Researchers trace dynamics of measles epidemics

December 12, 2001

University Park, Pa. - Infectious diseases and epidemics are the nightmares of bioterrorism, but a team of researchers is investigating historic measles epidemics to better understand the dynamics of disease spread.

"With bioterrorism today, there is enhanced concern about epidemics and containment," says Dr. Ottar N. Bjornstad, assistant professor of entomology and biology. "But, recent outbreaks of animal diseases like foot and mouth disease showed the incredible intense efforts necessary to obtain spatial containment. Even under normal circumstances, influenza travels around the world almost every year. Infectious diseases are still a major problem in all parts of the world." Bjornstad and Bryan T. Grenfell and Jens Kappey, University of Cambridge, U.K., studied the childhood disease measles because it is a classic airborne virus and one of the most contagious diseases known.

"Before 1967 when a vaccine was introduced, measles was an economic problem causing fatalities and severe complications in survivors," Bjornstad says. "Measles still kills more than a million people per year in the Third World."

Because of the contagious nature of measles and the periodic epidemics, by the middle of the 20th century, most Western countries required that physicians report cases to the government. Beginning in 1944, weekly information from every location in Great Britain was collected centrally, creating a very complete database of the epidemics. In general, before vaccination, epidemics occurred every two years.

"We wanted to understand to what extent an epidemic in a given city or village is independent of neighboring areas or regions," says Bjornstad. "To what extent different cities are coupled epidemiologically."

The researchers used new statistical techniques that allowed them to pinpoint for each time series, where a municipality was in the two-year cycle of each epidemic. For example, if the statistics showed that in the 35th week of the epidemic, London reached its peak number of cases, then the researchers could look at neighboring areas and see that they were still weeks away from their peaks. The team could then plot the rise and fall of an epidemic through time and space.

"We can see which areas lead the epidemic and how the contagion spreads," says Bjornstad. "The dominant waves were those associated with London and the northwestern industrial center, and a hierarchical spread of infection is a consistent feature," the researchers reported in today's (Dec. 13) issue of the journal Nature. "An interesting feature of the pattern is that, although London - the biggest city - leads the epidemics in surrounding areas, the major epidemic appears on average 4 to 6 weeks before London in the urban northwest, moving from Liverpool and Manchester into the north Midlands." The epidemics are seasonal, corresponding to the school year. Large cities catch on more rapidly than small ones because they have more susceptible individuals. Measles confers permanent immunity, so each epidemic removes large numbers of people from the susceptible pool. From the large cities, the disease moves outward in waves at about 5 kilometers per week, probably carried by school children visiting family or friends. The researchers found that very isolated communities could be experiencing their measles bout on the off epidemic years, falling out of the biennial pattern.

According to Bjornstad, measles are almost, but not quite, eradicated in the British Isles with less than 1,000 cases per year. Previous to vaccination, there could be as many as half a million cases per year. Vaccination reduced the susceptible population so low that even in the largest cities, the disease can completely fade out. Subsequent flare-ups of measles require reinfection from an outside source, creating a checkerboard of disease and completely decoupling the biennial cycle and the sequential waves from large cities.

Measles belongs to an ecological class of diseases that includes all the traditional childhood illnesses - mumps, rubella, chicken pox, whooping cough - that are extremely contagious, but short-lived in the air. Smallpox, before eradication, was considered one of these diseases as well. Others that fall into this category are Ebola and, to a smaller extent, influenza, although because influenza mutates so rapidly, each year brings a slightly different virus to infect even those who have contracted previous strains.

"Anthrax is not in this category," says Bjornstad. "Containment of anthrax is a matter of containing spores. Containment of communicable diseases is a matter of containing people. Human quarantine is the method of preventing widespread Ebola epidemics, for example."

Penn State

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