Module 3: Environmentally-Related Infectious Diseases

People living in low-income countries have a higher burden of disease and are more likely to be in poor health than people living in high income countries. This higher morbidity and mortality are typically due to lack of access to healthcare and greater exposure to environmental conditions that increase the risk of disease and injury. Several World Health Organization studies have examined the collective disease burden attributed to environmental risks globally and regionally, quantifying the amount of death and disease caused by factors such as unsafe drinking water, poor sanitation, and indoor/outdoor air pollution. In 2016, the WHO issued a report that estimated that one in four global deaths can be attributed to unhealthy environments.(1)

Neglected tropical diseases (NTDs), a group of communicable diseases found in predominantly tropical and subtropical conditions, affect the poorest populations who often live in remote, rural areas, in urban slums, or in conflict zones. The CDC reports that one hundred percent of low-income countries are concurrently affected by at least five neglected tropical diseases. With little political voice, neglected tropical diseases typically have low status among public health priorities. In 2012, the WHO published a comprehensive plan that established targets for the reduction of NTDs, representing a shift in these priorities. In 2010, an estimated 2 billion individuals needed treatment for neglected tropical diseases. By 2016, the estimate decreased to 1.5 billion confirming the great gains have been through approaches in treatment and prevention.(2)(3)(4)

Treatment and Prevention

Many measures can be taken almost immediately to reduce the environmental disease burden, such as the promotion of safe household water storage, improved hygiene, and the control of the use of toxic substances in the home and workplace. Though many of these diseases can be treated with simple, inexpensive medications, global strategies to control NTDs also include preventive chemotherapy, intensified case management, vector control, veterinary public health measures and improved water quality and sanitation.(5)

In 2013, the WHO categorized seventeen diseases as neglected tropical diseases, namely buruli ulcer, Chagas diseases, dengue fever, guinea-worm, liver flukes, trachoma, tapeworms, yaws, soil-transmitted helminthiases, river blindness, rabies, lymphatic filariasis, leishmaniasis, schistosomiasis, sleeping sickness and hydatid disease. Recent additions include mycetoma and snakebite envenoming. This module provides a comprehensive review of a few of the major neglected tropical diseases.(6)(7)

Trachoma

Disease Overview

Trachoma is the leading cause of preventable blindness of infectious origins caused by the bacterium Chlamydia trachomatis. Trachoma is spread easily by contact with an infected person’s hands or clothing, or by flies that come in contact with the eyes or nose of an infected person. Infected individuals do not instantly go blind; instead, the disease manifests gradually. Scarring from repeated infections causes the eyelashes to turn inward and scratch the cornea, a condition known as trichiasis, leading slowly and painfully to complete blindness. Since trachoma is transmitted through close personal contact, it tends to occur in clusters, often infecting entire families and communities. Estimates suggest that 1.9 million people are blind due to trachoma while another 21 million are actively infected with the disease.(8)

Blindness from trachoma strikes adults in their prime years, hindering their ability to care for themselves and their families. Women, traditionally the caretakers of the home, are four times more likely than men to be affected by trichiasis due to increased contact with to infected children and limited financial agency in health-related decisions necessary for treatment and prevention.(9)

Treatment and Prevention

Trachoma is treatable and preventable with an approach known as the SAFE strategy.  Recommended by the World Health Organization, the SAFE strategy is a comprehensive public health approach that combines treatment (Surgery and Antibiotics) with prevention (Facial cleanliness and Environmental improvement). The strategy works as follows:

S= Surgery to correct advances of the disease
A= Antibiotics, such as azithromycin, to treat infected individuals
F= Face washing to reduce transmission
E= Environmental improvement through increased access to clean water and improved sanitation

Through the work of national governments and various partners, the implementation of the SAFE Strategy has led to measurable results in the elimination of trachoma. As of 2016, over 260,000 individuals received surgery for trichiasis and over 85 million antibiotic treatments were given worldwide.(10)(11)

River Blindness

Disease Overview

Onchocerciasis, also known as river blindness, infects approximately 18 million people, of which the majority live in sub-Saharan Africa. Over 120 million globally are at risk for contracting river blindness. Approximately 6.5 million individuals experience severe itching due to onchocerciasis and another 270,000 have been left permanently blind from the disease. People living near the fast-moving streams of Africa are at high risk since these areas are breeding grounds for the Simulium black fly. Infected flies carry larvae of the filarial parasitic worm Onchocerca volvulus from person to person. Larvae, which enter the skin at the black fly bite site, move throughout the body and then die, resulting in a variety of health problems including skin rashes and de-pigmentation, itching, and skin nodules. Visual impairment and blindness occur if larvae enter the eye.(12)(13)(14)

Treatment and Prevention

The severe socioeconomic impact of river blindness prompted the creation of the highly successful Onchocerciasis Control Program in 1974. This program focused on 11 countries in West Africa that aimed to make ivermectin treatment more accessible and included intensified spraying of black fly breeding sites with environmentally safe insecticides. The program ended in 2002 after the successful end to transmission of the disease in 10 of the 11 participating countries.(15) In 2016, over 133 million treatments were provided globally. Current elimination barriers include political uncertainty and competing health priorities, though progress has been made and four countries within Central and South America have successfully eliminated river blindness since 2013.(16)

Guinea Worm disease

Disease Overview

Guinea worm disease, formally known as Dracunculiasis, is caused by the parasitic worm Dracunculus medinensis. It is transmitted through ingestion of stagnant water that is contaminated with guinea worm larvae. In the human body, the larvae are released and migrate into body tissues, and develop into adult worms. Female worms move through the person’s tissue, causing intense pain, and eventually emerge through the skin, producing oedema, a blister and eventually an ulcer, accompanied by fever. Though rarely fatal, symptoms include joint pain, vomiting and severe burning. There are no drugs available for treatment of the disease and the worm is instead physically removed over a period of several weeks. Prevention is key in the eradication efforts of guinea worm, specifically by protecting water sources and filtering contaminated water.(17)

Treatment and Prevention

The global transmission of guinea worm disease has significantly been reduced with only three reported cases as of April 2018 within the endemic countries of Chad, Ethiopia and Mali. The fight to end guinea worm was greatly influenced by The Carter Center which led the Guinea Worm Eradication Program along with partnerships with local ministries of health and global organizations beginning in the 1980s. The program’s success was a direct result of community-based interventions that focused on water filtration and health education. Guinea worm disease is on track to be the first eradicated neglected tropical diseases due to these concerted efforts on a global and local level.(18)

Schistosomiasis

Disease Overview

Schistosomiasis is acquired through contact with parasitic worms that are released by freshwater snails.  Inside the body, the larvae develop into worms. Female worms release thousands of eggs which are passed out of the body and released into bodies of freshwater by urination or defecation They migrate to snails and begin the cycle again. Some eggs trapped in the body migrate to specific organs and inflict major damage such as scarring and tearing of the bladder and kidneys. Estimates suggest that over 205 million individuals required treatment for schistosomiasis in 2015. Schistosomiasis is prevalent in 78 countries, with 92 % of those requiring treatment living in sub-Saharan Africa. The most susceptible populations include children under age 14, as well as individuals with responsibilities and occupations near infested water sources.(19)(20)

Treatment and Prevention

Improved sanitation and hygiene are critical to the elimination of schistosomiasis. Additional measures include efforts to eliminate the snail species required for transmission. Preventative chemotherapy, where individuals are treated without being diagnosed, has been shown to be successful in decreasing the disease burden of schistosomiasis globally. Praziquantel is the only recommended drug to treat schistosomiasis and has also been shown to reduce the severity of symptoms in those re-infected with the disease.(21)

Soil Transmitted Helminth Infections

Hookworm, roundworm and whipworm make up the group of intestinal soil-transmitted infections and are among the most common infections in the world. These infections are associated with poverty and poor sanitation given their transmission through contaminated food, soil, and feces. There are an estimated 1.5 billion individuals infected by soil-transmitted helminths.

Disease Overview

Ascariasis, also known as roundworm, is an intestinal infection caused by the soil transmitted parasitic worm Ascaris lumbricoidesart. It is most prevalent in warm tropical and sub-tropical climates in Sub-Saharan Africa and Southeast Asia. The disease flourishes in areas with poor sanitation and poorly irrigated crops treated with wastewater. It is the most common human worm infection. Between 807 million to 1.2 billion people worldwide are infected with ascariasis. It is transmitted by accidental ingestion of ascaris eggs in contaminated food, water or soil. The larvae migrate into the bloodstream and cause severe coughing and wheezing.(22)

Whipworms, caused by the soil transmitted parasitic worm Trichuris trichiura affect approximately 604-795 million individuals. Individuals infected by only a few worms usually do not experience symptoms while those with a higher parasitic load may experience weakness, anemia, rectal prolapse and even hinder growth in children.(23)

Hookworm, caused by the soil transmitted parasitic worms Necator americanus and Ancylostoma duodenaleaffect approximately 576-740 million individuals globally. Infected individuals typically experience gastrointestinal symptoms.(24)

Prevention & Treatment

Albendazole and mebendazole are the recommended medications for intestinal parasitic worms. These medications are easily accessible through the ministries of health in endemic countries. The WHO adopted a resolution to eliminate the disease burden due to soil-transmitted helminth infections by 2020 which would require periodic deworming of 75% of all at-risk school children by 2020. The program has been able to achieve some success in reducing the severity of worm infections in some areas, particularly in Africa. In 2017, 485 million school age children were treated with deworming tablets. Education efforts aimed at prevention through proper sanitation, hand washing, and food preparation techniques are critical to reducing the incidence of the disease in regions of Africa.(25)(26)

Lymphatic Filariasis

Disease Overview

Lymphatic Filariasis, also known as elephantiasis is common in tropical and sub-tropical climates, in both rural and urban areas where infected mosquitoes tend to breed. Over 850 million people are at risk for becoming infected with lymphatic filariasis with approximately 40 million currently affected by the disease. Lymphatic Filariasis is transmitted when infected mosquitoes bite humans and pass microfilariae into the skin. Larvae migrate through the body and settle in the lymphatic system, where they lay eggs that pass into the blood. The worms can live for up to six years here. When they die, they can cause severe disfiguring ailments. Initially, no symptoms may be present, but as the disease progresses, the following may occur: swelling or hardening of the skin, severe swelling/lymphedema of extremities or genitals, and “filarial fevers” which are episodes of severe swelling in chronic patients. The disease has been associated with social stigma leading to emotional and mental suffering.(27)

Treatment and Prevention

Unfortunately, there is no cure or vaccine for the disease, but symptoms can be managed with a treatment plan. The treatment to reduce the symptoms of Lymphatic Filariasis includes a combination of albendazole, diethylcarbamazine (DEC), and ivermectin medicines. Evidence suggests that treatment with these medications concurrently can eliminate the microfilaria from the bloodstream in a few weeks times. Careful cleansing of the skin to remove bacteria can reduce or reverse skin or tissue damage. The 1997 WHO’s Global Program to Eliminate Lymphatic Filariasis was implemented to stop the spread of transmission and to lessen the severity of the disease in those already infected. Hygiene education efforts aimed at reducing acute episodes of filarial fevers have been effective at lessening the social stigmas associated with the manifestations of the disease. Since 1997, 18 countries have begun the process to confirm elimination while mass treatment is currently ongoing in 45 countries demonstrating great strides in the elimination of lymphatic filariasis.(28)

Footnotes

(1) Prüss-Üstün, A, et al. “WHO: Preventing disease through healthy environments: Towards an estimate of the environmental burden of disease.” May 2016. https://bit.ly/2Mlpm7v.

(2) CDC. “CDC’s Neglected Tropical Diseases Program.”https://www.cdc.gov/globalhealth/ntd/resources/ntd_factsheet.pdf.

(3) Furst, T, et al. “Global health policy and neglected tropical diseases: Then, now, and in the years to come.”https://bit.ly/2N6ceIW.

(4) WHO. “Accelerating work to overcome the global impact of neglected tropical diseases.”http://www.who.int/neglected_diseases/NTD_RoadMap_2012_Fullversion.pdf.

(5) WHO. “Working to overcome the global impact of neglected tropical diseases.” https://bit.ly/2QmVzyQ.

(6) Molyneaux, D. “Neglected tropical diseases,”https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3756642/.

(7) WHO. “Neglected tropical diseases.”http://www.who.int/neglected_diseases/diseases/en/.

(8) WHO. “Trachoma.”http://www.who.int/trachoma/en/.

(9) Trachoma Coalition. “Trachoma- A women and children’s health issue.”http://www.trachomacoalition.org/sites/all/themes/report-2016/PDF/Trachoma_A_women_and_children's_health_issue.pdf.

(10) WHO. “Trachoma: Strategy.”http://www.who.int/trachoma/strategy/en/.

(11) WHO. “Trachoma.”http://www.who.int/news-room/fact-sheets/detail/trachoma.

(12) WHO. “Onchocerciasis.”http://www.who.int/onchocerciasis/en/.

(13) WHO. “Onchocerciasis- river blindness.”http://www.who.int/mediacentre/factsheets/fs095/en/.

(14) WHO. “What is onchocerciases?”http://www.who.int/onchocerciasis/disease/en/.

(15) WHO. “Onchocerciasis Control Programme (OCP).”http://www.who.int/blindness/partnerships/onchocerciasis_OCP/en/.

(16) WHO. “River blindness: shifting from prevention to surveillance and elimination.”http://www.who.int/neglected_diseases/news/shifting_from_prevention_to_surveillance_elimination/en/.

(17) WHO. “Dracunculiasis eradication.”http://www.who.int/dracunculiasis/disease/en/.

(18) The Carter Center. “Guinea Worm Eradication Program.”https://www.cartercenter.org/health/guinea_worm/index.html.

(19) WHO. “Schistosomiasis.”http://www.who.int/news-room/fact-sheets/detail/schistosomiasis.

(20) Ibid.

(21) Ibid.

(22) CDC. “Parasites- Ascariasis.”https://www.cdc.gov/parasites/ascariasis/index.html.

(23) CDC. “ Parasites - Trichuriasis (also known as Whipworm Infection).”https://www.cdc.gov/parasites/whipworm/index.html.

(24) CDC. “Parasites- Hookworm.”https://www.cdc.gov/parasites/hookworm/index.html.

(25) WHO. “Neglected Tropical Diseases.”http://www.who.int/neglected_diseases/news/WHO_publishes_recommendations_for_large-scale_deworming/en/.

(26) WHO. “Intestinal worms.”http://www.who.int/intestinal_worms/strategy/en/.

(27) WHO. “Lymphatic filariasis.”http://www.who.int/news-room/fact-sheets/detail/lymphatic-filariasis.

(28) WHO. “Lymphatic filariasis: Global progress towards elimination.”http://www.who.int/lymphatic_filariasis/global_progress/en/.