Human Intestinal Parasites

Disease trends
Signs and symptoms
Parasitic Infections & Diagnosis
Prevention & Treatment

For the purpose of this discussion, parasitism refers to intestinal parasitic infestations that cause diseases in man. A parasite is an organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host. Parasitism refers to the characteristic behavior or mode of existence of a parasite or parasitic population.

Pinworm (Enterobius vermicularis) and roundworm (Ascaris lumbricoides) are common parasitic infections found in man. Other infections caused by parasites include giardiasis; trichinosis; balantidiasis; fascioliasis; tapeworm, whipworm and hookworm infections; strongyloidiasis; and amebiasis. In this day of international travel, special alertness to the possibility of parasitic disease is indicated.

Infection by Soil-transmitted helminths (Intestinal worms) has been increasingly recognized as an important public health problem, particularly in developing countries. Progress has been made in the understanding of ecology, epidemiology and related morbidity and in developing new tools for control.

In the 1993 World Development Report intestinal helminths rank first as the main cause of disease burden in children aged 5-14 years and also rank highly as the disease that can be efficiently controlled by cost-effective intervention.

Disease Trends

Intestinal parasitic and protozoan infections are amongst the most common infections world-wide. It is estimated that some 3.5 billion people are affected, and that 450 million are ill as a result of these infections, the majority being children. Each year, some 65 000 deaths are directly attributable to hookworm infections, and other another 60 000 to Ascaris lumbricoides (roundworm). Entoameoba histolytica which causes amoebiasis is estimated to cause severe disease in 48 million people, killing 70 000 each year. Multiple infections with several different parasites (e.g., hookworms, roundworms and amoebae) are common, and their harmful effects are often aggravated by co-existent malnutrition or micronutrient deficiencies.

About 44 million pregnant women have hookworm infections which cause chronic blood loss from the intestine and predisposes to the development of iron deficiency anaemia, sometimes of great severity, constituting a major public health problem.

The numbers are increasing, with cases occurring in all WHO regions. In 2025 more than half of the population in developing countries will be urbanized and, as a consequence, a large number of people will live in shanty towns where E. histolytica, Giardia intestinalis¸ A. lumbricoides and Trichuris trichiura will find a favourable ground for transmission.

About 2.4 million people are infected with Fasciola haepatica, a liver fluke and over 20 million with Paragonimus spp., a family of lung flukes. This latter infection is often confused diagnostically with tuberculosis.

Signs and symptoms

Gastrointestinal complaints such as pain, diarrhea, nausea, and perianal itching are common in many intestinal parasitic infestations. Pruritus ani is especially common in pinworm infection. Roundworm has been known to cause intestinal obstruction and appendicitis.

Soil-transmitted nematodes (Ascaris lumbicoides, Trichuris trichiura, Hookworms) cause morbidity in humans in different ways, by:
· affecting nutritional equilibrium
· inducing intestinal bleeding
· inducing malabsorption of nutrients
· competing for absorption of micronutrients
· reducing growth
· reducing food intake
· causing surgical complications such as obstruction, rectal prolapse and abscess
· affecting cognitive development

Confirmatory diagnostic procedures discussed below are useful in the diagnoses of parasitic infections.

Parasitic Infections & Diagnosis

The following list of parasites and what to look for, may be of help in identifying some of the most common parasitic infections:

Amebiasis: Amoebas cause irregular ulcers in the rectum with red borders and gray bases. They can be found in fresh stool specimens and on biopsy specimens taken from rectal ulcers.

Balantidiasis: Diarrhea, dysentery, and occasional ulceration of the large intestine characterize this disease. Bnlantidium coli cysts found in stools are diagnostic.

Fascioliasis: Fasciola organisms can cause an enlarged and painful liver accompanied by fever. Eggs of these organisms can be found in stools.

Giardiasis: Infection with Giardia lamblia can cause diarrheic and/or dysenteric symptoms. Cysts and trophozoites can be found in stools.

Hookworm: Severe symptoms such as iron-deficiency anemia, physical and mental retardation, and cardiac complication sometimes occur. Diagnosis is made by the finding of ova in stools.

Pinworm: Although the most common complaint caused by pinworms is perianal itching, they may also cause intestinal abscesses and bleeding. Eggs may be found in the perianal area; they can be transferred with a cellophane tape swab to a slide for microscopic inspection. Worms may also be seen in stools, and sometimes around the anus.

Roundworm: Severe symptoms-such as intestinal, biliary, or pancreatic obstruction; or appendicitis-may occur. Eggs may be diagnosed by a simple smear of stools.

Strongyloidiasis: The infectious agent is the nematode Strongyloides stercoralis. It causes diarrhea. Larvae can be found in stools.

Tapeworm: Segments of beef, pork, and fish tapeworms can be found in stools. Eggs can be swabbed from perianal areas and examined.

Trichinosis: Severe trichinosis infestation may result in death within six weeks. Muscle biopsy, to determine the presence of encysted larvae of Trichinella spiralis, is the only reliable diagnostic procedure, since neither eggs nor worms occur in stools.

Whipworm: Blood-streaked stools, rectal prolapse, iron-deficiency anemia, and malnutrition may occur. Stool examination for eggs and parasites is indicated for an accurate diagnosis.

Prevention & Treatment

Schistosomiasis and soil-transmitted helminth infections are invariably more prevalent in the poorest sections of the populations in endemic areas of the least developed countries. The goal is to reduce morbidity from schistosomiasis and soil-transmitted helminth infections to levels such that these infections are no longer of public health importance. An additional goal is to improve the developmental, functional and intellectual capacity of affected children. Highly effective, safe single-dose drugs can be dispensed through health services, school health programmes and community interventions directed at vulnerable groups. As these infections are endemic in poor communities, more permanent control will only be feasible where chemotherapy is supplemented by improved water supplies and sanitation, strengthened by sanitation education. In the long term, this type of permanent transmission control will only be possible with improved living conditions due to economic development.

Drug treatment is effective in controlling most parasites. New drugs are forever emerging, and the treatment is forever changing. Please check the most current prescribing index for the most up to date treatment available. One point to keep in mind is that re-infection is common in many parasitic infestations; therefore, all members of the household should be treated, and good hygiene is a must for all concerned.


World Health Organization, 2002,