A majority of ovine lentivirus (OvLv) infections seen on farms develop after long incubation and a slow progression of disease to death but in nature they may also have short latency and cause acute leukoencephalitis and/or acute arthritis and pneumonia in young kids or lambs with exceptionally high mortality. Histopathologically, OvLv diseases may be characterized by lymphoid infiltration, lymphoid hyperplasia with germinal centers and plasmocytosis in the lungs and/or in the CNS, joints and udder. Lymphoid hyperplasia in lymph nodes and spleen, as well as lymphoid infiltration in the kidneys, are almost always seen in advanced cases. In some cases, it shows similarities to lymphoproliferative diseases that are considered malignant. Alveolar epithelial hyperplasia in the lungs is generally also seen, especially in older goats with caprine arthritis encephalitis virus (CAEV), and proliferation of these epithelial cells may form acine and papillary structures and in some cases are histopathologically indistinguishable from tumor nodules seen in sheep pulmonary adenomatosis. Because of complexities in the host-lentovirus interaction, cell-associated transmission and extensive antigenic and genomic variation among infecting isolates, control of infection or prevention of spread are problematic by traditional methods and exploration of alternative control strategies employing selection and expansion of animals genetically resistant to OvLv or transgenic for certain viral genes, merits consideration. Interestingly, the pure Awassi sheep breed are susceptible to infection but do not develop the disease, as do European breeds or cross-breeds in Israel, ie they are infected but not diseased. It seems that the local Bedouin black goat breed is resistant to infection of CAEV under natural conditions.(ABSTRACT TRUNCATED AT 250 WORDS)