The feline retroviruses, FeLV and FIV, today are well recognized for their ability to cause profound immune-suppressive disease in cats throughout the world. Clearly among the most complex infections affecting the cat, a retroviral infection demands an immune response that is robust and sustained if the infected cat is to survive long-term. Both innate immune responses (neutrophils, macrophages, and antigen presenting cells [APCs]) as well as adaptive immunity (B-lymphocytes and T-lymphocytes) are critical.
In general terms, FeLV causes the most significant clinical infection, compared to FIV. In the past, FeLV was considered the most common cause of death from infectious disease. Today, however, the incidence of disease has clearly declined from what it was just 20 years ago. While vaccination accounts for some of that decline, the ability to perform rapid, accurate retrovirus testing of cats is recognized to be the principle reason behind the decrease in FeLV incidence. It's important to note that susceptibility to FeLV infection is greatest during the first 6 months of a cat's life. After that, "natural resistance" develops...which is attributed to maturation of the immune system in an adult cat and production of one of the interleukins. In the immunologically naïve kitten, exposure to FeLV is likely to result in life-long infection. With FeLV, 'life-long' may be months or it may be years...it's a disease that is very difficult to prognose due to the wide range of clinical outcomes possible once the virus reaches the infected cat's bone marrow.
Susceptibility to FIV infection, on the other hand, does not change with a cat's age. Risk among adults is similar to that in kittens. Furthermore, FIV infection is not associated with the same spectrum of clinical consequences seen in FeLV-infected cats. Although FIV does cause an acquired immunodeficiency syndrome, one that shares many similarities with HIV infection in humans, the prognosis of FIV infection is generally going to be better than that of FeLV, especially when level of medical care the cat receives during the course of infection is high. Opportunistic infections are common, but FIV-infected cats tend to live longer (given the opportunity to do so) than FeLV-infected cats. Many FIV-infected cats are known to die of age-related causes not linked to their retrovirus infection. And...as such, FIV has not had the impact on the feline population that FeLV has.