A review of: Colwell et al (2003). Reduction of cholera in Bangladeshi villages by simple fitration. PNAS, 100(3): 1051-1055.
If we were to cite WHO statistics alone, it would seem that the worldwide state of cholera cases was improving. However, several countries have become reluctant in sharing their data for fear of the economic and social consequences such as trade sanctions and loss of tourism. As a result of this, many countries which are known to suffer greatly from the disease are not included at all, with reported cases estimated to represent only around 5-10% of actual cases. One such country in which cholera is endemic is Bangladesh, which this study focuses on.
This paper is an eye opening but relatively simple account of a successful method used to reduce outbreaks of cholera. The method works on the past proven association between Vibrio cholerae and copepods. The copepods aid the pathogenic vibrio in survival, multiplication and transmission. A common cycle is seen in Bangladesh; phytoplankton blooms during spring and summer, which are followed by zooplankton blooms and in turn lead to cholera outbreaks. Bangladeshi villagers often depend on untreated water sourced from local rivers and ponds, especially since the alternative water from drilled wells is reported to be polluted with high levels of arsenic.
The most common method used to improve untreated water quality is filtration through cloth as it is possible in any conditions, and is not at all costly. This study used laboratory experiments to prove just how efficient this method can be. If a sari cloth is folded just four times it can provide a filter of ≈20-µm mesh size. Plankton and attached V. cholerae cells are retained and >99% of V. cholerae is removed from the water. This study continues to test this method throughout 65 villages in Matlab, Bangladesh, over three years, suggesting that if this filtration method is utilised, the occurrence of cholera should significantly decrease. Villages with a high rate of cholera were selected and were assigned differing treatments; nylon, sari, or no filter. Important findings of the study are as follows;
- High acceptance and compliance by villagers. Use of the sari cloth could have avoided cultural barriers. Only 0.6% of the population was noncompliant.
- Both nylon filters (P<0.02) and sari filters (P<0.005) provided significantly lower cholera rates than the control. Both filters removing copepods>150µm.
- Sari filtration was observed to be most successful in lowering the rate of cholera, but there was no significant difference between the results using nylon or a sari (P>0.3).
- The sari group had a cholera rate ≈52% of the control. This equals a reduction in cholera outbreaks of 48%.
This study is successful in proving that sari filtration can remove all zooplankton, most phytoplankton, and all particulates >20µm. Nylon filtration was also shown to be almost equally effective. Since V. cholerae cells attach to this plankton and cholera is dose dependent, this filtration method can successfully reduce numbers and also severity of cholera cases. Sari cloths are incredibly effective for this community since they are cheap and readily available. However, other similar materials are likely to be as effective in other parts of the world where cholera is endemic. Not only does this method reduce cholera cases, but it is also suggested in this study to reduce cases of other waterborne diarrheal diseases. This is to be further investigated following this study.
This paper is very well written, easy to follow, and investigates an important topic which everyone is aware of to some degree. The results present a very simple solution to cholera outbreaks worldwide where water treatment facilities aren’t available. They also spark investigation into the array of other waterborne infections that this method could help to reduce.
If we were to cite WHO statistics alone, it would seem that the worldwide state of cholera cases was improving. However, several countries have become reluctant in sharing their data for fear of the economic and social consequences such as trade sanctions and loss of tourism. As a result of this, many countries which are known to suffer greatly from the disease are not included at all, with reported cases estimated to represent only around 5-10% of actual cases. One such country in which cholera is endemic is Bangladesh, which this study focuses on.
This paper is an eye opening but relatively simple account of a successful method used to reduce outbreaks of cholera. The method works on the past proven association between Vibrio cholerae and copepods. The copepods aid the pathogenic vibrio in survival, multiplication and transmission. A common cycle is seen in Bangladesh; phytoplankton blooms during spring and summer, which are followed by zooplankton blooms and in turn lead to cholera outbreaks. Bangladeshi villagers often depend on untreated water sourced from local rivers and ponds, especially since the alternative water from drilled wells is reported to be polluted with high levels of arsenic.
The most common method used to improve untreated water quality is filtration through cloth as it is possible in any conditions, and is not at all costly. This study used laboratory experiments to prove just how efficient this method can be. If a sari cloth is folded just four times it can provide a filter of ≈20-µm mesh size. Plankton and attached V. cholerae cells are retained and >99% of V. cholerae is removed from the water. This study continues to test this method throughout 65 villages in Matlab, Bangladesh, over three years, suggesting that if this filtration method is utilised, the occurrence of cholera should significantly decrease. Villages with a high rate of cholera were selected and were assigned differing treatments; nylon, sari, or no filter. Important findings of the study are as follows;
- High acceptance and compliance by villagers. Use of the sari cloth could have avoided cultural barriers. Only 0.6% of the population was noncompliant.
- Both nylon filters (P<0.02) and sari filters (P<0.005) provided significantly lower cholera rates than the control. Both filters removing copepods>150µm.
- Sari filtration was observed to be most successful in lowering the rate of cholera, but there was no significant difference between the results using nylon or a sari (P>0.3).
- The sari group had a cholera rate ≈52% of the control. This equals a reduction in cholera outbreaks of 48%.
This study is successful in proving that sari filtration can remove all zooplankton, most phytoplankton, and all particulates >20µm. Nylon filtration was also shown to be almost equally effective. Since V. cholerae cells attach to this plankton and cholera is dose dependent, this filtration method can successfully reduce numbers and also severity of cholera cases. Sari cloths are incredibly effective for this community since they are cheap and readily available. However, other similar materials are likely to be as effective in other parts of the world where cholera is endemic. Not only does this method reduce cholera cases, but it is also suggested in this study to reduce cases of other waterborne diarrheal diseases. This is to be further investigated following this study.
This paper is very well written, easy to follow, and investigates an important topic which everyone is aware of to some degree. The results present a very simple solution to cholera outbreaks worldwide where water treatment facilities aren’t available. They also spark investigation into the array of other waterborne infections that this method could help to reduce.
5 comments:
A really interesting review, and a shockingly simple solution to such an important problem!
I remember Colin mentioning this method in lectures but didn't realise how effective it is. Also I wasn't aware (although maybe I should have been?!) that the vibrio attach to plankton. Do you know why they do this?
I agree, the success of this solution really isn't highlighted enough. All countries that cholera is endemic to should be educated about the method's success.
And i have an answer to your question :) ...
Vibrio species are well known to associate with plankton, but it wasn't much highlighted in our lectures and it wasn't explained why they associate. This study reports that 99% of V. Cholerae can be removed by filtering plankton from the water. This shows just how preferential the association can be over any other surface.
As vibrio species are heterotrophic they need to find a good supply of organic nutrients in order to grow and multiply. Plankton offers a really nutrient rich microhabitat, so where better for Vibrio cholerae to locate? This enables them to reach much greater densities than could be found in the surrounding water column. The plankton also acts as a pretty good mode of dispersal, transporting the cells through the water body.
Vibro species including Vibrio cholerae are also able to produce chitinase, which is why they most commonly associate with copepods within the zooplankton. The chitinase breaks down the glycosidic bonds in the chitin exoskeletons of the copepods, providing a source of carbon and nitrogen.
So... associating with plankton, especially copepods, gives Vibrio cholerae lots of nutrients to grow and multiply, and also acts as a vehicle for transmission.
Hope that makes good sense! :)
Nice post, sometimes a pragmatic simple answer is effective, probably the case in most instances, anyway, what if any pathogenic exposure is still an risk below <20 micrometers, I imagine many viruses would still be an issue?
On that note are phages still considered to trigger toxin production in V.cholerae? If so perhaps the application of microarrays may elucidate the up regulation of toxins, perhaps I have misunderstood this, care to comment Colin?
Well this method was only targeted at reducing the transmission of cholera. It is definitely possible that this method could be used for several other waterborne viruses. But of course it isn't the answer to preventing transmission of all viruses.
As for the phages... Colins last lecture sheds light on this concerning the CTX phage's involvement in assembly and secretion of the cholera toxin. But i haven't quite wrapped my head around this yet.. more reading to be done! :)
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