, 1991 33. Furuya T, Kawaguchi Y, Miyazawa T, et al: Existence of feline immunodeficiency virus infection in Japanese cat population since 1968. Nippon Juigaku Zasshi 52:891893, 1990 34. Gendelman HE, Phelps W, Feigenbaum L, et al: Trans-activation of the human immunodeficiency virus long terminal repeat sequence by DNA viruses. Proc Nat) Acad Sci USA 83:9759-9763, 1986 35. George JW, Pedersen NC: Hematologic and lymphocyte subset changes during the first 11 months of infection with FIV: A study of 61 cats. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 36. Grindem CB, Corbett WT, Ammerman BE, et al: Seroepidemiologic survey of feline immunodeficiency virus infection in cats of Wake County, North Carolina. J Am Vet Med Assoc 194:226-228, 1989 37. Gruffydd-Jones TJ, Hopper CD, Harbour DA, et al: Serological evidence of feline

188

38. 39. 40.

41.

42.

43.

44. 45. 46. 47. 48. 49. 50.

51. 52. 53.

54. 55. 56. 57. 58. 59.

SPARGER

immunodeficiency virus infection in UK cats from 1975-76. Vet Rec 123:569-570, 1988 Haase AT: Pathogenesis of lentivirus infections. Nature 322:130-136, 1988 Hara Y, Ishida T, Ejima H, eta!: Decrease in mitogen-induced lymphocyte proliferative responses in cats infected with feline immunodeficiency virus. Nippon Juigaku Zasshi 52:573-579, 1990 Harbour DA, Williams PD, Gruffydd-Jones TJ, et a!: Isolation of a T-Iymphotropic lentivirus from persistently leucopenic domestic cats. Vet Rec 122:84-86, 1988 Hardy WD, Zuckerman EE: Comparison of ELISA, IFA and immunoblot tests for detection of feline immunodeficiency virus infection. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 Hartmann K, Donath A, Beer B, et a!: Use of two virustatica (AZT,PMEA) in the treatment of FIV- and of FeLV-seropositive cats with clinical symptoms. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 Hopper CD, Crispin SM: Ocular findings in cats naturally infected with feline immunodeficiency virus. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 Hopper CD, Sparkes AH, Gruffydd JT, et al: Clinical and laboratory findings in cats infected with feline immunodeficiency virus. Vet Rec 125:341-346, 1989 Hosie MJ, Jarrett 0 : Serological responses of cats to feline immunodeficiency virus. Aids 4:215-220, 1990 Hosie MJ, Reid G, Neil JC, et al: Enhancement of FIV infection after vaccination. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 Hosie MJ, Robertson C, Jarrett 0: Prevalence of feline leukaemia virus and antibodies to feline immunodeficiency virus in cats in the United Kingdom. Vet Rec 125:293297, 1989 Hosie MJ, Willett BJ, Dunsford TH, et a!: Identification of an FIV receptor. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 Hutson CA, Rideout RA, Pedersen NC: Neoplasia associated with feline immunodeficiency virus. JAm Vet Med Assoc 199:1357-1361, 1991 Ishida T, Taniguchi A, Kanai T, et a!: Retrospective serosurvey for feline immunodeficiency virus infection in Japanese cats. Nippon Juigaku Zasshi 52:453454, 1990 Ishida T, Taniguchi A, Wahizu T, et a!: Immunology of feline immunodeficiency virus (FIV) infection. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 Ishida T, Tomoda 1: Clinical staging of feline immunodeficiency virus infection. Nippon Juigaku Zasshi 52:645-648, 1990 Ishida T, Washizu T, Toriyabe K, et a!: Feline immunodeficiency virus infection in cats of Japan. JAm Vet Med Assoc 194:221-225, 1989 Kiyomasu T, Miyazawa T, Furuya T, eta!: Identification of feline immunodeficiency virus rev gene activity. J Virol 65:4539-4542, 1991 Klotz FW, Cooper MD: A feline thymocyte antigen defined by a monoclonal antibody. J Immunol 136:2510-2514, 1986 Knowles JO, Gaskell RM, Gaskell CJ, et al: Prevalence of feline calicivirus, feline leukaemia virus and antibodies to FIV in cats with chronic stomatitis. Vet Rec 124:336-338, 1989 Lackner AA, Dandekar S, Gardner MB: Neurobiology of simian and feline immunodeficiency virus infections. Brain Pathol 1:201-212, 1991 Lane HC, Masur H, Edgar LC, et a!: Abnormalities of B-cell activation and immunoregulation in patients with the acquired immune deficiency syndrome. N Eng! J Med 309:453-458, 1983 Lappin MR, Greene CE, Winston S, et a!: Clinical feline toxoplasmosis. Serologic diagnosis and therapeutic management of 15 cases. J Vet Intern Med 3:139-143, 1989

CURRENT THOUGHTS ON FELINE IMMUNODEFICIENCY VIRUS INFECTION

189

60. Lin DS, Bowman DD, Jacobson RH, et al: Suppression of lymphocyte blastogenesis to mitogens in cats experimentally infected with feline immunodeficiency virus. Vet lmmunollmmunopathol 26:183-189, 1990 61. Lutz H, Egberink H, Arnold P, et al: Felines T-lymphotropes lentivirus (FTLV): Experimentelle infektion und vorkommen in einigen landern Europas. Klentierpraxis 33:455-459, 1988 62. Lutz H, Isenbugel E, Lehmann R, et al: Retrovirus serology in non-domestic felids. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 63. Mandell CP, Sparger EE, Pedersen NC, et al: Long term hematologic changes in cats experimentally infected with feline immunodeficiency virus (FIV). Comparative Haematology International, 1992, in press. 64. Meyer PR, Yanagihara ET, Parker JW, et al: A distinctive follicular hyperplasia in the acquired immune deficiency syndrome (AIDS) and AIDS related complex. Hematol Oncol 2:319-347, 1984 65. Miyazawa T, Fukasawa M, Hasegawa A, et al: Molecular cloning of a novel isolate of feline immunodeficiency virus biologically and genetically different from the original U.S. isolate. J Virol 65:1572-1577, 1991 66. Narayan 0: Biology and pathogenesis of lentiviruses. J Gen Virol 70:1617-1639, 1989 67. North TW, Cronn RC, Remington KM, et al: Direct comparisons of inhibitor sensitivities of reverse transcriptases from feline and human immunodeficiency viruses. Antimicrob Agents Chemother 34:1505-1507, 1990 68. North TW, Cronn RC, Remington KM, et al: Characterization of reverse transcriptase from feline immunodeficiency virus. J Bio! Chern 265:5121-5128, 1990 69. North TW, North GL, Pedersen NC: Feline immunodeficiency virus, a model for reverse transcriptase-targeted chemotherapy for acquired immune deficiency syndrome. Antimicrob Agents Chemother 33:915-919, 1989 70. Novotney C, English RV, Housman J, et al: Lymphocyte population changes in cats naturally infected with feline immunodeficiency virus. Aids 4:1213-1218, 1990 71. O'Connor TJ, Tanguay S, Steinman R, et al: Development and evaluation of immunoassay for detection of antibodies to the feline T-lymphotropic lentivirus (feline immunodeficiency virus). JClin Microbiol 27:474-479, 1989 72. O'Connor TP, Tonelli QJ, Scarlett JM: Report of the National FeLV/FIV awareness project. J Am Vet Med Assoc 199:1348-1352, 1991 73. Olmsted RA, Barnes AK, Yamamoto JK, et al: Molecular cloning of feline immunodeficiency virus. Proc Nat) Acad Sci USA 86:2448-2452, 1989 74. Olmsted RA, Hirsch VM, Purcell RH, et al: Nucleotide sequence analysis of feline immunodeficiency virus: genome organization and relationship to other lentiviruses. Proc Nat) Acad Sci USA 86:8088-8092, 1989 75. Pedersen NC, Ho EW, Brown ML, et al : Isolation of a T-lymphotropic virus from domestic cats with an immunodeficiency-like syndrome. Science 235:790-793, 1987 76. Pedersen NC, Yamamoto JK, Ishida T, et al: Feline immunodeficiency virus infection. Vet Immunol Immunopathol 21:111-129, 1989 77. Phillips TR, Lovelace K, Konings DAM, et al: Putative REV and REV responsive elements of feline immunodeficiency virus (FIV) . In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 78. Phillips TR, Talbott RL, Lamont C, et al: Comparison of two host cell range variants of feline immunodeficiency virus. J Virol 64:4605-4613, 1990 79. Podell M, Olmsted R, Krakowka S, et al: Neurodiagnostic evaluation in cats experimentally infected with feline immunodeficiency virus. In Proceedings in the First International Conference of Feline Immunodeficiency Virus Researchers. Davis, CA, 1991 80. Polk BF, Fox R, Brockmeyer R, et al: Predictors of the acquired immunodeficiency syndrome developing in a cohort of seropositive homosexual men. N Engl J Med 316:61-66, 1987 81. Ranki A, Weiss SH, Valle SL, et al: Neutralizing antibodies in HIV(HTLV-III) infection: correlation with clinical outcome and antibody response against different viral proteins. Clin Exp Immunol 69:321-329, 1987

190

SPARGER

82. Reid G, Rigby MA, McDonald M, et al: Immunodiagnosis of feline immunodeficiency virus infection using recombinant viral p17 and p24. AIDS 5:1477-1483, 1991 83. Reinacher M: Feline leukemia virus-associated enteritis- a condition with features of feline panleucopenia. Vet Pathol 24:1-4, 1987 84. Reinacher M, Holznagel E: Post mortem diagnosis of spontaneous FIV infection. In Proceedings in the First International Conference of Feline Immunodeficiency Virus Researchers. Davis, CA, 1991 85. Sabine M, Michelsen J, Thomas F, et al: Feline AIDS. Aust Vet Pract 18:105-107, 1988 86. Shelton GH, Grant CK, Cotter SM, et al: Feline immunodeficiency virus and feline leukemia virus infections and their relationships to lymphoid malignancies in cats: A retrospective study (1968-1988). J Acquir Immune Defic Syndr 3:623630, 1990 87. Shelton GH, Grant CK, Linenberger ML, et al: Severe neutropenia associated with griseofulvin therapy in cats with feline immunodeficiency virus infection. J Vet Intern Med 4:317-319, 1990 88. Shelton GH, Linenberger ML, Grant CK, et al: Hematologic manifestations of feline immunodeficiency virus infection. Blood 76:1104-1109, 1990 89. Shelton GH, McKim KD, Cooley PL, et al: Feline leukemia virus and feline immunodeficiency virus infections in a cat with lymphoma. J Am Vet Med Assoc 194:249-252, 1989 90. Shelton GH, Waltier RM, Connor SC, et al: Prevalence of feline immunodeficiency virus infections in pet cats. JAm Anim Hosp Assoc 25:7-12, 1989 91. Smyth NR, Bennett M, Goskell RM, et al: Treatment for FIV? [letter). Vet Rec 126:409-410, 1990 92. Sparger EE, Shacklett BL, Renshaw-Gegg L, et al: Regulation of gene expression directed by the long terminal repeat of the feline immunodeficiency virus. Virology 187:165-177, 1992 93. Steinman R, Dombrowski J, O'Connor T, et al: Biochemical and immunological characterization of the major structural proteins of feline immunodeficiency virus. J Gen Virol 71 :701-706, 1990 94. Swinney GR, Paule JVR, Wilks CR: Feline T-Iymphotropic virus (FTLV) (feline immunodeficiency virus) in cats in New Zealand. N Z Vet J 37:41-43, 1989 95. Talbott RL, Sparger EE, Lovelace KM, et al: Nucleotide sequence and genomic organization of feline immunodeficiency virus. Proc Nat! Acad Sci USA 86:57435747, 1989 96. Thomas JB, Robinson WF, Chadwick BJ, et al: Association of renal disease indicators with feline immunodeficiency virus infection. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 97. Tompkins MB, Gebhard DH, Bingham HR, et al: Characterization of monoclonal antibodies to feline T lymphocytes and their use in the analysis of lymphocyte tissue distribution in the cat. Vet Immunol Immunopathol 4:305-318, 1990 98. Tong-Starksen S, Luciw PA, Peterlin BM: Human immunodeficiency virus long terminal repeat responds to T-cell activation signals. Proc Nat! Acad Sci USA 84:6845-6849, 1987 99. Torten M, Franchini M, Barlough JE, et al: Progressive immune dysfunction in cats experimentally infected with feline immunodeficiency virus. J Virol 65:2225-2230, 1991 100. Walker BD, Chakrabarti S, Moss B, et ai: HIV-specific cytotoxic T lymphocytes in seropositive individuals. Nature 328:345-348, 1987 101. Wasmoen T, Armiger-Luhman S, Egan C, et ai: Transmission of feline immunodeficiency virus from infected queens to kittens. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991 102. Weymouth LA, Hammer SM, Gillis JM, et al: Isolation of human immunodeficiency virus and serum neutralizing antibody. Lancet 2:1158-1159, 1986 103. Wheeler DW, Mitchell TW, Gasper PW, et al: FIV infection associated with neurologic abnormalities. In Proceedings of the First International Conference of Feline Immunodeficiency Virus Researchers, Davis, CA, 1991

CURRENT THOUGHTS ON FELINE IMMUNODEFICIENCY VIRUS INFECTION

191

104. Willett BJ, Hosie MJ, Dunsford TH, et al: Productive infection ofT-helper lymphocytes with feline immunodeficiency virus is accompanied by reduced expression of CD4. AIDS 5:1469-1475, 1991 105. Witt CJ, Moench TR, Gittelsohn AM, et al: Epidemiologic observations on feline immunodeficiency virus and Toxoplasma gondii coinfection in cats in Baltimore, MD. JAm Vet Med Assoc 194:229-233, 1989 106. Yamamoto JK, Hansen H, Ho EW, et al: Epidemiologic and clinical aspects of feline immunodeficiency virus infection in cats from the continental United States and Canada and possible mode of transmission. J Am Vet Med Assoc 194:213220, 1989 107. Yamamoto JK, Okuda T, Ackley CD, et al: Experimental vaccine protection against feline immunodeficiency virus. AIDS Res Human Retroviruses 7:911-922, 1991 108. Yamamoto JK, Pedersen NC, Ho EW, et al: Feline immunodeficiency syndrome-a comparison between feline T-lymphotropic lentivirus and feline leukemia virus. Leukemia 204S-215S, 1988 109. Yamamoto JK, Sparger E, Ho EW, et al: Pathogenesis of experimentally induced feline immunodeficiency virus infection in cats. Am J Vet Res 49:1246-1258, 1988

Address reprint requests to Ellen Elizabeth Sparger, DVM, PhD Department of Medicine University of California, Davis School of Veterinary Medicine Davis, CA 95616-8737