In adult animals, secondary immunodeficiencies are those that result from viral infections, malnutrition, stress, old age, drugs, or toxins. Virus-induced secondary immunodeficiencies are the most important of these.
Virus-induced Immunodeficiencies
One way in which some viruses survive in infected animals is by suppressing the immune system of their host. For example, canine distemper virus infects and kills lymphocytes, causing a profound combined immunodeficiency. This infection is associated with a progressive decline in serum immunoglobulin levels and increased susceptibility to agents normally controlled by cellular immunity, such as Pneumocystis and Toxoplasma.
Feline panleukopenia, caused by a parvovirus, is characterized by a drastic and acute loss of leukocytes, as demonstrated in blood smears of infected cats. Parvoviral infection in both dogs and cats also causes a profound depression in the resistance to fungal infections such as aspergillosis, mucormycosis, and candidiasis.
Feline Leukemia Virus in Animals
Feline leukemia virus infection (FeLV) is associated with an increased susceptibility to secondary and opportunistic infections. Infected cats can develop neutropenia, decreased synthesis of antibodies (especially to bacterial antigens), decreased cellular immunity, and decreased complement levels. FeLV also appears to suppress immunity to the feline infectious peritonitis (FIP) coronavirus and may lead to reactivation of quiescent FIP.
Simian Type D Retrovirus in Animals
Simian type D retrovirus infection of macaques has a similar pathogenesis to that of FeLV infection of cats but can induce even more severe immunodeficiency. Type D retrovirus infection of macaques can cause disease in adolescent animals. Affected macaques may die within several months with fever, lymphadenopathy, and opportunistic infections of the CNS, respiratory tract, and intestines. Alternatively, they may become lifelong subclinical carriers or may recover fully.
Simian Immunodeficiency Virus in Animals
Simian immunodeficiency virus (SIV) is a lentivirus closely related to human immunodeficiency virus. Many species and strains of SIV exist. Their common hosts are African primates such as African green monkeys, sooty mangabeys, mandrills, baboons, and other guenons. Transmission between infected and noninfected monkeys is probably a result of bites or in utero exposure. SIV is not present in native populations of Asian primates, and it rarely causes disease in the host African species.
If infected animals are under stress, as in captivity, some may develop AIDS-like disease. SIV, especially of sooty mangabey origin, causes severe disease in macaques (rhesus, stump-tailed, pig-tailed, bonnet, etc). The immunosuppression associated with SIV can last for weeks or years. Encephalitis (usually subclinical except for wasting) and lymphomas are also frequent consequences of SIV infection in macaques.
Feline Immunodeficiency Virus in Animals
Feline immunodeficiency virus (FIV) has been identified in domestic and wild felids. The infection is endemic in cats throughout the world. The virus is shed in the saliva, and biting is the principal mode of transmission. As a result, free-roaming, male, and aged cats are at the greatest risk of infection. A transient fever, lymphadenopathy, and neutropenia follow infection. Most cats initially recover and appear to be clinically normal for many months or years before progressive immunodeficiency leads to secondary infections and deaths.
The virus targets lymphocytes, leading to a gradual loss of CD4+ helper T cells, which results in a loss of cell-mediated immunity. Cats with acquired immunodeficiency induced by FIV then develop chronic secondary and opportunistic infections of the respiratory, GI (including mouth), and urinary tracts, as well as the skin. FIV-infected cats have a higher than expected incidence of FeLV-negative lymphomas, usually of the B-cell type, and myeloproliferative disorders (neoplasia and dysplasias).
Bovine Immunodeficiency-Like Virus in Animals
Bovine immunodeficiency-like virus is a lentivirus that has been isolated from cattle with persistent lymphocytosis, hemolymphadenopathy, and bovine lymphosarcoma virus-negative lymphosarcomas. The overall prevalence in North American cattle appears to be ~1%, although in some herds it may be as much as 15%. The virus is very uncommon and does not appear to be a significant pathogen.
For More Information
Also see pet health information regarding immune-deficiency diseases in dogs and immune-deficiency diseases in cats.
