Kim Painter

Special for USA TODAY

The USA has the most measles cases in 20 years and the most since homegrown outbreaks were eliminated in 2000, the federal Centers for Disease Control and Prevention announced Thursday.

The confirmed case count for 2014, as of May 23, was 288 and growing, the CDC says. That number includes 138 cases from Ohio, where the biggest outbreak is ongoing – and where the actual count is 166 as of Thursday, according to the state Health Department.

The nationwide total is the highest for late May since 1994, when 764 cases were reported, the CDC says. It surpasses the 220 cases reported in all of 2011, which was the most in the post-2000 era.

"This is not the kind of record we want to break, but should be a wake-up call for travelers and for parents to make sure vaccination records are up to date," said Anne Schuchat, director of the CDC's National Center for Immunizations and Respiratory Diseases. Schuchat, who spoke during a telephone news conference, said this year's outbreaks are occurring among unvaccinated clusters of people exposed to travelers bringing the measles virus back from other countries — most notably the Philippines, where a large outbreak began in October 2013.

Though measles remains officially "eliminated" in the USA — because there have been no sustained homegrown outbreaks in recent years — "this is a reminder that we cannot let our guard down," Schuchat said.

Before the measles vaccine became available in 1963, the virus infected about 500,000 Americans a year, causing 500 deaths and 48,000 hospitalizations.

Cases this year have been reported in 18 states and New York City. Ninety percent have been among people who have not been vaccinated or have unknown vaccination status, according to the CDC. Most of the patients report religious, philosophical or personal reasons for avoiding vaccines.

In Ohio, all the cases this year have been in members of the Amish community, said Melanie Amato, public information officer for the state Department of Health. Amish missionaries brought the virus back from the Philippines, the department said.

Though vaccination rates among the Amish were low, members of those communities are lining up to get vaccinated now, Amato said. The state has shipped out more than 13,000 doses of measles, mumps and rubella vaccine to deal with the outbreak, and more than 8,000 doses have been used, she said.

Amato said there's no sign the momentum of the outbreak has been broken: "We don't think it will be over any time soon. We're looking for this to continue into the summer," she said.

The most cases in the USA — 100 — have been in Knox County, Ohio. "The Amish population in Knox County has been very cooperative in getting vaccinated or self-reporting (if they have symptoms) or staying home if they do get the measles," county Health Commissioner Julie Miller said in a statement.

She said, "We've been getting calls from people who want to know if it is safe to travel to Amish country. The easy answer is yes if you've been vaccinated. And if you haven't been vaccinated, you should be, regardless of where you are going."

Measles is seen so infrequently today that doctors may not always recognize symptoms. Those include fever, cough, runny nose, red eyes, sore throat and a red rash that appears after three to five days. The virus is highly contagious and spreads through the air by breathing, coughing or sneezing.