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Essay on Water Borne Diseases
Essay # 1. Introduction to Water Borne Diseases:
Water is non-substitutable, and a life supporting molecule. There can be no life without water. Our earth is the only planet in this solar system whose temperature allows liquid water to exist and life to evolve. Water constitutes 2/3rd of our own bodies. It is today at important crossroads of human health and environment. Water is both a multi-purpose good as well as a major vehicle for transmission of disease. Water-borne diseases constitute a wide spectrum, caused by a variety of pathogens, i.e., bacterial, viral and parasitic.
World Health Organisation (WHO) estimates suggest that nearly 22% of all infection related deaths are water and food or soil borne, 13% are insect borne, 0.3% animal borne and 65% are transmitted from person to person. Water-borne diseases continue to cause immense suffering, millions of deaths and grave economic losses especially in developing countries.
Contamination of drinking waters either directly or indirectly by human/animal excreta particularly feces, and use of such water for drinking, preparing food, contact during washing and bathing result in infection. Most of water-borne microorganisms differ in their pathogenicity. Some act as opportunistic, affecting only those people whose local or general natural defense mechanisms are impaired. Some others may lead to epidemics, e.g., cholera, typhoid.
The infective dose of most water-borne pathogens depend on:
(a) Probability of ingesting large number of pathogens-in grossly polluted water and food,
(b) Balance between host’s defense mechanisms like intestinal immunity and gastric acidity, and
(c) Factors aiding the microbes in their attempt to colonise the intestine.
Fecal-oral route is predominantly used as a mode of transmission, can be transmitted by a variety of routes that are further facilitated under conditions of poor domestic hygiene and sanitation. Water- borne diseases are often classified based on the type of organisms that are responsible such as bacteria, viruses, protozoa etc. (Table 14.1).
Fecal-oral route is predominantly used as a mode of transmission though water borne diseases can be transmitted by a variety of other routes. Most of endemic transmission of diseases is water- washed whereas epidemic diseases are mainly water-borne.
The best way of classifying water related diseases from public health point has been classified by Bradley in Water Supply Sanitation and Hygiene Education—UNDP World Bank Water and Sanitation Report Series 1. This has been depicted in Table 14.2.
Water, food and soil borne diseases are also best considered together as their modes of transmission are closely interlinked and are shown in Table 14.3.
Essay # 2. Viruses that Cause Water Borne Diseases:
Many other viruses enter the host via the gastrointestinal tract, e.g., enterovirus, rotavirus coronavirus, adenovirus and norwalk-virus. Almost all picorna-viruses are capable of producing enteric infections in humans except rhinovirus.
Rotavirus are the principal cause of infectious diarrhoea in infants. They are also an important nosocomial pathogen in pediatric hospital. In 1973, Bishop and colleagues observed by electron microscopy, in the duodenal epithelium of children with diarrhoea, a 70 nm virus, designated as rotavirus. Within 5 years, it was recognized as the most common cause of diarrhoea in infants and young children worldwide. Rotavirus infects virtually all children 3-5 yrs of age both in industrialized and developing countries. Rotavirus infections in adults are usually sub-clinical but compound to high infection rate in neonates.
Rotavirus, an icosahedral virus belongs to the family Reoviridae. It is a double stranded segmented (11 segments) RNA virus. Segments code for products of either structural viral proteins (VP) or non-structural proteins (NSP). Rotavirus has seven major groups (A—G), most human strains belong to group A. VP7 (the G-protein) and VP4, the protein cleaved or P-protein encoded by gene segment 4, determine the serotype.
Infection is generally confined to the intestinal mucosa, although it can be found in lamina propria and regional lymphatics. Protection against rotavirus diarrhoea correlates with serum antibody titre following natural infection of young children. Infected children are more protected against re-infection with similar rather than different serotypes.
Most available tests are based on ELISA methodology and, except for a significant rate of false positivity in the stools of babies under 3 months of age, are of a sensitivity and specificity similar or superior to electron microscopy has been initially described by de Zoysa et al. (1985). Rotavirus are shed in large number during episodes of diarrhoea and usually are detectable by antigen enzyme immune assays (EIA) up to 1 week after infection.
Parashar et al. (1998) have reported successful use of Rhesus-Human Reassortment Trivalent vaccine (RRV-TV) in United States of America. However, recommending rotavirus vaccines for universal immunization is more complex. Several factors that affect are younger age at infection, poor nutritional status of children, interference by other enteropathogens, presence of unusual strains of rotavirus and, above all, the cost. However, considering the need, incorporation of this vaccine in immunization programme will depend not only on economic considerations but also on their perceived value by national governments.
During acute poliovirus infection, the virus is found for about 2 weeks in the respiratory tract, for about 6 weeks in the stool, and in the presence of meningitis or poliomyelitis, for a few days in cerebrospinal fluid. Following trivalent live oral poliovirus vaccine, the virus is found for up to 6 weeks in feces.
The virus grows rapidly and produces cell destruction in multiple cells lines (Hep2, HeLa, Vero). More definitive identification requires neutralization with type-specific anti-sera or fluorescence staining with monoclonal antibodies. A rise in neutralizing antibody to one of the three serotypes between acute and convalescent sera is required in order to demonstrate infection.
These are 28 nm diameter positive stranded RNA viruses, and named ‘astro’ after their characteristic star like surface structures. Herman et al. (1991) could prove that, Astrovirus in human was found to be second most common cause of viral diarrhoea in young children. Transmission of virus occurs through fecal-oral route, by person to person contacts via consumption of contaminated food and water.
These infections are also seen in elderly patients. Out of many serotype, serotype 1 appears to be the most prevalent strain. Human astrovirus infection induces a mild watery diarrhoea that typically lasts for 2-3 days and are associated with vomiting, fever, anorexia and abdominal pain. Biphasic distribution (young and old age) of symptomatic infection suggests that antibody acquired in childhood may provide protection from illnesses. Viruses can be detected by electron microscopy, ELISA and RT-PCR.
It is a spherical, non-enveloped 27 nm RNA virus and is a major cause of epidemic non-bacterial gastroenteritis that occur in family and in community wide outbreaks. Predominantly affects school children and young adults. Black et al. (1982) found that most Norwalk virus outbreaks had a seasonal predilection and were considered to occur predominantly in the cooler months of the year. Clinical manifestations observed in various outbreaks have shown nausea (79%), vomiting (69%), diarrhoea (66%), associated with abdominal cramps, headache and fever. Bloody stools have not been reported.
This virus is non-cultivable, cannot be grown either in tissue culture or induce illness in a laboratory animal. Although electron microscopy was used exclusively for the diagnosis, early studies have been superseded by subsequent ELISA and Radioimmuno assay (RIA). Recently, reverse transcriptase polymerase chain reaction (RT-PCR) has been found more effective.
Although a significant proportion of this immense burden of disease is caused by ‘classical’ water-related pathogens, newly-recognized pathogens and new strains of established pathogens are being discovered that present important additional challenges to both the water and public health sectors.
Toroviruses have been associated with gastroenteritis in both animals and humans. Lodha et al. (2005) found in their studies that torovirus infection is often associated with neonatal necrotizing enterocolitis.
Noroviruses (formerly Norwalk-like viruses) are the most commonly identified cause of intestine diseases in Western Europe communities. These viruses account for an estimated 6% and 11% of all infections intestine disease. Lew et al. (2005) delineated certain factors that contribute to the considerable impact of disease caused by NVs i.e.- (a) large human reservoir of infection. (b) a very low infection dose. It produces mild and self-limiting in the absence of other factors. NVs affect all age groups.
Other viruses, such as coronaviruses and pestiviruses are increasingly identified as causative agents of diarrhoea and CMV with Inflammatory Bowel Disease.
Emerging pathogens that have been defined by WHO (1997) are those that have appeared in a human population for the first time, or have occurred previously but are increasing in incidence or expanding into areas where they have not previously been reported, whereas re-emerging pathogens are those whose incidence is increasing as a result of long-term changes in their underlying epidemiology.
Developments, in our understanding, of the relationships between water and human health have been characterized by the periodic recognition of previously unknown pathogens or of the water- related significance of recognized pathogens.
Since 1970, several species of micro-organism from human and animal feces and from environmental sources, including water, have been confirmed as pathogens. Examples include Cryptosporidium, Legionella, Escherichia coli 0157, hepatitis E virus and norovirus (formerly norwalk virus). Helicobacter pylori (H. pylori) is an example of a recently emerged pathogen that may be transmitted through water. Between 1972 and 1999, 35 new agents of disease were discovered and many more have re-emerged after long periods of inactivity.
Major etiological agents of infectious diseases identified since 1972 are given below:
There are many reasons why human pathogens emerge or re-emerge after a long period of inactivity, but most have a common theme and may be grouped under a few general headings – new environments, new technologies, scientific advances, and changes in human behaviour and vulnerability are the main contributory factors. McMichael (2000) has reported the paradigm shifts that are responsible for changing scenario.
i. New Environments:
The Legionella bacterium provides another example of the significance of new environments to emerging pathogens. In 1976, a large outbreak of pneumonia was reported amongst delegates at an American Legion Convention and only after an exhaustive microbiological investigation, the disease became known as Legionnaires’ Disease. Legionella bacteria are now known to be a normal part of the aquatic micro-flora.
The design of domestic hot and cold water systems, specialist leisure pools and water-cooled air-conditioning plants create conditions that suit the growth of Legionella bacteria. Many water systems produce fine aerosols at some stage in their use providing a dispersion mechanism that proved to be an effective route of infection.
ii. New Technologies:
New water treatment, storage and distribution technologies are being developed to improve and maintain the quality of drinking-water. Many of these technologies will have significant benefits, but unforeseen problems with a few may introduce new risks, such as harmful by-products or pathways of transmission that may lead to the re-emergence of water-related pathogens. Each time a risk is identified, systems are developed to eliminate or reduce the risk that may, in turn, increase or decrease other risks.
Chlorine is the most widely used drinking-water disinfectant. It was first used for treating water by John Snow (1845) as he intervened to control the outbreak of cholera in London, but it was not until 1897 that it was used to disinfect a potable water distribution main. The use of chlorine disinfection has made an immense contribution to the safety of drinking-water supplies, yet, recently, the limitations of chlorine and some disadvantages linked to its use have been widely publicized.
Although chlorine is effective against most vegetative bacteria and viruses when used at the normal concentration for treatment, it will not inactivate Cryptosporidium oocysts. Furthermore, chlorine has a very limited effect upon pathogens growing in biofilms (organism that build a film of them having quorum sensing mechanism and related virulence factors). So while its use reduces overall risks, it changes the relative impact of different pathogens.
Despite the treatment of source water and the use of chlorine disinfectant, contamination of piped water supply continue to occur, without necessarily causing large easy-to-recognize outbreaks, through leaks, or at other vulnerable parts of the system, and during maintenance work. Once in the system, bacteria, fungi and protozoa can attach to the inner surfaces of the pipes and some may grow to produce biofilms. Some biofilms have been shown to contain one or more species of emerging pathogen, including Mycobacterium avium complex (MAC).
iii. Scientific Advances in Water Microbiology:
By increasing our capacity to concentrate and detect micro-organisms in water samples, we can recognize new pathogens and associate known micro-organisms with diseases of unknown etiology.
In 1972 and 1973 the use of high-resolution microscopy, antibody techniques and genetic analysis identified two groups of viruses that were the causative agents of diarrhoea – small round structured viruses (caliciviruses) and rotaviruses. Possibly the most revolutionary advance in analytical technology during the last 30 years was the discovery of a simple, but effective method of amplifying very specific regions of an organism’s genetic material. Polymerase Chain Reaction (PCR), is now widely used in medical, forensic and environmental laboratories.
It can be used to detect extremely small quantities of the nucleic acid equivalent to a single micro-organism. Detection of nucleic acid by PCR does not necessarily indicate the viable infective organisms, but it is an excellent exploratory tool. The use of PCR methods for the analysis of pathogens in water has been fundamental to our understanding of the distribution of some of the most important water-related viral pathogens – for example noroviruses, rotaviruses and hepatitis E virus.
iv. Changes in Human Behaviour and Vulnerability and Breakdown in Public Health Measures:
Several pathogens have emerged or are re-emerging within the immunocompromised population, e.g., Cryptosporidium and MAC. The 1993 outbreak of cryptosporidiosis in Milwaukee, USA, involved up to 4,00,000 cases of serious illness and 100 deaths, principally among AIDS patients.
The cause of the outbreak appeared to be linked to two factors:
(1) the location of the water intake to the treatment works, and
(2) the experimental coagulation system which turned out to be ineffective, resulting in the turbidity limits for some of the filters being exceeded.
v. Human Migration:
Cholera provides a good example of a water-related pathogen that is readily transported over long distances by human migration. International Organisation of Migration (IOM) reports, migration of people between natural boundaries has been responsible for the emergence of several infectious diseases. Most notable are diseases that have emerged as humans have encroached upon forest regions, bringing people into closer contact with animal species carrying pathogens that can be transmitted across the species barrier.
In water distribution system for public consumption throughout any year, 95% of such samples should ideally be free from containing any coliform organisms in 100 ml. No sample should contain Escherichia coli (coliform) in 100 ml and no sample should contain more than 3 coliform organisms per 100 ml. Coliform organisms should not be detectable in 100 ml of any two consecutive samples.
The water for individual and small community supplies should also be maintained in similar manner. Acceptable standards for viral quality is one plaque forming unit per litre and fecal bacteriophages and enteropathogenic viruses should be completely absent.
Standards for toxic/chemical substances are shown in the Table 14.4.
Substances that affect the acceptability of water are as per the Table 14.5.
For radioactive substances, gross alpha activity should be within 3 pcil–1.
The above criteria takes up to think about the public health interventions for water related (borne) diseases on the lines of:
1. Supplying safe and potable water.
2. Supply of sufficient quantity of water.
3. Ensuring strict microbiological monitoring of the water samples.
4. Effective and efficient excreta and waste disposal, food hygiene and water storage styles.
5. Personal hygiene, specially hand washing.
1. Supplying Safe and Potable Water:
Safe and wholesome water is defined as water that is:
(i) Free from pathogenic agents,
(ii) Free from chemical substances,
(iii) Pleasant to taste, and
(iv) Usable for domestic purposes.
The primary objective of water treatment is to protect the consumer from microbes and impurities that reduce water safety for human consumption and that are offensive to health.
The water for human consumption can be made safe either on large scale or small scale.
(a) Purification of Water on Large Scale:
This process of water treatment combines many activities like coagulation, sedimentation, flocculation, filtration and finally disinfection.
(b) Purification of Water on a Small Scale:
(i) Household Level:
Boiling – Useful in epidemic situation; rolling boil for at least 5 minutes
Chemical disinfection – chlorine solution, bleaching powder, chlorine tablets
The amount of chemicals needs to be adjusted as per the volume.
Filters – Candle filters—surface coated with silver catalyst—can hold only bacteria but not viruses.
Modern gadget e.g., UV radiation and carbon activators can protect from viruses.
(ii) Community Level:
Bleaching powder is required:
(a) Using Horrock’s apparatus.
(b) 2.5 gram of good quality bleaching powder for 1000 liters.
Gives an approximate dose of 0.7 mg I–1 (free chlorine).
Similar procedures, though difficult, may have to be adapted based on the source of water and possibility of estimating the quantity of water in that source.
2. Supply of Sufficient Quantity of Water:
(a) Rural – 40 litres per capita daily (LPCD) and
(b) Urban – 40-250 LPCD, as per laid down criteria.
3. Monitoring of Microbial Contamination:
The microbial monitoring influences the water quality. Many epidemics are seen due to slackness in monitoring microbial contamination.