ASCARIS LUMBRICOIDES (The large round worm)

GEOGRAPHICAL DISTRIBUTION

The worm has a world wide distribution but prevalence is high in warmer climates in tropical region where personal and environmental sanitation are poor. It is prevalent in countries like India, Nepal, Pakistan and Sri Lanka.

MORPHOLOGY

Adult female worms are 20-40 cm in length while the adult male worms measure 15-30 cm. The tail end of male is curved.

LOCATION IN THE HOST

The worms are found free in the lumen of the small intestine.

LIFE CYCLE

The fertilized female lays about 200.000 eggs per day. The eggs are oval in shape (65*45µm). The egg has an outer mammilated, ailbuminous coat that appears brown due to bile pigments. Some eggs may be decorticated (with outer coat missing). A certain portion of eggs (15%) is unfertilized and appear longer and rectangular (90*40µm) with no embryo. The embryo is unsegmented when eggs are passed in faeces.

The eggs develop further in soil with the first moult taking place (within the egg) to form the 1^st stage larva in about 7 days. With the second moult the infective 2^nd stage larva is formed inside the egg in 2-3 weeks under optimal conditions of development such as moist, warm (25-30 ºC) clay type soil. The infective eggs can remain viable in suitable soil for long periods of time.

When ingested the eggs hatch in the upper part of the small intestine liberating the larvae which penetrate the intestinal mucosa to enter the blood stream (or the lymphatics). The larvae are carried into the lungs via the liver and the heart. The larvae develop further in the lungs moulting twice (5^th and 7^th days). They penetrate the alveolar walls to move along the bronchioles, bronchi and trachea and are swallowed. On reaching the small intestine they mature into males and females. The time taken for an egg to develop into a mature is about 60 days. Adult worms may live up to 2 years.

PATHOGENESIS AND CLINICAL FEATURES

Many infected persons remain asymptomatic. However the presence of a small number of worms may lead to complications. The pathogenesis and clinical features can be categorized into; pulmonary ascariasis: intestinal ascariasis: complications of ascariasis and allergy to Ascaris species.

1. Pulmonary ascariasis: An intense host reaction occurs in the lungs as a result of larval migration, 5-6 days after the ingestion of infective eggs. Larval antigens released by moulting larvae elicit an inflammatory reaction associated with moderate eosinophilia, pneumonitis, bilateral pulmonary infiltration, cough, dyspnea, substernal pain, fever, skin rashes and often asthma. These signs and symptoms are collectively called the “Loffler’s Syndrome”. The severity of the host reaction depends on the number of larvae migrating and the previous infection history: hypersensitive individuals show severe reactions with other allergic manifestations. Loffler’s syndrome is usually transient, lasted about 2-5 days. Pulmonary pathology is primarily immunological in nature.
2. Intestinal ascariasis: Adult worms in the small intestine may cause mild abdominal pain and restlessness. Some asymptomatic persons are known to be harbour large worm loads. However, the clinical consequences of the infection are generally dependent on the number of worms present (worm load). It is difficult to say whether ascariasis is a direct cause of malnutrion as the infection is common in areas where malnutrion co-exist. But the infection certainly precipitates severe malnutrition (even kwashiorkor or marasmus) in undernourished children. Ascariasis as also known to lead to vitamin A deficiency. Improvement of growth in areas where protein energy malnutrition is prevalent has been demonstrated following worm treatment. Similarly worm treatment has been shown to improve physical fitness and educational abilities of malnourished children.
3. Complications of ascariasis: The commonest complication of ascariasis is intestinal obstruction, particularly in persons with heavy worm loads. They may also cause intestinal perforation leading to peritonitis. Adult worms may wander into orifices such as the opening of the bile duct, pancreatic duct causing obstruction bile and pancreatic secretion respectively. Some drugs, chemicals and food items may cause worms to get abnormally activated and migrate to distant site.
4. Allergy to Ascaris species: Allergic manifestations are common during the acute phase of larval migration and due to the presence of adult worms in the small intestine. Many allergic and infected persons experience asthma. The relationship of asthma, allergy and ascariasis is not clearly known. However it is well known that adult Ascaris worms are highly allergic and their handling may cause serious reactions in sensitive individuals.

DIAGNOSIS

The simplest diagnostic method is the demonstration of characteristic eggs in the faeces.

PREVENTION

Vegetable that are usually eaten raw or undercooked should be washed thoroughly. Fruits fallen under trees should be washed before eating. Washing of hands after working with soil helps in the prevention.

CONTROL

Prevention of indiscriminate defaecation, provision of sanitary latrines, elimination of parasite reservoir by worm treatment and health education are important.

TREATMENT

Broad spectrum antihelminthetic drugs like albendazole and mebandazole can be used. Usual drug course is 500mg stat for adults and the dose for children depends on the weight and the age.