Ascaris lumbricoides

Ascaris lumbricoides is the largest of the parasitic roundworms and is probably equal to or second to Enteribious vermicularis as the most endemically occurring roundworm.

Disease & Pathology

Heavy infections can cause stunted growth in children due to malabsorption of nutrients. Heavy infections can also cause abdominal pain; intestinal obstruction that can lead to emergency surgical intervention; and intestinal perforation.

Worms may migrate resulting in occlusion of the biliary duct, and appendicitis. Nasopharyngeal emergence can occur which can be disconcerting to the host. These migratory events can increase due to a number of reasons such as an acute febrile illness, and result in an unpredictable dissemination of worms or larvae that can lead to complications depending on the organs or body site involved. Some of these can have dire consequences for the patient unless treated expeditiously.

Pneumonitis similar to that seen in Strongyloides or hookworm infections can occur (Loeffler’s syndrome). The sputum in these cases will contain high numbers of eosinophils. Other symptoms such as fever, dry cough, dyspnea, and radiological studies of the lungs that show scattered shifting mottling of the lungs suggestive of pulmonary tuberculosis or viral pneumonia. The latter will require ruling out.

Heavy infections have severely affect the development of young children due to nutritional impairment. Young children are also are more predisposed to developing biliary obstruction due to heavy infections and require surgical intervention to remove the worms from the common bile duct. Abscesses of the liver may form requiring drainage and antibiotics.

Light infections can be asymptomatic.

Location in the Host

Ascaris lumbricoides is located in the small intestine.

Geographic Distribution

Found worldwide although the incidence is significantly higher in sub-tropical and tropical environments. Pig farms are known to have a high incidence.

Life cycle

Eggs hatch in the intestine after ingestion and invade the intestinal mucosa. From there they enter the portal and then systemic circulation to the lungs. Maturation in the lungs takes 10-14 days. The parasite will eventually penetrate the alveolar walls and ascend the bronchial tree to the throat to be swallowed by the host. Once the parasites reach the small intestine they develop into adult worms. Adult worms can live 1-2 years

Morphology & Diagnosis

Ascaris lumbricoides eggs can be found in the feces during an ova & parasite examination. Many times they are corticated and can be overlooked as debris. The eggs will take up Lugol’s iodine very well and will therefore have a dark brown color. Fertile and infertile eggs can be seen. Fertile eggs are oval and measure between 55-73u by 35-50u. These fertile cells are in the one cell stage when passed in the feces and the developing parasite can easily be seen. Infertile cells are somewhat elongated and measure 85-96u by 43-47u. These infertile cells have thin cell walls, have a mammillated layer that can either be rough and globular or completely absent. Internally the parasite is mass of disorganized retractile granules.

Adult worms are at times passed in the feces and at times may migrate up the esophagus and be passed through the mouth or nose. This migration is many times due to heavy infections and because of their unpredictability can lead to serious complications in the host. The adult worms measure between 15-35cm in length with the females having a straight tail and the males having a curved tail.

The larval migration phase of the parasite in the host that progresses from the intestine to the liver and lungs can produce symptoms associated with the organ(s) affected. Lung migration of the larvae can produce a pneumonitis called Ascaris pneumonitis or Loeffler’s syndrome.


Ascaris lumbricoides eggs in formalin concentrate – Lugol’s iodine under oil immersion and high dry (Photos by W. Vientos)

Ascaris lumbricoides adult worm with head zoomed in. Note the worms are cream-pink colored and non-segmented – Illustration by W. Vientos