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Scientists in the United States think they have solved the mystery of why West Nile virus cases appear to peak in late summer and early fall -- rather than in the spring when infected mosquitoes first start biting.

For their study, researchers collected thousands of mosquitoes from several U.S. sites to determine their feeding pattern throughout the season. In particular, they did a detailed analysis of the insects' most recent "blood meals."

The results revealed that mosquitoes change their dining habits over the course of the year. In the spring, Culex mosquitoes -- the type largely responsible for spreading the disease -- feed mostly on birds. Then, in late summer and fall, they switch to people. That shift goes a long way toward explaining the late-season surge in human cases of West Niles. (You get the illness from the bite of a mosquito that has recently fed on an infected animal, most often a bird.)

However, the study also showed that mosquitoes are very finicky eaters. "For some weird reason, these mosquitoes seem to love robins," said Marm Kilpatrick, the lead research scientist with the Consortium for Conservation Medicine in New York.

At the sites where the researchers collected mosquito samples, robins represented just 4.5 per cent of the total bird population between May and June.

Yet almost half of the blood meals in the insects' bellies came from these particular birds during the spring months.

As summer progresses, and the robins leave their breading grounds, "the mosquitoes switch to their next-favourite host -- and, unfortunately, that appears to be humans," Dr. Kilpatrick said in a telephone interview.

The study, published on-line this week by the journal Public Library of Science Biology, also helps to explain why West Nile virus is more of a problem in North America, than Europe or Africa, where the disease was first identified. In Europe and Africa, the various breeds of mosquitoes feed almost exclusively on either animals or people -- but not both.

West Nile virus can produce a wide range of symptoms. In most people, it causes a mild flu-like illness. Yet in a small minority, it can be life-threatening.

Dying has its priorities

Canadians no longer want to die at home, according to a new study of end-of-life desires. A hospital room would suit most terminally ill patients just fine.

Researchers, led by Daren Heyland of Kingston General Hospital, have completed a list of what's important to dying patients and their families. Their findings indicate the location of death -- either home or hospital -- is no longer a top priority. "I think what we're seeing in reality is that with cutbacks and constraints, the funding and resources in the community are not sufficient, that it is quite a drain and a burden on family to look after these patients," Dr. Heyland told Canadian Press.

Indeed, one of patients' chief concerns is not being an emotional or physical burden to others, according to the study published this week in the Canadian Medical Association Journal.

Other top priorities included:

Not being kept alive on life support when there is little chance of meaningful recovery;

That the doctor communicates honestly about their condition;

Having time to put their affairs in order, settle disputes, and say goodbye to family and friends.

Obesity's painful burden People who are obese have enough health problems to worry about. But a new study suggests the extra weight could also make them more sensitive to pain.

Researchers at Ohio State University set out to measure pain responses in 62 patients with osteoarthritis of the knee. One-third were obese.

The researchers applied a mild electrical shock to the ankles of the volunteers while their leg reflexes were being measured. Obese patients experienced the greatest degree of muscle contraction -- an indication they are more sensitive to pain than the other volunteers.

"This kind of evaluation is, in some ways, a more objective way of measuring the body's response to pain, as opposed to simply asking someone if they feel pain," according to the lead researcher, Charles Emery.

In fact, when asked about the experience, the obese patients reported similar levels of pain as the other study participants. "It's possible that their self-reports . . . reflect what they think they should be feeling," Dr. Emery said in an e-mail interview.

Even so, he added: "We believe that obese individuals may have a lower tolerance for painful stimuli." He noted that the blood of obese people contains elevated levels of so-called "inflammatory markers," substances that also play a role in pain.

The researchers are presenting their findings tomorrow at the annual meeting of the American Psychosomatic Society in Denver.

Breast cancer fallacies

Although there has been a lot of public attention paid to breast cancer in recent years, a new survey shows that many women still have misconceptions about the potentially fatal disease.

A recent poll of 800 Ontario women found that 35 per cent of them believe heredity is the most important factor affecting breast cancer risk. However, medical evidence suggests only 5 per cent to 10 per cent of breast cancer cases can actually be linked to family history. What's more, women older than 50 felt they were less susceptible to breast cancer than younger women. The reality is that the risk of cancer tends to increase with age.

These results are worrisome, says Sharon Wood, executive director of the Ontario chapter of the Canadian Breast Cancer Foundation, which commissioned the survey. "A woman who underestimates her risk might not be going to [regular mammogram]screening," Ms. Wood said in an interview.