Scientists have found preliminary evidence suggesting that multiple sclerosis is an infectious disease caused or, perhaps, triggered by an previously unknown virus that appears related to one that causes a rare form of leukemia.
The evidence does not prove a viral cause but does agree with the long-held suspicion that multiple sclerosis (MS), whose victims suffer a variety of crippling neurological disorders, is caused or triggered by a virus.
The findings are being reported today in Nature, a British scientific journal, by 10 researchers at the Wistar Institute in Philadelphia, the National Cancer Institute (NCI) in Bethesda, the University of Lund in Sweden and the University of Miami.
Officials of the National Multiple Sclerosis Society and other MS researchers described the findings as "potentially very interesting" and "tantalizing" but cautioned that many other viruses have been tentatively linked to MS but have failed to be proven as a cause.
"We've had so many false starts and raised so many false hopes that I'd be very reluctant to make too much of this until we have more confirming studies," Dr. Richard Johnson, an MS researcher at Johns Hopkins University, said.
On the other hand, Dr. Robert Gallo, head of the NCI team that collaborated on the study, stressed that all the previous links have been to known viruses that caused other diseases. The new study, he said, points to a virus that has not been linked to any other disease and the traces of which appear only in MS patients.
"We're definitely not saying a virus is causing MS," Gallo said. "We're saying we see the footprints of a virus and that this is an important lead that needs to be followed."
If a virus is proven to be a cause of MS, it may open the possibility, years from now, of a vaccine to prevent the disease or a treatment to block the virus' action.
Multiple sclerosis afflicts about 250,000 Americans. It strikes about 8,800 each year, most of them young or middle-aged adults. Its effects, which usually take years to become severe, include loss of muscular coordination and control leading to tremors, difficulty in walking and talking and, in the worst cases, blindness and near-total paralysis.
MS is known to result from the progressive loss of the myelin sheath that surrounds nerves in the brain and spinal cord and that acts like the insulation on electrical wires. When the sheath is destroyed, adjacent nerves touch and nerve signals go awry. Bodily functions under the control of those nerves are lost.
No one knows why the sheath is destroyed, but experts suspect destruction results from attack by special white blood cells that are supposed to destroy invading bacteria and viruses. Somehow, it appears, these blood cells are induced to attack the myelin sheath.
The new findings are based on comparisons of MS patients and healthy people in which the scientists looked for two signs of virus infection -- a certain kind of antibody, the special molecule produced by the immune system to attack invading microbes, and traces of the genetic code carried by an invading virus that can splice its genes into those of a victim's cells.
Among 52 MS patients, according to Dr. Hilary Koprowski of the Wistar Institute, the researchers found that about 60 percent had the suspect antibodies in the blood or the fluid that circulates in the brain and spinal column, called cerebrospinal fluid. Among 104 healthy people, not one had the antibodies. Among 17 people with non-MS neurological diseases, about 18 percent carried the antibodies.
Presence of the antibodies suggests that the patients were infected at some time with a particular kind of virus. No one knows what the virus is, nor has any virus been found. The antibodies, however, are of a type detected by a test developed to find antibodies made to attack a leukemia virus known as HTLV-I.
HTLV stands for human T-cell lymphotropic virus, an unusual form of infectious agent that Gallo found several years ago. The HTLV-I virus is known to invade certain blood cells called T cells, making them multiply out of control. Subsequently, Gallo found a related leukemia virus and named it HTLV-II. When Gallo identified the virus that causes acquired immune deficiency syndrome (AIDS), he named it HTLV-III because it seemed similar to the others.
Because the MS patients had antibodies so similar to those formed in response to the leukemia virus, Koprowski and Gallo suspected that they had been infected with yet another member of the HTLV virus family. The findings do not indicate that MS patients are at any unusual risk of getting leukemia or AIDS. Gallo emphasized that the putative new virus is not the same as any other HTLV virus.
In addition to the antibody findings, Koprowski and Gallo tested a smaller number of people for the presence of the virus' genes in their T cells. The scientists did this knowing that all HTLV viruses are of a special type, called retroviruses, that can insert copies of their genes into those of an infected cell. The virus genes then stay in the cell's chromosomes as if they belonged there.
The scientists looked at the T cells of eight MS sufferers and found that five contained genetic sequences similar to those of HTLV-I. Only two normal people have been tested as yet, but both lacked the suspect sequences.
Gallo said that a search for the new virus is under way and that if it can be found and grown in the laboratory, it should be possible not only to prove whether it is a cause of MS but to develop diagnostic tests for the disease, thought to be acquired in childhood but to remain dormant until adulthood.