**2. Feline immunodeficiency virus**

The first isolation of feline immunodeficiency virus (FIV) has been described by Pedersen, 1987 in the city of Petaluma, USA. FIV is an important lentivirus that causes immune disorders in both domestic and nondomestic cats.

Like other retroviruses the FIV containing accessory genes in addition to *gag*, *pol* and *env*. The FIV *gag*, *pol* and *env* genes encode the capsid protein p24; protease, integrase, and reverse transcriptase proteins; the viral glycoprotein (gp120) and the transmembrane protein (gp41), respectively. Both gag and pol are relatively conserved between strains (Olmsted et al., 1989; Kenyon & Lever, 2011; McDonnel et al., 2012).

Species-specific strains of FIV circulate in many feline populations. Five genetically distinct subtypes or clades of FIV have been described: A, B, C, D and E, with considerable sequence diversity in the *env* gene. Studies have shown that is possible to use the env nucleotide sequence and the p17-p24 region of the gag gene for subtypes indentification but, once gag gene has a retention rate higher than the env gene, it is considered a good candidate for phylogenetic studies (Sodora et al., 1994; Kurosawa et al., 1999; Hosie et al., 2009; Kenyon & Lever, 2011).

#### **2.1. Epidemiology and transmission**

The seroprevalence of FIV is highly variable between geographic locations. Epidemiological studies show that FIV transmission is influenced by behavior: cats with free access to the streets, sick cats, and males are more susceptible to infection with FIV (Courchamp & Pontier, 1994; Sellon and Hatmann, 2006).

Feline immunodeficiency virus can be transmitted primarily by inoculation of virus present in saliva or blood, presumably by bite and fight wounds. Male cats with free access to streets are more susceptible to infection because males are territorialist, other ways such as via mucosal exposure, blood transfer, during mating, and vertically during prenatal and postnatal exposure, can also be responsible for transmission. In natural infections, the efficacy of colostral immunity is not known (Sellon & Hartmann, 2006; Medeiros et al., 2012).

#### **2.2. Pathogenesis, immunity and clinical symptoms**

358 Immunodeficiency

Lever, 2011).

The diagnosis of FIV and FeLV can not be done based solely on clinical signs, but should be based on the demonstration of anti-FIV and FeLV antigen in the serum of infected animals.

FeLV and FIV do not survive for long outside the host and are easily inactivated by disinfectants, heating and drying. As prophylaxis against infection of FIV and FeLV is recommended castration to reduce aggression and lessen the bite. The rapid and accurate diagnosis of any secondary diseases is essential. In shelters, infected cats should be housed individually to prevent infection. All animals should be tested before being placed in shelters and breeding. Vaccines are available for both viruses; however, identification and

Studies and research about these viruses are continuously necessary to define prophylactic,

The first isolation of feline immunodeficiency virus (FIV) has been described by Pedersen, 1987 in the city of Petaluma, USA. FIV is an important lentivirus that causes immune

Like other retroviruses the FIV containing accessory genes in addition to *gag*, *pol* and *env*. The FIV *gag*, *pol* and *env* genes encode the capsid protein p24; protease, integrase, and reverse transcriptase proteins; the viral glycoprotein (gp120) and the transmembrane protein (gp41), respectively. Both gag and pol are relatively conserved between strains (Olmsted et

Species-specific strains of FIV circulate in many feline populations. Five genetically distinct subtypes or clades of FIV have been described: A, B, C, D and E, with considerable sequence diversity in the *env* gene. Studies have shown that is possible to use the env nucleotide sequence and the p17-p24 region of the gag gene for subtypes indentification but, once gag gene has a retention rate higher than the env gene, it is considered a good candidate for phylogenetic studies (Sodora et al., 1994; Kurosawa et al., 1999; Hosie et al., 2009; Kenyon &

The seroprevalence of FIV is highly variable between geographic locations. Epidemiological studies show that FIV transmission is influenced by behavior: cats with free access to the streets, sick cats, and males are more susceptible to infection with FIV (Courchamp &

Feline immunodeficiency virus can be transmitted primarily by inoculation of virus present in saliva or blood, presumably by bite and fight wounds. Male cats with free access to streets are more susceptible to infection because males are territorialist, other ways such as via mucosal exposure, blood transfer, during mating, and vertically during prenatal and postnatal exposure, can also be responsible for transmission. In natural infections, the

segregation of infected cats remains the cornerstone for preventing new infections.

management, and therapeutic measures for stray, feral and owned cats.

**2. Feline immunodeficiency virus** 

**2.1. Epidemiology and transmission** 

Pontier, 1994; Sellon and Hatmann, 2006).

disorders in both domestic and nondomestic cats.

al., 1989; Kenyon & Lever, 2011; McDonnel et al., 2012).

FIV has a tropism for T cells, macrophages, dendritic cells and central nervous system cells. The major targets for FIV infection are activated CD4+ T lymphocytes (Fig. 1). These cells typically function as T helper cells, which have a central role in immune functions, facilitating the development of humoral and cell-mediated immunity (Fig. 1) (Sellon and Hartmann, 2006; Hosie et al., 2009; Simões et al., 2012) . FIV does not use CD4 as a primary *binding* receptor, its gp120 glycoprotein binds to the CD134 (OX40) a member of the tumor necrosis factor receptor (TNFR)/nerve growth factor receptor family of molecules as the binding receptor in conjunction with the chemokine receptor CXCR4 as a cofactor for infection (Yuan et al., 2003; de Parseval et al., 2005; Willett et al., 2006b; Elder et al., 2010). The CD134 co-stimulatory pathway has been shown to be critical for T, B and antigenpresenting cell (APC) cell activation. Studies have shown that antigen stimulation of infected B-cells is increased compared with non-infected cells. FIV-infection in cats also results in a sustained polyclonal activation of B-cells with the production of antibodies to a variety of non-viral antigens (Yuan et al., 2003; Willett et al., 2006b).

**Figure 1.** Diagram illustrating the stages of the pathogenesis of FIV. Modified from Sellon and Hartmann, 2006.

As for HIV, it has been classified some stages for FIV infection (Table 1 and Fig. 2 and 3): clinical symptoms during initial stages (acute phase) of infection include fever, leucopenia, gingivitis, lethargy, signs of enteritis, stomatitis, dermatitis, conjunctivitis, respiratory tract disease, neutropenia, and generalized lymphadenopathy (Sellon e Hartmann, 2006; Gunn-Moore & Reed, 2011; Hartmann, 1998; Hartamann, 2011; O'Brien et al., 2012). The progression of disease occurs in a manner similar to the HIV-1 infection in humans. In the first few days after infection, FIV replicates in dendritic cells, macrophages and CD4+ T lymphocytes, within 2 weeks appears in the plasma. FIV-specific CD8+ cytotoxic T cells are detected in the blood within one week of infection. The duration of the following asymptomatic phase varies but usually lasts many years, viral replication is controlled by the immune response, but there is a progressive decline in CD4+ T lymphocyte numbers, resulting in a decreased CD4+/CD8+ T lymphocyte ratio (Fig. 1) (Hartmann, 1998; Sellon e Hartmann, 2006; Gunn-Moore & Reed, 2011; Hartamann, 2011). The most infected cats exhibit an increase in CD8+ T cells along with a strong humoral antibody response which allows them to control this initial phase of the infection (Elder et al., 2010). After 2-4 weeks after infection, antibodies are detectable in plasma for FIV. Antibodies recognizing *env* related proteins appear earlier than those against the *gag* protein p24. Factors that influence the length of the asymptomatic stage include the pathogenicity of the infecting strain (also depending on the FIV subtype), exposure to secondary pathogens, and the age of the cat at the time of infection. In the last, symptomatic stage (FAIDS phase) of infection, clinical signs are a reflection of opportunistic infections, skin infections such as demodicosis and pediculosis, neoplasia such as B cell lymphosarcoma and squamous cell carcinoma, myelosuppression, and neurologic disease, similar to those observed in people infected with human immunodeficiency virus (HIV) (Hartmann, 1998; Sellon e Hartmann, 2006; Elder et al., 2010; Gunn-Moore & Reed, 2011; Hartamann, 2011; Korman et al., 2012; Sobrinho et al., 2012). The chronic gingivostomatitis is the most common clinical sign in infected cats and significantly degrade the quality of life of animals

Feline Immunodeficiency 361

**Figure 2.** FIV infected cats. (Courtesy Bruno Teixeira, Universidade São Paulo, USP).

**Figure 3.** Stomatitis and gingivitis. (Courtesy Bruno Teixeira, Universidade São Paulo, USP).

cats (Sellon e Hartmann, 2006; Gunn-Moore & Reed, 2011; Hartmann, 2011).

Pathologic abnormalities described in FIV positive cats with alterations in the morphology of lymph node: hyperplastic during acute phase, follicular involution in the terminal phase of infection; thymus: cortical involution, atrophy, lymphoid follicular hyperplasia and germinal center formation; intestinal tract: villous blunting, pyogranulomatous colitis, lymphoplasmacytic stomatitis; liver: periportal hepatitis; bone marrow: dysplastic changes, granulocytic hyperplasia and the formation of marrow lymphoid aggregates; kidney: tubulointerstitial infiltrates, glomerulosclerosis; central nervous system: Perivascular cuffing, gliosis, myelitis, loss and reorganization of neurons, axonal sprouting, vacuolar myelinopathy; lung: interstitial pneumonitis, alveolitis; skeletal muscle: lymphocytic myositis, myofiber necrosis, perivascular cuffing; reproductive failure occurs in FIV infected


**Table 1.** Stages of clinical sings of FIV infection.

**Figure 2.** FIV infected cats. (Courtesy Bruno Teixeira, Universidade São Paulo, USP).

significantly degrade the quality of life of animals

**Table 1.** Stages of clinical sings of FIV infection.

Acute Unspecific signs

Unspecific clinical signs

As for HIV, it has been classified some stages for FIV infection (Table 1 and Fig. 2 and 3): clinical symptoms during initial stages (acute phase) of infection include fever, leucopenia, gingivitis, lethargy, signs of enteritis, stomatitis, dermatitis, conjunctivitis, respiratory tract disease, neutropenia, and generalized lymphadenopathy (Sellon e Hartmann, 2006; Gunn-Moore & Reed, 2011; Hartmann, 1998; Hartamann, 2011; O'Brien et al., 2012). The progression of disease occurs in a manner similar to the HIV-1 infection in humans. In the first few days after infection, FIV replicates in dendritic cells, macrophages and CD4+ T lymphocytes, within 2 weeks appears in the plasma. FIV-specific CD8+ cytotoxic T cells are detected in the blood within one week of infection. The duration of the following asymptomatic phase varies but usually lasts many years, viral replication is controlled by the immune response, but there is a progressive decline in CD4+ T lymphocyte numbers, resulting in a decreased CD4+/CD8+ T lymphocyte ratio (Fig. 1) (Hartmann, 1998; Sellon e Hartmann, 2006; Gunn-Moore & Reed, 2011; Hartamann, 2011). The most infected cats exhibit an increase in CD8+ T cells along with a strong humoral antibody response which allows them to control this initial phase of the infection (Elder et al., 2010). After 2-4 weeks after infection, antibodies are detectable in plasma for FIV. Antibodies recognizing *env* related proteins appear earlier than those against the *gag* protein p24. Factors that influence the length of the asymptomatic stage include the pathogenicity of the infecting strain (also depending on the FIV subtype), exposure to secondary pathogens, and the age of the cat at the time of infection. In the last, symptomatic stage (FAIDS phase) of infection, clinical signs are a reflection of opportunistic infections, skin infections such as demodicosis and pediculosis, neoplasia such as B cell lymphosarcoma and squamous cell carcinoma, myelosuppression, and neurologic disease, similar to those observed in people infected with human immunodeficiency virus (HIV) (Hartmann, 1998; Sellon e Hartmann, 2006; Elder et al., 2010; Gunn-Moore & Reed, 2011; Hartamann, 2011; Korman et al., 2012; Sobrinho et al., 2012). The chronic gingivostomatitis is the most common clinical sign in infected cats and

**Phases Clinical sings Duration** 

Neutropenia, Fever, lethargy, peripheral, lymphadenopathy, weight loss

decline in CD4+ T lymphocyte numbers

Apathy, weight loss, lymphadenopathy, fever, anorexia, depression, chronic gingivostomatitis (difficulty eating), chronic rhinitis, lymph adenopathy, immune-mediated glomerulo nephritis, leucopenia, bahvioral abnormalities

clinical symptoms of immunodeficiency, neoplasia, neurological abnormalities and seizures.

Asymptomatic No clinical signs, but there is a progressive

FAIDS Above symptoms more opportunistic infections,

Weeks-months

Years

Several months

Several months

**Figure 3.** Stomatitis and gingivitis. (Courtesy Bruno Teixeira, Universidade São Paulo, USP).

Pathologic abnormalities described in FIV positive cats with alterations in the morphology of lymph node: hyperplastic during acute phase, follicular involution in the terminal phase of infection; thymus: cortical involution, atrophy, lymphoid follicular hyperplasia and germinal center formation; intestinal tract: villous blunting, pyogranulomatous colitis, lymphoplasmacytic stomatitis; liver: periportal hepatitis; bone marrow: dysplastic changes, granulocytic hyperplasia and the formation of marrow lymphoid aggregates; kidney: tubulointerstitial infiltrates, glomerulosclerosis; central nervous system: Perivascular cuffing, gliosis, myelitis, loss and reorganization of neurons, axonal sprouting, vacuolar myelinopathy; lung: interstitial pneumonitis, alveolitis; skeletal muscle: lymphocytic myositis, myofiber necrosis, perivascular cuffing; reproductive failure occurs in FIV infected cats (Sellon e Hartmann, 2006; Gunn-Moore & Reed, 2011; Hartmann, 2011).

#### **2.3. Diagnosis and management of FIV-infected cats**

The methods used currently for detection of FIV infection in domestic cats include virus isolation, immunological tests for detection of specific antibodies and molecular tests for detection of viral genomic sequences, associated with the clinical diagnosis. Virus isolation is a reliable diagnostic method but is laborious and not used routinely.

Feline Immunodeficiency 363

thymic cortical regeneration, replenishing the peripheral T cell pool in experimentally FIV-

Most of the antiviral drug for FIV is licensed for the treatment of HIV infections in humans, with AZT (3'-azido-2',3'-dideoxythymidine), however, many human antiviral, are ineffective or toxic to cats. AZT (3'-azido-2',3'-dideoxythymidine) is a nucleoside analogue (thymidine derivative) that blocks the retroviral reverse transcriptase and inhibits the replication of FIV in vivo and in vitro. As with HIV, FIV resistance may arise after 6 months

Some studies have shown promising results with the use of recombinant interferon for the treatment of FIV, increasing the survival of infected animals. Immunomodulators and interferon inducers are most utilized in infected animals, but there are controversies regarding the use, for nonspecific stimulation of the immune system, can also assist in an

Like the HIV, FIV belongs to the genus *Lentivirus* of the Retroviridae family. Since the discovery of human immunodeficiency virus type 1 (HIV-1) in 1982 there is an urgent need for animal models to study the pathogenesis of HIV-1 infection and possibilities for interventional strategies (Elder et al., 2008). FIV was descript in 1987, and since then, FIV has been proposed as a model for HIV studies, because, among non-primate vertebrates, FIV infection in the cat may be the closest model of HIV infection and acquired

FIV is phylogenetically (though not antigenically) related to HIV-1. FIV and HIV share many features in their genomes in comprising three major open reading frames (ORF), gag, pol and env, especially in the pol gene and FIV also has a very similar life cycle to that of

 **FIV HIV** 

Vif Yes Yes Ver Yes Yes Tat No Yes Vpr No Yes Vpu No Yes OrfA Yes No DU Yes No Nef No Yes

Gag,Pol,Env,LTRs Yes Yes

of treatment (Hosie et al., 2009; Doménech et al., 2011; Mohammadi & Bienzle, 2012).

**3. Human immunodeficiency virus type 1 (HIV-1) x Feline** 

HIV (Table 2) (Savarino et al., 2007; Elder et al., 2008; Elder et al., 2010).

**Table 2.** Comparative viral genes encoded. Modified from Elder et al., 2010.

infected cats (Hosie et al., 2009; Mohammadi & Bienzle, 2012).

increase in viral load.

**immunodeficiency virus (FIV)** 

immunodeficiency syndrome (AIDS).

**Viral genes encoded**

The preferred initial tests are ELISA or immunochromatographic test, which detect antibodies recognizing viral structural proteins (such as the capsid protein p24 and a gp41 peptide) and offer the advantage of speed and convenience. However, when the results are inconclusive by these tests a more specific test should be used, such as the Western blot (Hosie et al., 2009). The PCR can have doubtful results due to the great genetic variability of FIV, use of specific primers only for one subtype, low viral load during a long period of infection, and inadequate preparation of PCR components. The specificity and sensitivity of PCR may vary from 40-100% according to the methodology and standardization of the laboratory (House & Jarret, 1990; Hartmann et al., 2007; Hosie et al., 2009). There is also a technique of lymphocytes, for the visualization of CD4+/CD8+, however, due to the complexity of these tests are not used routinely.

The cats positive for FIV should not be euthanized based only on a positive test result for FIV. These animals have a long life as large as that of uninfected cats, however, should be subject to regular veterinary (at least every 6 months) queries including monitoring biochemistry, hematological, and weight routine, however, the euthanasia should be considered when the clinical problems relate to an advanced stage of FIV infection leading the animal to a poor living conditions.

However, it is important to isolate the animal infected with FIV from others non-infected, and maintain a good state of health of the animal infected, because other conditions can lead to the progression of immune deficiency. Positive cats should be neutered to reduce aggressive and laughter of contamination during fighting and copulation.

Vaccination is a controversial subject. Vaccines with inactivated viruses are available in the U.S.A, Australia, New Zealandand and Japan. These vaccines induce a rapid humoral response with antibodies indistinguishable from natural infection. However, there are several studies aimed at differential tests to indentify natural FIV infected animals from those vaccinated ones.

Early diagnosis is very important because it will enable early treatment of disease. There are treatments with corticosteroids and other immunosuppressive drugs in animals with chronic stomatitis, however, they cause many side effects. The Filgastrim (granulocyte colony-stimulation factor), a recombinant human cytokine (rHuG-CSF), has been used in cats with profound neutropenia, to increase the neutrophil count, however, can also increase the viral load. The recombinant human erythropoietin is also used successfully in cats with non-regenerative anemia, administration elevates blood cells, without increasing the viral load. Treatment with insulin-like growth factor-1 recombinant human induces thymic growth and stimulates T cell function, resulting in a significant increase in thymus size and thymic cortical regeneration, replenishing the peripheral T cell pool in experimentally FIVinfected cats (Hosie et al., 2009; Mohammadi & Bienzle, 2012).

362 Immunodeficiency

**2.3. Diagnosis and management of FIV-infected cats** 

complexity of these tests are not used routinely.

the animal to a poor living conditions.

those vaccinated ones.

is a reliable diagnostic method but is laborious and not used routinely.

The methods used currently for detection of FIV infection in domestic cats include virus isolation, immunological tests for detection of specific antibodies and molecular tests for detection of viral genomic sequences, associated with the clinical diagnosis. Virus isolation

The preferred initial tests are ELISA or immunochromatographic test, which detect antibodies recognizing viral structural proteins (such as the capsid protein p24 and a gp41 peptide) and offer the advantage of speed and convenience. However, when the results are inconclusive by these tests a more specific test should be used, such as the Western blot (Hosie et al., 2009). The PCR can have doubtful results due to the great genetic variability of FIV, use of specific primers only for one subtype, low viral load during a long period of infection, and inadequate preparation of PCR components. The specificity and sensitivity of PCR may vary from 40-100% according to the methodology and standardization of the laboratory (House & Jarret, 1990; Hartmann et al., 2007; Hosie et al., 2009). There is also a technique of lymphocytes, for the visualization of CD4+/CD8+, however, due to the

The cats positive for FIV should not be euthanized based only on a positive test result for FIV. These animals have a long life as large as that of uninfected cats, however, should be subject to regular veterinary (at least every 6 months) queries including monitoring biochemistry, hematological, and weight routine, however, the euthanasia should be considered when the clinical problems relate to an advanced stage of FIV infection leading

However, it is important to isolate the animal infected with FIV from others non-infected, and maintain a good state of health of the animal infected, because other conditions can lead to the progression of immune deficiency. Positive cats should be neutered to reduce

Vaccination is a controversial subject. Vaccines with inactivated viruses are available in the U.S.A, Australia, New Zealandand and Japan. These vaccines induce a rapid humoral response with antibodies indistinguishable from natural infection. However, there are several studies aimed at differential tests to indentify natural FIV infected animals from

Early diagnosis is very important because it will enable early treatment of disease. There are treatments with corticosteroids and other immunosuppressive drugs in animals with chronic stomatitis, however, they cause many side effects. The Filgastrim (granulocyte colony-stimulation factor), a recombinant human cytokine (rHuG-CSF), has been used in cats with profound neutropenia, to increase the neutrophil count, however, can also increase the viral load. The recombinant human erythropoietin is also used successfully in cats with non-regenerative anemia, administration elevates blood cells, without increasing the viral load. Treatment with insulin-like growth factor-1 recombinant human induces thymic growth and stimulates T cell function, resulting in a significant increase in thymus size and

aggressive and laughter of contamination during fighting and copulation.

Most of the antiviral drug for FIV is licensed for the treatment of HIV infections in humans, with AZT (3'-azido-2',3'-dideoxythymidine), however, many human antiviral, are ineffective or toxic to cats. AZT (3'-azido-2',3'-dideoxythymidine) is a nucleoside analogue (thymidine derivative) that blocks the retroviral reverse transcriptase and inhibits the replication of FIV in vivo and in vitro. As with HIV, FIV resistance may arise after 6 months of treatment (Hosie et al., 2009; Doménech et al., 2011; Mohammadi & Bienzle, 2012).

Some studies have shown promising results with the use of recombinant interferon for the treatment of FIV, increasing the survival of infected animals. Immunomodulators and interferon inducers are most utilized in infected animals, but there are controversies regarding the use, for nonspecific stimulation of the immune system, can also assist in an increase in viral load.
