Dracunculiasis guinea worm

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Occurrence:

Tropical Africa

Organism:

Dracunculus medmensis

Reservoir.

Humans

Transmission:

Water contaminated with cyclops

Control:

Filtration of water

Boiling of water

Provision of safe drinking water

The guinea worm Dracunculus medinensis has been known since ancient times. It results in the formation of skin ulcers, often in the ankles or legs, with extrusion of embryos on contact with water. The ulcers frequently become infected and are a cause of disability and school absenteeism.

The sexually mature female is up to 1 m long and 2 mm in diameter; the uterus, which occupies most of the body, contains millions of embryos. The male is small and its fate after copulation is not known.

LIFE CYCLE

See Figure 4.5.

EPIDEMIOLOGY

Contamination of fresh water with larvae from infected persons takes place when such persons share drinking water from shallow ponds (Plate 28) or wells. The water in these ponds, being stagnant with a high organic content, favours the presence of the vecta species of cyclops. In the dry seasons these ponds are much frequented since they often provide the only readily accessible source of water, thus creating a high cyclops:man contact ratio.

In other places, transmission may occur during the rainy season when surface pools exist which disappear in the dry season. Infection can also be contracted when drinking water while bathing in contaminated pools or during ritual washing of the mouth in the performance of religious ablutions.

CONTROL

Individual

Filtering the water through a gauze linen mesh prevent infection by holding back the cyclops

Mode Transmission Dracunculiasis
Figure 4.5: Life cycle of Dracunculus medinensis (guinea worm).

Community

The eradication of guinea worm infection was adopted as a subgoal of the Clean Drinking Water Supply and Sanitation Decade (1981-90). The concept was further enhanced by other aid agencies by making the provision of safe drinking water a priority in rural areas of Africa and Asia.

Other interventions include filtering or boiling of water, especially the former by the use of monofilament nylon nets, which are long lasting and have a regular pore size and are able to sieve out cyclops. Treating water sources with iemepl\o| added to ponds is a useful adjunct in certain circumstances.

An international commission fo the ce/itfication of dracunculiasis eradication monitors progrev: at regular intervals. The eradication of guvjea worm infection is nearing completion (Plate 29). TSv only countries where a substantial number of cases still occur are the Sudan and Nigeria.

Trichinosis

Occurrence:

Worldwide

Organism:

Trichinella spiralis

Reservoir.

Pigs

Tnansmisaon:

Eating inadequately cooked

pork or pork products

Control:

Adequate cooking of pork meat

Trichinosis is caused by encysted larvae of Trichinella spiralis. The disease is characterized by diarrhoea with abdominal pain, followed by a febrile illness with severe muscular pain.

APPLIED BIOLOGY

The adult worms are found in the small intestine of a number of carnivorous animals including the pig, bush-pig, rats, hyenas and other hosts. Their lifespan in the intestine is approximately 8 weeks.

After fertilization, the female worms bury themselves in the intestinal mucosa and each produces about 1500 larvae. The larvae migrate via the intestinal lymphatics to the thoracic duct and into the bloodstream, whence they are distributed to the muscles. Here they develop and become encysted between the muscle fibres in 4-7 weeks. Calcification occurs in about 18 months but the encysted larvae remain alive for many years. When food containing encysted larvae is ingested by a suitable host the larvae are released by the action of the digestive juices on the capsule and the cycle is repeated in the new host. In susceptible animals the larvae grow into sexually differentiated adults which on mating produce larvae which then invade striated muscle. In man, infection terminates at the cystic stage.

EPIDEMIOLOGY

This parasite is more prevalent in temperate than in tropical countries and is mainly confined to those countries where pork is eaten. Serological tests have shown that in many communities the incidence of infection is apparently higher than the number of clinically diagnosed cases, and it is obvious that many light infections pass unnoticed. In recent years small and large epidemics have occurred. Congenital trichinosis has been reported.

Reservoir and transmission

Pigs are the main 'reservoir' of infection although many other animals harbour the infection. Trichinosis in man results from eating raw or inadequately cooked pork or pork products, for example sausage meat. In Kenya, the bush-pig is a common source of infection. Pigs become infected chiefly from eating uncooked slaughterhouse refuse containing infected meat scraps; occasionally rats, which have a high natural infection rate of trichinosis, can be a source of infection when they are eaten by pigs.

LABORATORY DIAGNOSIS

One of the most constant, single, diagnostic aids in trichinosis is a rising eosinophilia of 10-40%.

Parasitological diagnosis is based on the finding of the encysted larval worms in a thin piece of muscle biopsy compressed between two glass slides and examined under a low magnification of the microscope. In light infections, when direct examination is negative, the biopsy specimen should be incubated overnight in an acid-pepsin mixture and the centrifuged deposit examined for larvae. Immunological tests such as ELISA and countercommunoelectrophoresis are positive as early as 2 weeks after infection.

The CFT can provide a diagnosis in the first week of the disease - the specificity of the test is high. The bentonite, latex, and cholesterol agglutination tests are excellent tests for diagnosing recent infections but are unreliable in chronic infections.

CONTROL

Adequate cooking of pork and other meats (e.g. hare, polar bear, dog, etc.) will essentially protect the individual. Legislation compelling all pig feed: containing meat to be cooked virtually stops transmission of trichinosis to the pig.

Mebendazole or albendazole kills or sterilizes trichinella adult worms ind may prevent trichinellosis.

Angiostrongyiiasis

Angiostrongyiiasis is an eosinophilic meningitis due to the nematode worm Angiostrongylus cantonensis.

APPLIED BIOLOGY

A. cantonensis is essentially a parasite of rats and only occasionally infects man. The eggs hatch in the faeces of the rat in which they are expelled, and the infective larvae invade certain snails or slugs. These are later eaten by rats, which thereby become infected. The life cycle in man is unknown, but young adult worms have been found in the cerebrospinal fluid and the brain where they measured 8-12 mm in length.

EPIDEMIOLOGY

Sporadic, small epidemics occur in certain Pacific Islands, including Tahiti and Hawaii, and in South East Asia including Vietnam and Thailand. It has been suggested that A. cantonensis originated in the islands of the Indian Ocean (Madagascar, Mauritius, Ceylon) and then spread eastwards to South East Asia and so to the Pacific area, and that the giant African snail Achatina fulica might have been instrumental in the spread of the parasite.

Reservoir and transmission

Rats are the reservoir. Human infection results from the accidental ingestion of infected snails, slugs and land planarians (worm-like creatures) found on unwashed vegetables, such as lettuce. The peak incidence of eosinophilic meningitis occurs in the cooler, rainy months between July and November, the period of highest consumption of lettuces and strawberries which when unwashed lead to infection. It is also the season when freshwater prawn may become infected from snails and slugs washed into rivers and estuaries. This was thought to be the main source of local, human infection in Tahiti.

Eating raw or pickled snails of the genus Vila is considered the mode of infection in Thailand; the percentage of positive snails for A. cantonensis infections varying from 1.8 to 72%. In Malaysia, the shelled slug M. malayanus has been shown to shed infective third-stage larvae, but no human cases have yet been reported.

Host factors

In Thailand, males are affected twice as frequently as females, the highest attack rate occurring in the second and third decade.

LABORATORY DIAGNOSIS

Examination of the cerebrospinal fluid reveals increase in protein and a strikingly high eosinophilia (60.-80%). Larval worms are sometimes found in the CSF and can be identified as A. cantonensis on microscopy.

CONTROL

The infection is prevented by not eating unwashed vegetables and strawberries and uncooked snails, slugs and prawns infected with larvae. Efficient rat control will reduce the reservoir of infection.

Gnathostomiasis

Occurrence:

Thailand

Organism:

Gnathostoma spinigerum

Reservoir.

Domestic and wild cats

Transmission:

Eating or preparing raw fish or

chicken

Control:

Adequate cooking

Gnathostoma infection may present as 'creeping eruption', transitory swellings or eosinophilic meningitis.

APPLIED BIOLOGY

The life cycle in the definitive animal liosr; (felines, dogs and foxes) is well known, >id involves two intermediate hosts - a cyclops aid a fish or an amphibian. Man is an unnatu al host and the immature worms may locate either in the internal organs or near the surface of the body. As th larvae

Angiostrongyiiasis

Occurrence:

South East Asia and Oceania

Organism:

Angiostrongylus cantonensis

Reservoir,

Rodents

Transmission:

Snails

Control:

Adequate cooking and cleaning

of vegetables

rarely develop into adults the life cycle in man is not known. Adults have, however, been reported in the intestine and ova have been found in human faeces.

EPIDEMIOLOGY

Reservoir

The normal hosts for Gnathostoma spinigerum are domestic and wild felines, dogs and foxes. Human infections have, however, been reported from Israel, the Sudan, India and the Far East. The majority of human cases to date have occurred in Thailand, where a substantial animal reservoir has been reported. The parasite has been isolated from cats, dogs, domestic pigs, freshwater fish, eels, snakes, frogs, leopards, chickens and fish-eating birds. One per cent of dogs in Bangkok is infected with gnathostomiasis. In Thailand, human infection usually results from eating fermented fish, which is a Thai delicacy much liked by women. The dish known as Somfak is made up of raw freshwater fish, cooked rice, curry, salt and pepper and then wrapped in banana leaves. Recently it has been shown experimentally that penetration of the skin by the third-stage infective larva can occur.

There is a possibility therefore that in addition to ingestion, human infection is possible during the preparation of raw fish dishes or raw chicken dishes by the third-stage larva penetrating the bare skin of the hands of individuals preparing these meals. Other ways for man to acquire the infection is by eating other forms of raw fish, frogs, and possibly snakes infected with encysted larvae.

Human infection has occasionally been attributed to G. hispidum, the definitive host in this instance is the pig.

LABORATORY DIAGNOSIS

Diagnosis in human infections depends on finding the immature worms and identifying them. Cutaneous tests with antigens from larval or adult worms as well as precipitin tests have been used for diagnosis. Eosinophilia is present.

CONTROL

The infection is prevented by not eating uncooked fish and meats of other animals infected with encysted larvae. Ancylol (disophenol) kills both the larval and adult forms of gnathostoma in dogs and cats. Unfortunately the compound is too toxic for man.

Cestode (tapeworm) infections

Taeniasis

Occurrence:

Worldwide

Organisms:

Taenia solium, T. saginata, Cysticercus

cellulosae

Reservoir.

Humans

Transmission:

Uncooked meat

Auto-human infection (cysticercosis)

Control:

Personal hygiene

Individual specific treatment

Sanitary disposal of faeces

Thorough cooking of meat

Taeniasis occurs in all countries where beef or pork are eaten. The larval stage of T. solium produces cysticercosis. Clinical features of taeniasis are often absent, the patients only becoming aware of the worm infection when segments are passed in the stool.

The life cycles of T. saginata and T. solium are shown in Figures 4.6 and 4.7.

Direct infection of man by larval worms of T. solium produces a condition known as cysticercosis. For this to occur, man must be exposed to a source of ova from water and food contaminated by faeces; unclean hands transferring eggs from the adult worm carrier; auto-infection by carrying eggs from the anus to the mouth on the fingers; and, rarely, by auto-infection by massive regurgitation of ova from the small intestine into the stomach.

The liberated larvae penetrate the intestinal mucosa and are then carried by the bloodstream to various parts of the body where they encyst, the commonest sites being the subcutaneous tissues, skeletal muscles and the brain. The ¿»-sticcreus takes about 4 months to develop and becomes enveloped in a fibrous capsule, which eventually calcifies and may be seen radiologically. Il

The life cycle of the cysticercus ranges from a few months up to 35 years f

Figure 4.6: Life cycle of Taenia saginata.

EPIDEMIOLOGY

The beef tapeworm Taenia saginata has a cosmopolitan distribution and is particularly common in the Middle East, Kenya and Ethiopia. The world incidence of T. saginata is much higher than that of T. solium and it is estimated that in some parts of Kenya infection rates of taeniasis in man may approach 100% and that 30% of cattle may harbour cysticerci.

The pork tapeworm T. solium is also widely distributed in Central and South America, Africa and South East Asia and its larval stage, Cysticercus cellulosae, produces cysticercosis in man. In several countries (e.g. Ecuador) the prevalence of human neurocysticercosis is high and constitutes a serious public health problem.

Reservoir and transmission

Humans are the only reservoir of infection. Humans acquire T. saginata infection by eating raw or partially cooked beef, while cattle arc infected by grazing on pastures fertilized by human faeces or which are flooded with scwagc-ladc;. water. The role of birds in the transmission ; / the disease ö not clear.

T. solium is spread by the insanitary dispoal of faeces, thus providing the pigs with a ready opportunity for infection when they ingest human

Tenia Solium Life
Figure 4.7: Life cycle of Taenia solium.

excreta. Man is infected when eating uncooked or insufficiently cooked pork (Plate 30).

Host factors

Taeniasis is uncommon in young children and the incidence increases with age. The sexes are equally susceptible.

LABORATORY DIAGNOSIS

'Direct smear' examination of the faeces occasionally reveals the typical taenia ova. The intact segments that are usually passed can be compressed between two glass slides and the branches of the uterus at their origin from the main uterine stem can be counted and a differentiation easily made between T. saginata (20-35 branches) and T. solium (7-12 branches). A coproantigen ELISA has recently been developed as well as a PCR technique for identification of T. saginata proglotids and eggs.

CYSTICERCOSIS DIAGNOSIS

Calcified cysts can be seen radiologically, wMe computerized axial tomography (CAT) and Magnetic resonance have replaced jiuer meth Js. Complementary procedures are I.i.lS \ and Western blotting (WT).

Biopsy of palpable subcutan diagnostic.

Biopsy of palpable subcutan diagnostic.

may be may be

CONTROL

Transmission of taeniasis and cysticercosis can be controlled by the sanitary disposal of human faeces, the thorough cooking of meat and raising the standards of personal hygiene.

The individual

All persons suffering from taeniasis should be dewormed with niclosamide (given in a single dose of 2 g) or praziquantel (a single dose of 5-10 mg/kg after a light meal). The thorough cooking of all beef and pork meat affords personal protection and health education should be carried out to raise the standards of personal hygiene, especially among persons harbouring T. solium.

The treatment of cerebral cysticercosis includes praziquantel, albendozole, steroids and surgery.

The community

Sanitary disposal of human excreta is essential. Untreated human faeces should not be used as fertilizer and if possible human faeces should be avoided altogether as a means of manuring crops.

Strict abattoir supervision resulting in adequate inspection of carcasses and condemnation of infected meat should be carried out. If meat containing cysticerci has to be consumed it should be thoroughly cooked under the close supervision of a health officer.

Recently, community-oriented chemotherapy of T. solium taeniasis with praziquantel has been introduced in order to prevent local endemic neuro-cysticercosis.

APPLIED BIOLOGY

The adult, which may be 10 m long, lives in the ileum of humans or other mammals, and may have as many as 4000 segments. The gravid segments disintegrate and the ova are passed in the faeces. On reaching water the ciliated embryo escapes and is swallowed by the first intermediate host - a freshwater crustacean {Cyclops or Diaptomus species) - in which it develops as a procercoid. When the infected crustaceans are swallowed by various freshwater fishes (salmon, pike, etc.) further development takes place in the musculature of these second intermediate hosts to form plerocercoids. When man and other animals eat raw fish the plerocercoid is liberated and attaches itself to the small intestine, where it grows into an adult in about 6 weeks.

EPIDEMIOLOGY

Diphyllobothriasis is more common in the temperate zones than in the tropics where it has been reported from the Philippines, Madagascar, Botswana, Uganda and southern Chile.

Reservoir and transmission

Man and a number of fish-eating mammals (e.g. dog, cat, fox, pig, bear, seal, etc.) are the reservoir of infection. Man is infected by eating raw or insufficiently cooked fish; the latter having acquired their infections in waters contaminated by faeces containing ova of D. latum. As with the other tapeworms, the adult fish tapeworm is long-lived. The export of raw fish may cause infection outside the endemic areas.

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  • vanessa
    Which worms are associated with autohuman infections?
    2 years ago

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