Toxoplasmosis, a cat-borne parasitic disease that up to now has been feared primarily for the damage it does to the unborn, has emerged as a major threat to people who have AIDS.

Declaring that "the time has come" to bring toxoplasmosis under control, the New England Journal of Medicine said in a Feb. 4 editorial that a severe brain disorder called "toxoplasmic encephalitis, once a rare disease even in immunocompromised adults, is now the most commonly recognized cause of opportunistic infection of the central nervous system in patients with AIDS."

This may be one reason, some public health experts believe, that dementia (loss of mental function) is noted in some AIDS patients long before that complication would be expected.

There are no good statistics on the incidence of "toxo," according to the Centers for Disease Control, and no firm data on how many AIDS patients have been affected by it. It is clear, however, that even people with immunity against this common parasitic ailment lose it when AIDS strikes.

Although people who get toxo usually recover fully after a few weeks of mononucleosis-like or flu-like symptoms and acquire lasting immunity, the parasites in their bodies never really go away. So when the immune system fails and its guard against silent invaders drops, the parasites are free to run riot -- and do.

Thus, everyone with AIDS -- there have been about 54,000 cases and more than 30,000 deaths since record keeping began -- is at risk of serious complications from what would be a trivial ailment in a healthy individual. The same is true of people whose immune systems have been compromised by other diseases or the treatments they undergo -- such as cancer chemotherapy -- against those diseases.

The New England Journal editorial estimated that 5 to 10 percent of AIDS patients in the U.S. "will contract toxoplasmic encephalitis," which is an inflammation of the brain caused by the toxo parasite.

The editorial pointed out, too, a chilling danger that could lie in wait for transplant recipients, "who may contract the disease after acquiring toxoplasma from the donor organ (for example, the heart) or from reactivation of the latent infection (for example, in the recipient of a bone-marrow transplant)."

While AIDS patients are the newest victims of toxo infection, the greatest pool of potential toxo cases in the United States remains children of women who contract the disease during pregnancy. Like German measles (rubella), toxo is rarely serious in older children and adults but can be disastrous to the unborn and to very small infants whose immune systems are not yet developed.

Dr. Robert McCabe of the Veterans Administration hospital in Martinez, Calif., one of the authors of the Journal editorial, said in an interview that somewhere around one out of every 400 babies born in the United States -- about 7,500 a year -- contract toxo while in the womb and are thus at risk for defects that are apparent at birth or show up later in childhood.

Early complications of congenital toxo, according to the well-known physician's reference, Merck Manual, are eye problems in about 10 percent of the cases, as well as liver and blood disorders, brain involvement, convulsions and other problems in another 20 percent. These and other complications can show up later -- even years later -- in some of the 70 percent of congenital toxo babies who appear completely normal at birth.

What triggered the Journal editorial was a research report from France showing that if women are screened for toxo early in pregnancy and given appropriate drugs if found to be newly infected, the treatment "reduces the severity of the manifestation of the disease" in the babies they are carrying. France requires blood tests early in pregnancy to determine whether a woman has had toxo in the past, and has thus acquired lifelong immunity, or has not had toxo, and thus is at risk of getting it while pregnant.

There is no jurisdiction in the United States where toxo screening is required, according to Dr. Ralph Brian of the CDC's epidemic intelligence service in Chamblee, Ga.

McCabe of the VA and his co-author Jack S. Remington of the Stanford University Medical School said in the editorial that the value of screening has been recognized abroad for almost 15 years. "Cost, logistics, differences in priorities and questions about the adequacy of serologic (blood) tests have hampered the institution of routine . . . screening in the United States," they stated.

Asked whether testing should be required in this country as it is in France, the CDC's Brian prefaced his answer with an admonition "to distinguish very carefully between my personal opinion and the official CDC position."

His personal opinion, Brian said, "is that I agree with McCabe and Remington that we do need to do systematic testing here and that obstetricians and gynecologists need to be better educated themselves and need to better educate their patients about toxoplasmosis." And the CDC? "We have no official position as yet," said Brian.The National Institute of Allergy and Infectious Diseases, which is the lead government agency in the battle agains AIDS, published in 1983 an eight-page booklet called "Toxoplasmosis." But one admitted shortcoming of the booklet is that it contains nothing about the risk of toxo in AIDS patients, acknowledged NIAID public-affairs officer Judy Murphy.

Nevertheless, the CDC's Brian said safety rules outlined in the booklet for pregnant women are good guidelines for people with AIDS, since new toxo infection in people with faulty immune systems can have serious effects. These rules are:

Avoid cat feces -- in other words, don't clean litter boxes if you can avoid it;

Wear disposable plastic or rubber gloves if you can't;

Clean the box daily because organisms in droppings do not become infective until about 24 hours after they are out of the cat;

Wear gloves while working in the garden where cats might have "gone";

Keep cats indoors if at all possible and don't let them mingle with outdoor cats;

Cook meats -- especially pork, lamb and beef -- well in order to destroy parasites they might have acquired while grazing.

William Hines is a Washington science writer.