Travellers’ diarrhoea (TD) is contracted by consuming contaminated food or water. The risk depends on a variety of factors as outlined in the previous article in this series (PJ 1999;263:571). Contrary to popular belief, food poses a much greater hazard to the traveller than water. The reason for this is that contamination of water by an organism is prone to a dilution effect, so that insufficient organisms are swallowed by an individual at any one time to cause an infection. However, some organisms, such as Shigella or Cryptosporidium, need only be present in small numbers to cause problems, but such infections are comparatively rare in travellers. Occasionally, if a sewage system breaks down, there is a risk of cholera epidemics.
In the case of food contamination, there is an opportunity for organisms to multiply quite rapidly, resulting in large enough numbers being present on a single piece of food to cause an infection if ingested.
In the area of food hygiene, pharmacists will have a general role in health promotion, supporting any information the traveller has received from other sources, such as leaflets, GP or practice nurse. For water purification, the pharmacist can play a leading role in assuring the correct use of any chemicals for purification sold in the pharmacy, as well as offering advice on general principles.
- Fresh foods: those that are peeled or cooked are the safest
- Avoid high risk foods like shellfish
- Boiling water is the best method of sterilisation
- Chemicals are useful for preparing safe drinking water but have their limitations
- Chlorine-based tablets are widely available and will be appropriate for most travellers
- More intrepid travellers and those planning to use surface waters as a drinking water source should use an iodine-based product
While food hygiene practices are heavily promoted to the travelling public, there is no particular need to miss out on experiencing the local cuisine. It is not necessarily the type of food that poses a danger, but how well it has been prepared and stored. This section of the article considers practical aspects of helping the traveller choose the safest types of foods.
It would logically be expected that travellers who follow the standard advice regarding food hygiene would experience fewer incidences of diarrhoea. However, apart from avoiding very high risk foods, eg, steak tartar or raw oysters,1
following hygiene advice has not been demonstrated to reduce the risk of TD in most formal studies.2,3
One study of expatriates and tourists in Nepal4
could find no association between eating high-risk foods and diarrhoea. The best way of avoiding problems appeared to be preparing food at home, rather than eating in restaurants. However, the study did identify some association with foods that had been prepared earlier in the day and left to stand at room temperature.
gives a graphic description of a couple on a “round the world” trip who went to the most extraordinary lengths to avoid risky food and yet both developed severe dysenteric disease: “One developed life-threatening giardiasis, two episodes of diarrhoea and loose stools that persisted for 30 months on returning to Britain.”
It may be tempting to conclude that strictly following dietary advice is unnecessary. However, following the recommendations on food hygiene is supported by a number of compelling arguments:
- It is likely that failure to avoid TD is more due to an inability of travellers to follow food hygiene advice, rather than the advice being ineffective. Only a few per cent of travellers in one study were willing to comply with advice given.6
- The trials examining this issue to date have been rather small and therefore not of sufficient power to link incidence to a particular dietary habit.
- Those who do not follow any of the advice given may still be at greater risk of contracting parasites or Shigella, which give a more serious diarrhoea and to which resistance is not developed. This is supported by studies which indicate that such travellers tend to have multiple episodes, and a more severe TD.7
General principles of food hygiene
There is a universally accepted mnemonic to help travellers avoid the riskiest situations: “peel it, boil it or forget it”.
This underlies the principle that cooked, piping hot food is considered to be the safest. A few other general points should be understood:
- When possible it is better to prepare food personally, rather than eating in restaurants and hotels. The quality of the restaurant seems to make little difference to the chance of contracting TD.8
Eating products from street vendors is best avoided unless served hot, eg, straight from a wok.
- Dry foods are safer as organisms require a moist environment in which to grow. The key message is that bacterial contamination of moist foods, left out in a warm climate, will very rapidly produce a large number of organisms, likely to cause a gastrointestinal infection if consumed.
- Infection can potentially be picked up from contaminated plates or cutlery. Some travellers will carry their own knives, forks and drinking mugs. As an extreme measure, some also use alcohol wipes for swabbing down suspect utensils.
- It is also important to maintain good personal hygiene. If backpacking or living in rough conditions, it is easy to let standards drop. Travellers may need reminding of the importance of clean hands and fingernails when eating or preparing food.
- Even if pre-travel advice does not reduce the incidence of TD, it may help individuals in their understanding of the condition. This was illustrated by a recent study, where those offered such advice tended to be less likely to seek medical help regarding TD.9
- Local foods are often safer than attempts by restaurants to produced more westernised foods in a style of cooking with which they are unfamiliar.
Particular types of food
|Panel 1: Choice of foods for the traveller
Freshly prepared and hot food
Food from street vendors unless fresh and hot
Poorly stored food
Unpeeled fruit and salad unless well washed in clean water
Broad leafed salad
Dried food and freshly baked bread
Rare meat and fish
Unpasteurised dairy products
Panel 1 indicates the types of foods and their relative safety for the traveller. These are now discussed individually.
These come quite high on the list of high-risk foods.10
The worst is considered to be broad-leafed vegetables such as lettuce. This is because of the large surface area where there is a potential to harbour many organisms, so it must be adequately cleaned. Added to this is the practice of using human excrement, known as “night soil”, as a fertiliser in some developing countries. Other salad items such as tomatoes or cucumber might present a lesser risk if well prepared.
If undertaken personally, preparation of salad or raw vegetables should always include scrubbing with clean water until all visible signs of dirt have been removed. Some travellers like to soak vegetables overnight in a chemical disinfectant. There is little evidence that this offers significant advantage over scrupulous cleaning, but it does no harm. Traditionally, solutions of potassium permanganate were used, but this tends to spoil the quality of the food. Iodine or chlorine used for water purification (as discussed later) can also be used for soaking and manufacturers of these products give recommendations for the appropriate concentrations.
Fruit and vegetables
The golden rule is that if fruit and vegetables are not cooked, opt for those that must first be peeled. Therefore, in terms of fruit, bananas and oranges would be ideal, but grapes would be far more risky. Also, any cooked vegetables that have been allowed to stand for some hours could represent a hazard.
Buffets and sauces
These are quite common sources of TD and food poisoning. In the case of open buffets, flies settle on food and can deposit organisms, which in a warm environment multiply very rapidly. Cold sauces that have been left to stand are also a potential breeding ground for micro-organisms.
Breads, rice and pasta
Bread, being a dry food, is relatively safe. Rice and pasta once cooked should be consumed immediately. Rice in particular may harbour a bacterium called Bacillus cerus that produces a toxin, causing a severe diarrhoea.
Fish and shellfish
On the list of foods best avoided, fish and shellfish come quite high. Shellfish in particular can present hazards due to their feeding method of filtering large quantities of water for plankton. This results in a high chance of contamination if such fish are exposed to sewage outlets in the shallow waters where they grow. Fish generally requires careful storage. Poisoning by specific toxins can be a problem, for instance, ciguatera poisoning is caused by ciguatoxin-producing (neuro and cardio toxin) dinoflagellate plankton, which enter the fish food chain.
Meat and poultry
These are not intrinsically any more of a hazard than vegetable dishes, provided that they are well prepared and not reheated or incorrectly stored. When eating out, these conditions may not always be known so they may represent a potential source of infection. Where possible, freshly and thoroughly cooked meat products should be chosen. It may also be wise to avoid more elaborate dishes that have required a lot of handling during preparation.
The major problem with dairy products is the potential that they have been made from unpasteurised milk, presenting a risk of brucellosis. Provided milk has been boiled, it can be considered safe. Therefore, Indian chai, which is a widely available brew of tea and milk boiled together in a large vat, presents little danger. Goat’s cheese is a particularly notorious cause of problems.
In many respects it is easier to arrange a source of clean drinking water than strictly following the recommendations given concerning food hygiene. In addition, many people in westernised countries now regularly use bottled water as a main source of fluid intake, a practice that can usually be followed when travelling.
The use of bottled water does still carry a risk because in some developing countries there is a trade in “counterfeit” bottled water that has simply been filled from a tap. Even sealed bottles can be no guarantee of safety. Partly for this reason, it is often recommended to choose carbonated water if available, as this is less likely to be counterfeit. Also, it is claimed that the relative acidity of carbonated water makes it a less hospitable environment for bacteria.11
It is generally recommended that ice in drinks be avoided. The biggest danger is ice that has been chipped from large blocks; in some parts of the world these are delivered to restaurants, sometimes being left out in a street and stored in unhygienic conditions. A myth that should be dispelled is that alcohol will sterilise fruit juice or water to which it has been added – the quantity required would not be achieved in an alcoholic beverage.12
There will be circumstances where the traveller needs to sterilise a supply of water and this will be discussed in some detail. Essentially, parasites tend to be harder to kill than bacteria. Giardia cysts are often considered to be the hardest of all to remove. Giardia is most likely to be encountered by trekkers using surface waters, eg, streams and lakes, as a source of drinking water.
There are three methods by which the traveller can prepare clean drinking water: boiling, chemical disinfection and filtration.
Boiling should always be recommended as the method of choice for sterilising water. At higher altitudes, water must be boiled for longer than at sea level because of the lower boiling point. To allow for this variable, it is a good idea to boil water for a full five minutes at any altitude. Boiled and cooled water tends to have a flat taste due to loss of oxygen content. Cooling with the pan covered and drinking the water cold can improve the taste.11
Boiling will kill all organisms and is the most reliable method against Cryptosporidium. The main drawback for the traveller is arranging the facilities to boil sufficient quantities. Heating elements are available that can be used to boil water in a suitable cup or mug.
Water purification tablets are often supplied through pharmacies and it is important that travellers know how to use them properly. The halogens, chlorine and iodine, are the most commonly used; the only other widely available product is katedyne silver. Their effectiveness will depend on four variables:
- Concentration of chemical used
- Contact time
- Water temperature
- Water quality (eg, pH, presence of organic matter)
If the water is clear of particulate matter and at an ambient temperature, it can be left for a shorter time before being safe to drink. However, if the water is heavily contaminated then a higher concentration of halogen should be used. Recommendations on manufacturers’ labels should be followed carefully. The concentration of halogen is usually calculated to allow sterilisation in less than half an hour.
In the UK, chlorination has long been used for the preparation of clean water by trekkers and travellers, whereas in the US iodine appears to be more popular. There is a long running debate concerning the choice between chlorine- and iodine-based products. Some of these issues will now be addressed.
Spectrum of activity
Both chlorine and iodine have a wide spectrum of activity and are effective against bacteria, viruses and parasites. For iodine, a concentration of around 8mg/L is quoted13
as desirable for removal of organisms within less than half an hour. This concentration is required for inactivation of Giardia cysts, but for bacterial or viral contamination a concentration of 0.5 mg/L would be sufficient. For chlorine, 8mg/L is also effective,14
with the most popular branded product (Puritabs) producing 10mg/L.
The debate over activity concerns the effectiveness of the two halogens against Giardia cysts, which are particularly likely to present a problem to trekkers using surface waters in the wild. Chlorine can inactivate Giardia cysts,15
but a study comparing different chlorine- and iodine-based products found that chlorine removed a lower percentage of cysts than the iodine products over any of the chosen contact times.16
Interestingly, even the iodine-based products were shown not to remove more than 90 per cent of the cysts unless left for 24 hours.
Stability at high pH
In laboratory studies, chlorine, but not iodine, tends to loose activity at high pH due to formation of less active hypochlorite ions,17
requiring an increase in concentration or contact time above pH 7. The relative importance of this effect when higher concentrations of chlorine are used in the field has not been well studied.
Both halogens are less active at lower temperatures13,14,15
and an increase in contact time and/or concentration is required, particularly to remove Giardia cysts.
Presence of organic matter
Activity of both halogens can be reduced by the presence of organic matter. If the water appears cloudy, it should always be pre-filtered. This can be achieved by passing it through a piece of cloth or muslin. Alternatively, a special canvas bag known as a Millbank bag (see photograph) can be used to produce large quantities of clear water. These bags are particularly popular with trekkers. If pre-filtering is not possible then a greater concentration of halogen could be used.
Side effect profile and contraindications
There are few problems associated with chlorination, an obvious advantage over iodine for which certain precautions must be observed:
- An excessive iodine intake could lead to effects on the thyroid. Goitre formation has been reported where iodinated water has been used exclusively for some months. The most recent of such reports occurred among peace corps workers whose purifier was yielding water with a concentration of iodine at 10mg/L.18
The goitre does resolve quite rapidly once iodine consumption is reduced.19
Furthermore, there are no reports of a clinical hyperthyroidism through this practice. It is unlikely that most travellers would use iodinated water exclusively for any length of time, but most products do carry a warning stating that they should not be used continuously for more than a few weeks.
- Because of the theoretical effects on the thyroid, iodination is best avoided in pregnancy and in young children.
- Those people with an allergy to iodine should obviously avoid this method of purification.
Both halogens impart a taste to the water that some find unacceptable. With the higher concentrations of iodine recommended for Giardia, the resultant water is particularly unpalatable. Ascorbic acid is the most convenient neutraliser, removing both the taste and brown coloration of iodinated water. Dispersible ascorbic acid tablets are available for this purpose, but a pinch of ascorbic acid powder would work equally well. Travellers should be aware that if a neutraliser is added to the water, then the iodine will be inactivated. Even a small amount of ascorbic acid left in a flask used to sterilise water will reduce the activity of the iodine. Therefore travellers should be advised only to add such neutralisers to the final drinking container, eg, a cup, just before consumption.
The taste of chlorine and iodine can be removed by sodium thiosulphate, although no commercial tablets are available for this purpose. Hydrogen peroxide could be used, but most travellers do not wish to carry bottles of hydrogen peroxide. The taste of chlorine is worse at extremes of pH.
Storage Once prepared, the presence of chlorine in water may be better at discouraging growth of contaminating organisms than iodine. In either case, it is usually best to consume treated water within 24 hours of preparation.
Conclusion – iodine v chlorine
(Panel 2) For most travellers a chlorine-based product would be the most suitable and easily obtainable. Some trekkers and more adventurous travellers may wish to use iodine because of the theoretical advantages in certain situations, as outlined. If the water is cold (below about 10C) it would be wise to let it stand for double the time recommended by manufacturers. Where the treatment time is not made explicit and Giardia is suspected, water should ideally be left overnight.
|Panel 2: Comparison of iodine and chlorine as water purification for travellers
Spectrum of activity
Bacteria, viruses and parasites (except Cryptosporidium)
Possibly more effective than chlorine against Giardia cysts
As for iodine
Stable at high pH
Not as active at high pH
Less active at low temperature
Less active at low temperature
Presence of organicmatter
Less active but more tolerant than chlorine
Some contraindications; not for continuous use
No special contraindications; can be used continuously
Chlorine based preparations
Sodium dichloroisocyanurate (Puritabs)
These are one of the market leaders and are sold through most pharmacies. They are available in different strength tablets to treat either one or 25 litres of water. If used according to the directions on the packaging, they should be effective against Giardia, although to date no studies directly investigating their use against this organism have been performed. If the water is heavily contaminated, two tablets should be used. For soaking vegetables, it is recommended to add a total of three tablets to the required quantity of water. A contact time of just 10 minutes is advised. To treat double the quantity of water with the same number of tablets, the time can be increased to 30 minutes.
This can be used provided it is free of additives or disinfectants. Ordinary laundry bleach usually contains 4 to 6 per cent of available chlorine and one or two drops could be added to a litre of water. This method is probably best reserved for emergency situations where no other products are available. It is also probably not desirable to travel with bottles of bleach as continuous shaking could result in loss of chlorine activity.
Iodine based preparations
Tincture of iodine (alcoholic iodine solution BP)
This is the most readily available and cost effective iodine-based preparation for water purification, containing 2 per cent available iodine. Iodide ions from potassium iodide in the solution are also present; these have no antibacterial activity but double the total iodine concentration.
Directions for using tincture of iodine are somewhat of an anomaly. Martindale20
recommends using five drops per litre, increasing to 12 drops if Giardia is suspected. However, the volume per drop would tend to vary to some extent, depending on the type of pipette used. I estimate that one drop of tincture from a standard glass dropper bottle is just 0.02ml. This would mean that five drops would result in 2mg/L, more than adequate for removing bacteria and viruses. However, the recommended 12 drops would yield just 5mg/L, which falls at the lower range of the concentration required for inactivation of Giardia cysts. A full 30 minutes or longer (Martindale advises one hour) at ambient temperatures should be allowed when using tincture of iodine at this dosage if Giardia is suspected.
The obvious disadvantage with iodine tincture would be the mess involved with any breakage as it is stored in a glass bottle. The tincture should not be dispensed in plastic bottles because of leaching of iodine. The BP formulation of iodine tincture, which is formulated with ethanol, should always be used. There have been anecdotal reports of other iodine tinctures containing industrial methylated spirits. Povidone iodine or other forms of aqueous iodine should not be used for water purification. There are a number of commercial pre-packed iodine tinctures in dropper bottles labelled with full instructions and warnings for use.
Tetraglycine periodide tablets
These are quite widely available in the UK (eg, Potable Aqua). Each tablet yields 4mg of available iodine. They are more convenient than the tincture and will give a more reliable dose. However, they do have some disadvantages:
- The tablets can take a long time to dissolve, particularly in cold water
- They rapidly lose potency once the bottle has been opened
- They are more expensive than the tincture
Potable Aqua is also sold in packs containing both tetraglycine periodide tablets and ascorbic acid tablets for neutralisation of iodine as mentioned earlier.
Most cost effective of all, and occasionally employed by the more intrepid trekkers, is the system known as the Kahn-Visscher method.17
About 5g of crystals of iodine are stored in a clear glass 30ml jar with a paper lined Bakelite cap. This is filled with water to form a saturated solution; the resulting concentration of iodine will vary to some extent with water temperature. The supernatant liquid is then carefully poured off taking care not to draw off any crystals.
This solution is then used to disinfect water by adding 15ml to each litre of water. The crystals can therefore generate very large quantities of water, between 250 and 500L, at little cost.
The procedure is somewhat fiddly, although the system is available in a special cup sold for the purpose (Polar Pur), which gives clear directions on the volume to be added at different temperatures. There is also the risk of inadvertently consuming iodine crystals.
Katedyne silver (Micropure)
The main advantage of katedyne silver tablets is that they impart no taste to water. In addition, water can be stored, once treated, for many months.
There are no particular contraindications to their use. The main drawback is that these tablets are only effective against bacteria and should not be used to treat water where parasites may be a problem. A further disadvantage is that, following addition of the tablets to water, it must be left for two hours before drinking. Little published data are available on the effectiveness of katedyne silver.
Pumps and devices
There are a plethora of water purification devices that are marketed as suitable for use by travellers. General principles only will be described in this article. There are two modes by which these devices are able to purify water: by simple filtration or chemically via iodine bound to a resin. Many systems use a combination of both methods.
If a simple filtration method is used, usually with a ceramic filter, the smallest pore size available is around 0.3Î¼m, which would be too large for removal of viruses (eg, hepatitis A is 0.03Î¼m). Therefore, most practical systems that can remove all organisms employ a two-stage device: a filter to remove larger organisms and an iodine-bound resin for viruses. Some also have a third carbon filter to remove chemical contamination and any excess iodine.
The filtering systems that are designed to remove smaller organisms do require a great deal of pumping. Such filters may rapidly become clogged unless water which is free of debris is used. Some devices allow cleaning of the filter system and replacement cartridges to be used. A potential problem is that users may be unaware that the chemical is exhausted, so some purifiers are designed to pass no further water when this stage is reached.
It should be noted that iodine resin could release a high concentration of free iodine, so systems which incorporate a carbon filter are useful.
Pharmacists supplying such devices should also note the following points:
- Although it is often stated how many litres a particular system can purify, this may be much reduced if the water is very dirty.
- Be aware of the rate at which water can be produced by the purification system. Some take a great deal of pumping for little reward.
- Although manufacturers may state which organisms are removed, they may not mention the organisms against which the device is not effective.
- For water contaminated by industrial effluent, a device with a carbon filter is required.
- Water produced by purifiers should be used within 24 hours.
Dr Goodyer is director of pharmacy practice research at King’s College London, and superindendent of Nomad Travel Pharmacy
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