Public health
"The greatest risk to consumers of drinking water are pathogenic microorganisms. Protection of water sources and treatment are of paramount importance and must never be compromised" Australian Drinking Water Guidelines (NHMRC 2004) Waterborne disease is one of the greatest causes of preventable death worldwide. The treatment of water to prevent disease has probably saved more lives than any other practice.
Protozoa pose a high risk of infection as they can have high survival rates in the environment and only require ingestion of a few particles or cysts to cause infection. The infectious dose for many viruses is as low as one particle and may cause a wide range of illness including gastroenteritis, diarrhoea, vomiting and respiratory disease.
The spread of infection by pathogenic organisms depends on the survival of the pathogen in the environment, the dose required to cause infection, and a person's exposure to the pathogen. People may be exposed to pathogens if they come into contact with faecal material from an infected person or animal. Transmission can occur by a number of routes, including person to person (especially in childcare facilities), contaminated food or beverages, contact with infected animals, contamination of swimming pools with faecal material, and carriage in drinking water.
Public health decisions about the safety of a drinking water supply are made based on:
- detecting the presence or absence of pathogens in drinking water
- the effectiveness of treatment processes (especially filtration) to remove them from the water
- the likelihood that the detection of these organisms represents a serious contamination event, or contamination from a natural source eg wildlife in the catchment
- considering the infectivity to humans and understanding the viability, strain, and source of the pathogens (see pathogen research).
Consideration is given, but not restricted to the following parameters when making public health decisions:
- recent occurrence of heavy rainfall
- reports of sewer overflows or sewage treatment plant discharges in the catchment
- presence of indicators of contamination in raw water eg E. coli, high turbidity measures
- measures of the effectiveness of filtration at treatment plants (usually turbidity of filtered water and ability to maintain an adequate chlorine concentration)
With regard to testing for Cryptosporidium and Giardia:
- number of cysts and oocysts
- volume of water in which cysts/oocysts are detected
- frequency of testing samples, and
- detection methodology and recovery efficiency of the testing procedure.
Immuno-compromised people
Infections in highly immuno-compromised people can be severe. People with HIV infection who also have severely depressed CD4 lymphocytes can experience chronic severe diarrhoea with profound weight loss. In people with HIV, cryptosporidiosis can be more serious and prolonged. NSW Health factsheet is available at http://www.health.nsw.gov.au/factsheets/infectious/cryptosporidiosis.html
Evidence from an outbreak of cryptosporidiosis in Milwaukee, USA in the early 1990’s, when slightly less than 8,000 people were hospitalised and nearly 100 HIV positive people died, indicated that people with HIV/AIDS were not at higher risk of becoming ill, but were at more risk of suffering prolonged illness (MacKenzie et al, 1995). This suggests the number of Cryptosporidium oocysts required to establish infection in people with HIV/AIDS is similar to the general population. However, the inability of the immune system to fight the infection leads to chronic illness in those with severely depressed CD4 lymphocytes counts.
The incidence of disease due to Cryptosporidium and Giardia
Both Cryptosporidium and Giardia infections are relatively common causes of gastroenteritis throughout Australia. Individuals with normal immune systems generally experience mild symptoms and fully recover from both infections.
Young children are most likely to be diagnosed with these infections, suggesting person-to-person transmission is the most common cause of infection. This is likely in childcare centres and swimming pools. A more even distribution across the population would be expected from waterborne transmission. There has not been an outbreak of either infection linked to the public (mains) water supply in Australia.
Cryptosporidiosis has been a notifiable disease in NSW under the Public Health Act 1991 since November 1996. Laboratories detecting the disease are required to notify NSW Health for the purposes of health surveillance, the detection of outbreaks, and to allow an appropriate level of public health response. Background levels of cryptosporidiosis in the community vary seasonally, with particularly marked variations during swimming pool outbreaks.
In the mid 1990s a large outbreak of cryptosporidiosis was linked to contaminated swimming pools in Sydney. In 1997-98 several more large outbreaks on the east coast of NSW, including Sydney, were traced to multiple contaminated swimming pools. In 1997 there were 159 cases of cryptosporidiosis notified in NSW and in 1998 (to end of September) there were 1,085 cases notified.
Active surveillance for cryptosporidiosis cases during the boil water incident in Sydney in 1998 did not find an increased rate of illness during that period. There was no evidence that the detection of these parasites in water at that time lead to illness in the community. The NSW Department of Health is responsible for overseeing the quality of drinking water supplies for the community. When problems occur the NSW Department of Health has the power to call a boil water alert (BWA).
For further reading see:
Managing a cluster of cryptosporidiosis associated with a public swimming pool - NSW Public Health Bulletin 2005; 17 (5–6) 80
Sydney water incident: July-September NSW Public Health Bulletin 1998; 9 (8-9) 91
Cryptosporidiosis in the immunocompromised: weighing up the risk Medical Journal of Australia 1999; 171: 426-428
Gastroenteritis
Gastroenteritis describes an inflammation of the intestinal lining. This may be caused by non-infectious factors or by infection by harmful micro-organisms. Gastroenteritis can result in significant loss of fluid and electrolytes (body salts) and can interfere with the absorption of nutrients. There are many different causes of infectious gastroenteritis, including a range of viruses, bacteria, and protozoa.
Risk of infection
The number of micro-organisms required to cause infection varies greatly, depending on the characteristics of the particular strain of micro-organism and the individual. Ingestion of pathogenic micro-organisms, may result in one of three outcomes:
- passage of the micro-organisms through the gut without infection or symptoms
- establishment of infection which does not cause symptoms of illness
- establishment of infection which causes symptoms of illness.
Studies of a range of micro-organisms with human volunteers show that the risk of infection increases as the ingested number of organisms increases. However, once a person is infected the probability of developing symptoms does not necessarily vary with the number of organisms ingested, but may depend on the particular strain of micro-organism. For some strains most infected people will become ill, while for others only a small minority of infected people will become ill.
Symptoms of gastroenteritis
The symptoms of gastroenteritis caused by many micro-organisms are very similar. They commonly include nausea, vomiting, diarrhoea and abdominal pain. The symptoms caused by all the different organisms are so similar that it is very difficult, if not impossible, to distinguish on the basis of symptoms which organism is responsible for a person's episode of gastroenteritis.
Protecting water catchments from contamination by human and animal waste is of utmost importance. Effective catchment management, particularly in the areas of effluent from farms, riparian buffer zones, and sewage management, is central to maintaining water quality. Any activities that contribute pollution should be controlled. Well designed and operated treatment systems incorporating filtration and disinfection reduce the risk from pathogens in the water supply.
Research task
A research task on drinking water and infectious disease is provided to improve your understanding.
