In the month and a half since public health officials in California confirmed the first case of measles linked to Disneyland, the measles outbreak has grown to at least 182 cases in 18 states and the District of Columbia, according to the latest count by CBS News. But state and federal health officials are still uncertain of the exact source of the outbreak.
However, investigators from the U.S. Centers for Disease Control and Prevention now say there is new-found evidence that links the growing number of incidences in the U.S. to a sizable measles outbreak that recently occurred in the Philippines. The CDC reports that specimens from 30 California patients are a direct genetic match to the strain of the virus in the Philippines. Both are classified as measles genotype B3.
"It's basically a fingerprint of the virus," Melissa Stockwell, an assistant professor of pediatrics and population family health at Columbia University Medical Center, told CBS News.
This is not the first time the Philippines has been linked to diagnosed cases of measles in the U.S. Last May, one of the largest U.S. outbreaks in decades spread through Ohio, mostly among Amish communities, where childhood immunization is relatively uncommon. That outbreak began after several missionaries contracted the virus while volunteering in the Philippines, where the disease is still widespread, and then returned to the U.S.
"It was a perfect storm for having a logical outbreak," Stockwell said of that outbreak.
However, this new information about the Disneyland cases is not the end of the investigation. The same virus genotype has also been identified in at least 14 countries and six U.S. states in the last 6 months.
Molecular analysis of specimens is a critical part of outbreak investigation. A global measles sequence database at the Health Protection Agency in London tracks genetic sequencing of specimens from outbreaks. The data is also distributed to the World Health Organization.
There are eight different measles genotypes and each one has subgroups. The measles vaccine works against all strains of measles -- unlike the flu vaccine, which must be tailored each year to the strains of influenza that are expected to dominate flu season.
Measles is highly infectious and can be transmitted through a cough or sneeze. The virus can also live on surfaces for about two hours. Experts say a crowded place like Disneyland, packed with children and tourists from all over the world, would be an easy place for it to spread.
The latest measles outbreak in the U.S. first came to public attention on Jan. 5, 2015, when the California Department of Health received reports about a child who was hospitalized with symptoms related to measles. Parents had taken the unvaccinated 11-year-old to the doctor when the child developed a rash right before the new year. The child had recently visited Disneyland in Orange County, California. State health officials were also notified of four other suspected measles cases, and two more children were diagnosed in Utah.
Health officials confirmed that all of these children had spent time at the theme parks between Dec. 17 and 20, 2014. The outbreak continued to grow as a result of secondary and household exposure. However, doctors and families are still unable to confirm the source of exposure for a number of patients.
The CDC says it still hasn't been confirmed which reported case could be considered patient zero. Stockwell says public health officials will use this new finding about the genetic materials to work on contact tracing, and see if any of the patients in early weeks of the outbreak may have visited the Philippines or had contact with someone who was in the country.
Stockwell says this genotype link reinforces the need for vaccine campaigns not only in the U.S., but also abroad. According to the WHO, measles is still one of the leading causes of death among children worldwide. In 2013 there were 145,700 measles deaths worldwide, which is equivalent to about 400 deaths each day.
A little less than 85 percent of children worldwide received at least one dose of the measles vaccine, and in some regions the figure is lower, which causes a significant international public health threat and makes subsequent outbreaks likely.