By April Frawley

As the Ebola outbreak in West Africa continues to spread, isolated cases will likely make their way to the United States, with new estimates showing there is a nearly 20 percent chance this will occur by the end of September, according to researchers from several institutions, including the University of Florida.

The analysis, published Sept. 2 in the journal PLOS Currents: Outbreaks, combined patterns of international travel with estimates of how fast Ebola is spreading to calculate how quickly the disease might move to different locations.

Although some unexpected cases in the U.S. are likely inevitable, researchers anticipate they will be quickly isolated, not reaching any sort of outbreak level currently found in West Africa. Currently, the U.S. has imported four cases knowingly for treatment.

“We would assume that the U.S. would have sufficient capacity to test people and treat them. We would not expect any real transmission in the U.S.,” said Ira Longini, Ph.D., a professor in the department of biostatistics in the UF colleges of Public Health and Health Professions and Medicine.

Initially, the disease is expected to spread to currently unaffected African countries, which further increases the likelihood of it spreading beyond the continent’s borders. There is not a high level of international travel among some of the affected countries, such as Liberia and Sierra Leone. But Nigeria, where the outbreak has also spread, is linked to many countries across the globe.

In fact, according to the study, as many as 6,000 passengers travel from Nigeria to the U.S. each week. In addition, many airline passengers go from Nigeria to other countries outside of Africa. Within the past week, Ebola also spread to Senegal, home to a major international hub in Dakar, its capital.

As the outbreak affects more metropolitan areas with international airports, the chances of unknowingly infected passengers bringing the virus with them to new locales is highly probable, said Longini, the co-director of the Center for Statistics and Quantitative Infectious Diseases at UF’s Emerging Pathogens Institute. Longini collaborated with senior author and physicist Alessandro Vespignani, Ph.D., of Northeastern University, and other colleagues on the study.

According to the World Health Organization, 1,841 people have died from Ebola in West African nations during the outbreak. In total, the WHO reports 3,685 confirmed and suspected cases of Ebola thus far. The epidemic is anticipated to reach 10,000 cases by the end of September, according to an unrelated report in the journal Science.

Ebola spreads from person to person through contact with an infected person’s bodily fluids. It can take anywhere from two to 21 days between infection and the onset of symptoms, according to WHO. Symptoms typically include a sudden fever, muscle weakness and sore throat, eventually progressing to include vomiting, rash and even bleeding. There is currently no vaccine for the disease and all treatments, beyond supportive care and rehydration, are experimental.

Although it seems logical to assume that stopping international air travel would stem the spread of the outbreak, the researchers’ analysis showed it actually would have little effect. For example, reducing air travel by 80 percent would only prevent Ebola from spreading temporarily.

“Studies have shown the quarantining of entire villages and countries is highly ineffective, and this analysis shows that yet again,” Longini said. “Surveillance and containment, which includes the isolation of cases and quarantine of close contacts, is the only intervention strategy that works that is available.”

The researchers describe the current risk that the virus will spread to additional countries as moderate, but as the outbreak continues to defy containment, these risks will continue to grow.

Longini and colleagues, who began analyzing the data this summer, are continuing to monitor trends and update their mathematical model.

The study was partially supported by the MIDAS (Models for Infectious Disease Agent Study) grant from the National Institute of General Medical Sciences.

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