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close this bookA Guide to the Development of on-site Sanitation (WHO; 1992; 246 pages)
View the documentPreface
close this folderPart I. Foundations of sanitary practice
open this folder and view contentsChapter 1. The need for on-site sanitation
close this folderChapter 2. Sanitation and disease transmission
View the documentDiseases associated with excreta and wastewater
View the documentHow disease is carried from excreta
open this folder and view contentsChapter 3. Social and cultural considerations
open this folder and view contentsChapter 4. Technical options
open this folder and view contentsPart II. Detailed design, construction, operation and maintenance
open this folder and view contentsPart III. Planning and development of on-site sanitation projects
View the documentReferences
View the documentSelected further reading
View the documentGlossary of terms used in this book
View the documentAnnex 1. Reuse of excreta
View the documentAnnex 2. Sullage
View the documentAnnex 3. Reviewers
View the documentSelected WHO publications of related interest
View the documentBack Cover
 

How disease is carried from excreta

Transmission of diseases

Humans themselves are the main reservoir of most diseases that affect them. Transmission of excreta-related diseases from one host to another (or the same host) normally follows one of the routes shown in Fig. 2.1. Poor domestic and personal hygiene, indicated by routes involving food and hands, often diminishes or even negates any positive impact of improved excreta disposal on community health. As shown in the figure, most routes for transmission of excreta-related diseases are the same as those for water-related diseases, being dependent on faecal-oral transmission (waterborne and water-washed) and skin penetration (water-based with an aquatic host; soil-based but not faecal-oral; and insect vector with vector breeding on excreta or in dirty water). Table 2.3 gives examples of excreta-related diseases and data on the number of infections and deaths per year.


Fig. 2.1. Transmission routes for pathogens found in excreta

 

WHO 91364

Table 2.3. Morbidity and mortality associated with various excreta-related diseases

Disease

Morbidity

Mortality
(no. of deaths per year)

Population at risk

Waterborne and water-washed

diarrhoea

1500 million or more episodes in children under 5 years

4 million in children under 5 years

More than 2000 million

poliomyelitis

204 000

25 000

 

enteric fevers (typhoid, paratyphoid)

500 000-1 million

25 000

 

roundworm

800-1000 million infections

20 000

 

Water-based

schistosomiasis

200 million

More than 200 000

500-600 million

Soil-based

hookworm

900 million infections

50 000

 

As Table 2.3 illustrates, diarrhoeal diseases and helminth infections account for the greatest number of cases per year although there is a considerable difference in the levels of debility they produce. Schistosomiasis has relatively high rates of infection and death. The socio-economic impact of these diseases should not be ignored or underestimated. To illustrate this further, schistosomiasis will be considered in greater detail.

Schistosomiasis

Schistosomiasis is acquired through repeated contact with surface water contaminated with human excreta (both urine and faeces) containing schistosomes (WHO, 1985). Contact can be via agriculture, aquaculture, leisure activities (particularly swimming), collection of water, washing and bathing. Of the parasitic diseases, schistosomiasis ranks second in terms of socioeconomic and public health importance in tropical and subtropical areas, immediately behind malaria.

In 1990, schistosomiasis was reported to be endemic in 76 developing countries. Over 200 million people in rural and agricultural areas were estimated to be infected, while 500-600 million more were at risk of becoming infected, because of poverty, ignorance, substandard hygiene, and poor housing with few, if any, sanitary facilities.

People with light infections as well as those with obvious symptoms suffer weakness and lethargy, which decrease their capacity for work and productivity.

As shown in Fig. 2.2, the parasite develops in snails, the intermediate hosts. The free-swimming stage of the parasite penetrates the skin of humans and, if infection is heavy, the disease develops. The incidence of diseases such as schistosomiasis should be much reduced by the provision of sanitation. However, for this disease, as for many others, additional measures including the provision of safe drinking-water can also interrupt transmission by reducing contact with infested water. People living in endemic areas can benefit greatly from health education aimed at increasing their understanding of their role in transmission, and the importance of the use of latrines. Since young children are often most heavily infected, early use of latrines, especially in schools, will promote healthy habits.


Fig. 2.2. The cycle of transmission of schistosomiasis

 

WHO 91365

Reuse of excreta and wastewater in agriculture

Sanitation is not always the only factor to be considered when relating excreta disposal to disease transmission within and between communities. The reuse of excreta (untreated or treated to differing extents) as a fertilizer, and reuse of wastewater (including sullage water) for many purposes, but especially for irrigation, may also contribute to the incidence of excreta-related diseases. In many countries where the demand for water is greater than the supply, use of wastewater for irrigation of crops for consumption by animals or humans can have a major impact on community health. This is especially important in areas with poor soils and insufficient income for purchase of commercial fertilizers and conditioners, where the use of human as well as animal excreta to condition and fertilize soil is actively encouraged. With such practices the degree of the hazard is dependent on several parameters including:

 

- the level (or lack) of treatment prior to reuse;
- the nature of the crop;
- the method of irrigation;
- the extent of reuse;
- the incidence and type of disease in the area;
- air, soil and water conditions.

The groups most at risk of infection will also depend on these factors and on other agricultural practices. The diseases that may show the greatest increase in incidence where reuse is practised are helminth infestations, particularly hookworm, roundworm and whip-worm; schistosomiasis may also increase markedly in some circumstances. Bacterial infections, such as cholera and diarrhoea, are affected to a much smaller degree, with the incidence of viral infections being least affected by these practices (Mara & Cairncross, 1989; WHO, 1989).

Epidemiological characteristics of pathogens

Pathogen survival

The survival times and other epidemiological characteristics of organisms in different media are given in Table 2.4; it should be noted that these periods are approximate, being dependent on local factors such as the climate and the number (concentration) and species of organisms.

Pathogen infectivity and latency

In addition to knowing how long the infectious agent may survive, i.e., its persistence, knowledge of the infectivity and latency of the organism is of value. Some pathogens remain infective for only short periods after being excreted, yet the incidence of associated disease is high. This may be attributable to the low infectious dose of the organism, e.g., protozoal cysts. The latency of an organism (i.e., the period between leaving a host and becoming infective) can vary from zero for some bacterial infections to weeks for some helminth eggs. For example, schistosome eggs have a latency of a few weeks during which time they develop in an intermediate host into the infective, free-swimming cercariae (Fig. 2.2); however, both the eggs and the cercariae have a persistence of only a few hours if they do not enter a new host (intermediate or human). In contrast, Ascaris eggs can become infective within ten days of being excreted (latency) but may remain in the soil for at least a year and still be infective (persistence).

Table 2.4. The epidemiological characteristics of excreted pathogensa

Pathogen

Latency period

ID50b

Survival times for pathogens in:

     

wastewater

soil

crops

Bacteria

0

>104

few days to 3 months

 

Vibrio cholerae

0

108

~1 month

<3 weeks

<5 days

 

Faecal coliform

0

~109

~3 months

<2 months

<1 month

Viruses

0

unknown

months

months

1-2 months

 

Enterovirusesc

0

100

~3 months

<3 months

<2 months

Protozoa (cysts)

0

10-100

few days to few weeks

 

Entamoeba spp

0

10-100

25 days

<3 weeks

<10 days

Helminthsd

variable

1-100

months

months

months

 

Ancylostoma spp

1 week

1

3 months

<3 months

<1 month

 

Ascaris spp

10 days

several

~1 year

many months

<3 months

 

Flukese

6-8 weeks

several

life of hostf

hoursf

hoursf

 

a Sources: Feachem et al. (1983); WHO (1987a).

b The ID50 is the number of organisms required to cause the development of clinical symptoms in 50% of individuals.

c Including coxsackieviruses, echoviruses and polioviruses.

d Eggs or larvae/cercariae.

e Excluding Fasciola hepatica but including Schistosoma spp.

f Outside the aquatic host, the pathogen survives for only a few hours. In the host, survival is for the life of the host.

Control of excreta-related diseases

If transmission is blocked at one or more points, excreta-related diseases can be controlled or possibly eradicated. Sanitation provides one such block. For example, water-seal slabs in latrines reduce the breeding sites for culicine mosquitos, vectors of filariasis; treatment of excreta prior to its disposal can kill the eggs and cysts of many human parasites (Ascaris, Entamoeba, and Schistosoma spp), thus preventing contamination of both ground and water.

Relationship of health to disposal method

The technical objective of sanitary excreta disposal is to isolate faeces so that the infectious agents in them cannot reach a new host. The method chosen for any particular area or region will depend on many factors including the local geology and hydrogeology, the culture and preferences of the communities, the locally available raw materials and the cost (both short-term and long-term).

The types of disease that are endemic in an area should also be considered. The survival of endemic pathogens (eggs, cysts, infectious agents) and the destination or possible reuse of different products of disposal/treatment can have a great effect on incidence of disease in that area and, possibly, adjacent areas.

The possible sites for both negative and positive impacts on health, taking all the above parameters into consideration, should be considered during the planning stages of development projects to improve sanitation. This should ensure that the projects achieve the greatest possible effect on the incidence of diseases related to excreta and wastewater in the community.

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