Vector Borne Disease

A vector-borne disease is caused by a pathogenic microorganism transmitted by an arthropod or other agent. Mosquitoes are one of the deadliest animals in the world, spreading diseases such as Dengue, Lymphatic Filariasis, Zika and Malaria. Vectors such as sandflies, black flies, tsetse flies and mosquitoes cause 17% of all infectious disease burden.

Transmission of vector-borne disease requires at least three different organisms:

  • Pathogenic agent (causes the disease e.g. a bacterium, virus or fungus)
  • Vector (carries the disease and disperses reproductive structure)
  • A human host

According to WHO, there are more than 1 billion cases and over 1 million deaths from vector borne diseases a year globally. These devastating diseases have an overwhelming impact in developing countries in the tropics.

Many different factors such as climate change, growing urbanization and changes in agricultural methods cause levels of disease transmission to fluctuate. People displaced and caught up in conflict or natural crises are even more vulnerable to VBD since little or no shelter and poor sanitation increases exposure to vector contact. Alongside malnutrition in the poorest communities, multiple infections decrease the ability of the immune system, meaning that the body cannot fight preventable diseases.

With our expertise and experience in vector borne disease, MENTOR works to educate others of the epidemiology of different types of diseases and how to prioritize key control measures. Through implementing evidence-based control programmes across the world, MENTOR provides those most in need with prevention, surveillance and control of vector borne disease.

Chikungunya

Chikungunya virus (CHIKV) is an enzootic virus and has been identified in over 60 countries in Asia, Africa, Europe and the Americas. The name “Chikungunya” derives from a word in the Kimakonde language, meaning “to become contorted”, and describes the stooped appearance of sufferers with joint pain (arthralgia) as a result of the disease.[1]

The virus is transmitted from human to human by the bites of infected female Aedes mosquitoes. Most commonly, the mosquitoes involved are of the species Ae. aegypti and Ae. albopictus. They can be found biting throughout daylight hours, though there may be peaks of activity in the early morning and late afternoon. Both species are found biting outdoors, but Ae. aegypti will also feed indoors.13,[2]

After the bite of an infected mosquito, onset of illness occurs usually between 4 and 8 days but can range from 2 to 12 days. Often symptoms in infected individuals are mild and the infection may go unrecognized, or be misdiagnosed in areas where dengue occurs. The disease is characterized by an abrupt onset of fever, frequently accompanied by joint pain. Other symptoms include muscle pain, headache, nausea, fatigue and rash. The joint pain can be debilitating and usually lasts for a few days or may be prolonged to weeks. As a result, the CHIKV can cause acute, subacute or chronic disease. Occasional cases of eye, neurological and heart complications have been reported, as well as gastrointestinal complaints.13,[3]

As for dengue fever, there is no specific antiviral treatment for chikungunya. Treatment is primarily focusing at relieving the symptoms. There is no commercial chikungunya vaccine.13

[1] World Health Organization (2017). Factsheets – Chikungunya Virus.

[2] Burt, F.J. et al. (2012). Chikungunya: a re-emerging virus. Lancet 2012; 379: 662–71.

[3] Centers for Disease Control and Prevention (2018). Chikungunya Virus – Symptoms, Diagnosis & Treatment.

Cholera

Cholera is an acute diarrhoeal infection caused by ingestion of food or water contaminated with the bacterium Vibrio cholerae and remains a global threat to public health and an indicator of inequity and lack of social development.[1] It has epidemic and pandemic potential[2] and researchers have estimated that every year, there are roughly 1.3 to 4.0 million cases, and 21,000 to 143,000 deaths worldwide due to cholera.[3]

Cholera transmission is closely linked to inadequate access to clean water and sanitation facilities. Typical at-risk areas include peri-urban slums, and camps for internally displaced persons or refugees, where minimum requirements of clean water and sanitation are not been met.[4] Mechanical vectors such as flies can also disseminate V. cholerae.[5],[6]

Cholera is extremely virulent disease and can cause severe acute watery diarrhoea. It takes between 12 hours and 5 days for a person to show symptoms after ingesting contaminated food or water[7]. Cholera affects both children and adults and can kill within hours if untreated.

Most people infected with V. cholerae do not develop any symptoms, although the bacteria are present in their faeces for 1-10 days after infection and are shed back into the environment, potentially infecting other people.38

Cholera is an easily treatable disease and the majority of people can be treated successfully through prompt administration of oral rehydration solution (ORS). Severely dehydrated patients are at risk of shock and require the rapid administration of intravenous fluids. These patients are also given appropriate antibiotics to diminish the duration of diarrhoea, reduce the volume of rehydration fluids needed, and shorten the amount and duration of V. cholerae excretion in their stool.38

Currently there are three WHO pre-qualified oral cholera vaccines (OCV): Dukoral®, Shanchol™, and Euvichol-Plus®. All three vaccines require two doses for protection, although they only offer incomplete protection.38 Therefore, vaccination should never take the place of standard prevention and control measures[8], which mainly consist of appropriate hygiene practices such as hand-washing with soap, safe preparation and storage of food and safe disposal of the faeces. These measures are also spread through hygiene promotion and social mobilisation campaigns in endemic and epidemic-prone regions to avoid outbreaks of cholera.38

[1] World Health Organization (2018). Fact Sheets – Cholera. Available at: https://www.who.int/news-room/fact-sheets/detail/cholera

[2] Mandal, S. et al. (2011). Cholera: a great global concern. Asian Pacific Journal of Tropical Medicine; 573-580.

[3] Ali, M. et al. (2015). Updated Global Burden of Cholera in Endemic Countries. PLoS Negl Trop Dis.; 9(6).

[4] World Health Organization (2018). Fact Sheets – Cholera. Available at: https://www.who.int/news-room/fact-sheets/detail/cholera

[5] Nazni, W.A.; Seleena, B.; Lee, H.L.; Jeffery, J.; Rogayah, T.A.R.; Sofian, M.A. (2005) Bacteria Fauna from the House Fly, Musca domestica (L.). Tropical Biomedicine, 22 (2).

[6] Fotedar, R. (2001). Vector potential of houseflies (Musca domestica) in the transmission of Vibrio cholerae in India. Acta Tropica, 78(1).

[7] Azman, A.S. et al. (2013). The incubation period of cholera: a systematic review. Infect.;66(5):432-8.

[8] Centers for Disease Prevention and Control (2018). Cholera – Vibrio cholerae infection – Vaccine. Available at: https://www.cdc.gov/cholera/vaccines.html

Dengue Fever

Dengue fever is a mosquito-borne disease caused by the dengue virus, which belongs to the family of arboviruses (arthropod-borne virus). It is transmitted by several species of female mosquitoes belonging to the Aedes type.[1] The principal carriers of the virus are Aedes aegypti and Aedes albopictus, the latter becoming increasingly more important because of its rapidly increasing global distribution.[2]

The symptoms include fever, headache, nausea, muscle and joint pains as well as a rash. A small percentage of cases can progress to Dengue Haemorrhagic Fever (DHF), which can be fatal if treatment is delayed and the patient develops shock or multi-organ failure.56,[3]

Before 1970, only 9 countries had experienced severe dengue epidemics. As of now, dengue is endemic in more than 120 countries. The actual numbers of dengue cases are underreported and many cases are misclassified. Recent estimates indicate around 390 million dengue infections per year, of which an estimated 96 million cases manifest clinically.56

There is no specific treatment for dengue fever and therefore symptom management is key. If the disease progresses to Dengue Haemorrhagic Fever, intensive unit care is necessary.[4]

The first dengue vaccine, Dengvaxia (CYD-TDV) by Sanofi Pasteur, was first registered in Mexico in December of 2015 and targets all four strains of the dengue virus. In 2018 the European Medicines Agency approved the vaccine for the use in Europe. Dengvaxia is only for use in people from 9 to 45 years of age who have been infected with dengue virus before and who live in areas where this infection is endemic.56

The proximity of mosquito vector breeding sites to human habitation is a significant risk factor for dengue as well as for other diseases that Aedes mosquitoes transmit. The main method to control or prevent the transmission of dengue virus is to combat the mosquito vectors. This is achieved through prevention and treatment of mosquito breeding in and around houses, schools and health facilities, personal protection, community engagement and active mosquito and virus surveillance.56

[1] World Health Organization (2019). Dengue and Severe Dengue – Key Facts. Available at: https://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue

[2] Kraemer, M.U.G. et al. (2019). Past and future spread of the arbovirus vectors Aedes aegypti and Aedes albopictus. Nature Microbiology, 4: 854–863.

[3] Wiwanitkit, V. (2010). Dengue fever: diagnosis and treatment. Expert Rev. Anti Infect. Ther. 8(7), 841–845.

[4] Centers for Disease Control and Prevention (2019). Dengue Fever – Symptoms and Treatment. Available at: https://www.cdc.gov/dengue/symptoms/index.html

Ebola

Ebola Virus Disease (EVD), is a severe and often fatal illness in humans, transmitted to people from wild animals and spreading in human populations through human-to-human transmission. The viral disease appeared in 1976 in 2 simultaneous outbreaks, one in what is now Nzara, South Sudan, and the other in Yambuku, DRC. The latter occurred in a village near the Ebola River, from which the disease takes its name. The average EVD case fatality rate is around 50%, and case fatality rates have varied from 25% to 90% in past outbreaks.[1]

Read about MENTOR’s response to the Ebola outbreak in Liberia. 

Ebola is introduced into the human population through close contact with the blood, secretions, organs or other bodily fluids of infected animals such as fruit bats, chimpanzees, gorillas, monkeys, or forest antelopes.32 Human-to-human EBOV transmission occurs via inoculation by injection of the virus into the bloodstream or via exposure of mucous membranes or nonintact skin to infectious blood or body fluids (urine, saliva, sweat, faeces, vomit, breast milk, and semen) of a person who is sick with or has died from EVD. [2],[3] Infectious viral particles can survive on inanimate surfaces for days or weeks and can survive even longer in conditions such as climate-controlled hospital settings.[4]

Symptoms may appear anywhere from 2 to 21 days after contact with the virus, with an average of 8 to 10 days. Many common illnesses can have these same symptoms, including influenza or malaria. Symptoms include: Fever, severe headache, muscle pain, fatigue, diarrhoea, vomiting, abdominal pain and both internal and external bleeding (for example, oozing from the gums, or blood in the stools).32,34

Symptoms of Ebola virus disease (Ebola) are treated as they appear. When used early, basic supportive care can significantly improve the chances of survival. These include: Providing fluids and body salt through infusion into the vein, offering oxygen therapy, using medication to support blood pressure, reducing vomiting and diarrhoea, managing fever and pain, and treating other infections, if they occur.34

Community engagement is key to successfully controlling outbreaks and good outbreak control relies on applying a package of interventions, namely case management, infection prevention and control practices, surveillance and contact tracing, a good laboratory service, safe and dignified burials and social mobilisation. Vaccines to protect against Ebola are under development and have been used to help control the spread of Ebola outbreaks in Guinea and in the Democratic Republic of the Congo (DRC).32

[1] World Health Organization (2019). Key Facts – Ebola Virus Disease. Available at: https://www.who.int/news-room/fact-sheets/detail/ebola-virus-disease

[2] Baselere, L. et al. (2016). The Pathogenesis of Ebola Virus Disease. Annu. Rev. Pathol. Mech. Dis. 2017. 12:15.1–15.32.

[3] Centers for Disease Control and Prevention (2019). Ebola (Ebola Virus Disease) – Transmission. Available at: https://www.cdc.gov/vhf/ebola/transmission/index.html

[4] Fischer, R. et al. (2015). Ebola virus stability on surfaces and in fluids in simulated outbreak environments. Emerg. Infect. Dis. 21:1243–46.

Leishmaniasis

Leishmaniasis is one of the world’s most neglected tropical diseases. An estimated 700 000 to 1 million new cases and some 26 000 to 65 000 deaths occur annually. Affecting some of the poorest people on earth, the disease is widely associated with population displacement, poor housing, malnutrition, a weak immune system and lack of financial resources and linked to environmental changes such as deforestation, building of dams, irrigation schemes, and rapid urbanization.[1]

Transmitted by the bite of female phlebotomine sandflies (over 90 sandfly species are known to transmit leishmaniasis) and caused by protozoan parasites from the genus Leishmania, leishmaniasis can be devastatingly disfiguring and lead to fatal outcomes. Just as mosquitoes, sandflies become infected with the Leishmania parasites when they suck blood from an infected person or animal and when the infectious female sandfly then feeds on a fresh source of blood, it inoculates the person or animal with the parasite, spreading the disease.

There are three types of Leishmaniasis:

  1. Visceral Leishmaniasis (VL): VL is also known as kala-azar and occurs when the Leishmania parasites infect internal organs such as the liver, spleen and bone marrow. The usual incubation is between 2 – 6 months and symptoms include prolonged fever, weight loss, malaise and fatigue, as well as enlargement of the spleen and/or the liver. If left untreated, VL is fatal in over 95% of cases.[2],[3]
  2. Cutaneous Leishmaniasis (CL): CL, also known as “Silent Sheikh”, “Oriental Sore” or “Aleppo Boil”, is the most common manifestation of leishmaniasis and causes skin lesions that can be self-resolving but, if left untreated, can lead to mucocutaneous lesions. The sores may start out as papules (bumps) or nodules (lumps) and may progress to ulcers at the site of a vector bite. Sometimes, an infection can present as silent, without any symptoms or signs. It is estimated that between 600,000 to 1 million new cases occur worldwide annually.2
  3. Mucocutaneous Leishmaniasis: Begins as primary cutaneous ulcer at the site of inoculation, which eventually heals. After one month to 24 years later, the disease can start to manifest as mucocutaneus leishmaniasis, and lesions can lead to partial or total destruction of the mucous membranes of the nose, mouth, throat cavities and surrounding tissues.2

[1] World Health Organization (2019). Fact Sheets – Leishmaniasis. Available at: https://www.who.int/news-room/fact-sheets/detail/leishmaniasis

[2] World Health Organization (2019). Fact Sheets – Leishmaniasis. Available at: https://www.who.int/news-room/fact-sheets/detail/leishmaniasis

[3] McGwire, B.S. & Satoskar, A.R. (2014). Leishmaniasis: clinical syndromes and treatment. Q J Med 2014; 107:7–14.

Lymphatic Filariasis

Lymphatic Filariasis is a painful and disfiguring neglected tropical disease and has devastating social and economic consequences due to its visibility and disabling manifestations. The World Health Organization estimates that over 880 million people in 52 countries worldwide live in regions endemic for the disease.[1] Roughly 43 million people live with LF[2] and as many as 36 million individuals are seriously incapacitated and disfigured by the disease.[3]

Caused by three species of filarial (thread-like) worms in humans (Wuchereria bancrofti, Wuchereria brugia, and Wuchereria malayi) and transmitted by Aedes, Anopheles and Culex mosquitoes, this parasitic infection can result in an altered lymphatic system and subsequent abnormal enlargements of body parts, such as the legs and genitalia.1

Adult worms are long-lived and can reproduce for about 5–8 years, producing millions of larval (microfilarial stages). The microfilariae migrate from the lymphatic system to the blood and show a periodical pattern of swarming to the peripheral blood vessels, reaching peak concentrations that coincide with the local feeding habits of their mosquito vector.[4]

Transmission of LF can be reduced by controlling the vector mosquitoes. Through methods such as Long-Lasting Insecticidal Nets (LLINs), Indoor Residual Spraying (IRS) or treating breeding sites with insecticides, people can be protected from infection. To stop the spread of LF, Mass Drug Administrations (MDA) of the drug albendazole combined with ivermectin or diethylcarbamazine citrate can clear microfilariae from the bloodstream, preventing the spread of parasites to mosquitoes.

[1] World Health Organization (2019) Factsheet – Lymphatic Filariasis.

[2] Herricks, J.R. et al. (2017). The global burden of disease study 2013: What does it mean for the NTDs? PLoS Negl Trop Dis 11(8).

[3] World Health Organization (2017). Weekly epidemiological record – Global programme to eliminate

lymphatic filariasis: progress report, 2016. Geneva: World Health Organization.

[4] Taylor, M.J., Hoerauf, A. & Bockarie, M. (2010). Lymphatic filariasis and onchocerciasis. Lancet 2010; 376: 1175–85.

Malaria

Malaria is a life-threatening disease caused by Plasmodium parasites that are transmitted to people through the bites of infected female Anopheles mosquitoes. In 2017, there were an estimated 219 million cases of malaria in 87 countries. The estimated number of malaria deaths stood at 435,000 in 2017. The WHO African Region carries a disproportionately high share of the global malaria burden. In 2017, the region was home to 92% of malaria cases and 93% of malaria deaths.[1]

There are five parasite species that cause malaria in humans (P. falciparum, P. vivax, P. ovale, P. malariae, and P. knowlesi). P. falciparum and P. vivax pose the greatest threat to humans and are the most widespread malaria parasites.[2]

The parasites accumulate in the salivary glands of mosquitoes and enter the bloodstream when a person is bitten. The parasites then migrate to the liver, where they multiply. After an incubation period of 7 to 30 days (depending on the type of parasite), the liver cells burst and release the merozoite form of the parasite, which then enter red blood cells (RBCs), where they undergo another process of multiplication. The RBCs rupture and release the parasite into the blood. Most merozoites go on to infect more blood cells, while some develop into gametocytes, which are taken up by mosquitoes when the infected person is being bitten again, where they reproduce and the cycle starts anew when the mosquito infects another person.[3]

The first symptoms of malaria are nonspecific and comprise headache, fatigue, abdominal discomfort and muscle and joint aches, usually followed by fever, chills, anorexia, vomiting and worsening malaise. At this early stage of the disease, a rapid and full recovery can be expected, provided prompt and effective antimalarial treatment is given. If treatment is delayed or not administered, particularly in P. falciparum malaria, the parasite burden continues to increase to a level where the patient may develop potentially lethal severe malaria. Disease progression to severe malaria may take days but can occur within a few hours.[4]

Some population groups are at considerably higher risk of contracting malaria, and developing severe disease, than others. These include infants, children under 5 years of age, pregnant women and patients with HIV/AIDS, as well as non-immune migrants, mobile populations and travellers.6

[1] World Health Organization (2019). World Malaria Report 2018. Geneva: World Health Organization.

[2] World Health Organization (2019). Factsheet – Malaria.

[3] Centers for Disease Control and Prevention (2018). About Malaria – Biology.

[4] World Health Organization (2015). Guidelines for the Treatment of Malaria – Third Edition. Geneva: World Health Organization.

Onchocerciasis

Onchocerciasis, commonly known as “river blindness”, is caused by a parasitic worm that is transmitted to humans through repeated bites of infected blackflies. A 2017 study estimated that there were 20.9 million prevalent onchocerciasis infections worldwide: 14.6 million of the infected people had skin disease and 1.15 million had vision loss. More than 99% of infected people live in 31 African countries. The disease also exists in some foci in Latin America and Yemen.[1]

Once matured, most adult female worms live in fibrous nodules under the skin, which form around the worms as part of the interaction between the parasite and the human. Inside the nodules the worms are relatively safe from the human immune response. As adults, female worms produce thousands of new larvae daily.[2],[3]

Most symptoms of onchocerciasis are caused by the body’s response to dead or dying larvae. Symptoms include severe itching, disfiguring skin conditions (such as skin thinning and discolouration), and visual impairment, including permanent blindness.53,54

People who are found to be infected should be treated in order to prevent the long-term skin damage and blindness. The recommended treatment is Ivermectin, which will need to be given every 6 months for the life span of the adult worms or for as long as the infected person has evidence of skin or eye infection. Ivermectin kills the larvae and prevents them from causing damage but it does not kill the adults.[4]

The core strategy for eliminating onchocerciasis is community-directed treatment with ivermectin, carried out in Mass Drug Administration (MDA) campaigns.52

[1] World Health Organization (2019). Onchocerciasis – Key Facts. Available at: https://www.who.int/news-room/fact-sheets/detail/onchocerciasis

[2] Centers for Disease Control and Prevention (2018). Parasites – Onchocerciasis (also known as River Blindness). Available at: https://www.cdc.gov/parasites/onchocerciasis/disease.html

[3] Taylor, M.J. e al. (2010). Lymphatic filariasis and onchocerciasis. Lancet 2010; 376: 1175–85.

[4] Centers for Disease Control and Prevention (2018). Parasites – Onchocerciasis (also known as River Blindness). Available at: https://www.cdc.gov/parasites/onchocerciasis/treatment.html

Rift Valley Fever

Rift Valley fever (RVF) is an acute, viral disease, caused by RVF virus (RVFV), a member of the genus Phlebovirus. It is most commonly observed in domesticated animals (such as cattle, sheep, goats, and camels) and was first reported in livestock in Kenya’s Rift Valley in the early 1910s. It also has the ability to infect and cause illness in humans.[1]

RVF is generally found in regions of eastern and southern Africa where sheep and cattle are raised, but the virus exists in most of sub-Saharan Africa, including west Africa and Madagascar. Outbreaks have also occurred in Saudi Arabia and Yemen.16

The majority of human infections result from direct or indirect contact with the blood or organs of infected animals through inoculation, for example via a wound from an infected knife or through contact with broken skin, or through inhalation of aerosols.

Human infections can also result from the bites of several species of mosquito, most commonly the Aedes and Culex. Female mosquitoes are also capable of transmitting the virus directly to their offspring via eggs leading to new generations of infected mosquitoes hatching from eggs.[2]

Most RVF patients suffer from a self-limiting, febrile illness, but a small percentage of patients develop a severe form of the disease, which usually appears as one or more of three distinct syndromes: Ocular (eye) disease (0.5–2% of patients, and some experience permanent loss of vision), meningoencephalitis (less than 1% of patients, residual neurological deficit are common) or haemorrhagic fever (less than 1% of patients, and a death rate of 50%).17,[3]

Most human cases of RVF are mild and of short duration, and as a result no specific treatment is required. The predominant treatment is general supportive therapy for more severe cases.17

[1] Centers for Disease Control and Prevention (2016). Rift Valley Fever.

[2] World Health Organization (2018). Factsheet – Rift Valley Virus.

[3] Ikegami, T. & Makino, S. (2011). The Pathogenesis of Rift Valley Fever. Viruses 2011, 3, 493-519.

Schistosomiasis

Schistosomiasis is an acute and chronic parasitic disease caused by trematode worms (blood flukes) of the genus Schistosoma. In terms of impact this disease is second only to malaria as the most devastating parasitic disease and is considered one of the neglected tropical diseases (NTDs). Schistosomiasis is prevalent in tropical and subtropical areas, especially in poor communities without access to safe drinking water and adequate sanitation. Transmission has been reported from 78 countries and estimates show that at least 220.8 million people required preventive treatment in 2017. At least 90% of those requiring treatment for schistosomiasis live in African countries.[1],[2]

The adult male and female worms live within blood vessels of infected humans, where they mate and produce fertilised eggs. The eggs are either shed into the environment through faeces or urine, or are retained in host tissues. Eggs that reach freshwater will hatch and release miracidia, a free-swimming larval stage, which can remain infective for 1-3 days and infect snail hosts. In a suitable snail, the parasite undergoes asexual replication and eventually sheds cercariae (the form infectious for human beings) into the water. After cercariae penetrate the skin of mammalian hosts (such as dogs, pigs, cattle, rodents and humans), the larvae need about 5–7 weeks before becoming adults and producing eggs.[3] Adult schistosomes live an average of 3–10 years, but in some cases as long as 40 years, in their human hosts.21,[4]

The morbidity caused by schistosomiasis is caused by the eggs rather than the adult worms themselves. As many eggs are not excreted, they become permanently lodged in the intestines, liver, the bladder and/or the urogenital system, causing chronic inflammations.

Intestinal schistosomiasis can result in abdominal pain, diarrhoea, and blood in the stool. Liver enlargement is common in advanced cases. In such cases there may also be enlargement of the spleen. In women, urogenital schistosomiasis may present with genital lesions, vaginal bleeding, and nodules. In men, urogenital schistosomiasis can damage seminal vesicles, the prostate and other organs. Schistosomiasis may also have other long-term irreversible consequences, such as infertility.19 Bladder and ureter issues as well as kidney damage are sometimes diagnosed in advanced cases. Bladder cancer is another possible complication in the later stages.19

The control of schistosomiasis is based on large-scale treatment of at-risk population groups, access to safe water, improved sanitation, hygiene education, and snail control.

Periodic large-scale treatment of affected populations with the drug praziquantel involves regular treatment of all at-risk groups and aims to reduce disease morbidity and transmission. Treatment targets school-aged children in endemic areas, adults considered to be at risk in endemic areas, and people with occupations involving contact with infested water, such as fishermen, farmers, and women whose domestic tasks bring them in contact with infested water.

[1] World Health Organization (2019). Factsheet – Schistosomiasis.

[2] Centers for Disease Control and Prevention (2018). Parasites – Schistosomiasis.

[3] Colley, D.G. et al. (2014). Human Schistosomiasis. The Lancet. 28; 383(9936).

[4] Chabasse, D. et al. (1985). Developmental bilharziasis caused by Schistosoma mansoni discovered 37 years after infestation. Bull Soc Pathol Exot. 1985; 78:643–47.

Soil-Transmitted Helminth

Soil-transmitted helminth infections are caused by different species of parasitic worms that are transmitted by eggs present in human faeces, which contaminate the soil in areas where sanitation is poor.[1] Soil-transmitted helminth infection is also found mainly in areas with warm and moist climates where sanitation and hygiene are poor, including in temperate zones during warmer months.[2] Approximately 1.5 billion people are infected with soil-transmitted helminths worldwide.49

Soil-transmitted helminths live in the intestine and their eggs are passed in the faeces of infected persons. If an infected person defecates outside or if the faeces of an infected person are used as fertilizer, eggs are deposited on soil, where roundworm and hookworm eggs become infective as they mature. People are infected with them when eggs are ingested. This can happen when hands or fingers that have contaminated dirt on them are put in the mouth or by consuming vegetables and fruits that have not been carefully cooked, washed or peeled. Hookworm eggs hatch in soil, releasing larvae that mature into a form that can penetrate the skin of humans and so hookworm infection is transmitted primarily by walking barefoot on contaminated soil.50,[3]

Morbidity is related to the number of worms that live in the intestines. People with infections of light intensity usually do not suffer from the infection. Heavier infections can cause a range of symptoms including intestinal manifestations (diarrhoea and abdominal pain), malnutrition, general malaise and weakness, and impaired growth and physical development. Infections of very high intensity can cause intestinal obstruction that should be treated surgically. In children especially STH infection can impair the nutritional status by increasing malabsorption of nutrients, loss of appetite and anaemia.50,51

Control is based on periodical deworming of at-risk populations such as pre-school and school-aged children and women of reproductive age to eliminate infecting worms, health education to prevent re-infection, and improved sanitation to reduce soil contamination with infective eggs.49,50

[1] World Health Organization (2018). Soil-Transmitted Helminths – Key Facts. Available at: https://www.who.int/news-room/fact-sheets/detail/soil-transmitted-helminth-infections

[2] Centers for Disease Control and Prevention (2018). Parasites – Soil-transmitted helminths. Available at: https://www.cdc.gov/parasites/sth/ index.html

[3] Jourdan, P.M. et al. (2018). Soil-transmitted helminth infections. Lancet 2018; 391: 252–65.

Trachoma

Trachoma still is the leading infectious cause of blindness worldwide and is caused by the bacterium Chlamydia trachomatis. Chlamydia trachomatis evolved with the dinosaurs, and all vertebrates have evolved with their own chlamydial strains.[1] The infection is transmitted by direct or indirect transfer of eye and nose discharges of infected people, particularly young children. These discharges can also be spread by several species of flies. Trachoma is hyperendemic in many of the poorest and most rural areas of 37 countries of Africa (which is the most affected continent), Central and South America, Asia, Australia and the Middle East and is responsible for the blindness or visual impairment of about 1.9 million people. It causes about 1.4% of all blindness worldwide.[2]

If infected, individuals are frequently asymptomatic or have only mild symptoms. If present, symptoms are similar to those associated with any chronic conjunctivitis: redness, discomfort, tearing, photophobia and scant discharge. The key mechanism behind trachoma-induced problems with sight and eventual blindness is the intense conjunctival inflammation with follicles (recognised as active trachoma). It is sustained by repeated episodes of reinfection and reflects a sustained immune-mediated response to chlamydial antigens. The ongoing inflammation causes scarring, distortion of the lid, and inturning of the lid, whereby the eyelashes touch the cornea (trichiasis), which leads to blindness.[3]

Visual impairment or blindness results in a worsening of the quality of life of affected individuals, who are normally already amongst the poorest of the poor. Women are blinded up to 4 times as often as men, most likely due to their close contact with infected children and their resulting greater frequency of infection episodes.

Environmental risk factors influencing the transmission of the disease include:

  • Inadequate hygiene
  • Crowded households
  • Inadequate access to water
  • Inadequate access to and use of sanitation 24

The disease is prevented and controlled by implementing the WHO-recommended SAFE strategy. This consists of:

  • Surgery to treat the blinding stage (trachomatous trichiasis)
  • Antibiotics to clear infection, particularly mass drug administration of the antibiotic azithromycin, which is donated by the manufacturer to elimination programmes, through the International Trachoma Initiative
  • Facial cleanliness; and
  • Environmental improvement, particularly improving access to water and sanitation 24

[1] Clarke, I.N. (2011). Evolution of Chlamydia trachomatis. Ann N Y Acad Sci; 1230: E11–18.

[2] World Health Organization (2018). Fact sheets – Trachoma. Available at: https://www.who.int/news-room/fact-sheets/detail/trachoma

[3] Hu, V.H. et al. (2010). Epidemiology and control of trachoma: systematic review. Tropical Medicine and International Health: 15(6) pp 673–691.

Typhoid Fever

Typhoid fever is a life-threatening infection caused by the bacterium Salmonella Typhi, which is usually spread through contaminated food or water and only lives in humans.[1] It is most common in regions with poor economic development and limited public health infrastructure.[2],[3] Urbanization and climate change have the potential to increase the global burden of typhoid, and increasing resistance to antibiotic treatment is making it easier for typhoid to spread through overcrowded populations in cities and inadequate and/or flooded water and sanitation systems.44,[4] An estimated 11–20 million people get sick from typhoid and between 128,000 and 161,000 people die from it every year.44

After infection, the incubation period may not always be followed by clinical symptoms and people can become asymptomatic carriers. Those who go on to develop typhoid become fatigued and develop a fever. If left untreated, the temperature will remain high (>39◦C), and associated symptoms generally include coughing, vomiting, headache, a rapid pulse and some patients may also have a rash. Typhoid can be difficult to distinguish clinically from other causes of fever, such as malaria.45,[5] Severe cases may lead to serious complications or even death.44

Typhoid fever can be treated with antibiotics although increasing resistance to different types of antibiotics is making treatment more complicated. Even when the symptoms go away, people may still be carrying typhoid bacteria, meaning they can spread it to others through their faeces. It is therefore important that people being treated for typhoid fever take the full course of antibiotics, wash their hands with soap and water after using the bathroom, and do not prepare or serve food for other people.44

Access to safe water and adequate sanitation, hygiene among food handlers and typhoid vaccination are all effective in preventing typhoid fever. Two vaccines have been used for many years to protect people from typhoid fever: an injectable vaccine based on the purified antigen for people aged over 2 years, and a live oral vaccine in capsule formulation for people aged over 5 years. These vaccines do not provide long-lasting immunity and are not approved for children younger than 2 years old.44

[1] World Health Organization (2018). Fact Sheets – Typhoid. Available at: https://www.who.int/news-room/fact-sheets/detail/typhoid

[2] Dougan, G. & Baker, S. (2014). Salmonella enterica: Serovar Typhi and the Pathogenesis of Typhoid Fever. Annu. Rev. Microbiol. 2014. 68:317–36.

[3] Parry, C.M. et al. (2010). Suitable disk antimicrobial susceptibility breakpoints defining Salmonella enterica serovar Typhi isolates with reduced susceptibility to fluoroquinolones. Antimicrob. Agents Chemother. 54:5201–8.

[4] Mogasale, V. e al. (2014). Burden of typhoid fever in low-income and middle-income countries: a systematic, literature-based update with risk-factor adjustment. Lancet Glob Health 2014; 2: e570–80.

[5] Parry, C.M. et al. (2002). Typhoid fever. N. Engl. J. Med. 347:1770–82

Yellow Fever

Yellow fever is an acute viral haemorrhagic disease (damaging the walls of tiny blood vessels and cause them to leak as well as interfering with the blood’s ability to clot) transmitted by infected mosquitoes. The “yellow” in the name Yellow Fever refers to the jaundice (yellowing of the skin and eyes) that affects some patients. Symptoms of yellow fever include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue. A small proportion of patients who contract the virus develop severe symptoms and approximately half of those die within 7 to 10 days. Forty-seven countries in Africa (34) and Central and South America (13) are either endemic for, or have regions that are endemic for, yellow fever.[1]

Humans are infected sporadically when bitten by mosquitoes that previously fed on a viremic monkey (so-called jungle yellow fever), but may also serve as the host for human-human transmission, mainly by Aedes aegypti, a species of mosquito that breeds in water-containing vessels inside dwellings or in close proximity to them (so-called urban yellow fever).[2]

Yellow fever is the prototypical viral hemorrhagic fever, and shares many pathophysiological features with unrelated diseases associated with a similar syndrome.27 The majority of people infected with yellow fever virus will either not have any symptoms, or have mild symptoms and completely recover. For people who develop symptoms, the time from infection until illness is typically 3 to 6 days.[3] If Yellow Fever manifests, it can be a severe acute illness with fever, nausea, vomiting, epigastric pain, hepatitis with jaundice, renal failure, and shock. Bleeding can occur from the mouth, nose, eyes or stomach. Death occurs in 20–60% of cases.27 There is no medicine to treat or cure viral infection from yellow fever. Resting, drinking fluids, and using pain relievers and medication to reduce fever and relieve aching are used to treat mild symptoms. People with severe symptoms need to be hospitalized for close observation and supportive care.28

Vaccination is the most important means of preventing yellow fever. A single dose of the vaccine provides life-long protection against the disease. The risk of yellow fever transmission in urban areas can be reduced through effective vector surveillance and the elimination of potential mosquito breeding sites. This can be done through applying larvicides to water storage containers and other places where standing water collects and mosquitoes breed.

[1] World Health Organization (2018). Fact Sheet – Yellow Fever. Available at: https://www.who.int/news-room/fact-sheets/detail/yellow-fever

[2] Monath, T.P. & Vasconcelos, P.F.C. (2015). Yellow Fever. Journal of Clinical Virology 64, pp. 160–173.

[3] Centers for Disease Control and Prevention (2019). Yellow Fever – Symptoms, Diagnosis & Treatment. Available at: https://www.cdc.gov/yellowfever/symptoms/index.html