Summer may be long gone, but cases of West Nile virus are still popping up – making this year's outbreak the worst so far in the U.S. since 2003.

New tallies released today by the Centers for Disease Control and Prevention bring the total case count for the year to 5,054, and the death toll to 228 — more than the past four years combined.

The mosquito-borne virus has struck some states harder than others. Roughly 80 percent of cases occurred in the twelve states – Texas, California, Louisiana, Mississippi, Illinois, South Dakota, Michigan, Oklahoma, Nebraska, Colorado, Ohio, and Arizona. Texas has borne the brunt of the outbreak with 1,684 cases (about one third of the national total), and Dallas took its mosquito control measures at the end of the summer to the next level with city-wide spraying.

Epidemiologists believe West Nile first arrived in the U.S. 13 years ago as a strain that originated in Israel. Symptoms can be as mild as the flu, but severe forms can cause paralysis and inflammation in the spinal cord and brain. Such "neuroinvasive" infections account for 51 percent of all cases, according to the CDC.

CDC officials said in September that they thought the worst was over. Now they're still asking: why so many cases this year? But at this point, it's too soon to pin it on any single factor.

One possible suspect is weather – a mild winter means more disease-carrying mosquitoes survive through the cold months.

On October 31, Maine reported its first confirmed human case of West Nile – ever. The virus had previously been detected in mosquitoes and birds. "As long as the temperatures remain above freezing there is potential for West Nile virus transmission," Stephen Sears, a state epidemiologist, said in a statement.

And some epidemiologists are wondering if this year's outbreak could be linked to larger patterns of climate change.

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