"Little is known about epidemics in the wild, and how they affect the long-term survival of populations," said Harding.
The 1988 epidemic took the lives of at least 18,000 seals. Mortality this year has been slightly lower in some regions because the infection began to strike populations five weeks later in the year than in 1988.
The risk of infection is highest during the summer months when harbor seals gather in groups on land; therefore, the earlier an epidemic begins, the more damage it can cause.
However, the total seal population in 2002 was larger than in 1988, so more seals have died this year. Computer models indicate that in some seal colonies more than 50 percent of the seals have succumbed to the infection.
The findings also suggest that if epidemic outbreaks occur at 14-year intervals and fewer than 80 percent of the animals perish, then each seal population has a chance to recover. However, if an epidemic was to recur in the next seven years, it could potentially push European populations to the brink of extinction.
Most harbor seal populations will probably continue to grow if epidemics come only every 14 years or so, agreed Paul Jepson of London's Institute of Zoology in England. "However," he added, "we'd have reason for concern if the frequency of future epidemics increases."
Prior to the 1988 epidemic, harbor seal populations were still recovering after over-hunting for fur, meat, and oil in the 19th and early 20th centuries had drastically reduced their numbers.
English Harbor Seals Threatened
Though the disease has now subsided in much of Europe, dead seals continue to be found at a high rate in England and Scotland, said Ian Robinson, veterinary manager at the Wildlife Hospital of the Royal Society for the Prevention of Cruelty to Animals located in Norfolk, England.
Robinson worries that the consequences could be even more severe for the small population of seals found on the eastern coast of England.
"The population growth rate [here] is one hell of a lot lower than the one used in the model," said Robinson.
Though Swedish seals recovered rapidly from the 1988 epidemic, English harbor seals had barely returned to pre-epidemic levels 12 or 13 years later. Another epidemic anytime soon might put the "extremely threatened" colony at a very high risk of extinction, said Robinson.
However, most of the United Kingdom's harbor seals are found in Scotland, said Robinson, and these colonies have experienced much lower death rates.
Other species of seal, such as the gray seal, Halichoerus grypus, which travels extensively, and the arctic harp seal, Phoca groenlandica, may be carrying PDV between different populations of harbor seals. Both species are far less susceptible to the infection than harbor seals.
One of the most intriguing aspects of this year's epidemic is that it was first discovered in Anholt, said Harding. The Danish island was home to the first colony infected in 1988.
"We don't know why," she said, "but it can't be a coincidence."
SOURCES AND RELATED WEB SITES