As if the presence of mice scurrying along the city streets and subway tracks was not enough to establish a sense of dread, there may now be a medical reason to call the exterminator.

Researchers at Our Lady of Mercy Medical Center in the Bronx announced yesterday that they had identified a possible link between a virus that some mice carry and both breast cancer and non-Hodgkin’s lymphoma.

The hospital’s study, which was published in the September 1 issue of the Journal of Clinical Cancer Research, replicated the results of several previous studies, adding credence to the link between the “mouse virus” and breast cancer in women.

More noteworthy is that the small Bronx institution also detected a relationship between the mouse virus – or the mouse mammary tumor virus, MMTV, as it is called – and non-Hodgkin’s lymphoma, a cancer that at tacks the body’s infection-fighting white blood cells, in patients who previously had breast cancer.

Researchers found evidence of the virus in human breast and lymphoma tissue from six of 12 cancer patients and in breast tissue from 40 of 120 cancer patients. Samples from people without cancer tested negative for the virus.

Though the findings are the first in the long and arduous process of isolating a gene, researchers at Our Lady of Mercy said they provide some direction on the research “road map.”

“We know the mechanism of how this virus works in the mouse,” the lead author of the study, Polly Etkind, told reporters yesterday in a conference room on the first floor of the hospital. “This gives us a lead now, now that we know the virus is there, in how to go about trying to see if it is working the same way in the human. We have some idea now of what to follow.”

Though lymphoma tissue could be extracted from only 12 subjects, the large proportion of samples that tested positive for the mouse virus surprised the Bronx researchers.

“We don’t want to let this go unstudied,” Ms. Etkind said. “This is information that we feel could be important. We know that the virus does cause this in the mouse, and we see evidence in humans, but definitely we don’t have any means of knowing as of now that this virus is really causing the disease.”

When asked about the panic that might ensue among women who’ve spotted a mouse scampering through their homes or offices, Ms. Etkind and her collaborators were quick to say the study did not find that mice cause cancer. It is unclear how or if the virus passes from mice to humans.

“We see these sequences in the breast tissues and in the lymphoma,” the director of the hospital’s Comprehensive Cancer Center, Peter Wiernik, said. “We don’t know if they are even doing anything – it could just be that these people are more prone to infections. Where it is coming from I have no idea. We have no way to say that it is causing the disease at all.”

Dr. Wiernik stressed, though, that it’s worth figuring out.

Past studies have found that non-Hodgkin’s lymphoma, the disease that killed Jacqueline Kennedy Onassis, is far more prevalent in women with breast cancer than in the general population. That there are women with both cancers is not surprising, but it adds to the evidence that the diseases may come from a viral origin.

Speculation about a viral cause for breast cancer has been aired for decades, but scientists have generally pursued other leads, such as environmental or hereditary factors.

In an editorial that accompanied the article in the Journal of Clinical Cancer Research, Drs. James Holland and Beatriz Pogo, both of Mount Sinai School of Medicine, said “robust evidence” has been presented in a series of papers that MMTV is present in 38% of American women with breast cancer.

The editorial says the most recent study offers a possible explanation for the connection between breast cancer and lymphoma, but stresses that future studies will need to show that the viral infections occurred before the cancers.

The study, which was subsidized by the New York State Empire Grant and by the Levine Family Foundation, also looked at the incidence of the two cancers in men and women.

Researchers said yesterday that while all inbred lab mice carry the virus, there is no way to know what percentages of common mice are infected. Ms. Etkind said her hope is that the new research brings scientists closer to identifying the gene causing the diseases, so that the cancer can be “turned off.”

And in the meantime, she said, there is nothing good to come from having mice around, anyway.