Threadworms

Threadworm (Enterobius vermicularis) infections are widespread throughout the world.  The worm affects children particularly.  Its life follows a cycle - eggs are swallowed and, as they pass into the intestine, they develop and mate.  The pregnant females move down into the bowel and make their way into the anus where they lay their eggs.  This causes severe itching, particularly in bed at night.  Children scratch their itching anuses, infect their fingers with ova and then put their fingers in their food and mouth and re-infect themselves.  Children also touch each other's hands and this is how the infection is easily spread in nursery school.

Adult worms can easily be seen moving out of the anus and over the skin to the genitals: they can also be seen in the motions (the male worm is 2-5mm long and the female is 8-12mm long).  In girls the worms may spread up from the anus into the vagina where they can cause severe irritation and ascending infection up into the womb and fallopian tubes.

Treatment

The important part of treatment is strict hygiene in the family and in nursery schools.  Infected children must wash their hands and scrub their nails after going to the toilet and before meals.  They should also take a bath or shower every morning to wash the worms and eggs from the anus and genitals.  They should try not to scratch themselves and put their fingers in their mouths.  The whole family should practise good hygiene and be treated.

Medical treatment includes:

mebendazole- a single dose by mouth: it should not be used in children under the age of 2 years

piperazine- one dose by mouth daily for 7 days (the dose varies according to age); it may be repeated after 1 week if no improvement

pyrantel- a single dose by mouth.

Roundworm

The roundworm Ascaris lumbricoides causes a disease known as ascariasis.  It is a large worm: adult females may measure 20-45cm in length.  The worms live for about 18 months in the body before they die.  The intestine may contain any number of worms from one to hundreds.  The female produces 200,000 eggs per day; they are passed in the faeces and after about 10 days in soil they become infective and contaminate water and food.

The eggs enter humans from food contaminated with human faeces.  The eggs hatch in the duodenum and the larvae penetrate the wall of the intestine into the blood stream where they are carried to the lungs. In the lungs they move out of the blood stream and climb up the bronchial tubes into the throat.  They are then swallowed into the intestine where they mature, to start producing eggs after about 2 months.  A large proportion of the world's population is infected with roundworms.

In the lungs they may produce fever, pain, cough, malaise and breathlessness, and in the intestine they may cause pain, diarrhoea, vomiting and loss of appetite.  They may also interfere with the absorption of nutrients and vitamins, producing malnutrition, especially in children.  A bunch of worms may block the intestine and cause a serious obstruction.

Treatment

Mebendazole, piperazine and pyrantel are all effective.  Containment of the infection must rely on better education and sanitation.

Tapeworms

There are many types of tapeworm.  Humans are the source (the host) of tapeworms where they live in the intestine attached by suckers in their heads.  They have no gut and absorb nutrients through their surface.  Tapeworms consist of segments and cross-fertilization takes place between segments. 

Eggs are produced, which are passed in the faeces.  These infected faeces contaminate foods that are eaten by animals (e.g. cattle and pigs) and by fish.  These animals are intermediate hosts in the development of the tapeworm.  Humans get infected by eating these infected animals (e.g. uncooked or undercooked beef, pork, and fish) and this allows the next stage of development in the life cycle of the tapeworm.

In the intermediate hosts the surfaces of the eggs are dissolved in the stomach juice and they hatch to release larvae into the intestine.  These pass through the intestine wall and circulate round the body where they settle in various tissues, particularly muscles where they form cysts around themselves.  These cysts (or cysticerci) are the cause of ‘measly’ beef and pork.

Beef tapeworm (Taenia saginata)

Humans normally get infected from eating uncooked or under-cooked beef (rare steaks and kebabs) containing cysts (cysticerci).  The cysts are dissolved by the stomach juices and the baby worm attaches itself to the wall of the intestine.

Usually only one tapeworm develops; it hangs on to the intestinal wall by its suckers and reaches a length of 4-8metres.  Tapeworms are usually in the upper part of the small intestine and cause no symptoms, but occasionally they cause irritation, producing wind, cramps or diarrhoea.  Segments (proglottides) appear in the stools.  Eggs may also appear in the stools and can be obtained from the anus by applying sticky tape.  These eggs contaminate animal food and are eaten by intermediate hosts (cattle) and so the cycle continues.

Pork tapeworm (Taenia solium)

Humans normally get infected from eating uncooked or under-cooked pork containing cysts (cysticerci).  The cysts are dissolved by the stomach juice and the baby worm attaches itself to the wall of the small intestine.

Usually only one tapeworm develops; it hangs on by its suckers and reaches a length of 2-4metres.  There are usually no symptoms, and eggs and segments are passed in the faeces.  These eggs may contaminate animal food and be eaten by intermediate hosts (e.g. pigs and sheep) and so the cycle continues.

The development of pork tapeworm cysts in humans

Humans may develop pork tapeworm cysts (cysticercosis) by eating food or water contaminated with human faeces infected with eggs of the pork tapeworm.  Also, eggs may be transferred from the anus to the mouth by the fingers, and sometimes eggs may enter the stomach from the intestine during a severe bout of vomiting.

Once in the stomach, the pork tapeworm's eggs are dissolved in the stomach juice.  This allows them to hatch and to release their larvae, which work their way through the wall of the intestine to circulate all around the body, and wherever they settle they make cysts around themselves.  The cysts may develop in any organ or tissue of the body.  Inside the cysts the larvae mature over a few months but eventually they die, leaving a hard cyst.  These cysts may cause no problems in the skin or muscles except for some aches and pain and fever in the early stages.  However, if they develop in the brain, they may cause problems such as water on the brain (hydrocephalus) by blocking the drainage of brain fluid.  They may also cause pressure and inflammation in the brain, resulting in epilepsy, paralysis and personality changes.  The cysts may lodge in the eyes and cause inflammation, blurred vision and blindness, and in the heart to produce heart failure.

Fish tapeworm (Diphyllobothrium latum)

These are the largest tapeworm and can reach 10 metres in length.  They attach themselves to the wall of the small intestine.  Segments and eggs pass out in the faeces and these can infect freshwater areas where copepods (a group of crustaceae) breed.  The larvae develop in the copepods, which are then eaten by pike, perch or salmon.  Humans become infected by eating these fish raw or undercooked.  Poor sewage disposal allows infected human faeces to enter freshwater lakes and so the cycle continues. 

The most harmful effect of fish tapeworm is that it can interfere with the absorption of vitamin B12, causing anaemia.

Medical treatment of tapeworms

One dose of niclosamide or praziquantel after a light breakfast, followed by a purgative after 2 hours.

Hydatid disease (echinococcosis)

The dog and certain wild canines are the source (host) of a small tapeworm (Echinococcus granulosus).  These animals pass the eggs in their faeces, which contaminate the ground.  The eggs may be taken in by other animals (e.g. sheep and cattle) during feeding, and hatch in the intestines of these animals.  The larvae then enter the blood stream and are spread round the body, to form cysts (hydatid cysts) in various tissues.

The eggs may also be taken in by humans directly from dogs' faeces or by drinking water contaminated with the eggs.  The eggs hatch in the small intestine and the larvae work their way through the wall of the intestine into the bloodstream, to be taken to the liver, where they form a cyst.  The cyst may enlarge and become hardened, or it may rupture, causing the release of many cysts.  Occasionally larvae travel to the lungs or brain, where they form cysts.  Symptoms will depend on the size and location of the cysts.  A rare form has a cycle between foxes and voles; it may affect humans and invade the liver.

Treatment

Treatment consists of surgical removal of the cyst, avoiding spillage of its contents.  Mebendazole may be of some value, particularly in people who cannot have surgery or whose lungs are affected.

Improved hygiene, worming of dogs and inspection of meat should help to reduce the spread.

Hookworms

Hookworms are roundworms with conspicuous teeth.  They are about 1 cm in length and attach themselves to the lining of the small intestine and drink blood.  This can lead to anaemia and a low blood protein in the infected person (the host).  The females produce thousands of eggs, which contaminate the faeces and hatch in soil within 48 hours.  The larvae can survive for several months in soil.  Upon coming into contact with the skin of humans, the larvae penetrate it, enter the blood in the veins and are carried to the lungs.  They then climb through the lung tissue and up the breathing tubes into the throat where they are swallowed into the stomach and then reach the small intestine where they hook on and start sucking blood – the cycle is now completed. 

Light infections produce few symptoms but heavy infections (thousands of worms) may cause hookworm disease – abdominal pain and diarrhoea, chronic blood loss, causing anaemia, and low blood proteins which may result in weakness, fatigue and retarded growth in children.  The severe anaemia may cause heart failure.

Treatment

Medicines used include bephenium, levamisole, mebendazole, pyrantel and thiabendazole.  Tetrachloroethylene is also used in some countries.

Schistosomiasis (or bilharziasis)

This is a chronic worm infection, which is estimated to affect over 200 million people in the world.  It is one of the most important public health problems facing tropical and sub-tropical countries.  The schistosomes are worms that live in the body of humans as parasites.  They are transmitted by freshwater snails.

Humans may be infected by three species of schistosomes – Schistosoma haematobium, S. japonicum and S. mansoni.  Each species is particular to a certain area of the world and causes a specific illness resulting in a substantial amount of suffering and many deaths.

The schistosome worms live and mate in the venous blood – S. haematobium live in the veins around the ureters and bladder; S. mansoni  live in the main veins that drain the intestine (the inferior mesenteric veins) and S.japonicum live in other main veins that drain the intestine (the superior mesenteric veins).

The female worms lay their eggs in the blood stream and, according to their location, the eggs then break their way out of the body by entering the bladder or intestine to be passed in the urine or faeces.  Those eggs that do not escape get trapped in the wall of the bladder or intestine and cause areas of inflammation.  They may also spread to the liver and cause inflammation in the liver.

The eggs are present in the urine and faeces of infected humans, so passing urine or emptying the bowels into freshwater lakes contaminates the water with the eggs, where they hatch in a few hours.  The baby worms swim near the surface of the water and are capable of infecting snails for up to 8 hours.  Once they enter the snail they multiply by the hundreds and schistosome worms develop in about 4-6 weeks.  Under certain conditions of light and temperature they mature enough to infect humans.  They emerge from the snails into the water and remain infective to humans for about 72 hours.  If a human paddles or swims in the infected water, the worms attach themselves to the skin by suckers.  Within minutes they have burrowed deep into the skin, helped by a special chemical they produce from a gland on their heads that penetrates the skin and by a vibratory movement of their bodies.  During the few minutes while they are penetrating the skin, they shake off their tails and change into a different shape so that they can easily lie in tunnels burrowed under the skin.  At this stage they no longer need oxygen and would die if they re-entered the water.  They remain in the skin for 1-3 days before entering and migrating along veins to the lungs and then to the liver in about 4-6 weeks.  In the liver they undergo a further development and finally they migrate to the areas where they mate.

Eggs can be seen in the urine and faeces within about 5-9 weeks of first being infected.  The worms live in the body for 3-10 years and continue to produce eggs at regular intervals.

The infection is more common in the young and in males and is probably related to the frequency with which they enter and swim in infected waters.  Three distinct disease syndromes are caused by schistosomiasis and are related to three distinct stages in the development of the worms: swimmer's itch, and acute and chronic schistosomiasis.

Swimmer's itch is an allergic dermatitis caused when the young worms enter the skin from infected water.  The disease may go no further than this.

Acute schistosomiasis (Katayama fever) is a serum-sickness-like syndrome which occurs 3-9 weeks after infection.  There is fever, headache, pains in the abdomen and an enlarged spleen and liver.  It is an acute allergic reaction that coincides with the production of eggs in the body.

Chronic schistosomiasis is caused by eggs that fail to be excreted in the urine and faeces.  They remain in the bladder or intestine and, along with the chemicals they produce to help their escape through the wall of the bladder or of the intestine, they trigger severe immune reactions.  This causes immune cells and chemicals to move in on the eggs, which results in the production of what are called schistosome egg granulomas.  A granuloma is a tumour containing granulomatous tissue which is the debris left after a severe immune/inflammatory reaction.  These tumours then increase in size because scar tissue is laid down on them.  They may become large enough to cause all kinds of problems; for example, blockage to the flow of urine through a ureter, which causes a back-pressure into the kidney, resulting in kidney damage or blockage to the flow of blood in the liver and spleen.  These lesions may occur in any tissue or organ of the body. 

NOTE: Several schistosome species use migratory birds as their definitive hosts and fresh-and salt-water molluscs as their intermediate hosts.  Swimming in infected waters can cause the swimmer's itch stage of the disease only.

Treatment

Praziquantel is effective against all types of human schistosomes.  Oxamniquine is active only against S. mansoni and metriphonate is effective against S. haematobium.

If treatment is given early enough during the course of the disease, it may be possible to not only kill the worms but also to reverse some of the damage.  In later stages, treatment will kill the worms and prevent them from causing further damage.

Control

The most cost-effective approach to the control of schistosomiasis at present is the mass treatment of communities, using a single dose of praziquantel combined with an all-out attack on snails using chemicals (molluscicides).

The medicine treatment causes a low egg count in the urine and faeces for years.  However, at the same time there is need for health education programmes as well as a need to dramatically improve living standards, to provide toilets, to improve agricultural practices of irrigating fields and to discourage washing, swimming and playing in infected waters.

Filariasis

Filaria are a genus of parasitic worm that infect tissues under the skin and in the lymphatics.  There are eight types that commonly affect humans and three of them are responsible for most diseases caused by filariae.  They are transmitted by biting insects (mosquitoes, black fly, horse fly and midges) and they go through complex life cycles in the body.  The adults are thin worms about 2-50cm long.

The type that live under the skin cause severe skin rashes and can affect the eyes.  The type that live in the lymphatic system cause allergic and inflammatory reactions that block the flow of lymph producing back-pressure of fluid into the tissues, causing the affected area to swell up.  The blockage may affect the drainage from the breasts, scrotum, legs or arms.  These areas can swell up to a tremendous size.  The leg can be so swollen that is looks like an elephant's leg, which is why it is called elephantiasis.  The scrotum can become so enormous that it has been known for one affected individual to push his scrotum round in a wheelbarrow!

Treatment

Diethylcarbamazine is effective against most types of infections.  It kills the worms but triggers a severe allergic reaction.  If the infection persists, suramin is used; it kills adult worms but may also damage the kidneys.  Ivermectin is another effective medicine.

Dracontiasis

Dracontiasis is caused by the female guinea-worm, which lives under the skin in humans.  It is transmitted from a small crustacean (cyclops) that lives in wells and ponds.  When the cyclops is eaten by humans the larvae penetrate the intestine wall and spread to settle down in tissue under the skin and elsewhere.  The mature female worms work their way through the surface of the skin to the surface to release their eggs; this causes acute allergic symptoms, severe blisters and inflammation of the skin and joints.

Treatment

Metronidazole and niridazole are both effective at killing the worms.  The worms should be removed from the skin and sterile dressings applied.