Public health aspects

In developing countries, excrete-related diseases are very common. Excreta and waste water contain correspondingly high concentrations of excreted pathogens - the bacteria, viruses, protozoa and helminths (worms) that cause diseases in humans. There are approximately thirty excreted infections of public health importance. One must consider these infections in schemes for use of excrete and waste water.

There has been a tendency to examine the infectious-disease risks of excrete use from a strictly environmental perspective and to assume that the potential risks presented by the presence of pathogens in excreta (both treated and untreated) will result in disease or infection. This reasoning has led, in some countries, to stringent regulations on the use of excrete. Much of the literature on the subject recommends restricting the free use of bio-effluents.

It is important, however, not to exaggerate these risks. An actual risk to public health from agricultural or aquacultural use of excreta requires a series of conditions. In many circumstances usage of adequately treated bio-effluents is quite safe. The sequence of events required to produce an actual health risk is summarized in Figure 2.

An infective dose of an excreted pathogen must reach the field or pond, or multiply in the field or pond to form an infective dose. This infective dose must reach and infect a human host, and this infection must cause disease or further transmission. If some of these conditions are not fulfilled, the risk to public health is only potential and not actual.

Figure 2: Influences on the sequence of events between the presence of a pathogen in excrete or waste water and measurable human disease attributable to excrete or waste water use Excreted Load latency multiplication persistence treatment survival Infective Dose Applied to Land/Water persistence intermediate host type of use practice type of human exposure Infective Dose Reaches Human Host human behavior pattern of human immunity Risks of Infection and Disease alternative routes of transmission Source: International Reference Centre for Waste Disposal (IRCWD) - WHO Collaboration Centre for Waste Disposal, Report 05/85

Table 1: Relative Health Risks from Use of Anaerobically Treated Excreta, by Retention Time

Pathogen	Retention Time (months)
	1st	2nd	3rd	4th	6th	8th	10th
	M	M	M	M	M	M	M
Enteric viruses	+	+	0	0	0	0	0
Salmonellas	+	+	0	0	0	0	0
Shigellas	+	+	0	0	0	0	0
Vibrio cholerae	+	0	0	0	0	0	0
Path.E. Coli	+	+	0	0	0	0	0
Entamoeba	0	0	0	0	0	0	0
Ascaris	++	++	++	++	+	+	+
Trichuris	++	++	+	+	+	+	0
Hookworms	+	+	0	0	0	0	0
Schistosoma	0	0	0	0	0	0	0
Taenia	++	++	++	++	+	+	+

0 probable complete elimination
+ probable low concentration
++ probable high concentration
Source: Warner Baumann (1980)

All pathogens will eventually die or lose viability after excretion and release into the extra-host environment. In general, the reduction of viable pathogens is rapid in the first few hours or days after excretion, with a reduced number surviving over an extended period.

Variations of this die-off pattern are found with a few bacteria (e.g. Salmonella), which may temporarily multiply outside the host. Most helminths have one or more non-infective intermediate development stages with different die-off periods. A further variation is found with trematodes (e.g. Schistosoma), which have a multiplication phase in intermediate hosts.

Basically, pathogen die-off follows the same pattern, independent of the environment (such as in soil, on crops, in sludge, or in excrete in a latrine or leaching pit). Particular environmental factors, however, determine the actual die-off rate and the number of organisms surviving within a given time period. This, in turn, determines the time required to obtain a "safe" or "reason ably safe" product. The relative levels of risk and main environmental factors which influence pathogen die-off are shown in Tables 1, 2, and 3.

After disposal on land, the process of pathogen die-off continues although at different rates for each type of pathogen.

Whether there is a potential risk of pathogen transmission through soil or crops in a particular situation depends on numerous factors. Pathogen survival or die-off is dependent on the organism's persistence and the adverse environmental effects such as sunlight, desiccation, and soil properties. Residual pathogen levels on root crops are expected to be higher than on crops growing above ground because the exposure of root crops to sunlight and desiccation is lower.

Potential contamination of crops is also dependent on the time and method of excreta application to the field. Contamination of leaf and fruit crops is likely to be minimal or nil if the excrete is applied prior to plowing, sowing, or transplanting. However, crops may become contaminated through rainfall splashing or if they fall on the ground before they are harvested. The interval at which night-soil is applied to the field also plays a role. The potential risk of pathogen transmission from night-soil to farmers and consumers is greater if disposed of repeatedly on the same field during the crop growing period than if night-soil fertilization takes place only during the initial phase of the growing season.

Table 2: Likelihood of Infection by Class of Pathogen

Class of Pathogen		Excess Frequency of Infection or Disease
1. Intestinal nematodes		high
	Ascaris
	Trichuris
	Ancylostoma
	Necator
2. Bacterial infections		lower
	Vibrio cholera
	Salmonella typhi
	Shigella dysenteriae
	Path. E. Coli
3. Viral infections		least
	viral diarrhoeas
	hepatitis A
4. Trematode & cestode infections		from high to nil, depending on the particular excreta, usage of effluent and other local circumstances
	Schistosomiasis
	Clonorchiasis
	Teariasis

Source: IRCWD News No. 23 (1985) "The Engelberg Report."

Finally, the potential risk to those working in the field depends on the risk behavior of the workers, i.e. protective measures at work such as the wearing of shoes or boots (particularly to protect against hookworms and schistosoma infection), and habits of personal hygiene such as the washing of hands and body.

As mentioned above, most of the pathogens are inactivated within the first few hours or days after excretion. A few organisms, however, remain alive and infective for prolonged periods of time. Thus, the length of the time between night-soil application to a field and cultivation or harvesting is decisive for the risk of pathogen transmission through either soil or crops. If night-soil is applied only at the beginning of the growing season, it is important to ask whether excreted pathogens will die off within the growth period of the vegetable, or whether large numbers of pathogens will remain infective even at the time of harvest.

Table 3: Environmental Factors Influencing Pathogen Die-Off

Environmental Factor	Effect
Temperature	Accelerated die-off with increasing temperature, longer survival at low temperature
Moisture content	Generally longer survival in moist environment and under humid weather conditions, rapid die-off under conditions of desiccation
Nutrients	Accelerated die-off if essential nutrients are scarce or absent
Elimination by other micro-organisms	Longer survival in an environment with few or no micro-organisms competing for nutrients or acting as predators
Sunlight (ultra-violet radiation)	Accelerated die-off if exposed to sunlight
ph	Neutral to alkaline ph tends to prolong survival of bacteria, acid ph tends to prolong survival of viruses

Figures 3 and 4 compare vegetable growth periods and excreted pathogen survival times in soil and on crops in warm climates. Figure 3 shows that Ascaris lumbricoides eggs have normal survival times in soil and tend to live past the period required by most vegetables to reach maturity. Excreta fertilized crops are, therefore, potential transmitters of A. Lumbricoides eggs where ascariasis is endemic. Hookworm larvae, though normally dying off substantially faster than A. Lumbricoides, have survival periods in soil which are of the same order of magnitude as the growth period of vegetables such as radish, spinach, and cucumber. They pose an occupational risk to those who perform weeding and thinning work in the night-soil fertilized fields. Ascaris eggs, apart from being transmissible via crops, may also be carried into homes by people who work in the fields.

As a rule, survival of pathogens on crops tends to be substantially shorter (by a factor of more than two) than survival in soil. This is not unexpected since pathogens are subjected to harsher environmental impacts (solar radiations, desiccation, temperature) on crops, notably high-growing crops, than in soil. The majority of pathogens exhibit survival periods which are normally shorter than the growth periods of most vegetables exceptions being the eggs of ascaris or taenia saginata, and the salmonella on root and low growing crops.

Figure 3: Pathogen Survival in Soils vs. Vegetable Growth Periods in Warm Climates¹

1 Determined under widely varying conditions
² Maturation period from transplanting or from sowing if not transplanted

Source: Strauss, M. (1990): Survival of excreted pathogens in excreta and faecal sludges. Report No. 04/85 of IRCWD.

Figure 4: Pathogen Survival on Crops vs. Vegetable Growth Periods in Warm Climates¹

1 Determined under widely varying conditions
² Maturation period from transplanting or from sowing if not transplanted

Source: Strauss, M. (1990): Survival of exacted pathogens in excreta and faecal sludges. Report No. 04/85 of IRCWD.