Senckenberg Research

Whales and Parasites

  

 

 Diving Minke whale
Diving Minke whale (Balaenoptera bonaerensis; Fam. Balaenoperidae) as a potential definitive host for Anisakis in the Southern Ocean (Antarctica).

Anisakis is a parasitic marine genus of nematodes that can infect humans through the consumption of fish. The primary hosts of this species are whales. Scientists are now engaged in research on the distribution of the Anisakis species. This will enable them not only to draw conclusions about regional infection risks, but also about the migratory behaviour and nutritional ecology of the whales.

Parasitism is a survival strategy by which one organism uses another as its habitat, and also as a source of nutrition. There are many examples of this form of existence that come from greatly varied classes of organisms. Typical of parasitic organisms are their diversity and their tendency to reproduce quickly and in large numbers, thereby enabling them to adapt to changes in environmental conditions.

In fact, researchers suspect that all other organisms are used as a host at least once by a parasite or one of its intermediate life stages. In aquatic and terrestrial biotopes, a wide range of systematically differing parasites can be found in various trophic stages, so that parasites contribute significantly to the biodiversity of a given ecosystem. Parasitism is regarded as the most successful survival strategy on our planet, and it developed amongst free-living organisms at a very early stage of evolution.

Leaping dolphins
Leaping dolphins (Delphinus delphis; Fam. Delphinidae) off
the coast of Namibia.

Fish are of great significance in view of the central role they play in aquatic ecosystems and also as providers of high-quality food, and they are therefore regarded as important subjects for parasitological investigations. Over 32,000 different fish species have been described throughout the world, of which about half live in the sea. Parasitic infestation of fish intended for human consumption has long been recognized as a problem. A television broadcast in 1987 (Monitor, ARD) which reported on live nematodes being found in ready-to-eat fish products led to the so-called nematode scandal, and the headlines throughout Germany had dramatic effects on the German fisheries industry. In the aftermath of this event, it became clear that basic knowledge concerning the existence and spread of parasites that are potentially harmful to humans was missing. Especially in the aquatic domain, the state of knowledge concerning the global diversity of medically relevant species is very inhomogeneous. As a response to this, researchers from the BiK-F Medical Biodiversity and Parasitology project division are investigating the wide range of medically significant parasites, their distribution and population dynamics, their ecology, genetics and life-cycle strategies, as well as the effects they may have on human health.

The Anisakis nematode can also affect humans

Professor Sven Klimpel and the biologist Thomas Kuhn are currently engaged in a project investigating the causes and distribution of the disease known as anisakiasis, which is brought about by nematodes of the Anisakis genus that parasitize whales. Together with the species of the closely related genera Pseudoterranova and Contracaecum, this group of marine parasites is found most frequently throughout the world. They are considered to account for most zoonotic infections (i. e. diseases transmitted from animals to humans). Although the basic life cycles of these species is known, the precise mechanisms by which they are transmitted within the aquatic-marine food web is still being intensively researched. It is undisputed that in the course of its complex life cycle, Anisakis undergoes four moults and larval stages while transferring between several hosts that represent the entire food web. The definitive hosts (i. e. the hosts in which the parasite reproduces) are whales, dolphins, and porpoises (Cetacea) that become infested with these parasites by consuming a wide variety of intermediate hosts (cephalopods, crustaceans and fish) that are themselves infested. Humans are not included in this life cycle, but may become so-called incidental hosts through the consumption of living larvae (e. g. in fish fillets), thus becoming temporarily infested with the parasite. When living parasites in various stages are consumed, there is a risk of the human gastro-intestinal tract becoming infested. The infestation is accompanied by clinical symptoms such as severe abdominal pain, nausea, diarrhoea, vomiting and fever. Some patients even suffer from severe allergic reactions.

An Icefish liver (Chaenocephalus aceratus)
An Icefish liver (Chaenocephalus aceratus)
heavily infested with anisakid nematodes.

Fish liver with Anisakis larvae.
Fish liver with Anisakis larvae.

Most incidences of infestation affect people living in coastal states of Europe (the Netherlands, Germany, France, Spain) as well as the United States and Japan, and also population groups who are dependent on fish and other seafood as an important source of protein (including the so-called developing countries). It is estimated that over 60% of the world’s population supplies about 40% of its protein needs through fish products. The reported global figures for human infestations with Anisakis or their nearest relations (within the Anisakidae family) are increasing and currently amount to 25,000 per year. However, the actual infestation rate may be a great deal higher, as the symptoms can be confused with other gastro-intestinal conditions.

Infestations in humans are only rarely lifethreatening, and thoroughly cooking or deepfreezing edible fish kills parasites in both the flesh and entrails. Nevertheless, we may expect the figures to increase as nutrition habits change (such as the tendency not to overcook meals, for instance), and infestation will no longer remain restricted to coastal states or regions with a cultural tradition of eating raw or only partly cooked fish (e. g. sushi, sashimi).

Research icebreaker RV Polarstern during unloading in the Antarctic shelf ice
Research icebreaker RV Polarstern during unloading in
the Antarctic shelf ice (to provide the Neumayer station III).

It is only within the last 20 years that developments in molecular biology have made it possible to split up what was considered to be a single species occurring throughout the world, Anisakis simplex, into several species. These species have specific definitive hosts of the whale order, and now nine species have been described for the genus. Their ecology and genetics differ greatly, although their external appearance is practically identical (such species are known as ‘cryptic’ species). Anisakis plays a role as an integral part of the marine food web, with closely interwoven trophic interactions between the intermediate and definitive hosts. For this reason, it is possible to use these nematodes as natural indicators through which conclusions can be drawn with respect to the definitive host’s habitat, which in turn supplies information about migratory behaviour or food ecology. The existence of morphologically very similar, but ecologically distinct and taxonomically discrete species in the sea probably provides an explanation for the unequally distributed occurrence of anisakiasis in different countries and regions, and underlines the significant influence of climatic zones on these parasites’ zoogeography.

In order to be able to obtain more precise information about the distribution of Anisakis and the likelihood of anisakiasis occurring in certain regions, Klimpel and Kuhn have adopted the approach of combining various aspects of existing methods for calculating distribution areas within a new and comprehensive model. To this end, they are using existing data concerning the occurrence of Anisakis together with modern molecular biological techniques for species identification. By studying the differences in various nuclear- coded and mitochondrial genes, they are attempting to achieve exact identification of the cryptic Anisakis species. The aim is to obtain a data basis for their calculations that is as reliable and comprehensive as possible. The model results clearly indicate the existence of species-specific distribution areas within the climate zones and oceans that can unequivocally be associated with the distribution and migratory behaviour of the respective whale species serving as a definitive host. The studies provide a means of assessing anisakiasis infestation risks, and also of obtaining information regarding the occurrence and population of certain whale species in clearly defined regions and climate zones.

 

Authors

 

Prof. Dr. Sven Klimpel

Prof. Dr. Sven Klimpel has headed the Medical Biodiversity and Parasitology project division since 2010. He studied biology at the University of Kiel and the Helmholtz Centre for Ocean Research (previously IfM-Geomar). In 2003, he obtained his doctorate at the Institute for Zoomorphology, Cell Biology and Parasitology of the Heinrich Heine University in Düsseldorf. There, he was Head of the ‘ Terrestrial and Marine Parasitology‘ research group from 2004 to 2010, first as Assistant Professor and then, following his habilitation (2008), as Associate Professor. In addition to this, he was the Interim Director of the Senckenberg German Entomological Institute until May 2013.


 

Thomas Kuhn

Following training as a biological technical assistant, Thomas Kuhn studied biology at the Heinrich Heine University in Düsseldorf, majoring in parasitology, zoology and genetics. Since 2010, he has been pursuing a Ph. D. course in the Medical Biodiversity and Parasitology project division under Prof. Dr. Sven Klimpel. His area of study involves the distribution of and interrelationships between human-pathogenic, marine nematodes that he has collected from a variety of fish species in the course of various fisheries research trips.

 

 

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