Lifelong, persistent infections in sheep occur in the lungs, lymph nodes, spleen, joints, central nervous system, and mammary glands;[2][5] The condition is sometimes known as ovine progressive pneumonia (OPP), particularly in the United States,[1] or Montana sheep disease.

[6] Attempts at vaccination against maedi-visna virus have failed to induce immunity, occasionally causing increased viremia and more severe disease.

Visna refers to the neurological form of the disease and predominantly causes meningoencephalitis in adult sheep.

This disease has inflicted many economic losses worldwide due to the long incubation period and the high mortality rate of sheep and goats.

The onset of the diseases is gradual resulting in relentless loss of weight in addition to breathing problems.

Cough, abortion, rapid breathing, depression, chronic mastitis and arthritis are also additional symptoms observed.

This causal lentivirus can be found in monocytes, lymphocytes and macrophages of infected sheep in the presence of humoral and cell mediated immune response and can also be detected by conducting several serological tests.

[10] Transmission of the disease occurs most commonly via the oral route caused by ingestion of colostrum or milk that contains the virus or inhalation of infected aerosol droplets.

Due to variation of the strains of MVV, some of the association clinical symptoms may be more pre-dominant in a flock relative to others along with differences in genetic susceptibility patterns.

[11] Visna Maedi virus (VMV) belongs to the small ruminant lentivirus group (SRLV).

Studies suggest that VMV gains entrance to the cell via mannosylated residues on its envelope proteins.

[7] Horizontal transmission plays an important role among livestock due to their often close quarters, especially during winter stabling.

Virions consist of an icosahedral capsid surrounded by an envelope derived from the host plasma membrane.

[citation needed] The term viral tropism refers to the cell types a virus infects.

Visna virus is generally known to target cells of the immune system, mainly monocytes and their mature form, macrophages.

[1] The visna viral genome encodes three structural genes characteristic of retroviruses, gag (group specific antigen), pol (polymerase), and env (envelope protein).

[25] The pol gene encodes five enzymatic functions: a reverse transcriptase, RNase H, dUTPase, integrase, and protease.

[31] The integrase enzyme exists inside the viral capsid, facilitating integration into the host chromosome after entry and virion uncoating.

[38] It has been postulated that the effects of maedi-visna infection in sheep are the "equivalent" of central nervous system disease and wasting syndrome found in human AIDS patients.

Nucleotide sequence analysis demonstrated that the AIDS virus was a retrovirus related to visna and provided early clues as to the mechanism of HIV infection.