Marine parasites of humans

Human marine parasites are parasites that live in seawater and can attack humans .

Well over 100 and probably over 200 species of eukaryotes infect humans. By far the most important are parasites, whose life cycles take place on land or in freshwater. However, there are numerous species of marine parasites that can infest humans, although not a single one of these species requires humans as a mandatory host. The individual infections, including the symptoms and preventive measures, are discussed by various authors in the standard work published by Rohde (Rohde 2005), on which this article is largely based.

Helminths (parasitic worms)

The largest number of types of marine parasites found in humans are helminths , i. . parasitic worms. These include roundworms (Nematoda), flukes (Trematoda) and tapeworms (Cestoda). The characteristics of the groups that are important for this contribution are as follows:

Most nematodes are sexually separated and go through four larval stages (L1 – L4) until sexual maturity is reached in the fifth stage. The stages result from a total of four skins, ie shedding the old cuticle and forming a new one. In some cases the cuticle of the previous stage remains as a sheath for a time.
Adult trematodes are generally hermaphroditic (that is, both male and female) and produce eggs in which an eyelash larva ( miracidium ) develops. This larva has to penetrate into intermediate hosts ( molluscs ), where it develops into a sporocyst, which in turn produces daughter sporocysts or rediae . Tailed cercariae develop in these larvae and leave the intermediate host to encyst either outside of one host or in a second intermediate host. The encysted larvae are the metacercariae . They develop into sexually mature worms when they are eaten by vertebrates. Species of the Schistosomatidae family have no metacercaria, but the cercaria actively penetrate through the skin into the ultimate host.
All marine ancestors found in humans belong to the family of Diphyllobothriidae , which become sexually mature in marine mammals. Eggs are excreted in the feces. In them, the first larval stage, the ciliated developed Coracidium that enters copepods and the so-called Prozerkoid developed larva. When fish eat infected copepods, a plerocercoid develops in their muscles. Mammals become infected by ingesting infected fish.

It is not uncommon for additional, so-called transport hosts (paratenic hosts) to be included in the life cycle of helminths, in which larvae survive but do not develop further.

Nematodes

Aniseed

Nematodes are especially important because they can cause the most severe symptoms. So cause larval roundworms family Anisakidae the syndrome of Anisakiose . Infection occurs through consumption of undercooked or raw fish or squid . The most common responsible species is Anisakis simplex (Anisakis type I larva). Even pseudo decipiens is not rare. Anisakis physeteris , Contracaecum osculatum and Hysterothylacium aduncum , on the other hand, rarely occur in humans.

The life cycle of Pseudoterranova decipiens ("seal" or "seal worms") is as follows: Adult worms live in the intestines of seals. Eggs are excreted in the stool and sink to the sea floor. The development in the eggs goes up to the third larval stage. The 0.2 mm long larva then leaves the egg, but remains in the cuticle of the second larval stage, which serves as a sheath. The larvae attach themselves with the tail end to the substrate and are eaten by various small crustaceans ( copepods , juvenile amphipods and hover shrimp ). The larvae now break through the vagina and invade the host's haemocoel . Copepods are eaten by larger macro-invertebrates (sexually mature amphipods, hover shrimp, multi-bristle ), in which the larvae can reach a length of more than 8 mm. Small fish become infected when they eat macro-invertebrates that have been infected in this way. Larger fish become infected when they eat invertebrates or smaller fish. Larvae (still in the third larval stage) penetrate the intestinal wall of the fish and grow up to a length of 30 to 60 mm in the muscles. Finally, seals become infected when they eat infected fish or infected macro-invertebrates. The last two moults take place in the seal's gut. Humans are not necessary for completing the life cycle. They play the role of a transport host for the third larval stage.

Species of the genus Anisakis do not use seals as final hosts, but whales, and are therefore also called "whale worms". The symptoms of anisakosis vary greatly depending on which organs or tissues the larvae are in. One speaks therefore of stomach, intestinal and extra gastrointestinal anisakosis. There is also an acute and chronic form. Acute gastric anisakiosis is primarily characterized by abdominal pain that sets in quickly (two to seven hours after infection), often accompanied by nausea and vomiting. In chronic mild gastric anisakiosis, the pain is less severe, but can last for over two years if the parasites are not surgically removed. Almost all cases of intestinal anisakiosis are acute, usually with severe abdominal pain, vomiting, constipation, and diarrhea. In most extra-gastrointestinal cases, symptoms are mild and localized.

Infection can easily be prevented by cooking the hosts, or by freezing (−20 ° C for one or more days).

Trichinellosis

Trichinellosis is caused by infections with species of the roundworm Trichinella . Serious infections can lead to death and were not uncommon in Germany in the 19th century, for example. The most common mechanism of infection is eating pork that has not been thoroughly cooked. Sexually mature worms only live briefly in the small intestine, where they copulate and produce live larvae that enter the blood system. First instar larvae (i.e., before molting) encapsulate in striated muscles. Some genotypes live freely in muscles without forming capsules. Marine mammals are only infested with genotype 2, the species Trichinella nativa . The species is freeze-tolerant, in contrast to the forms transmitted through pork. Cases of Trichinella in marine mammals are restricted to the circumpolar Arctic. Polar bears (60% infection rate in some areas), walrus (also up to 60%) and, less often, whales and seals are carriers.

Trichinella acquired from marine mammals are important pathogens in humans in the Arctic. Walrus and then polar bears are the main sources of infection. The symptoms mainly depend on the strength of the infection. It is important that even freezing at a temperature of −20 ° C for four years did not kill all larvae.

Angiostrongylus infections (rat lungworm)

The rat lungworm Angiostrongylus cantonensis infects various rodents, including rats, as “normal” final hosts. The female reaches a length of about 33 mm, the male is much smaller. The parasite is not a true marine parasite, but uses land and freshwater animals in its life cycle, but some marine invertebrates serve as transport (paratenic) hosts that can transfer the parasite to other hosts in which, however, no development takes place. Humans are abnormal hosts who “host” third instar larvae. In general there is no development to sexual maturity in humans. The parasite was originally restricted to the Indo-Pacific region but is now found in many tropical and subtropical regions through introduction.

Sexually mature worms live in the pulmonary arteries of rodents (also in the right ventricle if the infection is strong). They produce eggs from which larvae hatch in the pulmonary arteries and capillaries, which travel up the trachea, are swallowed and excreted with the feces. Land and freshwater mollusks eat the larvae or become infected through external invasion of the larvae. In a few weeks, the larvae develop to the third stage, which is infectious to rodents. The eaten larvae reach the surface of the brain in the rodent's blood or nervous system. After a few weeks, the larvae migrate into the pulmonary arteries.

Various brackish water and marine fish as well as invertebrates could be infected experimentally. Aquaculture shrimp belonging to the Palaemonidae and Penaeidae families are probably the main transport hosts in the sea, and oysters and sea clams ( Mercenaria ) are suitable intermediate hosts.

The syndrome caused is eosinophilic meningoencephalitis (or eosinophilic meningitis ), an inflammation of the meninges with accumulation of eosinophilic white blood cells. In extreme cases, insanity and death can result. Very severe headache, paralysis, vomiting, and fever are just a few of the other possible symptoms. Freezing and heating kill the larvae.

Schistosomatidae

Schistosoma are the known pathogens of schistosomiasis that are spread via freshwater.

Larvae of various other species of the Schistosomatidae trematode family , which generally infect birds and (non-human) mammals, try to penetrate human skin in certain bodies of water (see also bathing dermatitis ). Since humans are abnormal hosts, they die without reaching sexual maturity, but can often cause severe inflammatory reactions. In the sea, species of the genera Austrobilharzia , Ornithobilharzia and Gigantobilharzia and probably other unidentified genera are responsible. They all have birds as "normal" final hosts. Austrobilharzia ( A. terrigalensis , A. variglandis ) with numerous mollusc species as intermediate hosts is of particular interest .

Infections mainly occur in calm coastal waters such as lagoons or estuaries, which is explained by the behavior of the schistosome larvae. The larvae of the schistosomes swim to the surface of the water and wait there until a suitable host appears. They then penetrate the skin, often causing tingling irritation that lasts for about an hour. Also, urticaria (hives) other skin rashes and reactions that may develop into fluid-filled blisters, are characteristic. Secondary infections are possible, and lesions are often pigmented and can last ten days or even weeks or months. First infections generally lead to only weak reactions, repeated infections are much more serious and can also cause general symptoms such as fever and edema.

Orally ingested trematodes (flukes) and cestodes (tapeworms)

Humans become infected with a large number of species by ingesting larvae (metacercariae of flukes, plerocercoids of tapeworms) with infected, undercooked marine animals. Raw, salted, marinated or poorly cooked fish and invertebrates such as mollusks are traditionally very popular delicacies in many countries (including Korea, Japan, Hawaii and other Pacific islands). Sushi and sashimi , originally Japanese delicacies but now popular worldwide , are widespread .

Trematodes transmitted by marine or brackish aquatic animals use gastropods (snails) as the first, and molluscs or fish containing the metacercariae as second intermediate hosts , without exception . Infections are restricted to estuaries and coastal areas, and all species are small. With a few exceptions, the species belong to the family Heterophyidae (more rarely also to the Echinostomatidae and Gymnophallidae ). The infection rates can be very high. For example, in one Korean village, 75% of the population was infected.

Most species cause little or no symptoms, although the severity of the infection is sometimes enormous. Almost 70,000 worms of three types were found in a Korean patient who had only relatively mild symptoms: occasional slight epigastric pain, diarrhea, and poor digestion. On the other hand, worms have also been found in patients with serious symptoms such as pancreatitis , although it is not clear whether the parasites were the cause of the symptoms. In rare cases, the small worms also leave the intestinal tract and they, or more often their eggs, cause embolisms in the brain, spinal cord and heart, sometimes fatal.

Cestodes (tapeworms)

The tapeworms belong to the Diphyllobothriidae , most of the species to the genus Diphyllobothrium , some also to Diplogonoporus . Natural final hosts are whales and seals . Life cycles include copepods first, and fish (which harbor plerocercoid larvae, which are infectious to humans) second intermediate hosts. Transport hosts can also be involved. The most important species for humans are Diphyllobothrium pacificum and Dipogonoprus spp. Symptoms include diarrhea, abdominal pain, anorexia, and general weakness.

Protista (unicellular eukaryotes )

The species that infect humans in the sea belong to the Microsporidia (a division of the fungi in WP ) and Apicomplexa , and possibly to the flagellates .

Microsporidia occur in almost all phyla of the invertebrates and in five classes of vertebrates. More than 1,300 species have been described to date, but this is only a very small percentage of the existing species as most potential hosts have never been studied. The microsporidia are intracellular and form spores used for infection. At least 14 species in six genera (only some of them in the sea) infect humans, but most only when the immune system is weakened, as for example in AIDS patients. Pleistophora , for example, parasitizes cold-blooded animals, especially marine and freshwater fish, but has also been found in humans with defective immune responses. An increasing number of microsporidia similar to species in fish are found in immuno-incompetent humans. Fish and also crabs must be viewed as possible sources of infection.

Apicomplexa in the sea are exclusively coccidia . Infection occurs through ingestion of oocysts. Immune-deficient people are particularly affected. The species Cryptosporidium parvum (and related species of the same genus) causes cryptosporidiosis in humans. Oocysts are ingested with contaminated water and cause serious diarrhea. Although the species are typical freshwater organisms, infectious oocysts can be washed up in coastal waters and survive there for up to a year. Sea clams and oysters can accumulate the oocysts and the consumption of such potential carriers should therefore be avoided by immune-deficient persons.

The flagellate Giardia infects various vertebrates, including humans, usually also in fresh water. However, resistant dormant stages have been isolated from marine bivalves. The possibility of human infection through eating infected mollusks in the sea can therefore not be ruled out. Symptoms include severe diarrhea (Freeman in Rohde 2005).

swell

For more details, see the sections by Blair, Forbes, Freeman, McClelland, Nagasawa, Overstreet, and Walker in Rohde (2005).
Rohde, Klaus (ed.) (2005): Marine Parasitology, CSIRO Publishing, Melbourne and CABI Publishing, Wallingford, Oxon. xxiii +565 pp. ISBN 1845930533 : [1]