Soil-transmitted helminth infections

● The soil-transmitted helminths (STH) are a group of parasitic nematode worms causing human infection through contact with parasite eggs or larvae that thrive in the warm and moist soil of the world’s tropical and subtropical countries.

● As adult worms, the soil-transmitted helminths live for years in the human gastrointestinal tract (chronic infection).

● Worldwide an estimated 2 billion people are infected with at least one species.

Of particular worldwide importance are the:

roundworms (Ascaris lumbricoides)

whipworms (Trichuris trichiura)

hookworms (Necator americanus and Ancylostoma duodenale).

They are considered together because it is common for a single individual, especially a child living in a less developed country, to be chronically infected with all three worms.

And children experience the greatest STH infection-related morbidity: malnutrition, growth stunting, intellectual retardation, cognitive and educational deficits.

It is now accepted that parasitic disease is a major contributor to the etiology of the malnutrition-infection complex:

STH and HIV

Helminth infection in HIV-infected individuals may increase HIV RNA levels and increase the rate of progression of HIV to AIDS.

Studies have also shown that successful treatment of helminth co-infection (as documented by clearance of helminth eggs in stool) led to a significant decrease in HIV plasma viral load, which, in turn, means a reduced risk for sexual transmission of the HIV.

Burden of disease in Kenia

Some studies evaluated the prevalence of STH infections in preschool-aged and primary school-aged children in Kenya reporting values varying from 16% to 68%.

Therefore, wide variation may exist in the prevalence of helminth infections within proximate geographic areas. These differences may be environmental (e.g., warmer and more humid areas would probably have higher prevalence) or socioeconomic (e.g., the prevalence of Ascaris and Trichuris infections among school children living in overcrowded conditions would probably be higher than that among children attending rural schools). 

Characteristics of the soil-transmitted helminths

● STH can live for several years in the human gastrointestinal tract.

● Human beings are regarded as the only major definitive host for these parasites, although in some cases ascaris infections can also be acquired from pigs.

● After mating, each adult female produces thousands of eggs per day, which leave the body in the faeces.

● STH do not reproduce within the host and there is no direct person-to-person transmission from fresh faeces because eggs passed in faeces need about 3 weeks in the soil before they become infective.

Age distribution of STH infections

● For A. lumbricoides and T. trichiura, the most intense infections are in children aged 5–15 years, with a decline in intensity and frequency in adulthood. Whether such age dependency indicates changes in exposure, acquired immunity, or a combination of both, remains controversial.

● Although heavy hookworm infections also occur in childhood, frequency and intensity commonly remain high in adulthood, even in elderly people.

Host-parasite interactions

● Despite their large size and ability to elicit potent immune responses, soil-transmitted helminths are refractory to host immunity, establishing chronic infections during the host’s life, and, in the case of hookworm, intensity of infection actually rises with the age of the host.

● STH are thought to survive within the host not just by warding off immune attack, but instead by aggressively subverting the host immune response to create niches that optimise successful residence, feeding, and reproduction. STH induce a constellation of responses sharing key features with the allergic response and known as the T-helper-2 (Th2) immune response.

Clinical features

The clinical features of soil-transmitted helminth infections can be classified into:

➔ acute manifestations associated with larval migration through the skin and viscera

➔ acute and chronic manifestations resulting from parasitism of the gastrointestinal tract by adult worms

Clinical features: early larval migration

● Ascaris larvae that die during migration through the liver can induce eosinophilic granulomas.

● In the lungs, ascaris larval antigens cause an intense inflammatory response consisting of eosinophilic infiltrates that can be seen on chest radiographs. The resulting verminous pneumonia is commonly accompanied by wheezing, dyspnoea, a non-productive cough, and fever, with blood-tinged sputum produced during heavy infections. Children are more susceptible to pneumonitis, and the disease is more severe on reinfection.

Pneumonitis resulting from hookworm larvae is not as great as in ascaris infection (cough).

Several cutaneous syndromes result from skin penetrating larvae.

● Repeated exposures to hookworm 3rd-stage larvae result in ground itch, a local erythematous and papular rash accompanied by pruritus on the hands and feet.

● By contrast, when zoonotic hookworm 3rd-stage larvae (typically A. braziliense) enter the skin, they produce cutaneous larva migrans, which is characterised by the appearance of serpiginous tracks on the feet, buttocks, and abdomen.

● Oral ingestion of A. duodenale larvae can result in Wakana syndrome, which is characterised by nausea, vomiting, pharyngeal irritation, cough, dyspnoea, and hoarseness.

The symptoms of STH intestinal parasitism are frequently nonspecific including nausea, tiredness, abdominal pain and loss of appetite.

Infections of moderate and high intensity produce more severe symptoms and each of the major soil-transmitted helminths produces characteristic disease syndromes.

Ascariasis

● The presence of large numbers of adult ascaris worms in the small intestine can cause abdominal distension and pain. They can also cause lactose intolerance and malabsorption of vitamin A and possibly other nutrients, which might partly cause the nutritional and growth failure.

● Adult ascaris worms also tend to move in children with high fever, resulting in the emergence of worms from the nasopharynx or anus.

● In young children, adult worms can aggregate in the ileum and cause partial obstruction because the lumen is small. Intussusception, volvulus, and complete obstruction can ensue, leading to bowel infarction and intestinal perforation.

● Adult worms can enter the lumen of the appendix, leading to acute appendicular colic and gangrene of the appendix tip, resulting in a clinical picture indistinguishable from appendicitis.

● Hepatobiliary and pancreatic ascariasis results when adult worms in the duodenum enter and block the ampullary orifice of the common bile duct, leading to biliary colic, cholecystitis, cholangitis, pancreatitis, and hepatic abscess. Hepatobiliary and pancreatic ascariasis occurs more commonly in adults (especially women) than in children, presumably because the adult biliary tree is large enough to accommodate an adult worm.

Tichuriasis

● Adult whipworms live preferentially in the caecum, although in heavy infections, whipworms can be seen throughout the colon and rectum. The adult parasite lives with the anterior end embedded in epithelial tunnels within the intestinal mucosa and the posterior end located in the lumen. Inflammation at the site of attachment from large numbers of whipworms results in colitis. Longstanding colitis produces a clinical disorder that resembles inflammatory bowel disease, including chronic abdominal pain and diarrhoea, as well as the sequelae of impaired growth, anaemia of chronic disease, and finger clubbing.

● Trichuris dysentery syndrome is an even more serious manifestation of heavy whipworm infection, resulting in chronic dysentery and rectal prolapse.

Hookworm infection

● The major pathology of hookworm infection results from intestinal blood loss as a result of adult parasite invasion and attachment to the mucosa of the small intestine. When the blood loss exceeds the nutritional reserves of the host, iron-deficiency anaemia develops.

Thus the clinical manifestations of hookworm disease resemble those of iron-deficiency anaemia from other causes.

Because children and women of reproductive age have reduced iron reserves, they are at particular risk of hookworm disease. The severe iron-deficiency anaemia that can arise from hookworm disease during pregnancy can have adverse results for the mother, the fetus, and the neonate.

● The chronic protein loss from heavy hookworm infection can result in hypoproteinaemia and anasarca.

Impact of STH infections during childhood

➔ Restricted growth. Loss of appetite, vitamin A deficiency and anaemia all interfere with a child's ability to grow healthily and to his or her full potential. Moreover, a child with heavy worm burden is less resistant to other infections. Altogether, this means a child infected with STHs will be sickly and, if untreated, will grow up to be an unhealthy adult.

➔ Reduced ability to learn. Worm-infected children are less able to concentrate or memorize information. They score less well in school tests and therefore their only chance to gain a few years of schooling in their lives is compromised.

Diagnosis

● Faecal examination: Several egg concentration techniques (eg, formalinethyl acetate sedimentation) can detect even light infections. Some methods (Kato-Katz faecal-thick smear and McMaster method) are used to measure the intensity of infection by estimating the number of egg counts per gram of faeces.

● Complete blood count: eosinophilia is a common presenting finding, especially of hookworm infection

● Ultrasonography and endoscopy are useful for diagnostic imaging of the complications of ascariasis, including intestinal obstruction and hepatobiliary and pancreatic involvement.

Treatment

The treatment goal for soil-transmitted helminth infections is to remove adult worms from the GI tract. The drugs most commonly used are mebendazole and albendazole.

Important therapeutic differences affect their use in clinical practice:

● Both agents are effective against ascaris in a single dose, but for both trichuriasis and hookworm infection, several doses of benzimidazole anthelmintic drugs are commonly needed.

● Mebendazole is poorly absorbed from the GI tract so its therapeutic activity is confined to adult worms. Albendazole is better absorbed, especially when ingested with fatty meals, and it has a high distribution in the tissues. So albendazole is used for the treatment of disorders caused by tissue migrating larvae.

Both pyrantel pamoate and levamisole are regarded as alternative drugs for the treatment of hookworm and ascaris infections, although they are not effective for the treatment of trichuriasis and they are administered by bodyweight.

Morbidity control through deworming

Benzimidazole anthelmintic drugs are now used also for large-scale morbidity reduction in endemic communities.

➔Regular treatment in school-age children reduces and maintains the worm burden below the threshold associated with disease. The benefits of regular deworming include improvements in iron stores, growth and physical fitness, cognitive performance, and school attendance.

➔In younger children, studies have shown improved nutritional indicators such as reduced wasting, malnutrition, and stunting, and improved appetite. Administration of anthelmintic drugs to children infected with soil-transmitted helminths from 1 year of age is now deemed appropriate.

➔In areas where hookworm infections are endemic, anthelmintic treatment is recommended during pregnancy (except in the 1st trimester). If women are treated once or twice during pregnancy, there are substantial improvements in maternal anaemia, birthweight, and infant mortality at 6 months.

Recommended schedules:

·      albendazole 400mg (200mg for children aged 12–23 months)

·      mebendazole 500mg administered once or twice per year

Ineligible population

Children in the 1st year of life; pregnant women in the 1st trimester of pregnancy.

An important factor in treatment is reinfection.

After community-wide treatment, rates of hookworm infection reach 80% of pretreatment rates within 30–36 months. A lumbricoides infection reached 55% of pretreatment rates within 11 months and T. trichiura infection reached 44% of pretreatment rates within 17 months. Despite reinfection, however, regular treatment to reduce the worm burden consistently could prevent some of the sequelae associated with chronic infection.

The best solution would be to prevent these diseases rather than treat or cure them.

Effort should be made in order to improve:

-  clean water supply

- adequate sanitation measures

- less crowded living conditions

- higher levels of education

- better access to health care

Fr Beppe Gaido & Dottoressa Nadia Chiapello