Feline Immunodeficiency Virus (FIV) is a common viral infection that causes immunosuppression in cats. It belongs to the lentivirus family, which is closely related to the human immunodeficiency virus (HIV) that causes AIDS. However, they have different antigenicity and FIV does not transmit to humans. FIV weakens a cat's immune system over time, making them vulnerable to various infections and illnesses.

FIV transmission generally occurs during fights and bites, where the virus is transmitted through saliva into wounds. Transmission from an infected mother cat to kittens through milk is also possible, while an infected mother cat may rarely pass the infection to her kittens. FIV transmission among cats is not primarily through sexual contact.

Symptoms and outcomes

Most clinical symptoms are not directly caused by FIV itself but result from secondary infections. FIV infection symptoms are similar to human HIV infection. Clinically, it initially enters an acute phase, then transitions into an asymptomatic carrier phase before ultimately developing into the progressive phase, resulting in various symptoms triggered by secondary infections.

During the first few weeks to months after FIV infection, there may be brief clinical symptoms lasting from several days to weeks, including mild fever, lethargy, and temporary lymph node enlargement. However, adult cats may exhibit milder symptoms or none at all. Clinical signs often remain absent for an extended period, lasting several years or even the cat's entire life, before symptoms related to immune deficiency become apparent. Therefore, clinical diseases typically appear in the later years.

Immune deficiency and/or immune stimulation most commonly manifest in the form of gingivostomatitis, chronic rhinitis, lymphadenopathy, immune-mediated blood disorders, cancer, and weight loss.

Diagnosis

The most commonly used method for diagnosing FIV infection is serology. Antibodies can be detected as early as 2 to 4 weeks after infection, and they can be detected throughout the entire infection process. Only a small number of cats take more than a year to produce antibodies. Serological tests include immunofluorescence, ELISA, Western blot, with ELISA being the most commonly used for diagnosis. 

Molecular diagnosis is another commonly used method, using the highly sensitive polymerase chain reaction (PCR) test, which detects the viral DNA itself directly rather than the antibodies against the virus.