Parasitology

ASCARIASIS (caused by Ascaris lumbricoides)

Written by MicroDok

Ascariasis is a parasitic disease that is caused by intestinal roundworms or nematodes in humans. The disease is widely distributed, and it is a common infection in places where sewage treatment and environmental sanitation is poor. The nematodes that cause ascariasis are usually maintained in children than in adults because children defecate indiscriminately, and they pick up the eggs of intestinal roundworms in the process especially when they play in contaminated soil. Children’s outdoor activities with their folks especially in faecally contaminated soil are among the risk factors necessary for the acquisition of intestinal worms. Humans usually become infected with causative agents of ascariasis following the ingestion of food or vegetables contaminated with eggs of the parasite. Roundworm infection (i.e., ascariasis) in children can cause clinically significant malnutrition and retardation especially in cases where there is heavy worm infection. It is noteworthy that countless number of healthy humans harbours intestinal nematodes but they remain asymptomatic. Ascariasis or roundworm infection is caused by roundworms particularly Ascaris lumbricoides.  

Type and morphology of Ascaris lumbricoides

Ascaris lumbricoides basically exist in two morphological forms i.e. as eggs which are ellipsoidal in shape and adult female and male worms. The eggs of A. lumbricoides are either fertilized or unfertilized. A. lumbricoides is a large worm, and the parasite has pointed ends with curved tails. They resemble earthworm in morphology, and the eggs of A. lumbricoides are resistant to environmental stress and they remain viable for a long period of time in the soil. The egg of A. lumbricoides embryonate in the soil (especially in warm and moist shady soil) for about three weeks before they become infectious; and it is only then that they can parasitize humans.

Vector, reservoir and habitat of Ascaris lumbricoides

Ascaris lumbricoides has no insect or animal vector. Humans are the only definitive or final host of the parasite. No intermediate host exists for A. lumbricoides; and the soil is the normal habitat of the parasite i.e. after they have been egested from infected human hosts.

Clinical signs and symptoms of Ascaris lumbricoides infection

The clinical signs and symptoms of ascariasis may include unclear abdominal cramp, fever, chronic cough, vomiting, weight loss, nausea, hypersensitivity reaction (e.g. asthma) and diarrhea. Most cases of ascariasis is usually asymptomatic and this is occurs when the worm load in the individual is low. Migrating A. lumbricoides worms can cause appendicitis; and perforation of the intestines may occasionally occur in severe cases due to heavy worm infection. The passing out of adult worms in the stool of infected persons especially children as well as the vomiting or coughing up of worms raises suspicion for ascariasis or roundworm infection. In children, ascariasis may cause growth retardation, malnutrition and intellectual impairment. Children who have the disease in endemic regions have pain and discomfort in their anus, and this causes them to always scratch the anal region and they may also miss school.

Pathogenesis of Ascaris lumbricoides infection

A geo-helminthic parasite, A. lumbricoides enters the body via the consumption or ingestion of food and vegetables contaminated with embryonated or viable eggs of the parasite from soil. A. lumbricoides is majorly transmitted to humans via the faecal pollution of the soil. Faecally-contaminated hands containing the viable eggs of A. lumbricoides (especially in infected children) could also serve as route via which the infection spreads. After ingestion of viable eggs, the infective eggs hatch in vivo to larval forms which migrate through the liver and the lungs. Larva are passed up the trachea and swallowed again before they eventually become mature worms in the small intestine. A. lumbricoides have similar lifecycle with hookworms but the former does not suck blood from the small intestine like the latter. Adult worms of A. lumbricoides produce numerous infective eggs which are latter shed in the faeces of infected persons (Figure 1). When infected individuals especially children defecate indiscriminately, the soil become polluted and human infection can easily occur via the ingestion of viable eggs of A. lumbricoides. It is noteworthy that adult worms of A. lumbricoides can as well pass through the anus of infected individuals. However, worm expellation from the anus is common in children, and it can also be experienced after mass de-worming of children especially in roundworm endemic regions.

Figure 1: Life cycle of Ascaris lumbricoides. 1. Eggs of A. duodenale (known as rhabditiform larvae) are passed in the stool of a human host. 2. The released rhabditiform larvae grow in the feces and/or the soil under favorable conditions (such as optimal moisture, warmth, and shade). The larvae hatch in 1 to 2 days. 3. After 5 to 10 days (and two molts) of hatching they become filariform (third-stage) larvae that are infective. 4. Filariform can survive 3 to 4 weeks in favorable environmental conditions. On contact with the human host, the larvae penetrate the skin and are carried through the blood vessels to the heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are eventually swallowed. 5. The larvae reach the small intestine, where they reside and mature into adult worms. Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant blood loss by the human host. Most adult worms are eliminated in 1 to 2 years after infection, but the longevity of the disease may reach several years. Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle). In addition, infection by A. duodenale may probably also occur by the oral and transmammary route (i.e. via breastfeeding). However, certain Ascaris species such as Necator americanus, however, requires a transpulmonary migration phase to initiate an infection. CDC

Laboratory diagnosis and treatment of Ascaris lumbricoides infection

Ascariasis is diagnosed in the laboratory by microscopical examination of fresh stool specimens for the eggs of A. lumbricoides (Figure 2). Expelled A. lumbricoides worms resemble earthworms, and they can also be identified in cases where the worms are expelled via the anus of infected persons. Ascariasis or roundworm infection can be treated with mebendazole, albendazole and pyrantel pamoate.

Figure 2: Eggs of A. lumbricoides. A. lumbricoides produces two types of eggs viz: fertilized egg (A) and unfertilized egg (B). Eggs of A. lumbricoides can remain viable in the soil for a long period of time which spans into years (e.g. up to 5 years or even more); and they are produced in thousands by female A. lumbricoides worms. Harsh environmental conditions such as high temperature and drying can reduce the viability of the eggs. CDC

Control and prevention of Ascaris lumbricoides infection

Ascariasis like hookworm disease is a preventable public health infection. Improvement in the health the general public, their nutrition, improvement in environmental sanitation and proper sewage management could help in containing and stopping the spread of the disease. Proper washing of hands with soap and running water especially after defecation and engaging in outdoor activities that involves contact with the soil helps in preventing A. lumbricoides infection in humans. Provision of adequate sanitation and toilet facilities in rural areas and in public places will help to prevent the disease. The use of human feaces as manure in farms should be discouraged as this could lead to the contamination of vegetables and other crops in the farm by the infective eggs of the parasite. Children living in endemic areas should be de-wormed on a regular basis, and infected persons should also be properly treated to avoid any possible re-infection.

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