Dog virus seals fate of European seals.

Dog virus seals fate of European seals

Over the last five months, harbor seals in the North and Baltic Seas have died in vast numbers from unknown causes. The deaths have occurred in Scandinavian, German and Dutch waters, and the British population of seals, the largest in Europe, now appears threatened. Although scientists have isolated two viruses from dead seals, they have been unable to establish a causal link to the fatal disease. Others have blamed persistent pollutants, such as PCBs or dioxins -- which are known to affect immune response -- in the deaths of an estimated 8,000 to 9,000 seals.

Now a Dutch research group says it has discovered the apparent cause: another virus, canine distemper virus (CDV) or a close relative. CDV belongs to the morbillivirus family, which includes the human measles virus. The work was led by Alfred D.M.E. Osterhaus of the Dutch National Institute of Public Health and Environmental Protection in Bilthoven and ElizabethJ. Vedder of the Seal Orphanage in Pieterburen.

Osterhaus and others initially isolated a herpesvirus from the lungs and other organs of 8 of 35 dead seals from the afflicted areas, and a picornavirus from the lungs of 20 of 22 seals investigated. In a letter in the July 28 NATURE, he proposed that one or both of these viruses "probably caused" the outbreak.

But when the scientists attempted to confirm the hypothesis with blood studies, they found "no correlation" between disease symptoms and a rise in antibody levels against either virus -- although virus-neutralizing antibodies were present in the blood of the seals coming from afflicted waters. On this basis, and after failed efforts to immunize young seals in the orphanage, the scientists searched for other viruses.

Meanwhile, Swedish workers at the National Veterinary Institute in Uppsala observed that tissue damage in diseased seals resembled that in dogs with distemper, leading the Dutch to test for CDV. Of 23 wild seals brought to the orphanage and tested between July 3 and Aug. 8 -- all apparently healthy and without CDV antibodies -- 22 developed disease symptoms and CDV antibodies within 14 days, Osterhaus and Vedder report in the Sept. 1 NATURE. These and further blood studies on various seal populations -- gray seals living in the orphanage during the outbreak who survived the disease, and harbor seals from Denmark and Germany at different stages of the disease -- "clearly show" that the primary cause is CDV or a close relative, they say.

Further evidence implicating CDV comes from the similarity of the seal and canine symptoms: respiratory, gastrointestinal, skin and central nervous system ailments and secondary infections.

The Dutch scientists are trying to isolate the virus and develop a vaccine. A preventive vaccine for use in seal sanctuaries is urgently needed, though chances of protecting the wild harbor-seal population are remote, the researchers say. Osterhaus told SCIENCE NEWS he thinks the canine vaccine is "quite likely to be effective in seals," but "the use of live vaccines is not generally advisable in wild animals, because you don't know what it's going to do once it starts spreading in animal species."

Some speculation concerning the epidemic's origin centers on the virus' possible spread from dogs to harbor seals. There is also "some concern that overall ocean degradation may be playing an important role in the seal deaths in Europe," sayd Douglas B. Inkley of the National Wildlife Federation in Washington, D.C. "Exactly what the causes and links are is difficult to assess, but you have to look beyond the virus and ask what's making the seals susceptible. Widespread epidemics like this are not the norm."