What is visna?

Visna is virus causing a "slow disease" of sheep. Visna becamea known when it caused an epidemic in sheep in Iceland between 1930 and 1950. Visna causes pneumonia and inflammation of the central nervous system that may progress to total paralysis. The virus results in a slowly progressive disease with prolonged periods of apparent health. The sheep may eventually die of a lack of nourishment; the word "visna" is Icelandic for "wasting". If helped with food and water, however, sheep can live more than 10 years with the disease. (The disease is also know as maedi-visna; "maedi" is Icelandic for "labored breathing".)

Experimental inoculation of sheep with the visna virus results in persistent infection and slowly progressive disease with incubation periods of months to years. Some animals remained infected for life but never became ill.

Generally, visna infection is widespread but few animals become ill, usually those infected 2-3 years earlier. The explosive epidemic that occurred in Iceland was rare.

Visna is a retrovirus. Bjorn Sigurdsson found the virus in 1949. Visna is the original member of the lentivirus family; "lenti" is Latin for "slow". HIV was later found to also be a lentivirus. Click here for information on how HIV and visna are related.

References:

Fields, Virology, Chapter 55.

Sonigo et al, "Nucleotide sequence of the visna lentivirus: relationship to the AIDS virus", Cell 42, August 1985, pp 369-382.