Virus epidemics of 1988, 2002 and 2007

In 1988 and 2002, the harbour seals in the North Sea and wadden region were infected with a virus. The virus had been unknown till 1988. Almost 52% of the population of harbour seals in the Dutch Wadden Sea were killed by this virus epidemic. In the spring of 1989, there were a little over 400 Dutch seals alive. The situation in Germany and Denmark was even more serious, where approximately 80% had perished. The number of seals counted in the whole wadden region after the epidemic had ended was 4500 specimen.

Studies showed that the disease in 1988 was caused by a virus from the group Morbilli viruses. These viruses are related to those which can also infect dogs. It was later discovered that several harp seals, which had wandered into the Skagerak and the Danish wadden region, were the carriers of the disease.

The first recovery began quickly, with the birth of many young pups in the summer of 1989. The autumn count in 1989 was 535 Dutch seals. The population kept growing till the summer of 2002, when the next epidemic took place. Before the disease had taken affect in 2002, the population had grown to 20,975 for the whole region, of which 3600 were counted in Dutch waters. All in all, the seal population had recovered much more quickly than one had ever dared to hope. The virus epidemic probably caused a severe natural selection: only extremely vital animals with a high resistance and a good reproduction could survive.

From June till December 2002, the harbour seals in the North Sea waters suffered a second major virus epidemic. This virus was closely related to the morbilli virus from 1988. However it was now referred to as phocine distemper virus, or PDV. It is an extremely contagious disease affecting the seals' immune system. PDV spreads through coughing and has an incubation period of 10 to 14 days. Most of the contaminated animals died either from the disease or from secondary infections which the weakened animal caught.

The virus is not dangerous for people, but it is contagious for dogs. If dogs are vaccinated for rabies then they are also protected from PDV. If you find a sick or dead seal, it is advisable to stay away from it and keep dogs leashed and far away. In addition to the PDV virus, dead animals can carry other contagious germs, which could affect the health of man as well.

Just like the epidemic in 1988, the first contaminated animals in 2002 were also found in Denmark. It did not reach the Netherlands until the end of June. The Dutch population was estimated at 6000 seals at the beginning of the epidemic. 2240 eventually died. The epidemic ended for the entire wadden region in December 2002. The total number of seals found in the summer of 2003 in the Wadden Sea were 10,800: 1160 in Denmark, 4235 in Sleeswijk Holstein, 3050 in Niedersaksen and 2365 in the Netherlands. By 2004, the population had increased to 12,803. Once again, this shows that the harbour seal population is able to recover quickly.

The epidemic was also active in other countries. In the United Kingdom and Ireland, around 3540 animals had died by December 2002, including at least 540 grey seals. In the Netherlands, death among the grey seals was not greater than normal. In Belgium and France, around 20 dead seals had washed ashore.

Successive epidemics of a virus disease are usually caused by a virus which differs slightly from the causer of the previous epidemic. Therefore, one speaks of a family or group with varying roots. One can compare this to families and species of plants and animals, the difference being that the creation of new virus species occurs much more rapidly. The appearance of varying roots makes it much more difficult to get insight in an epidemic and have time to combate it. A well-known example is the various forms of flu epidemics by people.

In 2007, another virus epidemic threatened when dozens of dead seals in Denmark were found. Seals in sanctuaries were vaccinated as a precautionary measure. After two months, it turned out to be a different virus. In addition, the damage remained limited to around 100 dead animals in the vicinity of the Kattegat.

Source: de Vleet, Ecomare