Motor neuron diseases and viruses: poliovirus, retroviruses, and lymphomas.

The viral theory of motor neuron disease (MND) has been rejuvenated in the last 5 years for several reasons. First, it is now recognized that enteroviruses and picornaviruses similar to poliovirus can persist and induce immune-mediated diseases. In some picornavirus animal models, the immune-mediated disease can occur and continue long after the infectious virus has been cleared, and in some cases of human MND an immune-mediated disease may occur. Second, the human retroviruses human immunodeficiency virus (HIV) and human T-cell lymphotrophic virus (HTLV) have caused isolated MND syndromes. Neither of these two specific viruses appear to be 'the amyotrophic lateral sclerosis (ALS) retrovirus', because they cause a plethora of neurologic syndromes unrelated to MND. Retroviruses of mice, however, can cause MND and lymphomas, and because there is an increased incidence of lymphomas in ALS patients, it has been suggested that retroviruses are another possible viral agent of human MND.