The following points highlight the nine major human diseases caused due to infestation with the nematodes. The human diseases are: 1. Ascariasis 2. Ancylostomiasis 3. Enterobiasis 4. Trichuriasis 5. Trichinosis 6. Strongyloidiasis 7. Filariasis or Elephantiasis 8. Loiasis 9. Onchocerciasis.
Human Disease # 1. Ascariasis:
Ascariasis is a highly prevalent disease caused by the largest nematode (roundworm) Ascaris lumbricoides. It resembles an ordinary earthworm.
When fresh from the intestine, it is light brown or pink in colour but it gradually changes to white. It is most frequently seen in the stool of children. The male of A. lumbricoides measures about 15 to 25 cm in length, while the female is longer and stouter measuring 25 to 40 cm in length.
The egg-laying capacity of mature female Ascaris has been found to be enormous, liberating about 200,000 eggs daily.
The eggs liberated by a fertilised female pass Out of the human host with the faeces and may remain alive for several days. Infection is effected by swallowing ripe eggs (embryonated eggs) with raw vegetables cultivated on a soil fertilised by infected human excreta. Infection also occurs by drinking contaminated water.
Among children playing in the contaminated soil, there is also hand to mouth transfer of eggs by dirty fingers. Infection may also occur by inhalation of desiccated eggs in the dust reaching the pharynx and swallowed. A rhabditiform larva is developed from un-segmented ovum within the egg-shell in 10 to 40 days in the soil.
The ripe egg containing the coiled-up embryo is infective to man.
When ingested with food, drink or raw vegetables, the embryonated eggs pass down to the duodenum where the digestive juices weaken the egg-shell. Splitting of the egg-shell occurs and the rhabditiform larvae are liberated in the upper part of the small intestine. The newly hatched larvae burrow their way through the mucous membrane of the small intestine and are carried by the portal circulation to the liver.
Finally they pass out of the liver and via right heart enter the pulmonary circulation. Breaking through the capillary wall they reach the lung alveoli. From the lung alveoli the larvae crawl up the bronchi and trachea, they are propelled into larynx and pharynx and are once more swallowed.
The larvae pass down the oesophagus to the stomach and localize in the upper part of the intestine, their normal abode. The larvae on reaching habitat grow into adult worms and become sexually mature in about 6 to 10 weeks’ time. Four moultings of the larva occur-one outside while within the egg-shell, two in the lungs and one in the intestine.
The symptoms attributed to Ascaris infection may be divided into two groups:
(i) Those produced by migrating larvae, and
(ii) Those produced by the adult worms.
(i) Symptoms due to the migrating larvae:
In heavy infections typical symptoms of pneumonia such as fever, cough and dyspnoea may appear. Urticarial rash and eosinophilia are seen in such cases. Disturbances have been reported due to their presence in the brain, spinal cord, heart and kidneys.
(ii) Symptoms due to the adult worms:
With the adult worms inhabiting the intestine the patient complains of abdominal pains, vomiting, headache, irritability, dizziness and night terrors. Sometimes there is a diarrhoea and salivation. Often the patient grits his teeth in the sleep. When the adult worms migrate through the intestinal wall they cause severe peritonitis.
Wandering Ascaris may enter the lumen of the appendix, causing appendicitis. Obstructive jaundice and acute haemorrhagic pancreatitis have been known to occur when the worm has entered into the biliary passage. At times it penetrates high up in the liver causing one or more abscesses.
The treatment of human ascariasis has been fairly successful through the oral administration of piperazine citrate syrup (two spoonful twice a day for one week, followed by another course after a gap of one week) and hexyl-resorcinol tablets (10 mg taken at bed time with water).
Other drugs which are known to have specific action on Ascaris include the following tetramisole, pyrantel pamoate, bephanium hydroxynaphthoate, diethylcarbamazine (Hetrazan), thiobendazole and mebendazole.
Human Disease # 2. Ancylostomiasis:
Ancylostomiasis is caused by two hookworms Ancylostoma duodenale and Necator americanus. Both the hookworms are parasites within the intestine. The adult worms live in the small intestine of man particularly in jejunum, less often in duodenum and rarely in ileum.
They are most frequent in rural areas. Female hookworms produce 5000 to 10,000 eggs per day which pass out in the stools. Man acquires infection when the eggs hatch and the larvae penetrate through the skin of the hands and feet. Infection occurs when man walks bare-foot on the faecally contaminated soil. The filariform larvae penetrate directly through the skin with which they come in contact.
The most common sites of the entry are:
(i) The thin skin between the toes;
(ii) The dorsum of the feet, and
(iii) The inner side of the soles.
Infection may also occur by accidental drinking of water contaminated with filariform larvae.
The filariform larvae enter the blood vessels and are carried to the lungs. Now they make their way to one of the bronchi, trachea and larynx, crawl over the epiglottis to the back of the pharynx and are ultimately swallowed. The growing larvae settle down in the small intestine, undergo moulting and develop into adult worms.
The characteristic symptoms of ancylostomiasis are ancylostome dermatitis or ground itch, and creeping eruption by ancylostome larvae, and gastro-intestinal disorders, and severe anaemia by adult worms. Gastro-intestinal manifestations produce dyspeptic troubles associated with epigastric tenderness stimulating duodenal ulcer.
Due to severe anaemia the skin becomes pale yellow in colour and the mucous membrane of the eyes, lips and tongue becomes extremely pale. The face appears puffy with swelling of lower eyelids and there is oedema of the feet and ankle. The general appearance of the patient is a pale plumpy individual with protruded abdomen and dry lustreless hair.
For the treatment of hookworm infection the following steps are to be taken:
(i) Expulsion of worms by antihelminthic drugs and
(ii) Treatment of anaemia.
Human Disease # 3. Enterobiasis:
Enterobiasis is caused by Enterobius vermicularis commonly called pinworm, threadworm or seatworm.
These worms are small and white in colour. Male worm measures 2 to 4 mm and female worm measures 8 to 12 mm in length. Adult worms (gravid females) live in the caecum, colon and vermiform appendix of man. The females migrate out through the colon and rectum and enormous number of eggs in the skin folds about the anus, where they cause intense itching.
Each of the egg, newly laid in perianal skin, containing a tadpole-like larva completes its development in 24 to 36 hours time, in the presence of oxygen.
Infection occurs by the ingestion of these eggs. When the skin about the anus is scratched, eggs are easily picked upon the fingers and under the nails from where they find their way to food and are swallowed. The egg-shells are dissolved by digestive juices and the larvae escape in the small intestine where they develop into adult worms.
The pinworm infection is more frequent in children than in adults. The symptoms of enterobiasis include severe itching around the anus, loss of appetite, sleeplessness and sometimes inflammation of the vermiform appendix. Enterobiasis is treated with antihelminthics such as piperazine citrate, pyrevinium pamoate (Povan), pyrental pamoate, stibazium iodide, thiobendazole and mebendazole.
Human Disease # 4. Trichuriasis:
Trichuriasis is caused by Trichuris trichura, commonly known as whipworm. The adult worms live in the large intestine of man, particularly in the caecum; also in vermiform appendix.
The worm resembles a whip in shape and general appearance. Male measures 3 to 4 cm and female measures 4 to 5 cm in length. The females lay enormous number of eggs daily that pass in the stool. Development proceeds slowly in water and damp soil.
A rhabditiform larva develops within the egg in the course of 3 to 4 weeks in tropical countries. The embryonated eggs are infective to man. Man is infected when the embryonated eggs are swallowed with food or water. The egg-shell is dissolved by the digestive juices and the larva emerges.
The liberated larvae pass down into the caecum, their site of localisation. They grow directly into adult worms and embed their anterior parts in the mucosa of the intestine. The worms become sexually mature within a month from the time of ingestion of the eggs and gravid females begin to lay eggs. The cycle is then repeated.
The patient suffering from trichuriasis (whipworm disease) shows the symptoms of acute appendicitis. In heavy infections the patient often complains of abdominal pain, mucous diarrhoea often with blood streaked stool and loss of weight. Prolapse of rectum has occasionally been observed in massive trichuriasis.
The drugs at present most commonly used for the treatment of trichuriasis are stibazium iodide, deftarsone, thiobendazole and mebendazole.
Human Disease # 5. Trichinosis:
Trichinosis is caused by Trichinella spiralis, the trichinia worm. It is one of the smallest nematodes infecting man. The male measures 1.4 to 1.6 mm and female measures 3.0 to 4.0 mm in length. This disease is common and widespread in Europe and America. Although it prevails in areas where pork is eaten. Humans become infected by eating undercooked or raw meat containing encysted larvae mainly pork.
The cysts, located in striated muscles, are digested liberating larvae that mature to adult worms that attach to the wall of small intestine. Female worms there liberate larvae that invade the intestinal wall, enter the circulation and penetrate the striated muscles, where they encyst and remain viable for years. Usually one larva is present in a single cyst.
The early symptoms of trichinosis is eosinophilia. The invasion of muscle by larvae is associated with muscle pain, swelling of the eyelids and facial oedema, eosinophilia and pronounced fever. Respiratory and neurologic manifestations may appear. On invasion of the muscle layer the larvae cause inflammation and destruction of muscle fibres.
The most frequently involved muscles are those of limbs, diaphragm, tongue, jaw, larynx, ribs and eyes. Larvae in other organs, including the heart and brain cause oedema and necrosis.
The diagnosis is made by identifying larvae in muscle biopsies or by serological tests. Antihelminthic drugs remove adult worms from the intestine. Promising results have been obtained in the treatment of trichinosis by thiobendazole (Botero, 1965). Corticosteroids have been found to be useful in alleviating clinical symptoms.
Human Disease # 6. Strongyloidiasis:
Strongyloidiasis is an infection caused by the nematode Strongyloides stercoralis, commonly called threadworm.
It is found worldwide but is most common in tropical countries. S. stercoralis is a complex organism that has three life cycles which are as follows:
(i) Parasitic pathogenic females live in the human small intestine and lay eggs that hatch in the mucosal epithelium, releasing rhabditiform larvae. These larvae become infective filariform larvae in the intestine or on the perianal skin and invade human host directly (the autoinfection cycle).
(ii) The rhabditiform larvae pass in the faeces, become infective filariform larvae in the soil and later penetrate human skin (direct development cycle).
(iii) The rhabditiform larvae passed in the faeces become free-living adults in the soil and eventually produce infective filariform larvae. These infective larvae penetrate the skin, enter blood vessels and pass to the lungs, where they invade alveoli. They ascend the trachea, descend the oesophagus and mature to become parthenogenic females in the small intestine.
Invading larvae cause dermatitis. Larvae migrating through lungs may provoke cough, haemoptysis and dyspnoea, severe infection of the intestine causes vomiting, diarrhoea, and constipation. Female worms and rhabditiform larvae living in jejunum crypts cause mild eosinophilia and chronic inflammation.
By contrast patients with hyperinfection may have ulceration, oedema, congestion fibrosis and severe inflammation of the intestine. The diagnosis is made by identifying larvae in the stool.
The most specific antihelminthic drug for treatment of strongyloidiasis is thiobendazole.
Human Disease # 7. Filariasis or Elephantiasis:
Filariasis is caused by Wuchereria bancrofti commonly called the filaria worm. The adult worms inhabit lymphatic vessels, most frequently those in the lymph nodes, testes and epididymis. The female worm discharges microfilariae that circulate in the blood. Humans are the only definitive host of these worms.
Insect vectors which serve also as intermediate hosts, include 80 species of mosquitoes of the genera Culex, Aedes, Anopheles and Mansonia. Filariasis is endemic in large regions of Africa, coastal areas of Asia, Western Pacific islands and coastal areas and islands of the Caribbean basis.
In India, it is distributed chiefly along the sea coast and along the banks of big rivers (except Indus); it has also been reported from Rajasthan, Punjab, Uttar Pradesh and Delhi. Following copulation the female worm delivers larvae called microfilariae. These, at night, get in the blood capillaries of the skin to be sucked up by the mosquito with blood meal.
When the infected mosquito bites a human being, the microfilariae are not directly injected into the blood but are deposited on the skin near the site of puncture. Later, attracted by the warmth of the skin, the microfilariae either enter through the puncture wound or penetrate through the skin on their own.
After penetrating the skin, microfilariae reach the lymphatic channels, settle down at some spot (inguinal, scrotal, or abdominal lymphatics) and begin to grow into adult forms.
Features of acute infection include fever, lymphangitis, lymphadenitis, orchitis, epididymitis, urticaria, eosinophilia and microfilaremia. Chronic infection is characterised by enlarged lymph nodes, lymphoedema, hydrocele and elephantiasis. Filariasis also causes tropical eosinophilia which is characterised by cough, wheezing, eosinophilia and diffuse pulmonary infiltrates.
The infection of the filaria worms also causes enlargement of the limbs, scrotum and mammae. The swelling takes place due to blockage of the lymph circulation by the parasitic worms resulting into the inflammation of lymph vessels and lymph glands. The diagnosis is usually made by identifying the microfilariae in the blood.
There is no effective drug for the eradication of the filaria worm. The drug of choice is diethylcarbamazine (Hetrazan) which kills microfilariae and possibly adult worms.
Human Disease # 8. Loiasis:
Loiasis is an infection caused by the filarial nematode Loa loa, the African eyeworm or loa- worm. It inhabits the rain forests of Central and West Africa. Humans and baboons are definitive hosts and infection is transmitted by mango flies (Chrysops species).
The adult L.loa migrates in the skin and occasionally crosses the eye beneath the conjunctiva, making the patient actually aware of his infection. Gravid worms discharge microfilariae that circulate in the blood stream during the day but reside in capillaries of the skin, lungs and other sense organs at night.
Most infections are symptomless but persist for years. Ocular symptoms include swelling of lids, congestion, itching and pain. Female worms, and rarely male worms may be extracted during their migration beneath the conjunctiva. Systemic reactions include fever, pain, itching, urticaria and eosinophilia.
The diagnosis is made by identifying microfilariae in the blood films taken during the day, by removal of adult worm from conjunctiva, or by identifying microfilariae or adult worms in biopsy specimen. Diethylcarbamazine (Hetrazan) is an effective remedy for loiasis, causing a quick disappearance of microfilariae from the peripheral blood and even death of adult worms in some cases.
Human Disease # 9. Onchocerciasis:
Onchocerciasis is the infection caused by the filarial nematode Onchocerca volvulus. It is one of the world’s major endemic diseases, afflicting an estimated 40 million people, of whom about 2 million are blind. Man is the only known definitive host. Onchocerciasis is transmitted by several species of black flies of the genus Simulium, which breeds in fast flowing streams.
There are endemic regions throughout the tropical Africa and in focal areas of Central and South America.
The adult worms live singly and as coiled entangled masses in the subcutaneous tissues of man. The gravid female worms produce millions of microfilariae which migrate from the nodule into the skin, eyes, lymph nodes and deep organs causing the onchocercal lesions. The diagnosis is made by identifying the microfilariae in tissue sections of skin and the adult worms in the subcutaneous nodules.
The cardinal manifestations are subcutaneous nodules, dermatitis and eye disease. Nodulectomy removes adult worms in palpable nodules. Suramin kills adult worms but has dangerous side effects. Oral diethylcarbamazine (Hetrazan) kills microfilariae. A new drug, ivermectin, kills microfilariae but with a lesser allergic reaction than diethylcarbamazine.