A deadly bacterium found in a large group of stranded porpoises

Lonneke IJsseldijk (left) and his colleague Darryl Leydekkers examine one of the porpoises left behind.Credits: Bas Niemans, Utrecht University

Last summer, within just 10 days, about 190 porpoises were washed ashore on the islands of the Wadden Sea in the Netherlands. Considering the annual average of about 600 stranded porpoises across the Dutch coast, this is a very high number. In the past few months, researchers at Utrecht University and Wageningen University & Research (consigned by the Ministry of Agriculture, Natural Foods and Quality) have investigated the possible causes of death for these porpoises. Their most notable finding is the presence of Erysipelothrixrhusiopathiae, a bacterium that can cause sepsis in cetaceans.

Of the 22 porpoises investigated, 16 were adult females. “All porpoises were in a similar degradation state, so they are very likely to have died at about the same time,” explains Utrecht University biologist Lonneke IJsseldijk.

“Sudden death seems obvious”

Harbor porpoises were tested (especially) for their physical condition, the presence of parasites, and blood samples were taken. This was followed by laboratory studies on the contents of the stomach and the presence of algal toxins and possibly deadly microorganisms. This generally showed that the porpoise was reasonably healthy before it died. IJsseldijk said, “They have a healthy fat layer, relatively few parasites, and many adult females have recently become pregnant. Sudden death It seems natural. “


Despite being relatively healthy, no prey debris was found in the stomachs of 11 porpoises. Others had only a small amount of food in their stomach. “Therefore, the animals involved in this mass stranded were hungry for some time before they died,” says Mardik Leopold, a marine biologist at Wageningen University and Research. “This is consistent with a severe acute illness.”


Specific bacterial strains were found in more than three-quarters of the porpoises investigated: Erysipelothrixrhusiopathiae.These bacteria occur in almost all Animal species Can cause erysipeloid: A common infection in pigs and turkeys. The disease can also occur in reptiles, fish and humans. In porpoises, bacteria have been found to be present in multiple organs, indicating sepsis.

Harbor porpoises can be infected with Erysipelothrix via infected water, infected fish, or intercommunication. According to IJsseldijk, it is unclear if and how this resulted in the deaths of dozens of porpoises. It is not known if this bacterium normally occurs in porpoises, but the fact that this bacterium has never been found in porpoises during autopsy is certainly noteworthy. “

Exclude options

“Overall, we were able to rule out many possible causes of death through this study,” says Leopold. “You can rule out sudden mass mortality from underwater explosions, offshore wind farms, climate change, and overfishing. However, special weather conditions may have contributed to mass stranded animals, which are far away at sea. Must have died in. “

Nevertheless, there are still some open endings. Researchers at Erasmus University are still investigating the possible existence of the virus. And a small amount of saxitoxin (STX) was found in the porpoise. STX is a powerful biological toxin produced by algae that affects the functioning of the nervous system. Due to the poor condition of the stranded porpoise, it is still unclear what the STX concentration was just before death and whether this concentration was high enough to have affected mass mortality.

Food quality and health that have been shown to be essential for successful breeding of porpoises

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A deadly bacterium found in a large group of stranded porpoises

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