Chapter 2-Nematodes

Nematodes-Refer to powerpoint for lifecycles and tables.

INTESTINAL HELMINTHS
Ascaris lumbricoides (roundworm),
Trichinella spiralis (trichinosis),
Trichuris trichiura (whipworm),
Enterobius vermicularis (pinworm),
Strongyloides stercoralis (Cochin-china diarrhea), Ancylostoma duodenale and
Necator americanes (hookworms) and
Dracunculus medinensis
E. vermicularis and T. trichiura are exclusively intestinal parasites.

Ascaris
Epidemiology The annual global morbidity due to Ascaris infections is estimated at 1 billion with a mortality of 20,000.
Ascariasis can occur at all ages, but it is more prevalent in the 5-9 years age group. The incidence is higher in poor rural populations.
Morphology-The female worm larger than male.

Ascaris Life Cycle
The infection is man--man. Auto infection can occur. Ingestion of food with infective eggs which hatch in the upper small intestine.
The larvae penetrate the intestine and enter the venules or lymphatics.
The larvae pass through the liver, heart and lung to reach alveoli in 1-7 days.
They migrate up the bronchi, ascend the trachea to the glottis, and pass down the esophagus to the small intestine where they mature in 2-3 months.
A female may live in the intestine for 12-18 months (capacity of producing 25 million eggs at an average daily output of 200,000) .
The eggs are in feces, and, at 21 to 30 C, moist, aerated environment, infective larvae are formed in the egg. The eggs are resistant to disinfectant but are killed by heat (40 C for 15 hours).

Ascaris
Symptoms Symptoms are related to the worm burden.
The commonest complaint is vague abdominal pain. I
In more severe cases, listlessness, weight loss, anorexia, distended abdomen, intermittent loose stool and occasional vomiting.
During the pulmonary stage, brief cough, wheezing, dyspnea and sub-sternal discomfort. Most symptoms are due to the physical presence of the worm.
Treatment and Prevention Mebendazole is effective. Good hygiene is the best preventive measure

Trichinella spiralis (Trichinosis)
Epidemiology Consumption of poorly cooked pork. About 2 % of the population is infected. The mortality rate is low.
Morphology

Trichinella
Life cycle Infection occurs by ingestion of larvae, in poorly cooked meat, which immediately invade intestinal mucosa and sexually differentiate within 18 -24 hours.
The female, after fertilization, burrows deeply in the small intestinal mucosa, whereas the male is dislodged (intestinal stage).
By day 5, eggs begin to hatch and young larvae are deposited in the mucosa where they reach the lymphatics, lymph nodes and the blood stream (larval migration).
Larval dispersion occurs 4 -16 weeks after infection. The larvae are deposited in muscle fiber and, in striated muscle, where they form a capsule which calcifies to form a cyst.
In non-striated tissue, such as heart and brain, the larvae do not calcify; they die and disintegrate. The cyst may persist for several years. One female worm produces approximately 1500 larvae. Man is the terminal host. The reservoir includes most carnivorous and omnivorous animals .

Trichinella
Symptoms Trichinosis symptoms depend on the severity of infection: mild infections may be asymptomatic. A larger bolus of infection produces symptoms according to the severity and stage of infection and organs involved.
Pathology and Immunology Trichinella pathogenesis is due the presence of large numbers of larvae in vital muscles and host reaction to larval metabolites.
The muscle fibers become enlarged edematous and deformed. The paralyzed muscles are infiltrated with neutrophil, eosinophils and lymphocytes. Splenomegaly is dependent on the degree of infection.
The worm induces a strong IgE response which, in association with eosinophils, contributes to parasite death.

Trichinella
Diagnosis Diagnosis is based on symptoms, recent history of eating raw or undercooked meat and laboratory findings (eosinophilia, increased serum creatine phosphokinase and lactate dehydrogenase and antibodies to T. spiralis).
Treatment and Control Steroids are use for treatment of inflammatory symptoms and Mebendazole is used to eliminate worms.
Elimination of parasite infection in hogs and adequate cooking of meat are the best ways of avoiding infection.

Trichuris trichiura (whipworm)
Epidemiology Trichuriasis is a tropical disease of children (5 to 15 yrs) in rural Asia (65% of the 500-700 million cases). It is, however, seen in the two Americas, mostly in the South and is concentrated in families and groups with poorer sanitary habits.

Morphology The female organism is 50 mm long with a slender anterior (100 micrometer dia,eter) and a thicker (500 micrometers diameter) posterior end. The male is smaller and has a coiled posterior end. The Trichuris eggs are lemon or football shaped and have terminal plugs at both ends.

Trichuris
Life cycle Infection occurs by ingestion of embryonated eggs in soil. The larva escapes the shell in the upper small intestine and penetrates the villus where it remains for 3-10 days.
Upon reaching adolescence, the larvae pass to the cecum and embed in the mucosa.
They reach the ovipositing age in 30-90 days from infection, produce 3000 -10,000 eggs per day and may live as long as 5 to 6 years.
Eggs passed in feces embryonate in moist soil within 2-3 wks.
The eggs are less resistant to desiccation, heat and cold than Ascaris eggs. The embryo is killed under desiccation at 37 degrees C within 15 minutes. Temperatures of 52 degrees C and -9 degrees C are lethal.

Trichuris
Symptoms Symptoms are determined largely by the worm burden: less than 10 worms are asymptomatic.
Heavier infections (e.g., massive infantile trichuriasis) are characterized by chronic profuse mucus and bloody diarrhea with abdominal pains and edematous prolapsed rectum.
The infection may result in malnutrition, weight loss and anemia and sometimes death.

Trichuris
Diagnosis Diagnosis is based on symptoms and the presence of eggs in feces
Treatment and Control Mebendazole, 200 mg, for adults and 100 mg for children, for 3 days is effective.
Accompanying infections must be treated accordingly. Improved hygiene and sanitary eating habits are most effective in control.

Enterobius vermicularis (pinworm)
Epidemiology Enterobiasis is by far the commonest helminthic infection in the US (18 million cases at any given time). The worldwide infection is about 210 million.
It is an urban disease of children in crowded environment (schools, day care centers, etc.).
Adults may get it from their children.

Morphology The female worm measures 8 mm x 0.5mm; the male is smaller. Eggs are ovoid but asymmetrically flat on one side.

Enterobius
Life cycle Infection occurs when embryonated eggs are ingested from the environment, with food or by hand to mouth contact.
The embryonic larvae hatch in the duodenum and reach adolescence in jejunum and upper ileum.
Adult worms descend into lower ileum, cecum and colon and live there for 7 to 8 weeks.
The gravid females, containing more than 10,000 eggs migrate, at night, to the perianal region and deposit their eggs there.
Eggs mature in an oxygenated, moist environment and are infectious 3 to 4 hours later.
Man-to-man and auto infection are common. Man is the only host.

Enterobius
Symptoms Enterobiasis is relatively innocuous and rarely produces serious lesions.
The most common symptom is perianal, perineal and vaginal irritation caused by the female migration.
The itching results in insomnia and restlessness. In some cases gastrointestinal symptoms (pain, nausea, vomiting, etc.) may develop.

Enterobius
Diagnosis Diagnosis is made by finding the adult worm or eggs in the perianal area, particularly at night. Scotch tape or a pinworm paddle is used to obtain eggs.
Treatment and Control Two doses (10 mg/kg; maximum of 1g each) of Pyrental Pamoate two weeks apart gives a very high cure rate.
Mebendazole is an alternative. The whole family should be treated, to avoid reinfection.
Bedding and underclothing must be sanitized between the two treatment doses. Personal cleanliness provides the most effective in prevention.

Strongyloides stercoralis (Threadworm)
Epidemiology Threadworm infection, also known as Cochin-China diarrhea, estimated at 50 to 100 million cases worldwide, is an infection of the tropical and subtropical areas with poor sanitation. In the United States, it is prevalent in the South and among Puerto Ricans.
Morphology The size and shape of threadworm varies depending on whether it is parasitic or free-living. The parasitic female is larger than the free-living worm.

Strongyloides
Life cycle The larvae penetrate the skin of man, enter the venous circulation and pass through the right heart to lungs, where they penetrate into the alveoli.
From there, the adolescent parasites ascend to the glottis, are swallowed, and reach the upper part of the small intestine, where they develop into adults.
Ovipositing females develop in 28 days from infection. The eggs in the intestinal mucosa, hatch and develop into rhabditiform larvae in man.
These larvae can penetrate through the mucosa and cycle back into the blood circulation, lung, glottis and duodenum and jejunum; thus they continue the auto infection cycle.
Alternatively, they are passed in the feces, develop into infective filariform larvae and enter another host to complete the direct cycle. If no suitable host is found, the larvae mature into free-living worm and lay eggs in the soil.
The eggs hatch in the soil, produce rhabditiform larvae which develop into infective filariform larvae and enter a new host, or mature into adult worms to repeat the free-living cycle.

Strongyloides
Symptoms Light infections are asymptomatic.
Skin penetration causes itching and red blotches.
During migration, the organisms cause bronchial verminous pneumonia and, in the duodenum, they cause a burning mid-epigastric pain and tenderness accompanied by nausea and vomiting.
Diarrhea and constipation may alternate. Heavy, chronic infections result in anemia, weight loss and chronic bloody dysentery.
Secondary bacterial infection of damaged mucosa may produce serious complications.

Strongyloides
Diagnosis The presence of free rhabditiform larvae in the feces is diagnostic. Culture of stool for 24 hours will produce filariform larvae.
Treatment and control Ivermectin or thiabendozole can be used effectively. Direct and indirect infections are controlled by improved hygiene and auto-infection is controlled by chemotherapy.

Necator americanes and Ancylostoma duodenale (Hookworms)
Epidemiology Hookworms parasitize more than 900 million people worldwide and cause daily blood loss of 7 million liters.
Ancylostomiasis is the most prevalent hookworm infection and is second only to ascariasis in infections by parasitic worms.
N. americanes (new world hookworm) is most common in the Americas, central and southern Africa, southern Asia, Indonesia, Australia and Pacific Islands. A. duodenale (old world hookworm) is the dominant species in the Mediterranean region and northern Asia.
Morphology Adult female hookworms are larger than the males. N. americanes has a pair of curved cutting plates whereas A. duodenale has one or more pairs of teeth.

Hookworm
Life cycle The life cycle of hookworms is identical to that of threadworms, except that hookworms are not capable of a free-living or auto-infectious cycle. Furthermore, A. duodenale can infect also by oral route.
Symptoms Symptoms of hookworm infection depends on the worm site and worm burden. Light infection may not be noticed
Hookworm
Diagnosis Diagnosis is made by identification of hookworm eggs in fresh or preserved feces.
Species of hookworms cannot be distinguished by egg morphology.
Treatment and control Mebendazole, 200 mg, for adults and 100 mg for children, for 3 days is effective.
Sanitary disposal of fecal material and avoidance of contact with infected fecal material best method of control.


Dracunculus medinensis (Guinea worm; fiery serpent of the Israelites)
Epidemiology There have been dramatic efforts to eradicate Dracunculus. Guinea worm disease is expected to be the next disease after smallpox to be eradicated and presently there may be as few as 1000 cases worldwide.
Morphology The adult female worm measures 50-120 cm by 1 mm and the male is half that size.

Life Cycle of Dracunculus
The infection is caused by ingestion of water contaminated with water fleas (Cyclops) infected with larvae.
The rhabtidiform larvae penetrate the human digestive tract wall, lodge in the loose connective tissues and mature into the adult in 10-12 weeks.
In about a year, the gravid female migrates to the subcutaneous tissue of organs that normally come in contact with water and discharges its larvae into the water.
The larvae are picked up by Cyclops, in which they develop into infective form in 2 to 3 weeks.

Dracunculus
Diagnosis
From the local blister, worm or larvae. The outline of the worm under the skin may be revealed by reflected light.
Treatment Treatment includes the extraction of the adult guinea worm by rolling it a few centimeters per day or preferably by multiple surgical incisions under local anaesthesia.
Metronidazole is effective in killing the worm. Protection of drinking water from being contaminated with Cyclops and larvae are effective preventive measures.

Ancylostoma braziliensis (cutaneous larva migrans, creeping eruption)
Disease is prevalent in many tropical and subtropical countries and in the US especially along the southern Atlantic states.
The organism is a hookworm of dogs and cats but the filariform larvae in animal feces can infect man and cause skin eruptions.
The larvae have a tendency to move migrate in the skin.
The duration of larval persistence ranges from 2 to 10 weeks.
Light infection can be treated by freezing the area. Heavier infections are treated with Mebendazole.

Toxocara canis and T. catti (visceral larva migrans)
These are roundworms of dogs and cats but they can infect humans and cause damage of the visceral organs.
Eggs from feces of infected animals are swallowed by man and hatch in the intestine.
The larvae penetrate the mucosa, enter the circulation and are carried to liver, lungs, eyes and other organs where they cause inflammatory necrosis.
Symptoms are due to the inflammatory reaction at the site of infection.
The most serious consequence of infection may be loss of sight if the worm localizes in the eye.
Treatment includes Mebendazole to eliminate the worm and prednisone for inflammatory symptoms.


BLOOD AND TISSUE HELMINTHS
The major blood and tissue parasites of man are microfilaria. These include:
Wuchereria bancrofti
W. (Brugia) Malayi,
Onchocerca volvulus
Loa loa (eye worm)

Wuchereria bancrofti and W. (Brugia) malayi (elephantiasis)

Epidemiology W. bancrofti is strictly a human pathogen and is distributed in tropical areas worldwide, whereas B. malayi infects a number of wild and domestic animals and is restricted to South-East Asia. Mosquitoes are vectors for both parasites.

Morphology These two organisms are very similar in morphology and in the diseases they cause . Adult female W. bancrofti found in lymph nodes and lymphatic channels are larger than the males which are only half that size.
Filariform Life Cycle
Filariform larvae enter the human body during a mosquito bite and migrate to various tissues.
They may take up to a year to mature and produce microfilaria which migrate to lymphatics and, at night, enter the blood circulation.
Mosquitos are infected during a blood meal. The microfilaria grow 4 to 5 fold in the mosquito in 10 to 14 days and become infective for man.

Microfilariae
Diagnosis Diagnosis is based on history of mosquito bites in endemic areas, clinical findings and presence of microfilaria in blood samples collected at night.

Treatment and control Diethylcarbamazine quickly kills the adults worms or sterilizes the female. It is given 2 mg/kg orally for 14 days. Steroids help alleviate inflammatory symptoms. Cooler climate reduces the inflammatory reaction.

Onchocerca volvulus (Blinding filariasis; river blindness)
Epidemiology Onchocerciasis is prevalent throughout eastern, central and western Africa, where it is the major cause of blindness.
In the Americas, it is found in Guatemala, Mexico, Colombia and Venezuela. The disease is confined to neighborhoods of low elevation with rapidly flowing small streams where black flies breed. Man is the only host.
Morphology Adult female is larger than the male. Infective larvae of O. volvulus are 500 um by 25um.

Onchocerca
Life cycle Infective larvae are injected into human skin by the female black fly (Simulium damnosum) where they develop into adult worms in 8-10 mo.
The adults usually occur as group of tightly coiled worms (2 to 3 females and 1 to 2 males).
The gravid female releases microfilarial larvae, which are usually distributed in the skin.
They are picked up by the black fly during a blood meal.

Onchocerca
Symptoms Onchocerciasis results in nodular and erythematous lesions in the skin and subcutaneous tissue due to a chronic inflammatory response to persistent worm infection. During the incubation period of 10-12 months, there is eosinophilia and urticaria. Ocular involvement consists of trapping of microfilaria in the eye leading to photophobia, lacrimation and blindness.

Onchocerca
Diagnosis Diagnosis is based on symptoms, history of exposure to black flies and presence of microfilaria in nodules.

Treatment and control Diethylcarbamazine is effective in killing the worm. Destruction of microfilaria produces extreme allergic reaction which can be controlled with corticosteroids. Prevention measures include vector control, treatment of infected individuals and avoidance of black fly.

Loa loa (eye worm)
Loasis is limited to the areas of African equatorial rain forest. The incidence in endemic areas varies greatly (8 to 75 percent).

The life cycle of Loa loa is identical to that of onchocerca except that the vector for this worm is the deer fly.

The infection results in subcutaneous (Calabar) swelling, measuring 5 to 10 cm in diameter, marked by erythema and angioedema, usually in the extremities. The organism migrates under the skin at a rate of up to an inch every two minutes. Consequently, the swelling appears spontaneously, persists for 4 to 7 days and disappears, and is known as fugitive or Calabar swelling.

The worm usually causes no serious problems, except when passing through the orbital conjunctiva or the nose bridge. The diagnosis is based on symptoms, history of deer fly bite and presence of eosinophilia. Recovery of worms from the conjunctiva is confirmatory