Three important vector-borne diseases have been discussed previously in this Module: malaria in Study Sessions 5–12 and relapsing fever and typhus in Study Session 36. In this study session, you will learn the causes, modes of transmission, clinical manifestations, prevention and control of four other vector-borne diseases of public health importance in Ethiopia. They are schistosomiasis, leishmaniasis, onchocerciasis and lymphatic filariasis. A better understanding of these diseases will help you to identify patients and refer them quickly to a health centre or hospital for specialist treatment.
You will also learn about the health education messages that you need to communicate to members of your community, so they can reduce their exposure to the vectors of these diseases and apply appropriate prevention measures. As you will see in this study session, prevention of all of these diseases includes controlling the vectors with chemicals and/or environmental management, using personal protective clothing or bed nets to reduce exposure to the vectors, and rapid case detection and referral for treatment. Early treatment prevents serious complications and can save lives, and it also reduces the reservoir of infectious agents in the human population.
When you have studied this session, you should be able to:
37.1 Define and use correctly all of the key words printed in bold. (SAQs 37.1 to 37.5)
37.2 Identify the vectors and modes of transmission of schistosomiasis, leishmaniasis, onchocerciasis and lymphatic filariasis. (SAQs 37.1, 37.2 and 37.3)
37.3 Describe the distribution and impact of schistosomiasis, leishmaniasis, onchocerciasis and lymphatic filariasis in Ethiopia. (SAQs 37.1, 37.3, 37.4 and 37.5)
37.4 Describe the symptoms of schistosomiasis, leishmaniasis, onchocerciasis and lymphatic filariasis, and how you would diagnose and refer cases to a higher level health facility. (SAQs 37.3 and 37.4)
37.5 Describe how you would apply prevention and control measures against schistosomiasis, leishmaniasis, onchocerciasis and lymphatic filariasis. (SAQs 37.1, 37.3 and 37.5)
Schistosomiasis is pronounced ‘shy-stoh-soh-my-assis’. It is described as chronic because the symptoms develop gradually, become progressively more serious, and last for a long time unless treatment is given.
Schistosomiasis is a chronic communicable disease caused by parasitic flatworms (also known as trematodes, or blood flukes), which affect the blood vessels in the intestines or in the urinary tract of infected people. In some places, the disease is known by its alternative name – bilharzia. Two species of Schistosoma parasites are common in Ethiopia: Schistosoma mansoni (Figure 37.1) which causes disease mainly in the intestines, and Schistosoma haematobium, which causes disease mainly in the bladder and sometimes also in other parts of the urinary tract such as the kidneys.
The WHO estimates that more than 207 million people worldwide are infected with Schistosoma parasites – and 85% of them are in Africa. Approximately 200,000 people die every year in Africa as a result of the complications caused by these parasites. Rural communities living near water bodies such as rivers, lakes and dams may be highly affected by the disease, because the worms have a complex lifecycle in which they spend part of their development living in freshwater snails. You will learn more about their lifecycle later in this section. First, as a Health Extension Practitioner, you need to know where the disease is common in Ethiopia.
Schistosoma mansoni is widespread in several parts of Ethiopia, usually at an altitude of between 1,200 to 2,000 metres above sea level. Some of the common places include Ziway (Figure 37.2), Hawassa, Bishoftu, Wonji, Haromaya, Jimma, Bahir Dar and some places in Gojam, Dessie and Tigray. In many of these locations, more than 60% of schoolchildren are infected with Schistosoma mansoni. A high burden of the disease in children has severe adverse effects on their growth and performance at school.
Schistosoma haematobium is limited to some lowland areas, including the swampy land and floodplains of the Awash and Wabe Shebele valleys and along the Ethiopian–Sudan border.
Let us now focus on the mode of transmission of schistosomiasis. The major reservoirs of Schistosoma parasites are infected humans (the primary hosts) and freshwater snails (the intermediate hosts).
What do you understand about the term reservoir in the context of communicable diseases? (Think back to Study Session 1 of this Module.)
A reservoir is any location where infectious agents live before they infect new human hosts, and which is important for the survival of the disease-causing organism. Reservoirs can be living (e.g. infected humans or other animals, such as dogs, cows, insects or snails), or non-living things in the environment (e.g. water, food).
Figure 37.3 shows how Schistosoma parasites are transmitted from infected people to new human hosts, via the intermediate hosts – freshwater snails. Figure 37.4 shows the lifecycle of the parasites in more detail, highlighting the immature forms that can be found in the water.
The immature form of the parasite penetrates the skin of a new host when he or she is swimming, washing or standing in infected water. They pass to the liver, where they mature into adult worms. Male and female adult worms mate (look back at Figure 37.1) and deposit their eggs in the blood vessels of either the intestine (Schistosoma mansoni) or bladder (Schistosoma haematobium). The eggs pass out into the water in either the faeces or urine, to continue the infection cycle.
The infected person’s immune system reacts against the parasites’ eggs in their blood vessels, which are recognised as ‘foreign bodies’. The immune reaction causes an acute inflammation around the eggs, which can lead to chronic symptoms (see Box 37.1). Note that the clinical manifestations of schistosomiasis are mainly related to the immune response against the eggs in the intestine or bladder – the symptoms are not due to the worms themselves. The adults can survive in the person’s body for up to 20 years, releasing around 300 eggs every day.
The clinical manifestations (described above) should lead you to suspect cases of schistosomiasis. Asking children if they have seen any blood in their urine is an important way of detecting whether Schistosoma haematobium is common in the area. The diagnosis of schistosomiasis is confirmed in a laboratory by direct observation of the parasite eggs in samples of faeces or urine examined under the microscope (Figure 37.5).
You will meet these prevention and control categories again when we discuss the other vector-borne diseases later in this study session.
Several prevention and control strategies should be integrated to reduce the burden of schistosomiasis. You have an important role as a Health Extension Practitioner to teach community members in affected areas how to apply the major prevention and control measures, which can be described in five general categories:
What actions would you educate community members to take to protect themselves and their children from schistosomiasis?
In particular, you should encourage people to build and use latrines and avoid urinating or defaecating in water, in order to reduce contamination by Schistosoma eggs. Also they should wear protective clothing when standing in infected water, and seek early diagnosis and treatment for any suspected cases.
Leishmaniasis is a chronic parasitic disease, which exists in two forms: visceral leishmaniasis (also known as kala-azar), which affects the internal organs such as the liver and spleen, and cutaneous leishmaniasis, which affects the skin. The infectious agents are protozoa (single-celled organisms, Figure 37.6). There are four major species of Leishmania protozoa in Ethiopia:
Leishmaniasis is pronounced ‘lye-sh-man-eye-assis’. Visceral is pronounced ‘viss-urr-al’ and cutaneous is pronounced ‘kute-ay-nee-ous’.
Around 12 million people in 88 countries around the world are currently thought to be infected with Leishmania parasites and the WHO estimates that one to two million new cases occur each year. The vectors (and intermediate hosts) for these parasites are sandflies. The lifecycle will be described later in this section.
During your work in the community, you should know the common places where leishmaniasis is present. Visceral leishmaniasis affecting the internal abdominal organs such as the liver and spleen is widely distributed in the lowlands of Ethiopia. Important endemic locations include Konso woredas (Lake Abaya and Segen Valleys), the Lower Omo plains, the Metama and Humera plains and Adiss Zemen. Cutaneous leishmaniasis occurs in Meta-Abo, Sebeta, Kutaber in Wello, and in some places in South West Ethiopia such as Jimma Zone.
Leishmaniasis is transmitted through the bite of female phlebotomine sandflies (Figure 37.7), which bite humans and some animals, and take blood meals to feed the development of their eggs. Phlebotomine means ‘blood-sucking’ and is pronounced ‘fleb-otto-meen’. There are about 30 species of sandflies that can transmit Leishmania parasites to humans found throughout the tropical and temperate regions of the world. The females lay their eggs in many locations, including the burrows of rodents, old tree bark, cracks in buildings and rubbish heaps – anywhere that is warm and humid enough for their eggs to develop into flies.
When sandflies take blood meals from an infected person, they also become infected with the protozoa that cause leishmaniasis. The protozoa develop inside the sandfly and are passed on when the sandfly takes a blood meal from a healthy person. The Leishmania protozoa multiply inside the white blood cells of the healthy person and cause disease (Figure 37.8).
The clinical presentation of the two forms of leishmaniasis are very different. Cutaneous leishmaniasis normally produces skin ulcers on the exposed parts of the body such as the face, arms and legs (Figure 37.9). The disease can produce a large number of ulcers – sometimes up to 200 – which may result in physical disability (e.g. in using the hands). The visible ulcers are a source of social stigma, which can leave the patient suffering mental distress and rejection in their community.
If you suspect a case of visceral leismaniasis, the patient should be immediately referred to a higher health facility.
Visceral leishmaniasis (also known as kala azar, which means black fever in Hindu) is a life-threatening disease characterised by irregular episodes of fever, rapid and extensive weight loss, huge swelling of the spleen and liver (Figure 37.10), and anaemia. If left untreated, up to 100% of patients die within two years of infection. For this reason, visceral leishmaniasis is said to have a high case-fatality rate.
You can identify patients with leishmaniasis by the clinical manifestations of the disease. For confirmation of the diagnosis, laboratory investigations should be done in health centres or hospitals, where the protozoan parasites can be detected in blood smears viewed with a microscope (look back at Figure 37.6).
Several prevention and control measures are available for leishmaniasis. The general principles will already be familiar to you from the earlier discussion of schistosomiasis.
What actions would you educate community members to take to protect themselves and their children from leishmaniasis?
You would encourage them to use ITNs at night and not to sleep unprotected out of doors, so as to avoid sandfly bites; they should welcome spraying teams to treat their houses with insecticide, and eliminate rubbish heaps and other locations where sandflies like to lay their eggs. They should seek early diagnosis and treatment of any suspected cases.
Onchocerciasis is pronounced ‘onk-oh-serk-eye-assis’.
Onchocerciasis is a parasitic vector-borne disease caused by a worm that affects the skin, lymph nodes and the eyes of infected people. It is also called river blindness. The WHO estimates that worldwide there are about 500,000 people who are blind due to onchocerciasis. The disease is caused by a tiny worm called Onchocerca volvulus (Figure 37.12), which is transmitted from person to person in the bite of blackflies.
Onchocerciasis is found in the western part of Ethiopia, where there are many rapidly flowing rivers and streams, with vegetation along the banks that provide good habitats for the blackflies that transmit the parasite. The most affected areas include Keffa, Illubabor, Gambella and Wollega. Cases of onchocerciasis have been also reported from Pawe, Humera and Metema.
The parasites that cause onchocerciasis are transmitted from human to human through the bites of blackflies, which belong to Simulum species (Figure 37.13). Blackflies breed in fast-flowing rivers and streams, with good vegetation nearby. Unlike mosquitoes and sandflies, they bite during the day when people are active in the area.
What type of water does the blackfly need to breed? How does this differ from the water required by the mosquito vectors of malaria?
Blackflies need fast-running water to breed, unlike Anopheles mosquitoes, which breed in shallow stagnant water collections.
The adult worms mate in the infected person, and the eggs hatch into microscopic worms called microfilaria, which burrow through the body tissues. The person’s immune system attacks the microfilaria, causing inflammation and damage in the surrounding tissues. Sight defects and eventually blindness develops when the microfilaria are embedded in the person’s eye. When a female blackfly bites an infected person during a blood meal, the microfilaria are transferred from the person to the fly (Figure 37.14a). Over the course of one to three weeks, the microfilaria develop inside the blackfly to form infective larvae (Figure 37.14b). These are then passed on to other people when the blackfly takes another blood meal (Figure 37.14c). The microfilaria migrate to the skin, lymph nodes and eyes of the infected person, causing inflammation and tissue damage.
In the human host, the larvae migrate into the skin, and nodules (swellings) form around them. They slowly mature into adult worms, which can live for 15 years in the human body. After mating, the female worm releases around 1,000 microfilaria a day into the surrounding tissue. Microfilaria live for one to two years, moving around the body. When they die, they cause an inflammatory response which leads to the clinical manifestations and complications such as blindness.
The clinical manifestations of onchocerciasis are the result of inflammation against the dead microfilaria. The most common clinical manifestations include skin rashes, lesions, intense itching, loss of the colour of the skin, and nodule formation (Figure 37.15a). Microfilaria also migrate to the eye, and causes scarring of the cornea (the covering of the eyeball), which leads to sight defects and ultimately blindness (Figure 37.15b).
The itching and disfiguring nodules and blindness are sources of great distress to patients, who may be stigmatised and rejected by their communities.
Diagnosis of onchocerciasis is made by clinical examination. If you suspect that a patient may be infected, you should make a referral for laboratory confirmation and treatment. Microscopic investigation of a skin snip (taking samples from the skin) can identify the microfilaria and confirm the diagnosis.
The WHO believes that onchocerciasis can be eliminated through the application of effective prevention and control methods, which are summarised below:
In this final section, you will learn about the definition, mode of transmission, clinical manifestations, and methods of prevention and control of lymphatic filariasis. It is also known as elephantiasis because of its effects on the legs of infected people. Lymphatic filariasis is a parasitic disease caused by a worm that invades the lymphatic system – the network of vessels that exists throughout the body, connecting the lymph nodes, spleen and other organs, and where white blood cells are primarily found (Figure 37.18).
The WHO estimates that over 120 million people worldwide are currently infected with the worm (species name Wuchereria bancrofti, Figure 37.19) which is responsible for 90% of all cases.
Like onchocerciasis, lymphatic filariasis is common in western Ethiopia, such as Illubabor, Keffa, Jimma, Wollega, Gambella and Pawe. Though the disease is not fatal, it is responsible for considerable disability and distress, causing social stigma among men, women and children. You will learn more about the social consequences of lymphatic filariasis and a non-infectious cause of swelling in the legs (podoconiosis) in Study Session 39.
The parasites that cause lymphatic filariasis are transmitted from human to human through the bites of Culex and Anopheles mosquitoes. The female mosquitoes take the microscopic forms of the parasitic worm (microfilaria) from an infected person during a blood meal (Figure 37.20a). The microfilaria develop into larvae, and when the mosquito feeds on another person, the larvae enter the skin punctured by the mosquito bite (Figure 37.20b). The larvae travel via the lymphatic vessels, where they develop into adult worms all over the body (Figure 37.20c). After mating, the females lay millions of eggs which develop into microfilaria, completing the lifecycle.
The clinical manifestations of the disease are as a result of the inflammation and damage to the lymphatic vessels caused by the person’s own immune response trying to reject the worms, and when vessels become blocked by clusters of worms. The overall effect is to disrupt the lymphatic system, which normally collects tissue fluids draining from the body’s cells and returns the fluid to the blood stream. If the lymphatic drainage is blocked, the lower limbs and sometimes also the genitals become hugely swollen with fluid – a condition called lymphoedema (pronounced ‘limf-ee-deem-ah’). Most infections do not produce symptoms, but in people where the lymphatic drainage is badly damaged the common symptoms include hydrocele (swelling of the scrotum, pronounced ‘hy-droh-seel’), swelling of the legs and feet, and thickening of the skin into folds (Figure 37.21). Infection of the swollen skin folds by bacteria is a frequent cause of very painful attacks. Patients suffer from episodes of fever and around 40% develop kidney damage.
Suspected cases of lymphatic filariasis should be referred to the health centre.
You can suspect lymphatic filariasis from the clinical manifestations, but the diagnosis can only be confirmed by laboratory tests to reveal the microfilaria in blood smears viewed with a microscope. Adult worms blocking the lymphatic vessels or nodes are difficult to reach. Therefore, if you live in an endemic area and you suspect a case of lymphatic filariasis, you should refer the patient to the nearest health centre for further testing and treatment.
Lymphatic filariasis is one of the few communicable diseases that the WHO believes could be eradicated (totally removed from all populations in the world, never to return) with the currently available prevention and control measures. These are:
The methods described here are also used to reduce the swelling due to podoconiosis (non-infectious elephantiasis, Study Session 39).
Patients with lymphoedema and thickened skin folds (for example, as in Figure 37.21) can be empowered to manage their symptoms and reduce their discomfort and pain through simple, but rigorous, hygiene techniques. You should educate them to wash the affected parts carefully every day, especially between the folds of thickened skin, and gently dry the area with a clean cloth. They should elevate (raise) swollen legs as much as possible whatever they are doing during the day and raise the foot of the bed or sleeping mat at night (Figure 37.22). Advise the patient to exercise the limbs any time and anywhere, as often as possible, to help the fluid to exit from their swollen limbs.
In the next study session, you will learn about two more diseases of public health importance in Ethiopia – rabies and taeniasis (tapeworm disease) – which are spread to humans by warm-blooded animals (dogs and cattle).
In Study Session 37, you have learned that:
Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering the following questions. Write your answers in your Study Diary and discuss them with your Tutor at the next Study Support Meeting. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.
How many communicable diseases can you name that can be prevented by integrated vector control methods? (Think about all the vector-borne diseases you have learned about in this Module – not just in this study session!)
Malaria, relapsing fever, typhus, schistosomiasis, leishmaniasis, onchocerciasis and lymphatic filariasis are all vector-borne diseases prevented by integrated vector control methods. Did you remember all seven of these conditions?
Complete Table 37.1 by writing the common name of the vector in the second column beside the disease that it transmits.
The completed table appears below.
|Lymphatic filariasis||Mosquitoes (Culex and Anopheline females)|
Imagine that you have been assigned to Humera and you see a 25-year-old man who has signs of severe weight loss, fever and a hugely enlarged abdomen.