A new study suggests that the timing of outbreaks of rotavirus -- the leading cause of severe diarrhea in infants and young children -- may have more to do with differences in birth rates across the United States than with environmental factors such as climate.

According to the authors of the study, rotavirus infection kills more than 600,000 children per year. The illness is milder in the United States, where about 30-40 children under the age of 5 years die of it each year, and about 60,000 are hospitalized. In the United States, epidemics of rotavirus start in the southwest in late fall and end in the northeast by winter, suggesting that perhaps climate differences were responsible.

However, In our study, we use a mathematical model to show that the pattern of rotavirus epidemics in the U.S. -- in which epidemics tend to peak in the southwest 2 to 3 months earlier than they do in the northeast -- can be attributed to variation in birth rates, Virginia Pitzer, from Penn State University in State College, Pennsylvania, told Reuters Health.

Since infants often have diarrhea and can be very infectious when they get infected with rotavirus, she explained, they are the ones who tend to drive the epidemics. Thus, outbreaks of rotavirus happen sooner in states and years in which more infants are born.

In a report published late last week in Science, Pitzer and colleagues note that birth rates have traditionally been about 30% higher in the southwestern part of the US than in the northeast, and it appears that this larger pool of susceptible children fuels the start of rotavirus epidemics in the southwest.

However, over the past decade, birth rates have declined by 10% to 25% in the southwest and, as a result, the geographic and seasonal variation of rotavirus outbreaks have been less pronounced, they report.

The introduction of the rotavirus vaccine into the routine vaccination schedule of US infants in 2006 has had an effect similar to a decline in the birth rate, Pitzer said, since vaccination also reduces the number of infants vulnerable to infection.

Our model is able to explain why the rotavirus outbreak in 2007-2008 was much smaller and later than expected, Pitzer told Reuters Health. We show that vaccination against rotavirus has an impact that extends beyond just those who receive the vaccine, and can lead to changes in the pattern of infection and epidemics in the population as a whole.