Soil-transmitted helminthiasis

Female Ascaris lumbricoides

Ascariasis of STH is caused by the large roundworm A. lumbricoides. It is estimated to be the most widespread STH, affecting approximately 1 billion people. The victims constitute about half of the populations in tropical and subtropical areas. Most conditions are mild and often show little or no symptoms. Heavy infections however are debilitating, causing severe intestinal blockage and impair growth in children. Children, compounded with malnutrition, are most infected, with the most common age group being 3- to 8-year-olds, with an annual death of about 20,000. Children are more susceptible due to their frequent exposure to contaminated environment such as during playing, eating raw vegetables and fruits, and drinking wastewater.[7]

Hookworm

Hookworm infection of STH is caused by N. americanus and A. duodenale. Mild infections produce diarrhoea and abdominal pain. More severe infections can create serious health problems for newborns, children, pregnant women, and malnourished adults. In fact it is the leading cause of anaemia and protein deficiency in developing nations, affected an estimated 740 million people. N. americanus is the more common hookworm, while A. duodenale is more geographically restricted. Unlike other STHs, in which school-age children are most affected, high-intensity hookworm infections are more frequent in adults, specifically women. Roughly 44 million pregnant women are estimated to be infected. The disease causes severe adverse effects in both the mother and infant, such as low birth weight, impaired milk production, and increased risk of mortality.[7]

Trichuris trichiura egg

Whipworm (Trichuris trichiura) is the third most common STH-causing nematode in humans. According to current estimate, nearly 800 million people are infected, the majority of them children. Heavy infections could lead to acute symptoms such as diarrhoea and anaemia, and chronic symptoms such as growth retardation and impaired cognitive development. Medical conditions are more often serious since coinfection with protozoan parasites such as Giardia and Entamoeba histolytica, and with other nematodes is common.[7] Predominantly a tropical disease of developing countries, trichuriasis is quite common in the United States.[10]

Most conditions of STH have a light worm burden and usually have no discernible symptoms. Heavy infections however cause a range of health problems, including abdominal pain, diarrhoea, blood and protein loss, rectal prolapse, and physical and mental retardation. Severe ascariasis is typically a pneumonia, as the larvae invade lungs, producing fever, cough and dyspnoea during early stage of infection.[14][15] Hookworm infections insinuate a skin reaction (dermatitis), increased white blood cells (eosinophils), a pulmonary reaction (pneumonitis), and skin rash (urticarial).[15][16]

Iron deficiency anaemia due to blood loss is a common symptom.[17]

STH is often associated with malnutrition in children as it worsens their nutritional status in multiple ways.[2] The worms can induce intestinal bleeding, competition for nutrients (malabsorption of nutrients), frequent anaemia and diarrhoea.[18] Soil-transmitted helminths can also cause loss of appetite.[2] These nutritional "knock on" effects of STH can have a significant impact on the mental and physical development of children. In endemic countries, communities remain suppressed due to malnourishment, cognitive disability and physical weaknesses as a result of heavy infections.[11][12]

One strategy to control the disease in areas where it is common is the treatment of entire groups of people regardless of symptoms via mass drug administration. The World Health Organization recommends mass treatments to all at risk groups in endemic communities, especially women of childbearing age, pre-school aged children and school-aged children. Mass treatments can also be provided to pregnant women in their second and third trimesters, and breastfeeding women.[22] This known as mass deworming.[2][23] While testing and treating children who are infected looks like it is effective, there is significant evidence that concludes that routine deworming, in the absence of a positive test, does not improve nutrition, haemoglobin, school attendance or school performance.[24]

For this purpose, broad-spectrum benzimidazoles such as mebendazole and albendazole are the drugs of choice recommended by WHO. These anthelminthics are administered in a single dose are safe, relatively inexpensive, and effective for several months. Mebendazole can be given with a single dose twice a day for three consecutive days. Albendazole is given at a single dose. WHO recommends annual treatment in areas where between 20 and 50% of people are infected, and a twice a year treatment if it is over 50%; and in low risk situation (i.e. less than 20% prevalence) case-by-case treatment.[17][25] In addition to these, pyrantel pamoate is also equally effective on ascaris. However, it has been reported that albendazole, mebendazole, and pyrantel pamoate are not entirely effective against T. trichiura with single oral doses in population-based control.[26]

In cases of coinfection, combination therapy with ivermectin and diethylcarbamazine is advocated. However coinfection with malaria and HIV, especially among African women, does not respond well to the current combination therapies.[27] It is more pressing for trichuriasis that the recommended drugs fail to provide positive results.[28] A novel drug tribendimidine, which was approved in China by the CCDC for human use in 2004, has been subjected to clinical trials showing that they are highly effective against major human flukes, ascaris (>90% cure rate) and hookworm (>82%); however with low cure rate for whipworm (<37%).[29]

In some severe cases of ascariasis the numbers of Ascaris worms can cause intestinal obstruction requiring emergency surgery.[30] The obstruction may be due to a collected mass of worms or to a twisting of the intestine.[30] During the surgery the worms may be manually removed.[30]

Infections are widely distributed in tropical and subtropical areas, with the greatest numbers occurring in sub-Saharan Africa, the Americas, China and east Asia.[2]

The World Health Organization estimates that globally more than 1.5 billion people (24% of the total population) have a soil-transmitted helminth infection.[2] Over 270 million preschool-age children and over 600 million school-age children live in areas where these parasites are intensively transmitted, and are in need of treatment and preventive interventions. Latest estimates indicate that more than 880 million children are in need of treatment from STH infections.[3][18][31]

By type of parasitic worm the breakdown is:[32]

• approximately 807–1,121 million with ascaris
• approximately 576–740 million with hookworm
• approximately 604–795 million with whipworm

Latest estimates indicate that the total annual death toll which is directly attributable is as high as 135,000.[3][18][31] The death toll due to the malnutrition link is likely to be much higher.[33][34]