A measles poster is seen at Venice Family Clinic in Los Angeles, California, Feb. 5, 2015. Lawmakers in several U.S. states are backing proposals to make it harder for parents to opt out of school vaccinations based on personal beliefs, as health officials fight a growing measles outbreak that has sickened more than 140 people in 17 states. Reuters

Scientists may have pinpointed the origin of the U.S.’ ongoing measles outbreak after lab tests showed that the virus linked to California’s Disneyland theme park is genetically very similar to the type usually found in the Philippines, where measles is a much larger public health problem affecting tens of thousands of people per year. Officials haven’t been able to trace the U.S. outbreak that has infected dozens of people across several U.S. states to any one individual, however the findings, announced Tuesday, suggest the virus probably came from a person or family who visited the theme park from overseas and not from immigrants coming to the U.S. illegally, as some have claimed.

Health officials earlier this month genotyped the virus from 30 California measles patients and found that all specimens were measles B3, which was recently responsible for a large outbreak in the Philippines. "The genotype is really the genetic fingerprint of the virus, and so it certainly tells us the number of cases that we have been seeing in California likely all relate to that individual that began at Disneyland," Matt Zahn, medical director of epidemiology for Orange County Public Health, told the Los Angeles Times.

Measles genotype B3 has also been found in at least 14 other countries in the last six months, meaning health officials cannot say for sure that the U.S. outbreak came from the Philippines, although the likelihood is much higher given the scope of the Philippines’ measles problem, according to the U.S. Centers for Disease Control and Prevention (CDC.) Nearly 24 million people walk through Disneyland’s turnstiles every year, many of them international travelers coming from countries where measles hasn’t been eradicated.

In the Philippines alone there were over 21,000 confirmed cases of measles, 36,000 suspected cases, and 110 deaths in 2014. Last year, 25 U.S. travelers, most of whom were unvaccinated, who returned from trips to the Philippines became sick with measles, the CDC reported in January.

As of this week there have been a total of 141 confirmed cases of measles in the U.S. across 17 states since the first cases were reported in early January, the CDC reported Tuesday. Of the outbreak’s first 34 measles patients, only five had received two doses of the measles vaccine, highlighting the growing trend of parents opting not to vaccinate their children because of their religious or personal beliefs, according to the California Department of Public Health. California’s measles outbreak has sparked a national debate over vaccinations, with some parents arguing that the vaccines themselves could cause health complications in their children while public health officials maintain that the vaccines are harmless and are the best defense against possibly deadly outbreaks.