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Parasitic Worms - Potential Hazards to Mariners

Introduction

Parasitic worms - correctly called helminths - that cause diseases in humans, are not related to earthworms, in spite of the appearance of some of them. Many different species are found in various parts of the world. Mariners may therefore be at risk, not only when they go ashore but also from those that parasitise some of the animals carried as cargo or as shipboard pets.

The life cycles of some helminths are complicated, requiring more than one host to develop from egg to worm and to egg again. The adult worms may be male or female, or both. The transmission from host to host may be through the soil, by species of insects or snails. There are over 20 species. Some parasitise the intestine, others the blood, muscles or other organs. Only the former are included here.

Self-diagnosis may be possible where the worms are passed in the stools. Otherwise microscopic examination of stools is necessary, possible at sea only if the ship's company includes a medical officer and a microscope.

Broadly speaking, there are two kinds of parasitic helminths that may infect the human intestine - round worms (nematodes) and tapeworms (cestodes).

No vaccines are available for the prevention of diseases (which, strictly speaking are infestations rather than infections) caused by these worms. Treatment is usually with tablets or powders. Some of these are available across the counter but a few are issued only under a 'named patient' prescription (See British National Formulary and Useful web sites).

Introduction

Parasitic worms - correctly called helminths - that cause diseases in humans, are not related to earthworms, in spite of the appearance of some of them. Many different species are found in various parts of the world. Mariners may therefore be at risk, not only when they go ashore but also from those that parasitise some of the animals carried as cargo or as shipboard pets.

The life cycles of some helminths are complicated, requiring more than one host to develop from egg to worm and to egg again. The adult worms may be male or female, or both. The transmission from host to host may be through the soil, by species of insects or snails. There are over 20 species. Some parasitise the intestine, others the blood, muscles or other organs. Only the former are included here.

Self-diagnosis may be possible where the worms are passed in the stools. Otherwise microscopic examination of stools is necessary, possible at sea only if the ship's company includes a medical officer and a microscope.

Broadly speaking, there are two kinds of parasitic helminths that may infect the human intestine - round worms (nematodes) and tapeworms (cestodes).

No vaccines are available for the prevention of diseases (which, strictly speaking are infestations rather than infections) caused by these worms. Treatment is usually with tablets or powders. Some of these are available across the counter but a few are issued only under a 'named patient' prescription (See British National Formulary and Useful web sites).

Round worms

Round worms (nematodes)

Ascaris, the common round worm

The adults, male and female, live in the small intestine, are between 10 and 30 cm long, are cylindrical and have tapered ends. They may occur anywhere in the world, but are found mainly in the tropics.

How does one become infested?

The female produces large numbers of eggs, which are passed in the stools. Infestation in humans may follow eating food, e.g. vegetables grown in soil that has been fertilised by human waste or by the hand-to-mouth route in insanitary and unhygienic conditions. The eggs hatch in the intestine and the larvae migrate via the blood system to the lungs and the throat back to the intestine where they develop into adult worms.

What are the symptoms?

While the larvae are in the lungs they may cause inflammation of the lungs, fever and laboured breathing. In the intestine the adults may cause no symptoms or only discomfort, but if they congregate in a mass they may cause a blockage of the bowels, causing vomiting, abdominal distension and pain. The worms may also block the bile ducts, leading to jaundice and other complications. Medical attention is then a matter of urgency.

Worms may be passed in the stools, but usually diagnosis is made by looking for the eggs in the stools under a microscope.

How is infestation treated?

Mebendazole tablets and Piperazine sachets are available without prescription and 100 mg of Mebendazole, twice daily for 3 days is usual but if there is reinfestation a second course is given after 2 or 3 weeks. A single 4-4.5 mg dose of Piperazine may be repeated once a month for 3 months if there is further risk of reinfestation.

Medical advice should be sought before either of these is given to pregnant women or infants. (See British National Formulary)

How to prevent infestation?

Good standards of food hygiene, proper cooking, hand washing and the avoidance of crops grown in places where human waste is used as manure should minimise the risk of infestation.

Pin worms and threadworms

Pin worms parasitise the lower bowel. They are about 1 cm long and resemble headless pins or small pieces of cotton thread. Although they commonly infect young children they may affect seamen who live in unhygienic conditions.

They occur mainly in temperate climes, mostly Western Europe and North America.

How does one become infested?

The females crawl out of the anus at night and lay their eggs on the surrounding skin and thus contaminate fingers, clothing and bedding. Disturbance, as in dressing or bedmaking, may release them into the air, when they may contaminate carpets, furniture, etc. They are transferred directly to the mouth from the skin by fingers and from contaminated bedding, etc.

Cramped quarters, poor ventilation, communal use of bunks and bedding and low levels of personal hygiene may all contribute to spread among seamen.

What are the symptoms?

There is an intense itching on the skin around the anus, where the eggs have been deposited.

Diagnosis is made by examining the stools for the worms and by looking for the eggs with a microscope. Doctors also use Sellotape to remove eggs from the skin for microscopy.

How is infestation treated?

A single 100 mg dose of Mebendazole tablets is usual but if there is reinfestation a second dose is given after 2-3 weeks. An alternative is one 4 mg dose of Piperazine, repeated after 14 days if there is a risk of reinfestation. All those sharing quarters with the sufferer may be treated.

Medical advice should be sought, however, before either of these is given to pregnant women or infants. (See British National Formulary)

How to prevent infestation?

Personal hygiene and frequent washing of clothing and bedding will reduce the possibility of infestation.

Hookworms

These round worms are about 1 cm long and attach themselves to the intestinal wall from which they extract blood.

They are widely distributed in S. Europe, Africa, S. and Central America, India, Sri Lanka and the Pacific islands.

How does one become infested?

The eggs are passed in the stools, hatch into larvae in the soil and enter the body usually through bare feet. The larvae migrate via the blood, lungs and throat to the intestines where they develop into adults.

What are the symptoms?

The loss of blood leads to anaemia. There may be breathlessness, headache and depression. Diagnosis is made by finding the eggs in the stools by microscopic examination.

How is infestation treated?

The recommended treatment is with 100 mg Mebendazole tablets, twice daily for 3 days. (See British National Formulary)

How to prevent infestation?

Shoes or other enclosed footwear should always be worn, especially in areas that may be used by the native population for defaecation or the deposition of excrement.

Whipworms

Whipworms are about 5 cm long. They are found all over the world but especially in moist tropical countries.Human whipworm - trichuris trichiura

How does one become infested?

The eggs are passed in the stools and develop into larvae in the soil. Humans become infected if their food or drink is contaminated, e.g. by eating unwashed salads or fruit that has been in contact with soil.

What are the symptoms?

There may be none if the infestation is light, but a heavy infestation leads to chronic diarrhoea. Diagnosis is made by finding the eggs in the stools by microscopic examination.

How is infestation treated?

The recommended treatment is with Mebendazole tablets, at the rate of 100 mg twice daily for 3 days. (See British National Formulary) Medical advice should be sought for infants under two years of age.

How to prevent infestation?

Infestation may be avoided by good kitchen hygiene, proper cooking, hand washing and the avoidance of crops grown in places where human waste is used as manure.

Tapeworms (cestodes)

As the name suggests, these are ribbon-like, rather than round and vary in length, according to species, from a few centimetres to several metres. They consist of a head, which has suckers or hooks for attachment to the intestinal wall, and a series of segments that grow from it, maturing as they are pushed further and further away from the head. Each segment absorbs nutrients from the intestinal contents and when mature has both male and female sexual apparatus. Eggs are released and passed in the stools. Mature segments may become detached and also be passed in the stools.

The beef tapeworm

This may grow as long as 5 metres in the human intestine. Eggs, passed in the stools contaminate grassland. When ingested by cattle they develop into larval forms that migrate into muscle tissue.

It is distributed worldwide but especially in Africa and the Middle East.

How does one become infested?

If humans eat uncooked or improperly cooked meat containing any larval forms present, these will develop into adult worms in the intestine.

Diagnosis is made by finding the segments, which may be 1-2 cm long in the stools, or looking for eggs in the stools by microscopic examination.

What are the symptoms?

These vary from none to abdominal discomfort, digestive disturbance, loss of, or excessive appetite, and loss of weight.

How is infestation treated?

Two drugs are available on prescription and on a 'named patient' basis. Single doses of 2 g Niclosamide tablets, and 10-20 mg Praziquantal are given after a light breakfast. (See British National Formulary)

How to prevent infestation?

Meat inspection, which will reveal the larval forms, and adequate cooking should prevent infestation.

The pork tapeworm

This resembles the beef tapeworm in many respects but differs in that the larval form can also infest humans. When it does the disease is called cysticercosis.

The pork tapeworm is distributed worldwide but especially in Africa and the Middle East. It is rare in the UK.

How does one become infested?

The usual way in which the parasite enters the human body is by eating inadequately cooked pork.

What are the symptoms?

These vary from none to abdominal discomfort, digestive disturbance, loss of, or excessive appetite, and loss of weight. In cysticercosis there are headaches, fever, and pain in the muscles where the larvae form cysts. Cysts in the brain may cause epilepsy and be life-threatening.

Diagnosis is made by finding the segments, which may be 1-2 cm long in the stools, or looking for eggs in the stools under a microscope. Diagnosis of cysticercosis requires x-ray and CT scans.

How is infestation treated?

Two drugs are available on prescription and on a 'named patient' basis. Single doses of 2 g Niclosamide tablets, and Praziquantal at a rate of 10-20 mg/kg body weight, are given after a light breakfast. (See British National Formulary) Cysticercosis, however, diagnosed by x-ray and/or CT scan, requires surgical intervention.

How to prevent infestation?

Pork should be inspected for the larval forms and should be adequately cooked.

The dwarf tapeworm

This tapeworm is only about 3 cm long and also differs from those above by requiring only one host for its life cycle. The eggs hatch in the small intestine and the larvae penetrate the intestinal wall and then develop into the adult worms. This worm is distributed worldwide but mostly infests children.

How does one become infested?

Eggs passed in the stool contaminate food and water. When these are consumed the worms develop in the intestine.

What are the symptoms?

There is diarrhoea and pain but there may be convulsions and neurological complications. Diagnosis is made by looking for eggs in the stools under a microscope.

How is infestation treated?

Two drugs are available on prescription and on a 'named patient' basis. Single doses of 2 g tablets of Niclosamide, and Praziquantal at a rate of 25 mg/kg body weight, are given after a light breakfast. (See British National Formulary) All those sharing quarters with the sufferer may be treated.

How to prevent infestation?

This is difficult as infection is passed from person to person. Treatment of close contacts of the sufferer may be recommended.

The fish tapeworm

This is the longest of the tapeworms that may infest humans, reaching up to 10 metres. It has three hosts: freshwater fish, water fleas and humans.

It is found mostly in temperature climes, but also in tropical Africa and N and S America.

How does one become infested?

Humans are infested by eating infested freshwater fish that have been inadequately cooked. Eggs are passed in the stools of humans and if these enter water they are ingested by water fleas where they spend part of their life cycle. In turn the water fleas are eaten by fish where they develop further.

What are the symptoms?

There is weakness, fatigue, diarrhoea and, especially important, anaemia.

Diagnosis is not easy because it is often difficult to find the eggs by microscopy of stools. Several samples may have to be examined.

How is infestation treated?

As with the beef tapeworm, two drugs are available on prescription and on a 'named patient' basis. Single doses of 2 g tablets of Niclosamide, and 10-20 mg Praziquantal. are given after a light breakfast. (See British National Formulary)

Useful websites

Useful websites

For further information on these helmiths see:

The current edition of the British National Formulary gives information on current therapy
Division of Parasitic Diseases
C H Collins MBE MA DSc FRCPath

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