Ascaris/Ascariasis

General Characteristics and Epidemiology

General features of Ascaris

Ascaris is a genus of parasitic nematodes (roundworms).

Adults: 

• Features:
  • Striated cuticle
  • 3 small lips
  • Apical mouth
  • Lifespan: 1–2 years
• Males: 
  • Diameter: 2–4 mm
  • Length: 15–30 cm long
  • Ventrally-incurvated end
• Females: 
  • Diameter: 3–6 mm
  • Length: 20–50 cm 
  • Straighter end
  • Can lay up to 200,000 eggs per day

Eggs: 

• Oval
• Thick, lumpy outer shell
• Unfertilized eggs are not infective.

Clinically relevant species

Ascariasis is caused by: 

• A. lumbricoides (most common)
• A. suum

Epidemiology

Ascariasis is one of the most common parasitic infections in humans.

• Approximately 1 billion people are infected worldwide.
• Majority of cases occur in:
  • Asia
  • Africa
  • South America
  • Rural > urban
  • Tropical, wet climates
• Children > adults

Pathogenesis

Reservoirs

A. lumbricoides:

• Humans
• Eggs in soil

A. suum:

• Pigs
• Humans

Transmission

• Fecal–oral route (A. lumbricoides and A. suum)
• Ingesting uncooked pig or chicken liver (A. suum)

Life cycle

• Embryonated eggs are ingested through contaminated food or water.
• Larvae hatch → penetrate the intestinal mucosa
• Larvae are carried through the portal system → systemic circulation → pulmonary circulation → lungs 
• Larvae mature over 1–2 weeks in alveoli.
• Migration up the trachea → coughed up and swallowed into the GI tract
• Within the intestines, larvae mature into adult worms → females lay eggs
• Eggs are excreted in feces → embryonate in the soil → cycle continues

Pathophysiology

Effect of larvae migration in the lungs → Löffler syndrome:

• Respiratory symptoms
• Eosinophilic pneumonitis 

Effect of adult worms in the GI tract:

• Competition for nutrients and impaired absorption → malnutrition (particularly in children)
• A tangled mass of worms in heavy infections → bowel obstruction
• Aberrant migration → obstruction of: 
  • Biliary tract → cholecystitis
  • Pancreatic ducts → pancreatitis
  • Appendix → appendicitis

Clinical Presentation

General

• Incubation period: 4–16 days after ingestion
• Most patients with ascariasis are asymptomatic.
• Symptomatic disease can present in 2 stages:
  • Early phase (pulmonary stage)
  • Late phase (intestinal stage)

Early phase

Symptoms of Löffler syndrome include:

• Fever
• Cough
• Blood-tinged sputum 
• Dyspnea
• Wheezing
• Substernal chest pain
• Urticaria

Late phase

• Abdominal pain
• Bloating
• Nausea and vomiting
• Anorexia
• Intermittent diarrhea
• Adult worms passed in the stool

Complications

• Malnutrition in children: 
  • Poor weight gain and failure to thrive
  • Short stature
  • Impaired cognitive development
• Bowel obstruction 
• Cholecystitis
• Acute cholangitis
• Liver abscess 
• Pancreatitis 
• Appendicitis

Diagnosis and Management

Diagnosis

Definitive:

• Microscopic examination of the stool for eggs
• Presence of worms in the stool

Supporting evaluation:

• CBC: eosinophilia (more commonly seen in the early phase)
• Imaging:
  • Chest X-ray: patchy infiltrates → Löffler syndrome
  • Abdominal X-rays: 
    • If concern for bowel obstruction
    • Barium contrast may demonstrate filling defects due to the presence of worms.
  • CT or MRI: 
    • May demonstrate worms in the bowel
    • “Bull’s eye” appearance on a cross-section of the worm
  • Ultrasound: evaluation of hepatobiliary complications

Management

• Albendazole (preferred)
• Mebendazole
• Pyrantel pamoate

Prevention

• Wash hands.
• Wash raw vegetables and fruits.
• Cook food properly.
• Sanitize properly.
• Do not use human or pig feces as fertilizer.

Comparison of Roundworms

Differential Diagnosis

• Pneumonia: infection of the lung parenchyma most often caused by a bacteria or virus. Patients present with fever, dyspnea, and productive cough. Chest X-ray findings usually show lobar consolidation; however, multifocal infiltrates can be seen in some cases. Management usually involves empiric antibiotics, which can be tailored if the causative organism is identified. Antivirals are used in cases where a viral cause is suspected.  
• Asthma: an inflammatory respiratory condition characterized by bronchial hyperresponsiveness and airflow obstruction. Unlike ascariasis, the condition is chronic. Patients typically present with wheezing, cough, and dyspnea. The diagnosis is confirmed with a pulmonary function test showing a reversible, obstructive pattern. Management includes bronchodilators and inhaled corticosteroids for control of inflammation. Biologic agents may be used for severe, persistent asthma.
• Allergic bronchopulmonary aspergillosis (ABPA): a hypersensitivity reaction to Aspergillus. Patients may have symptoms of airway obstruction such as dyspnea, wheezing, productive cough, and fever. Bronchiectasis and pulmonary fibrosis may occur if left untreated and eosinophilia may be seen. The diagnosis is made with imaging, IgE levels, skin-prick testing, and serology. Management includes steroids and antifungal therapy.
• Gastroenteritis: inflammation of the stomach and intestines, which is commonly caused by bacteria, virus, or parasite infection. Clinical features include abdominal pain, diarrhea, vomiting, fever, and dehydration. Diagnostic testing with stool analysis or culture is not always required but can help determine the etiology in certain circumstances. The majority of cases are self-limited; therefore, management is typically supportive. Antibiotics are indicated in severe cases.
• Acute cholecystitis: inflammation of the gallbladder resulting from sustained gallstone impaction of the cystic duct. Patients present with colicky, upper abdominal pain, nausea, and vomiting. Inflammation of the gallbladder and gallstones is seen on ultrasound or CT. Adult worms are not seen on imaging, which rules out ascariasis. Management includes IV fluids, antibiotics, and cholecystectomy. 
• Acute pancreatitis: inflammation of the pancreas. Patients present with sudden-onset, severe epigastric pain, which is typically sharp and radiates to the back. The diagnosis results from characteristic abdominal pain, lipase elevation, or imaging revealing pancreatic edema. Management includes bowel rest, pain control, and IV fluid hydration.