The Mystery of Why Urinary Tract Infections Peak in Summer
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Severe urinary tract infections spike in the summertime, especially among younger women, a new study finds.

Researchers analyzed information on people who were hospitalized for urinary tract infections (UTIs) from 1998 to 2011 in the United States. There are more than 7 million UTIs in the United States each year, but only a small fraction of severe cases require hospitalization.

Overall, the number of hospitalizations for UTIs nearly doubled during the study period, from about 136,000 nationally in 1998 to nearly 244,000 in 2011. UTI hospitalizations increased at twice the rate among women compared with men.

But in addition to this overall increase, the researchers also found a seasonal pattern: UTI hospitalizations peaked every summer, and dipped every winter, among both women and men.

When the researchers looked at this seasonal effect in different age groups, they found that the summer peak was seen mainly among younger women, under age 44. For older women, UTI hospitalizations were about the same year-round.

"We found — really surprisingly — for women, that seasonality was the highest in these youngest women, between 18 and 40," said study researcher Jacob Simmering, of the University of Iowa's Department of Pharmacy Practice and Science. "As age went up, seasonality diminished," he said. The seasonality of UTI among men did not differ by age group. [7 Common Summer Health Concerns]

There are several theories as to why UTIs could be more common in the summer for younger women. Dehydration is more common in the summer, and it may increase the risk for UTI. Younger people might also be more likely to experience dehydration if they are outside more, Simmering said.

Sexual activity — another risk factor for UTI among women— may also increase in the summer among people in younger age groups, the researchers said.

The overall rise in UTI hospitalizations may be tied to an increase in antibiotic resistant bacteria that cause these infections. In previous years, most UTIs could be treated by taking antibiotics, but more recently, an increasing number of people with UTIs need different antibiotics, which can only be delivered by an IV, requiring hospitalization, Simmering said.

While it's hard to change this upward trend in UTI hospitalizations, doctors may be able to reduce the summertime peak, Simmering said. For example, they could make more recommendations for preventing UTIs in the summer, aimed at women in younger age groups, Simmering said.

Simmering presented the findings this month at IDWeek, a meeting of several organizations focused on infectious diseases.

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