Ascariasis

[Ascaris lumbricoides]

Parasite Biology Image Gallery Laboratory Diagnosis Resources

Causal Agents

Ascaris species are very large (adult females: 20 to 35 cm; adult males: 15 to 30 cm) nematodes (roundworms) that parasitize the human intestine. A. lumbricoides is the primary species involved in human infections globally, but Ascaris derived from pigs (often referred to as A. suum) may also infect humans. These two parasites are very closely related, and hybrids have been identified; thus, their status as distinct, reproductively isolated species is a contentious topic.

Life Cycle:

View Larger

Adult worms live in the lumen of the small intestine. A female may produce approximately 200,000 eggs per day, which are passed with the feces . Unfertilized eggs may be ingested but are not infective. Larvae develop to infectivity within fertile eggs after 18 days to several weeks , depending on the environmental conditions (optimum: moist, warm, shaded soil). After infective eggs are swallowed , the larvae hatch , invade the intestinal mucosa, and are carried via the portal, then systemic circulation to the lungs . The larvae mature further in the lungs (10 to 14 days), penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed . Upon reaching the small intestine, they develop into adult worms. Between 2 and 3 months are required from ingestion of the infective eggs to oviposition by the adult female. Adult worms can live 1 to 2 years.

Hosts

Humans and swine are the major hosts for Ascaris; see Causal Agents for discussion on species status of Ascaris from both hosts. Natural infections with A. lumbricoides sometimes occur in monkeys and apes.

Occasionally, Ascaris sp. eggs may be found in dog feces. This does not indicate true infection but instead spurious passage of eggs following coprophagy.

Geographic Distribution

Ascariasis is the most common human helminthic infection globally. The burden is highest in tropical and subtropical regions, especially in areas with inadequate sanitation. This infection is generally rare to absent in developed countries, but sporadic cases may occur in rural, impoverished regions of those countries. Some cases in these areas where human transmission is negligible have direct epidemiologic associations to pig farms.

Clinical Presentation

Although heavy infections in children may cause stunted growth via malnutrition, adult worms usually cause no acute symptoms. High worm burdens may cause abdominal pain and intestinal obstruction and potentially perforation in very high intensity infections. Migrating adult worms may cause symptomatic occlusion of the biliary tract, appendicitis, or nasopharyngeal expulsion, particularly in infections involving a single female worm.

Ascaris lumbricoides unfertilized eggs.

Fertilized and unfertilized Ascaris lumbricoides eggs are passed in the stool of the infected host. Fertilized eggs are are rounded and have a thick shell with an external mammillated layer that is often stained brown by bile. In some cases, the outer layer is absent (known as decorticated eggs). Fertile eggs range from 45 to 75 µm in length. Unfertilized eggs are elongated and larger than fertile eggs (up to 90 µm in length). Their shell is thinner and their mammillated layer is more variable, either with large protuberances or practically none. Unfertile eggs contain mainly a mass of refractile granules.

Figure A: Unfertilized egg of A. lumbricoides. Note the prominent mammillations on the outer layer.

Figure B: Unfertilized egg of A. lumbricoides in an unstained wet mount, 200x magnification.

Figure C: Unfertilized egg of A. lumbricoides in an unstained wet mount of stool.

Figure D: Unfertilized egg of A. lumbricoides in a wet mount of stool. Note this specimen lacks the mammillated layer (decorticated).

Figure E: Infertile, decorticated egg of Ascaris lumbricoides. Image courtesy of The Leiden University Medical Center, The Netherlands.

Figure F: Infertile, decorticated egg of Ascaris lumbricoides. Image courtesy of The Leiden University Medical Center, The Netherlands.

A. lumbricoides fertilized eggs.

Figure A: Fertilized egg of A. lumbricoides in unstained wet mounts of stool, with embryos in the early stage of development.

Figure B: Fertilized egg of A. lumbricoides in unstained wet mounts of stool, with embryos in the early stage of development.

Figure C: Fertilized egg of A. lumbricoides in an unstained wet mount of stool, undergoing early stages of cleavage. Image taken at 200x magnification.

Figure D: Fertilized egg of A. lumbricoides in an unstained wet mount of stool.

Figure E: Fertilized egg of A. lumbricoides in an unstained wet mount of stool, 200x magnification. A larva is visible in the egg.

Figure F: Fertilized egg of A. lumbricoides in an unstained wet mount of stool.

A. lumbricoides fertilized, decorticated eggs.

Figure A: A. lumbricoides decorticated, fertile egg in wet mounts, 200x magnification.

Figure B: A. lumbricoides decorticated, fertile egg in wet mounts, 200x magnification.

Figure C: A. lumbricoides decorticated, fertile egg in a wet mount, 200x magnification. The embryo has advanced cleavage.

Figure D: The same egg as in Figure C, but at 400x magnification.

**Larvae of A. lumbricoides hatching from eggs.**

Figure A: Larva of A. lumbricoides hatching from an egg.

Figure B: Larva of A. lumbricoides hatching from an egg.

**Adults of A. lumbricoides.**

Adults of Ascaris lumbricoides are large roundworms. Females measure 20–35 cm long with straight taisl; males are smaller at 15–31 cm and tend to have curved tails. Adults of both sexes possess three “lips” at the anterior end of the body.

Figure A: Adult female A. lumbricoides.

Figure B: Adult female A. lumbricoides. Image courtesy of the Orange County Public Health Laboratory, Santa Ana, CA.

Figure C: Close-up of the anterior end of an adult A. lumbricoides. Note the three 'lips.' Image courtesy of the Orange County Public Health Laboratory, Santa Ana, CA.

Figure D: Posterior end of a male A. lumbricoides, showing the curled tail.

Figure E: Cross-section of an adult female A. lumbricoides, stained with hematoxylin and eosin (H&E). Note the presence of the prominent muscle cells (MU), gravid uterus (UT), intestine (IN) and coiled ovary (OV).

Figure F: Cross-section of the cuticle of an adult A. lumbricoides, stained with H&E. Shown here are the cuticle (CU), and immediately below the cuticle, the thin hypodermis (HY). Also shown are the prominent muscle cells (MU) and one of the lateral chords (LC).

A. lumbricoides in tissue specimens.

Ascaris lumbricoides in tissue specimens, stained with hematoxylin and eosin (H&E).

Figure A: L3 larvae of A. lumbricoides in lung tissue, stained with H&E. Image taken at 400x magnification.

Figure B: Higher magnification (1000x) of the specimen in Figure A. Note the prominent alae (AL), intestine (IN) and excretory ducts (EC).

Figure C: Eggs of A. lumbricoides in an appendix biopsy, stained with H&E. This image was taken at 200x magnification.

Figure D: Eggs of A. lumbricoides in an appendix biopsy, stained with H&E. This image was taken at 400x magnification.

Figure E: Eggs of A. lumbricoides in an appendix biopsy, stained with H&E. Image taken at 400x magnification.

Laboratory Diagnosis

Microscopic identification of eggs in the stool is the most common method for diagnosing intestinal ascariasis. The recommended procedure is as follows:

Collect a stool specimen.
Preserve the specimen in formalin or another fixative.
Concentrate using the formalin–ethyl acetate sedimentation technique
Examine a wet mount of the sediment.

Where concentration procedures are not available, a direct wet mount examination of the specimen is adequate for detecting moderate to heavy infections. For quantitative assessments of infection, various methods such as the Kato-Katz or quantitative fecal flotation can be used.

Larvae can be identified in sputum or gastric aspirate during the pulmonary migration phase (examine fixed organisms for morphology). Adult worms are occasionally passed in the stool or through the mouth or nose and are recognizable by their macroscopic characteristics (e.g., presence of three “lips”).

Molecular methods for the detection of egg/worm DNA in human stools are increasingly used in research settings, often in multi-parallel formats for detecting this and other soil-transmitted helminths.

Laboratory Safety

Standard precautions for the processing of stool samples apply. Ascaris spp. eggs require weeks in the environment to develop to an infective stage; thus infectious eggs are not likely to be encountered in clinical specimens.