Ascaris lumbricoides

Ascaris lumbricoides
An adult female Ascaris worm.
Scientific classification
Kingdom:	Animalia
Phylum:	Nematoda
Class:	Secernentea
Order:	Ascaridida
Family:	Ascarididae
Genus:	Ascaris
Species:	A. lumbricoides
Binomial name
Ascaris lumbricoides Linnaeus, 1758

Ascaris lumbricoides is the giant roundworm of humans, belonging to the phylum Nematoda. An ascarid nematode, it is responsible for the disease ascariasis in humans, and it is the largest and most common parasitic worm in humans. One sixth of the human population is estimated to be infected by Ascaris lumbricoides or another roundworm . Ascariasis is prevalent worldwide and more so in tropical and subtropical countries.

It can reach a length of up to 35 cm.

Life cycle

Ascaris lumbricoides, or "roundworm", infections in humans occur when an ingested fertilised egg becomes a larval worm that penetrates the wall of the duodenum and enters the blood stream. From here, it is carried to the liver and heart, and enters pulmonary circulation to break free in the alveoli, where it grows and molts. In 3 weeks, the larvae pass from the respiratory system to be coughed up, swallowed, and thus returned to the small intestine, where they mature to adult male and female worms. Fertilization can now occur and the female produces as many as 200,000 eggs per day for a year. These fertilized eggs become infectious after 2 weeks in soil; they can persist in soil for 10 years or more.

The eggs have a lipid layer, that makes them resistant to the effects of acids and alkalis as well as other chemicals. This resilience helps to explain why this nematode is such a ubiquitous parasite.

Morphology

Fertile egg as can be seen in a microscope

Fertile egg in human faeces (detail)

Infertile egg

Ascaris lumbricoides is characterized by its great size. Males are 2–4 mm in diameter and 15–31 cm long. The males' posterior end is curved ventrally and has a bluntly pointed tail. Females are 3–6 mm wide and 20–49 cm long. The vulva is located in the anterior end and accounts for about a one third of its body length. Uteri may contain up to 27 million eggs at a time with 200,000 being laid per day. Fertilized eggs are oval to round in shape and are 45-75 micrometers long and 35-50 micrometers wide with a thick outer shell. Unfertilized eggs measure 88-94 micrometers long and 44 micrometers wide.

Epidemiology

More than 2 billion people are affected by this infection. In the United States there is a reported prevalence of 0.8% of the total population as of 1987. Ascaris lumbricoides eggs are extremely resistant to strong chemicals, desiccation, and low temperatures. The eggs can remain viable in the soil for several months or even years.

Eggs of A. lumbricoides have been identified in archeological coprolites in the Americas, Europe, Africa, the Middle East, and New Zealand, the oldest ones being more than 24,000 years old.

Infections

Infections with these parasites are more common where sanitation is poor and raw human feces are used as fertilizer.

Symptoms

Often, there are no symptoms with an A. lumbricoides infection. However, in the case of a particularly bad infection, symptoms may include bloody sputum, cough, fever, abdominal discomfort, passing worms, etc.

Prevention

Preventing any fecal-borne disease requires educated hygienic habits/culture and fecal treatment systems once a year. This is particularly important with ascaris because its eggs are one of the most difficult pathogens to kill (second only to prions), and the eggs commonly survive 1–3 years. Ascaris lives in the intestine where it lays eggs. Infection occurs when the eggs, too small to be seen by the unaided eye, are eaten. The eggs may get onto vegetables when improperly processed human feces of infected people are used as fertilizer for food crops. Infection may occur when food is handled without removing or killing the eggs on the hands, clothes, hair, raw vegetables/fruit, or cooked food that is (re)infected by handlers, containers, etc. Bleach does not readily kill Ascaris eggs but it will remove their sticky film, to allow the eggs to be rinsed away. Ascaris eggs can be reduced by hot composting methods, but to completely kill them may require rubbing alcohol, iodine, specialized chemicals, cooking heat, or "unusually" hot composting (for example, over 120 degrees Fahrenheit for 24 hours [1]).

Details of infection process

Infections happen when a human swallows water or food contaminated with unhatched juveniles. The juveniles hatch in the duodenum (1st section of small intestine). They then penetrate the mucosa and submucosa and enter venules or lymphatics. Next they pass through the right heart and into pulmonary circulation. They then break out of the capillaries and enter the air spaces. Acute tissue reaction occurs when several worms get lost during this migration and accumulate in other organs of the body. The juveniles migrate from the lung up the respiratory tract to the pharynx where they are swallowed. They begin producing eggs within 60–65 days of being swallowed. These are produced within the small intestine where the juveniles mature. It might seem odd that the worms end up in the same place where they began. One hypothesis to account for this behavior is that the migration mimics an intermediate host, which would be required for juveniles of an ancestral form to develop to the third stage. Another possibility is that tissue migration enables faster growth and larger size, which increases reproductive capacity.

Diagnosis and treatment

Most diagnoses are made by identifying the appearance of the worm or eggs in feces. Due to the large quantity of eggs laid, physicians can diagnose using only one or two fecal smears.

Infections can be treated with drugs called ascaricides. The treatment of choice is Mebendazole. The drug functions by binding to tubulin in the worms' intestinal cells and body-wall muscles. Nitazoxanide and ivermectin can also be used.

External links

• Ascaris lumbricoides Video - DAVE Project