Ascaris lumbricoides

Prepared by: MBLOGSTU

1. Introduction

Ascaris lumbricoides, one of four soil‑transmitted helminths (nematodes), is among the most prevalent parasitic infections in humans. Approximately one‐fifth of the world’s population is infected. In developed countries, ascariasis may also occur due to Ascaris suum, found in domestic swine reared under unhygienic conditions.

The high prevalence and broad geographic spread of A. lumbricoides can be attributed to the robust nature of its eggs, the large number of eggs produced per female worm, and conditions of poor socioeconomic status that facilitate transmission.

Ascariasis persists in areas where fecal sanitation is inadequate, as infected individuals contaminate the environment, ensuring ongoing transmission.

2. Epidemiology

An estimated 73% of A. lumbricoides infections occur in Asia, with approximately 12% in Africa and 8% in Latin America. However, the infection is cosmopolitan, affecting both temperate and tropical regions.

Poor socioeconomic conditions and inadequate sanitation are key risk factors. In many developing countries, high population density in slums and shanty towns facilitates transmission. In addition, cultural practices such as geophagia (soil-eating) contribute to infection.

Infections may cluster within households, with heavier worm burdens in families with many members.

3. Morphology

Adult Worms

• Appearance: Small, greyish white, cylindrical worms; when freshly passed, they appear reddish brown due to ingested blood.
• Size: Males are approximately 8 mm in length; females measure about 12.5 mm.

Eggs

• Shape: Oval or elliptical, with a thin shell and a clear space between the developing embryo and the shell.
• Size: Approximately 65 μm in length and 40 μm in breadth.
• Characteristics: Colorless (non bile-stained), surrounded by a transparent hyaline membrane, and usually contain a segmented ovum with 4 blastomeres.
• Behavior: The eggs float in saturated salt solution.

4. Life Cycle

The life cycle of Ascaris lumbricoides begins when embryonated eggs are ingested from contaminated food, water, or soil.

1. Ingestion: Humans ingest the infective eggs from the environment.
2. Larval Hatching: In the small intestine, the eggs hatch into larvae.
3. Tissue Migration: The larvae penetrate the intestinal wall and migrate through the bloodstream, eventually reaching the lungs. Here, they mature and may cause respiratory symptoms.
4. Swallowing: From the lungs, larvae ascend the trachea and are swallowed, returning to the intestine.
5. Maturation: In the small intestine, larvae mature into adult worms.
6. Reproduction: The adult worms reproduce; females produce thousands of eggs that are passed in the feces, continuing the cycle.

5. Pathogenesis

The pathogenesis of Ascaris lumbricoides infection is linked to both the mechanical damage caused by larval migration and the host’s immune response.

• Larval Migration: As larvae migrate through tissues, they cause mechanical injury and lysis of host cells by releasing digestive enzymes.
• Granuloma Formation: Invasion by larvae can trigger granulomatous inflammation, with eosinophils, neutrophils, and macrophages accumulating at the infection site.
• Intestinal Obstruction: Although most infections are mild, even a single worm can cause significant morbidity if it migrates into a sphincter or duct (e.g., the common bile duct, leading to obstruction).
• Immune Response: Ascaris infection results in both a specific and a nonspecific increase in circulating IgE levels.

6. Clinical Manifestations

The majority of patients with Ascaris infections remain asymptomatic. When symptoms do occur, they are generally proportional to the worm burden:

• Intestinal Involvement: May range from mild discomfort and occasional abdominal pain to intestinal obstruction and malabsorption. In severe cases, a single worm migrating into the bile duct can cause obstructive symptoms.
• Respiratory Manifestations: During the larval migration phase, pulmonary symptoms (such as cough, dyspnea—sometimes referred to as Ascaris pneumonia or Loeffler’s syndrome) may be observed.
• Systemic Manifestations in Children: In heavy infections, children can develop malnutrition and growth retardation.

7. Laboratory Diagnosis

Laboratory diagnosis is primarily based on stool examination:

• Direct Microscopy: Identification of characteristic eggs in stool via direct wet smears or concentration methods (sedimentation or flotation).
• Egg Morphology: Eggs are characterized by their oval shape, thin shell, and clear space between the developing embryo and the shell. Preservation is important; if the stool is left unpreserved for over 24 hours, eggs may continue to develop or hatch.
• Radiography: In cases of heavy infection, imaging may reveal the alignment of two or more adult worms in the intestine, sometimes described as “trolley car lines.”
• Sputum Examination: In the larval migration phase, larvae may occasionally be recovered from sputum, though this is uncommon.

8. Treatment

The primary treatment for ascariasis involves the use of anthelmintic medications:

• Mebendazole
• Albendazole

The intensity of treatment is generally proportional to the worm burden, with heavy infections requiring repeated courses.

9. Prevention and Control

Effective prevention of ascariasis depends on addressing the underlying socioeconomic and sanitation issues:

• Improve fecal sanitation and ensure proper disposal of human waste.
• Promote public health education regarding personal and food hygiene.
• Provide access to effective anthelmintic drugs in endemic areas.
• Long-term reduction in prevalence depends on alleviating poverty and improving housing and community services.

10. References

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