Helminth Infections in Pregnant Women

Helminth Infections

Helminths are parasitic worms with a worldwide geographic distribution. These worms typically infect residents of developing nations and, without treatment, create chronic, lifelong morbidity in children and adults.^[1,2,3] Helminths are categorized into:

• Nematodes (roundworms): these include intestinal worms, such as Strongyloides stercoralis, A lumbricoides, Trichuris trichiura, N americanus, and A duodenale, and filarial worms, such as W bancrofti, B malayi, Onchocerca volvulus, and Loa loa;

• Trematodes (flatworms), including Clonorchis, Opisthorchis, Paragonimus, and Schistosoma species; and

• Cestodes (tapeworms), including Taenia solium.^[4]

Many of these infections are highly stigmatizing in the community, particularly when they cause changes in physical appearance and infertility.

In endemic areas, a large burden of helminth disease may affect women of childbearing age and pregnant women.^[1] Approximately 8 billion people in the world are infected with helminths, although the proportion who are pregnant is unknown.^[5] Hookworm alone, however, infects approximately 44 million pregnant women each year.^[6] Helminths are typically isolated from women of younger age, with fewer educational opportunities, lower socioeconomic backgrounds, and baseline anemia and malnutrition, all of which can lead to devastating morbidity and mortality.^[2,7]

Helminth Infections and Pregnancy

Helminth infections in general affect both mother and fetus. Although data suggesting increased susceptibility to helminth infection in pregnant women are conflicting, they have shown that severe anemia, malnutrition, and concurrent infectious processes increase the risk for maternal and infant morbidity and mortality.^[2,8]

Anemia in these highly endemic regions is common among pregnant women and often multifactorial. Anemia in pregnancy has been associated with stillbirth, prematurity, and low birth weight.^[9,10] Helminth infections, such as hookworm, trichuriasis, and schistosomiasis, have been shown to directly contribute to severe anemia in patients through blood loss and micronutrient deficiencies.^[7,11] Hookworm is the leading cause of pathologic blood loss in endemic areas.

Malnutrition is another complication of a high worm burden, particularly from intestinal nematodes, such as Ascaris, hookworm, and Trichuris. Helminth-infected women are typically found with protein-energy malnutrition and deficiencies of micronutrients, such as iron and zinc. These nutritional deficiencies in the setting of pregnancy -- a state of increased nutritional needs -- can have devastating effects on both mother and the fetus. Chronic malnutrition in the mother secondary to chronic helminth infection increases the maternal risk for future parasitic infections and adverse pregnancy outcomes.

Infants born to malnourished mothers are often delivered prematurely, with low birth weight, and may have poor growth and development throughout childhood.^[11] Mouse models have shown that intrauterine exposure to helminths affects fetal linear growth, birth weight, growth of lymphoid tissues and bone, and infant mortality.^[7]

It has also been postulated that intrauterine or perinatal exposure to parasites promotes infant immune system down regulation, not only to future helminth infections but to infections in general, via immunoregulatory mechanisms that make infants more susceptible to infection.^[3,12] In the Entebbe Mother and Baby Study (EMaBS) cohort in Uganda, infants born to 2507 women with symptomatic and asymptomatic hookworm parasitemia during their second or third trimesters had increased likelihood of childhood malaria.^[12]

Despite this evidence, more studies are needed to determine both the mechanism and the significance of maternal helminth infection for the developing fetus and its long-term outcomes on the growth and development of the child.

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