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"Consider the global impact of a waterborne infectious disease (which may be caused by a bacterium or parasite) and discuss current methods for detection, treatment and prevention."
Amongst metazoans, humans are considered as the most complex organisms due to their interlinking mechanisms within the body that aids in their well being i.e. immune system. Over the period of years it is known in the medical field that humans are susceptible to several infectious diseases associated with bacteria, viruses, protozoan and fungi. Infectious diseases can be transmitted to humans via different modes including vector borne diseases such as malaria and dengue. Another mode of transmission is airborne, however little data is accumulated about airborne diseases. Infectious airborne pathogens include influenza and rhinoviruses. Finally, the mode of transmission which has over the years lead to many human deaths worldwide is waterborne diseases (Lipp et al., 2002). The diseases associated with waterborne are considered to have a significant global impact i.e. Cholera as they are life threatening and often cause many human deaths worldwide (Faruque et al., 2004).
The concept of waterborne infectious disease is the theme of this essay and will focus on cholera as an example of waterborne disease. To build our understanding it's important to know that waterborne diseases are caused by pathogenic micro-organisms which are transmitted directly when an individual consumes contaminated drinking water. Also, contaminated water can be used to prepare food, hence, could be source to food borne disease due to the ingestion of same micro-organism (Colwell, 1996). In recent times, World Health Organizational (WHO) has reported diarrheal diseases caused by waterborne pathogens contributes estimated 4.1% towards the total daily global burden of disease and accounts for 1.8 million worldwide deaths of humans every year. In addition, WHO suggests 88% of that burden is due to unsafe drinking water, sanitation and hygiene and is mostly children's in developing countries who are victims of waterborne diseases. Importantly, waterborne infectious diseases can be caused by viruses, protozoa, intestinal parasites and bacteria (WHO, 2008).
History of Cholera
As mentioned earlier, the example of waterborne infectious disease this essay will give insights to is bacterial caused Cholera. Pacini in 1854 in Florence, Italy, during a cholera epidemic was first to describe the comma-shaped gram-negative Vibrio, the "comma bacillus", as a causative agent for cholera, which was later by Robert Koch termed as Vibrio cholerae. In the same period of time whilst Pacini did his investigation on cholera, the disease engulfed Western Europe, John Snow constructed a linked between drinking water source in London and cholera, hence, discovering that cholera is a waterborne disease. Although V. cholerae is the causative agent of acute intestinal infection and severe diarrheal disease, it is known that V. Cholerae is also an inhabitant of the microbial aquatic community (Lipp et al., 2002). All Vibrio spp. can survive and replicate in contaminated waters with increased salinity and at temperatures of 10-30°C. V. Cholerae, V. parahaemolyticus and V. vulnificus have a broad temperature 18-37°C and pH range between 7.0-9.0 for growth on media (Rodighiero et al., 1998)
Vibrio, is a diverse species that is under intense research worldwide of the Vibrionaceae family. Due to diversity Vibrio has been of significant interest in relations to taxanomy and systematics of Vibrio spp. (Faruque, 2002). Amongst many bacteria, Vibrio spp. are commonly isolated in marine and estuarine waters (Lipp et al., 2002). They drastically change the nutrient cycle within these environments and often form a major part of the natural flora (Lipp et al., 2002). Importantly, several members of the genus have been found to be pathogenic either for marine animals or humans (Colwell R., 1996).
Mode of transmission for Vibrio cholerae
Vibrio cholerae could be transmitted to human possibly in three ways i.e. water, food and person to person contact. Water contamination is linked to large sudden outbreaks of cholera worldwide (Faruque, et al., 2004). Food is also known to be other important source for the transmission of V. cholerae, especially, seafood which is raw or uncooked shellfish that is harvested from sewage contaminated environments where V. cholerae origins. In addition, V. Cholerae grows healthily on moist and alkaline foods. Fruits and vegetables grown in sewage areas are at times consumed in parts of world without cooking therefore could facilitate the transmission of V. Cholerae (Cotter, 2000). Lastly, person to person transmission has not provided the means for disease to spread, however, WHO suggests it could still be a source of infection (WHO, 2008).
Upon entry within host through ingestion, Vibrio cholerae targets the epithelial lining of small intestine for its colonization (Jiunn, et al., 2008). The incubation period for this bacterium is 1-5 days and patients are symptomatic for 2 to 7 days. V. Cholerae produces enterotoxin, Cholera Toxin, which induces copious, painless, watery diarrhea and can lead to severe dehydration. In critical cases, individual can lead to death due to hypovolemic shock within 2-4 hours of colonization (Beyhan, et al., 2008). There are two type of cases associated with Cholera i.e. Mild and Severe. Approximately 90% of mild cases resembles similar characteristic to normal diarrheal diseases, whereas, severe cases 10% are related with painless, watery diarrhea, with about 20 L day-1 fluid loss (Cotter,2000). As a result of serious dehydration an individual can experience loss of skin turgor, muscle cramps, weak pulse and scaphoid abdomen (Cotter, 2000). Furthermore, cholera induce diarrheal onset allowing rapid distribution of copious quantities of Vibrio cholerae into the environment, potentially infecting other individuals (Faruque, et al., 2004).
As mentioned above V. Cholera is a gram negative bacterium, comprised of 2 chromosomes which are completely sequenced. Also, contain polar monotrichous, and is asporogenous which means it does not sporulate (Faruque,. et al., 1998).
Figure 1: Flow chart to show classification scheme of V. Cholera
Division into ribotypes
Each O1 biotype can have 3 serotypes
Division into 2 biotypes
Toxigenic V. Cholera
Division into 2 epidemic serotypes.
A, B, C antigens
A & B antigens
A & C antigens
Over 200 serotypes have been recorded to date of V. cholerae, however only O1 and O139 have been associated with major epidemic outbreaks (Faruque et al., 2004). Importantly, it is often observed that in any epidemic, one strain predominates. Moreover, the genes encoding Cholera Toxin (CT) are rarely found in serogroups besides O1 or O139 (Lipp et al., 2002). The O1 strain predominated as the primary epidemic strain until 1992(Lipp et al., 2002). From the above classification scheme of V. cholera, we see that serogroup O1 is further divided into 2 biotypes, Classical and El Tor. The classical biotype was the causative agent for the first six pandemics until it was largely replaced by El Tor biotype in 1961. Furthermore, the biotypes Classical and El Tor are further divided into three ribotypes depending on the antigens that are presents. The toxigenic V. cholera O139 serotypes replaced the O1 serotype as the main pandemic strain when it first emerged in south-eastern India and Bay of Bengal in year 1992 (Colwell,1996). Several studies have shown that O139 isolates may have resulted from the genetic exchange i.e. horizontal gene transfer, particularly transduction with non-O1 strains as well as clinical strains of O1 (FAaruque, et al., 2004).
The known virulence of V. cholera include integrons, Toxins i.e. Cholera Toxin (CT), HA Protease, RTX Toxin, ACE and Zot and also have adherence/adhesins factors (Pierro,2001). Cholera toxin is an enterotoxin, exerting its effects on cells in large part through the ADP-ribosylation of guanine nucleotide binding proteins. Cholera Toxin (CT) is an A-B type toxin consisting of two subunits, one that inactivates ribosomes and another that binds to galactose, allowing for internalization into the cell. Structurally and functionally CT is similar to enterotoxigenic E. coli (ETEC) heat liable toxic (LT) (Fasano, 2002). Within the apical membrane of small intestine, the B subunit of CT binds to GM1 ganglioside receptors. Reduction of disulfide bond in A-subunit activates A1 that ADP-ribosylates guanosine triphosphate (GTP) binding protein (Gs) by transferring ADP - ribose from nicotinamide adenine dinucleotide (NAD). ADP-ribosylated GTP binding protein activates adenyl cyclase leading to an increased cyclic AMP(cAMP) level and hyper-secretion of fluids and electrolyes. Activation of cAMP results in the secretion of chloride ions, bicarbonate and water, causing a typical diarrheal response (Jiunn et al., 2008).
Figure 2: Mechanism of Action of Cholera Toxin taken from Todd Primm (2002) Cholera toxin Mechanism of Action [WWW] http://184.108.40.206/ASM/276-Introduce.htm [Accessed: 29/10/2010]
Microbiological and molecular methods of Detection
The detection methods for V. cholerae include microbiological culture based methods using faecal or water samples or rapid tests such as dark field microscopy, rapid immunoassays and molecular methods PCR (Wang et al., 2006). For the microbiological culture based detection methods, culture from faecal or water samples should be carried out in a following manner i.e.
Start culture from fecal matter in TCBS (thiosulphate citrate bile salts support the growth of Vibrios but suppresses most other organisms) and allow it to grow for 18 hours.
Start culture of fecal matter in peptone water, a high pH enrichment broth. After incubation in peptone water for 6-12 hours, inoculate a second TCBS plate and allow it to grow for 18 hours.
V. cholerae appears as smooth yellow colonies with slightly raised centers. Appearance of these colonies gives a presumptive positive. Samples must be sent to the appropriate regional reference laboratory for conformational testing (WHO, 2010) and (Yamazaki et al., 2008).
The rapid tests including dark field microscopy which requires inoculating a wet mount of the faecal specimen and then observing the appearance of microbes that are arrested by the addition of antiserum either O1 or O139 (Merrell, et al., 2002). Rapid colorimetric immunoassay such as Cholera SMART â„¢ II (Sensitive Membrane Antigen Rapid Test) is used as an adjunct to culture methodology for presumptive detection of V. cholerae 01 in human faeces for the diagnosis of cholera in symptomatic patients when cholera is prevalent or pandemic (Nato et al., 2003). The Cholera SMART â„¢ II utilizes a monoclonal antibody - polyclonal antibody based sandwich format. The monoclonal antibody is specific for the A antigen of 01 lipopolysaccharide (LPS) of V. cholerae 014 (Wang, et al. 2006). The Cholera SMART â„¢ II test is simple, and can be performed in approximately 15 minutes. Dipsticks tests permit rapid diagnosis of cholera based on the principle of immune-chromatography. The test is performed by immersing the dipstick into a stool sample and read after 2 to 15 minutes. If two red lines appear on the dipstick, then this would indicate that patient has cholera, if only one red line appears, the test is negative (New Horizon, 2003) and (Qadri, 1995).
Furthermore, there are also ready to use PCR detection kit for cholera. The PCR based methods can isolate and detect V. cholera within 3 hours, provide high quality concentrated DNA and also have high sensitivity and specificity for V. cholerae (Yamazaki, et al., 2008). In addition, if theres a need to distinguish between the biotypes of O1 i.e. Classical and El Tor can be determined by using tests i.e. Polymyxin B sensitivity, Hemolysin is mostly produce by El Tor and Phage sensitivity reveals classical to be sensitive to phage IV whereas El Tor is not (Nair, 2002) and (Bhuiyan, 2003).
Individuals suffering with cholera can be successfully treated by oral administration of rehydration salts. In severe cases where an individual is seriously dehydrated therefore would require intravenous fluids administration. Same individuals would also need appropriate antibiotic treatment to reduce volume of rehydrating fluids needed and they also decrease the volume and duration of diarrheal and the period of Vibrio excretion. Importantly, prescribed antibiotics should be ones to which the infective strain is susceptible due to resistance among strains of V. cholerae is growing problem worldwide (Sack et al., 2004).
Furthermore, using protocol standard disk diffusion test or through broth microdilution the susceptibility of infectious strains should be established. The most often prescribed antibiotic in severe cases is tetracycline. Other prescribed antibiotics include cotrimoxazole, doxycycline, erythromycin, furazolidone and chloramphenicol. It is recommended to avoid mass administration of antibiotics as it has no effect on the transmission of cholera. It has been suggested that cholera treatment centre (CTCs) should be set up amongst affected populations as this would provide timely access to treatment, therefore, the case fatality rate (CFR) should be expected below 1% (WHO, 2008).
The main antibiotic used against cholera is Tetracycline, which functions to inhibit cell growth by inhibiting translation. Once inside the cell, tetracycline binds to 30S ribosomal subunit, hence, blocks the binding of aminoacyl-tRNA to acceptor site on mRNA-ribosome complex. This results in the inhibition of protein synthesis, thus resulting in bacteriostatic effect. The adverse effects of using tetracycline include photosensitivity and discoloration of teeth (Sack et al., 2004).
Currently, there are three oral cholera vaccines available. All three vaccines have demonstrated to be safe, immunogenic and effective in protecting individuals. In addition, according to WHO 60 countries hold the license for administrating these vaccines and most importantly they are used by travellers (WHO, 2008).
Dukoral (WC/rBS) vaccine:
One vaccine consists of killed whole-cell V. cholerae O1 with purified recombinant B-subunit of cholera toxoid (WC/rBS). This vaccine provides short term protection against O1Inaba and Ogawa, Classical & El Tor among all age groups at 4-6 months following after administration of two doses, one week apart. Dukoral is orally administrated with 150ml of safe water (WHO, 2008)
Variant WC/rBS vaccine:
Due to advance in technology it is now possible to transfer genes within humans; an alternative of the WC/rBS vaccine has been developed and tested. The recombinant B-subunit is absent in this vaccine and is administered in two doses, one week apart (Sack et al., 2004).
Orochol (CVD 103-HgR) vaccine:
This is an attenuated live oral cholera vaccine, containing genetically manipulated V. cholerae O1 strain (CVD 103-HgR). Placebo-controlled trials in number of countries have displayed safety and immunogenicity of a single dose of CVD 103-HgR. The efficacy of this oral vaccine has been investigated in adult volunteers in the United States of America, where it has been found that a single dose confers high protection (95%) against V. cholera (Calain, et al., (2004).
The above vaccine are directed against O1 strains, therefore the need to develop vaccine against O139 is underway. WHO suggest that vaccine called Shanchol provides longer-term protection against V. cholerae O1 and O139 in children less than five years of age, however, approval from WHO still remains pending. Importantly, Shanchol promises three advantages over Dukoral. First, it does not require administration with a buffer, thereby greatly simplifying its use under field conditions, including in refugee camps and other post-crisis situations. Second, it will be available to governments and international agencies at low cost. Third, a large efficacy trial taking place in India is showing that the vaccine is more effective and lasts longer in young children (1-5 years old) than Dukoral (WHO, 2008).
Furthermore, immunization with currently available cholera vaccines if used in conjunction with control measures in areas where cholera is endemic as well as in areas at risk of outbreaks could be more effective. Vaccines provide a short term effect while longer term activities like adequate supply of water and hygienic disposal of human waste could be more advantageous to human health.
Cholera undoubtedly is a waterborne infectious disease, presenting a great threat to developing countries worldwide of cholera outbreak or the danger of cholera epidemic. Therefore implementation of control measures by developing countries could halt the progression of disease if occurred.