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D I S E A S E A N D D I S E A S E T R A N S M I S S I O N
Chapter 2
Disease and disease transmission
An enormous variety of organisms exist, including some which can survive and even develop in the body of people or animals. If the organism can cause infection, it is an infectious agent. In this manual infectious agents which cause infection and illness are called pathogens. Diseases caused by pathogens, or the toxins they produce, are communicable or infectious diseases (45)^. In this manual these will be called disease and infection.
This chapter presents the transmission cycle of disease with its different elements, and categorises the different infections related to WES.
2.1 Introduction to the transmission cycle of disease
To be able to persist or live on, pathogens must be able to leave an infected host, survive transmission in the environment, enter a susceptible person or animal, and develop and/or multiply in the newly infected host.
The transmission of pathogens from current to future host follows a repeating cycle. This cycle can be simple, with a direct transmission from current to future host, or complex, where transmission occurs through (multiple) intermediate hosts or vectors.
This cycle is called the transmission cycle of disease, or transmission cycle. The transmission cycle has different elements:
The pathogen: the organism causing the infection The host: the infected person or animal ‘carrying’ the pathogen The exit: the method the pathogen uses to leave the body of the host Transmission: how the pathogen is transferred from host to susceptible person or animal, which can include developmental stages in the environment, in intermediate hosts, or in vectors
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The environment: the environment in which transmission of the pathogen takes place. The entry: the method the pathogen uses to enter the body of the susceptible person or animal The susceptible person or animal: the potential future host who is receptive to the pathogen
To understand why infections occur in a particular situation, and to know how to prevent them, the transmission cycles of these infections must be understood. The rest of this chapter looks at the elements of the transmission cycle in more detail.
Figure 2.1. The different elements of the transmission cycle of disease
The environment
Transmission
The host
The pathogen
The pathogen leaves the host
Entry of the pathogen
The susceptible person or animal
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Usually immunity only develops against the specific pathogen that caused the infection. If there are different types (serotypes or strains) of the same pathogen (e.g. in dengue fever and scrub typhus), immunity will often only develop against the particular type which caused the infection. The person or animal can still develop the illness when infected with another serotype or strain of the patho- gen (3).
Table 2.1 presents the different categories of pathogenic organisms with some of their characteristics, including latency, persistence, and immunity. The informa- tion is general, and exceptions can occur.
2.3 The host
The host is the person or animal infected by the pathogen. The importance of the host in the transmission cycle is its roles as both reservoir and source of patho- gens.
There are two types of host: definitive and intermediate host. The definitive host is the person or animal infected with the adult, or sexual, form of the pathogen. In the infections covered here, people are usually the definitive host. To keep things simple the definitive host is called just ‘the host’.
The intermediate host is an animal or person infected by a larval, or asexual, form of the pathogen (3)^. Cysticercosis and hydatid disease are the only infections covered here for which people are the intermediate host. Where intermediate host is meant, this term is used. Of the infections covered here, only helminths have both definitive and intermediate hosts. All other pathogens only have definitive hosts, although vectors function technically as intermediate hosts for protozoa.
Zoonosis: transmission from animal to person Some pathogens are specific to humans, others to animals. Many pathogens are less specific and can infect both people and animals. Infections that can naturally be transmitted from animal to person are called zoonoses (3)^. Zoonoses are very common; over half of the infections covered in this manual are zoonoses. Many of these infections normally occur in an animal cycle, with people being infected by chance.
The problem with zoonoses is that a continuous reservoir of pathogens exists outside humans. Even if all human infections were cured and transmission to people stopped, the presence of an animal reservoir would remain a continuous risk to people.
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Table 2.1. Categories of pathogenic organisms and their characteristics
Pathogen
Virus
Rickettsiae
Bacteria
Fungi
Protozoa
Helminths (worms)
Description
Particles invade living cells. The pathogen needs structures in these cells to reproduce. (45)
Organisms resemble bacteria. (45)^. However, similar to viruses, the pathogen needs to develop inside the cells of the host. (2)
Bacteria are single cell organisms. They are considered more primitive than animal or plant cells. (45) A group of organisms which include yeast, moulds, and mushrooms. (45) Protozoa area single cell organisms. (45)
Helminths are worms (roundworms, flukes or tapeworms) (45)^. Often male and female must meet in host to reproduce, and sometimes they multiply in intermediate hosts.
Latency
The pathogens are non-latent.
n/a
The pathogens are non-latent.
n/a
The pathogens are non-latent.
The pathogen is latent. It often has a complex lifecycle with a development in the environment or intermediate hosts. (73)
Persistence
Viruses can survive for months in tropical temperatures. (28)
n/a
Persists up to several weeks. (16,73) (^). Can multiply outside the host. (3) n/a
Forms a resistant cyst which can survive for months. (3,44)
The pathogen is persistent and some may survive for years in the environment. (3, 16)
Additional information
Where vector- borne, transmission to offspring is possible (3)^. The immunity is often long- lasting. (73) Transmission of the pathogen to the offspring of the vector occurs. (73)^. The immunity is usually long- lasting. (3) The immunity developed is often incomplete or short-lived. (3) The duration of the immunity is variable. (3)
The immunity is only maintained by repeated infections or vaccinations. (73)
Usually no immunity is built up against the pathogen. (3)
n/a: Not applicable as the pathogens are not excreta-related.
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Other reservoirs of pathogens Besides hosts, there are several other pathogen reservoirs that can play a role in the transmission of disease. Some pathogens are very resistant, and can survive in the environment for considerable time. Though this will normally be an excep- tion, roundworm eggs can remain viable in soil for years (3)^.
Intermediate hosts may be important reservoirs of pathogens, and several helminths can even multiply in the intermediate host.
Vectors are usually infectious for life, and several pathogens can be transmitted to the offspring of the vector over several generations (2). A soft tick, for example, can survive for more than five years and can pass to its offspring the pathogen which causes tick-borne relapsing fever (73)^.
Some pathogens can live their entire lifecycle outside the host. These include threadworm and several faecal-oral bacteria which cause bacillary dysentery, (para)typhoid, and salmonellosis (3).
Animal hosts, asymptomatic carriers, and other potential reservoirs of pathogens can be important sources of infection, and this must be taken into account when trying to control disease. The whole population at risk may have to be treated, or animal hosts controlled. With several diseases these preventive measures will have to be maintained over a long period before a reduction in the occurrence of the infection will be noticeable.
2.4 Transmission of disease
To survive as a species, pathogens must infect new people or animals. To do this, they must leave the body of the host, find their way to a new susceptible person or animal, and enter the body of that person or animal. As the exit, transmission, and entry of the pathogens are closely associated, we will cover them together.
Water and environmental sanitation interventions that aim to improve the health of a population usually try to reduce the risk of transmission of infection. To do this appropriately, the WES specialist needs to be familiar with the pathogens’ transmission route(s). It is this understanding that enables the specialist to deter- mine which control measures will be most effective in a particular situation.
As many infections are linked to WES, it is useful to categorise the different diseases.
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For a water and sanitation specialist the most useful categorisation is based on the transmission cycles of the infections. Generally speaking, diseases with similar transmission cycles can be controlled by similar preventive measures, and will occur in similar environments.
The infections are categorised and their transmission routes described at the same time. More information on the transmission routes and potentially effective preventive measures of specific diseases can be found in Annexe 1.
Some terms relating to the transmission or classification of infections are defined here:
Food-borne infections: infections which can be transmitted through eating food containing the pathogen.
Vector-borne infections: infections transmitted through vectors. We use vector- borne infections only for infections with a biological vector , that is a vector in which the pathogen goes through a development before further transmission is possible (e.g. mosquitoes, tsetse fly, body louse). We do not classify as vector- borne those infections which are transmitted by mechanical vectors , that is the animal is only a vehicle for transporting the pathogen (e.g. domestic flies, cockroaches).
Water-borne infections: infections which can be transmitted through drinking- water which contains the pathogen.
Water-washed infections: infections caused by pathogens whose transmission can be prevented by improving personal hygiene.
Infections can have either direct or indirect transmission routes.
2.4.1 Infections with direct transmission A pathogen with a direct transmission route can infect a susceptible person or animal immediately after leaving the host. The pathogen does not need to develop in the environment, in an intermediate host, or in a vector.
In faecal pathogens these are the non-latent infectious agents.
This group contains three disease-groups: faecal-oral infections, leptospirosis, and infections spread through direct contact.
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2.4.1.2 Leptospirosis The main reservoir of leptospirosis is normally rats, though many other animals can potentially transmit the infection. The pathogen leaves the animal host through urine. People are usually infected through direct skin contact with water, moist soil, or vegetation contaminated with urine from infected animals. Other ways of transmission are direct contact with body tissues of infected animals or ingesting food contaminated with urine. Transmission from person to person is rare (3).
2.4.1.3 Infections of direct contact All the diseases covered in this manual that fall into this category are infections which affect the skin or eyes. Pathogens are present on the skin or in the discharges of affected body parts or eyes. The pathogens are transmitted directly through contaminated hands, clothes, domestic flies, or any other contaminated material.
The pathogen enters the body through skin or mucous membranes such as the eyes. These infections are associated with poor personal hygiene and are water- washed.
Few of these infections have animal hosts. The diseases in this category include conjunctivitis, trachoma, yaws, and scabies.
2.4.2 Infections with indirect transmission A pathogen with an indirect transmission route must go through a development phase outside the host before it can infect a new susceptible person or animal. This development will take place in a specific intermediate host, vector, or type of environment.
This need to go through a particular organism or environment gives the transmis- sion route a focus, which preventive measures can target, for example by vector control or improved food preparation.
In the faecal pathogens these are the latent infectious agents.
The disease-groups with indirect transmission are soil-transmitted helminths, water-based helminths, beef/pork tapeworm infection, Guinea-worm infection, and vector-borne infections.
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2.4.2.1 Soil-transmitted helminths These worms leave the body through faeces as eggs or larvae. After excretion they have to develop in soil. They can be further divided based on how the pathogen enters the human body.
Entrance by penetration of the skin: the pathogen enters the body through skin which is in direct contact with contaminated soil. This is the method used by hookworms and threadworms.
Entrance by ingestion: if either contaminated soil, or food or hands contami- nated with polluted soil come into contact with the mouth, the pathogen can be transmitted. These infections can be food-borne and water-washed. This method is used by roundworms and whipworms.
The infections covered here do not have animal hosts. Figure 2.3 presents the transmission routes of the soil-transmitted helminths.
Figure 2.3. The transmission routes of soil-transmitted helminths (16,73)
Faeces
Mouth (Roundworm infection)
Food
Soil
Fingers
Skin in direct contact with contaminated soil (Hookworm disease)
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Figure 2.4. The transmission routes of water-based helminths (3)
Faeces (urine for urinary schistosomiasis)
Mouth Water-based helminths with 2 intermediate hosts
Freshwater snail (freshwater copepod for diphyllobothriasis)
Freshwater fish (freshwater crab-crayfish for lung fluke disease, freshwater plants for fasciolopsiasis and fascioliasis)
Skin in direct contact with contaminated water
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are the only host to this infection. Figure 2.5 shows the transmission route of Guinea-worm.
2.4.2.5 Vector-borne diseases These infections are transmitted by vectors. Vectors are arthropods (insects, ticks, or mites) which can transmit infections from host to future host (73)^. The pathogen exists in the blood or skin of the host. The vector becomes infected when it feeds on a host. The pathogen develops and multiplies inside the vector, which then becomes infectious. People are usually infected through the bite of an infectious vector, though other ways of entry are possible. With several vector-borne dis- eases animal hosts are important reservoirs. Vector-borne diseases include yellow fever, malaria, sleeping sickness, plague, epidemic louse-borne typhus fever, and louse-borne relapsing fever.
Figure 2.5. The transmission route of Guinea-worm (3)
Worm in blister in skin
Freshwater copepod
Mouth
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In warmer climates children are also likely to play in surface water, where they can be infected with schistosomiasis.
2.5.2 The landscape The landscape consists of the larger physical structures in the environment. These structures are usually natural, but can be man-made. They include mountains, deserts, rivers, jungle, artificial water reservoirs, and deforested areas. Aspects of the landscape that would influence disease transmission most are the micro- climate, the presence of water, and types of vegetation.
Man-made modifications of the landscape often increase the risk of disease transmission by creating a habitat favourable to vectors or intermediate hosts. Large artificial water reservoirs frequently increase the occurrence of malaria and schistosomiasis (6)^ , for example, and introducing irrigation schemes can increase the occurrence of schistosomiasis (15)^.
Although the WES specialist working in the field must recognise the risk-factors linked to the landscape, he or she will normally not be able to modify the landscape to reduce the risks of disease transmission.
2.5.3 The human surroundings Landscape and human surroundings are closely linked, and it is difficult to divide the two clearly. The difference is one of scale; while the landscape normally cannot be modified or improved by individual people, individuals can modify the human surroundings.
Although the landscape will normally be similar for all people living in an area, the human surroundings may be very different for people living in the same region, village, or even household. Many infections are linked to specific circum- stances, and people with specific occupations, socio-economic status, gender, or religion may be far more at risk than others. While the father of an African family may be exposed to leptospirosis and plague because he works in sugarcane fields and regularly traps rats, the mother may be exposed to sleeping sickness as she goes to the river to wash clothes, and the children may be exposed to schisto- somiasis while playing in the local pond.
The human surroundings are created by a combination of natural elements and how people have modified these elements.
People adapt their surroundings to their needs. If these adaptations are well done, they can help to prevent the transmission of disease. In practise they often
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encourage the transmission of disease, however, as people do not have the space, motivation, understanding, time, energy, or financial or material means to do them properly.
In relation to the WES aspects, human surroundings are concerned with water supply, proper handling of excreta, removal of unwanted water, adequate manage- ment of solid waste, and control of vectors or intermediate hosts through modifi- cation of the environment or change in behaviour.
Waste products like excreta, wastewater, and refuse are disposed of in the human surroundings. These wastes must be properly managed to prevent them becoming a health risk.
The WES specialist working in the field will have to know what aspects of the human surroundings increase the risk of disease transmission. This will enable him or her to determine which aspects play an important role in the transmission of disease in a specific situation. Based on this, an intervention can be planned which will reduce the health risks to the population. More on the health risks relating to the human surroundings, and the components from WES interventions can be found in Chapter 5.
2.5.4 Human behaviour People behave in a certain way because they believe that they are making the most of their lives. Human behaviour is complex. It is influenced by culture, for example religion, attitudes, and traditional beliefs; by social position, such as gender or age; by availability of means, for example money, energy, time, or material; and by politics.
One type of handpump may be acceptable in one culture, but unacceptable in another. One type of latrine may be preferred by men, while women or children might prefer another. People may not accept things from a government they despise, or from an insulting development worker.
Having access to a safe water supply, or technically adequate sanitation, does not automatically mean people will use them (25)^. If people do not regard structures as acceptable, appropriate, or as an improvement to their quality of life, they will not be used, or will not be used to their full potential.
Interventions that have only focused on structural improvements have often given poor results in controlling infections. Studies in disease prevention indicate that the most important factor in reducing the transmission of diseases related to WES
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2.6.2 The barriers of the body against pathogens The body has a range of mechanisms that prevent a pathogen from causing infection.
The skin and mucous membranes have anti-microbial substances, and the stom- ach is acid to act as the first barriers against pathogens. Low acidity in the stomach or an open wound (e.g. insect bite, cut, abrasion) can make this barrier ineffective.
The next barriers are mechanisms that react to the pathogen, and try to counter its development. These barriers are not specific to the pathogen, and the body does not need to have been in contact with the pathogen for them to be effective. These mechanisms are the host’s resistance against pathogens (41)^. Resistance is lowered if someone is suffering from other infections (73)^ , or is malnourished, stressed, or fatigued (41)^. Women have a higher risk of infection when pregnant (73)^.
An individual’s immune system may have experienced a pathogen through an earlier infection or immunisation (vaccination) with inactivated pathogens. When the pathogens enter the person’s body, their immune system will recognise the pathogen and make antibodies which will attack the pathogen. This is called active immunity (45)^. The effectiveness of active immunity depends on the patho- gen, and the length of time since the body has been in contact with the pathogen. Active immunity is effective only against that particular pathogen. The effective- ness against bacteria and viruses usually lasts for years (3).
Passive immunity is created by introducing foreign antibodies into the body. An unborn baby receives antibodies from the mother through the placenta, which will protect it for some time after birth. Vaccination with antibodies is another way of creating passive immunity (73)^. The foreign antibodies will slowly disappear from the body, and passive immunity will usually only last days or months (3)^.
Table 2.3. Examples of infective doses of faecal-oral diseases
Disease
Bacillary dysentery (shigellosis) (16) Giardiasis (16) Rotaviral enteritis (16) Cholera (73) Typhoid (73)
Infectious dose (in number of pathogens)
10 to 100 10 to 100 100 to 10, Usually 10 6 to 10 8 10 3 to 10 9
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A person or animal who lacks effective barriers (has a poor resistance and/or a low immunity) against a pathogen is susceptible to this infectious agent (45)^.
Two important practical points define the susceptibility of a population:
A population that is weakened because of poor nutrition or a high occurrence of disease, fatigue, or stress has an increased risk of disease. When a pathogen is very common in a population, or the population is immunised, most people will have some form of immunity against it. In this case the disease will attack mainly children. If the same pathogen is introduced into a population which has low immunity, there is a risk of an outbreak (an epidemic) which can attack all ages.
2.6.3 The infection over time When a pathogen is introduced in sufficient numbers, and overcomes the resist- ance and immune system of a person or animal, infection will follow. The time between entrance of pathogen and appearance of the first signs of disease or symptoms is called the incubation period. As mentioned earlier, not all infections will result in disease, and for many infections asymptomatic carriers are common.
Figure 2.6. Communicability and disease over time in one person (adapted from 73)
Initial infection Onset of disease Full recovery
Incubation period Period of disease Disease
Communicability Latent period Period of communicability
Incubating carrier Convalescent carrier
Time
