Característiques principals

2.3.3 Pathogens to Watch

Burkholderia pseudomallei Highlighted B. pseudomallei is a major cause of water- and soilborne disease in tropical countries. The CDC lists Africa, the northern part of Australia, Cambodia, India, Laos, Malaysia, the Middle East, Myanmar, the South Pacific, Thailand, and Vietnam as areas with endemic melioidosis, the disease caused by this pathogen. Sporadic cases have also occurred in Brazil, Ecuador, Guyana, Haiti, Panama, Peru, and the United States (Hawaii and Georgia) (CDC, 2008b).

The ease of environmental transmission, through direct contact with contaminated soils or water, has made this pathogen a potential candidate for bioterrorism. Infec- tion can occur through exposure of open skin wounds, ingestion, or inhalation of aerosolized bacteria. Disease manifestations range from localized skin infections to acute pulmonary and bloodstream infections that, in turn, can lead to chronic sup- purative infections of major organs. Although seldom fatal, a widespread epidemic of melioidosis has the potential to disrupt health care services on a major scale and incapacitate a large proportion of a population.

Like V. cholerae and E. coli O157:H7, B. pseudomallei are genetically diverse and appear to be evolving rapidly (Holden et al., 2004). The B. pseudomallei genome is relatively large, 7.2 megabase pairs in size, and consists of two chromosomes. The genome contains a large collection of genes, allowing for survival in a variety of different niches. The genetic fluidity of B. pseudomallei potentially poses challenges for study and future control of pathogenesis of the organism as well as for vaccine development.

In terms of control, to date there are no available vaccines and the ease of exposure would make prevention extremely difficult without significant behavioral changes, for example, in agricultural practices. The infections are, however, generally treatable with antibiotics. Treatment is somewhat complicated by the fact that B. pseudomallei are inherently resistant to certain antibiotics (Cheng and Currie, 2005). For severe melioidosis, current treatment and management protocols include an initial, intensive course of intravenous antibiotic administration followed by an eradication course of oral antibiotics. Relapse is sometimes observed even with aggressive treatment, however.

2.3.4 New World Diseases

In the more developed countries, considerable interest has revolved around the pro- tozoan pathogen Cryptosporidium parvum and the calicivirus group of viruses, most specifically norovirus. Are they a major cause of waterborne disease in developing countries? This is not a trivial issue in relation to “old world” diseases such as cholera, due to the exceptional disinfection resistance of C. parvum and the high environmen- tal stability of the norovirus (Poschetto et al., 2007). C. parvum are clearly present in water supplies of developing countries and can be isolated from stool samples of patients presenting with diarrhea (Snelling et al., 2007). There is less information on the potential burden of norovirus infections, although traveler’s diarrhea is often caused by norovirus that has clearly been contracted in developing countries (Chapin et al., 2005). As water treatment improves in any given region, we should anticipate seeing greater morbidity from such organisms as C. parvum and norovirus. Today, these infections are seldom diagnosed in developing countries but are undoubtedly present and contributing

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to the overall burden of waterborne disease. Given that the source of exposure is often untreated or poorly treated water that is subject to both human and animal wastes, it is likely that exposed individuals are infected with multiple pathogens (Albert et al., 1999). In these cases, identifying the contribution to morbidity (and mortality) from C. parvum and norovirus would be extremely difficult. Both infectious agents can cause severe diarrhea and can therefore contribute significantly to dehydration and potentially mortality, particularly among the immunocompromized, the very young, the pregnant, or the elderly.

A similar argument can probably be used for the environmental pathogens that are of concern in treated drinking water: the Mycobacterium avium complex of organisms, Legionella pneumophila, and Helicobacter pylori . The burden of the diseases caused by these bacteria will probably never be fully assessed for developing countries (Kumar et al., 2006).

2.4 CASE STUDIES

Flooding is the most frequent form of catastrophic disaster, happening relatively pre- dictably in countries such as Bangladesh (e.g., Hashizume et al., 2008) as the result of monsoon rains, and less predictably in other countries, due to unanticipated heavy rains, storm surges, tsunamis, snowmelt, and perhaps in the future, sea levels rise from melting polar caps (IPCC, 2007).

In all cases of flooding, waterborne disease outbreaks on different scales are almost inevitable. The most common illnesses associated with floods described in the literature are diarrhea, cholera and typhoid, hepatitis (jaundice), and leptospirosis. Unusual ill- nesses such as tetanus have also been reported. Cholera in particular has been associated directly with flooding in Africa. It is instructive to examine two different scenarios and the approaches to disease prevention, control, and mitigation. Almost every monsoon season in Bangladesh is accompanied by outbreaks of waterborne disease; however, the severity of flooding and the consequent severity of the outbreaks is not easily pre- dicted. Major flooding events occurred in 1988, 1998, 2004, and 2007, accompanied by mass outbreaks of waterborne disease (Schwartz et al., 2006; ICDDR,B, 2007). The most commonly isolated pathogen is V. cholera (estimated 35% of cases in August 2007) followed by rotavirus (estimated 12% of cases in August 2007). Infections with enterotoxigenic E. coli, Shigella, and Salmonella species also increase during floods in Bangladesh, but the burden of identified disease is clearly dominated by cholera.

Flooding and disease have long been a part of Bangladeshi life, and aggressive rehydration therapy, even during mass waterborne disease events, is credited with saving large numbers of lives— no deaths from diarrhea or dehydration occurred among patients admitted to the Dhaka hospital in August 2007 (21,401 patients). However, tens of millions of people are thought to have been displaced in the 2007 floods (ICDDR,B, 2007), and it would be almost impossible to estimate the true morbidity and mortality from these events.

The International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR,B), established in 1978 by the government of Bangladesh, is a world leader in diarrheal disease research. Through promotion of household treatment, vaccination programs, and support for aggressive rehydration therapies, the center has undoubtedly