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Water treatment issues of infectious disease start primarily with the concerns of emerging pathogens. What is an emerging pathogen? Emerging pathogens 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, usually over the last 20 years (WHO, 1997). Re-emerging pathogens are those whose incidence is increasing as a result of long-term changes in their underlying epidemiology (Woolhouse,2002).

By these criteria, 175 species of the infectious agent from 96 different genera are classified as emerging pathogens. Of this group, 75% are zoonotic species. Improved methods of surveillance, epidemiological studies, and the continuous development of more advanced methods of diagnosis have allowed us to detect new pathogenic species of a micro-organism or to associate a known micro-organism with a new or atypical set of disease symptoms. Furthermore, the agents of several diseases that were thought to have been controlled are re-emerging as a result of adaptive changes in the pathogen, changes to the immunological status of the population normally affected.

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. Several studies have confirmed that water-related diseases not only remain a leading cause of morbidity and mortality worldwide but that the spectrum of disease is expanding and the incidence of many water-related microbial diseases is increasing. 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 O157 (E. coli O157), rotavirus, hepatitis E virus, and norovirus (formerly Norwalk virus).

Furthermore, the importance of water in the transmission of recognized pathogens are being continually assessed as new tools become available through advances in science, technology, and epidemiology. Helicobacter pylori (H. pylori) is an example of a recently emerged pathogen that may be transmitted through water. Similarly, water-related vector-borne pathogens have been (re-) emerging over the past 20 years.

To a large extent this has been caused by the emergence and spread of drug-resistant parasites (for example, the Plasmodium species causing malaria) There is a strong link between H. pylori infection and gastric cancer in many countries, but there are large intercountry variations in the incidence of gastric cancer and H. pylori seroprevalence seen among many Asian countries. For example, the prevalence of H. pylori infection is high in India and Bangladesh, but low gastric cancer rates have been reported. Factors that may influence the etiology of gastric cancer include the genetic diversity of the infecting H. pylori strains and differences in the host genetic background in various ethnic groups.

These factors, in addition to environmental factors, such as personal hygiene and dietary habits, reflect the multifactorial etiology of gastric cancer (Miwa, Sakaki & Sugiyama, 2002). A number of studies have demonstrated that H. pylori survive in water although isolation of H. pylori from water systems has been shown to be difficult. Changing environments linked to such trends as intensified water resources development and urbanization, and the accompanying demographic changes, have created conditions where vector-borne diseases can gain new strongholds. International travel has contributed to the spread of pathogens to areas where the vector was already present but so far innocuous (for example, West Nile virus in North America).

Major etiological agents of infectious diseases identified since 1972:

1972 Small round structured viruses Diarrhoea   1989 Hepatitis C virus Parenterally transmitted non-A, non-B hepatitis
1973 Rotaviruses Infantile diarrhoea   1990 Human herpesvirus-7 Exanthema subitum
1975 Astroviruses Diarrhoea   1990 Hepatitis E virus Enterically transmitted non-A, non-B hepatitis
1975 Parvovirus B19 Aplastic crisis in chronic haemolytic anaemia   1991 Hepatitis F virus Severe non-A, non-B hepatitis 1992 Vibrio cholerae O139:H7 New strain associated with epidemic cholera
1976 Cryptosporidium parvum Acute enterocolitis   1992 Bartonella henselae CAT-scratch disease, bacillary angiomatosis
1977 Ebola virus Ebola haemorrhagic fever   1993 Sin nombre virus Hantavirus pulmonary syndrome
1977 Legionella pneumophila Legionnaires' disease   1993 Hepatitis G virus Non A-C hepatitis
1977 Hantaan virus Haemorrhagic fever with renal syndrome   1994 Sabia virus Brazilian haemorrhagic fever
1977 Campylobacter spp. Diarrhoea   1994 Human herpesvirus-8 Kaposi's sarcoma
1980 Human T-cell lymphotropic virus-1(HTLV-1) Adult T-cell leukaemia/ HTLV-1 associated myelopathy   1995 Hendravirus Castleman's disease
1982 HTLV-2 Hairy T-cell leukaemia   1996 Prion (BSE) Meningitis, encephalitis
1982 Borrelia burgdorferi Lyme disease   1997 Influenza A virus New variant Creutzfeldt-Jakob disease
1983 HIV-1, HIV-2 Acquired immunodeficiency syndrome   1997 Transfusion-transmitted virus 1997 Enterovirus 71 Epidemic encephalitis
1983 Escherichia coli O157:H7 Haemorrhagic colitis; haemolytic uremic syndrome   1998 Nipah virus Meningitis, encephalitis
1983 Helicobacter pylori Gastritis, gastric ulcers, increased risk of gastric cancer 1988 Human herpesvirus-6 Exanthema subitum   1999 Influenza A virus Influenza (Hong Kong)
1989 Ehrlichia spp. Human ehrlichiosis   1999 West Nile-like virus Encephalitis (New York) (Desselberg, 2000)

An outbreak of arboviral encephalitis was first recognized in New York City in 1999. The cause of the outbreak was confirmed as a West Nile-like virus. Before and concurrent with this outbreak, local health officials observed increased fatalities among New York City birds, especially crows. Tissue specimens from these birds with pathologic evidence of encephalitis were reported as positive for West Nile-like virus sequence by genomic analysis, implying these as the vectors. Four human deaths occurred among elderly persons. One case patient with onset in late August reported a history of travel to Africa completed in June 1999. Vector control measures were initiated to control the host-seeking adult Culex pipiens mosquito population (MMWR, 1999).

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