CDC Yellow Book 2024Preparing International Travelers
Waterborne diseases are a risk for international travelers who visit countries where access to safe water, adequate sanitation, and proper hygiene is limited, and for wilderness visitors who rely on surface water in any country, including the United States. In both high-income and low- and middle-income countries, lack of potable water is one of the most immediate public health problems faced after natural disasters (e.g., earthquakes, hurricanes, tsunamis), or in refugee camps. The list of potential waterborne pathogens is extensive and includes bacteria, viruses, protozoa, and parasitic helminths.
Most of the organisms that cause travelers’ diarrhea can be waterborne. Many types of bacteria and viruses can cause intestinal (enteric) infection through drinking water. Common waterborne protozoa include Cryptosporidium, Entamoeba histolytica (the cause of amebic dysentery), and Giardia. Parasitic worms are not commonly transmitted through drinking water, but drinking water is a potential means of transmission for some. Respiratory viruses, including coronaviruses like severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), can be passed in feces, but the risk for fecal transmission, including through water, is considered low; for more details, see the US Centers for Disease Control and Prevention (CDC) website on the National Wastewater Surveillance System (NWSS).
International travelers and wilderness visitors have no reliable resources to evaluate local water system quality. Substantial progress has been made toward the goal of safe drinking water and sanitation worldwide, particularly in Asia and Latin America. Seven hundred and eighty million people (11% of the world’s population), however, still lack a safe water source; 2.5 billion people lack access to improved sanitation, and >890 million people still practice open defecation.
Where treated tap water is available, aging or inadequate water treatment infrastructure might not effectively disinfect water or maintain water quality during distribution. Some larger hotels and resorts might use additional onsite water treatment to generate potable water. Where untreated surface or well water is used, and no sanitation infrastructure exists, the risk for waterborne infection is high.
All international travelers—especially long-term travelers and expatriates—should become familiar with and use simple methods to ensure safe drinking water. Bottled water has become the convenient solution for most travelers, but in some places, bottled water might not be superior to tap water. Moreover, plastic bottles create an ecological problem because most low- and middle-income countries do not recycle them. Water disinfection methods that can be applied in the field include use of heat, clarification, filtration, chemical disinfection, and ultraviolet radiation (UVR). Several of these methods are scalable, and some can be improvised from local resources, allowing adaptation to disaster relief and refugee situations. Table 2-10 compares the advantages and disadvantages of the different methods. Additional information on water treatment and disinfection methods can be found at CDC’s Water Treatment Options when Hiking, Camping, or Traveling website.
Table 2-10 Water disinfection techniques: advantages & disadvantages
|Chemical disinfection: halogens & electrolytic solutions||
|Chemical disinfection chlorine dioxide||
|Ultraviolet readiation (UVR)||
Field Techniques for Water Treatment
Common intestinal pathogens are readily inactivated by heat. Microorganisms are killed in a shorter time at higher temperatures, but temperatures as low as 140°F (60°C) are effective when a longer contact time is used. Pasteurization uses this principle to kill foodborne enteric pathogens and spoilage-causing organisms at temperatures between 140°F (60°C) and 158°F (70°C), well below the boiling point of water (212°F [100°C]).
Boiling is not necessary to kill common intestinal pathogens, but boiling is the only easily recognizable end point that does not require a thermometer. All organisms, except bacterial spores (which are rarely waterborne enteric pathogens), are killed within seconds at boiling temperature. In addition, the time required to heat the water from 140°F (60°C) to boiling works toward heat disinfection. Any water brought to a boil should be adequately disinfected; if fuel supplies are adequate, however, CDC recommends that travelers boil water for a full minute to account for user variability in identifying boiling points and to add a margin of safety.
Although the boiling point for water decreases with increasing elevation, at common terrestrial travel elevations, the temperature needed to achieve boiling is still well above the temperature required to inactivate enteric pathogens. For example, at 16,000 ft (≈4,900 m) elevation, the boiling temperature of water is 182°F (≈83°C). In hot climates with sunshine, a water container placed in a simple reflective solar oven can reach pasteurization temperatures of 150°F (≈65°C).
Travelers with access to electricity can bring a small electric heating coil, and many hotels have electric water pots to brew tea or coffee. When possible, travelers should avoid using water from the hot water tap for drinking or food preparation, because hot tap water can contain higher levels of metals, like copper and lead, that leach from the building’s water heater and pipes.
Clarification refers to techniques that reduce the cloudiness (turbidity) of water caused by the presence of natural organic and inorganic material. Clarification can markedly improve both the appearance and taste of the water. Decreasing turbidity is an indicator that microbiological contamination will also be reduced, but not enough to ensure water potability; clarification techniques facilitate disinfection by filtration or chemical treatment.
Coagulation & Flocculation
Large particles like silt and sand will settle by gravity (sedimentation). Cloudiness due to dissolved substances or smaller particles that remain suspended in water can be improved by using chemical products that coagulate and flocculate (i.e., cause clumping). This process removes many, but not all, microorganisms unless the product also contains a disinfectant.
Alum, an aluminum salt widely used in food, cosmetic, and medical applications, is the principal agent for coagulation/flocculation. Travelers should add one-fourth teaspoon (1/4 tsp) of alum powder to 1 quart (32 oz; .95 L) of cloudy water; stir frequently for a few minutes and add more powder as necessary until clumps form. Allow the clumped material to settle into the bottom of the container, and then pour the water through a coffee filter or clean, fine cloth to remove the sediment. Since most microbes are removed but not all, travelers must use a second disinfection step. Some commercially available tablets or powder packets combine a flocculant with a chemical disinfectant. Travelers should check their product to determine whether they need additional disinfection.
Portable hand-pump or gravity-drip filters with various designs and types of filter media are commercially available to international travelers. Filter pore size is the primary determinant of a filter’s effectiveness (see Figure 2-01). Manufacturers claiming a US Environmental Protection Agency (EPA) designation of water “purifier” for their products must conduct their own testing to demonstrate their filters can remove at least 106 bacteria (99.9999%), 104 viruses (99.99%), and 103 Cryptosporidium oocysts or Giardia cysts (99.9%). The EPA does not independently test the validity of these claims.
Filter Pore Size
Most portable filters are microfilters with a pore size <1 µm, which should readily remove bacteria and protozoan parasites like Cryptosporidium and Giardia. Travelers should not expect portable microfilters to effectively remove enteric viruses (e.g., norovirus) with an average size of 0.03 µm (see Table 2-11).
For areas with high levels of human and animal activity in the watershed or in places with poor sanitation, travelers should use higher levels of filtration or other techniques to remove viruses. If using a microfilter, travelers can pretreat water with chlorine to remove viruses. Progressively finer levels of filtration, known as ultrafiltration, nanofiltration, and reverse osmosis, all can remove viruses (see Figure 2-01). Ultrafilters with pore size of 0.01 µm should be effective for removing viruses, bacteria, and parasites. Other available portable ultrafilters use hollow-fiber technology that operate by gravity, hand-pump, or drink-through methods. Nanofilters have rated pore sizes of 0.001 µm and will remove chemicals and organic molecules from water. Reverse osmosis filters have a pore size of ≤0.0001 µm (0.1 nm) and will remove monovalent salts and dissolved metals, achieving water desalination. Progressively smaller pore size filters are available; however, these filters are both more costly and require greater pressures to push water through the filter, often at a slower rate. For these reasons, small hand-pump reverse osmosis units can be a challenge for land-based travelers to use, but they are a viable survival aid for ocean voyagers; military and refugee camps use larger, powered devices.
Activated Charcoal, Clay, Sand & Gravel
Many household and field filters include granular activated charcoal (GAC), which further treats water by adsorbing organic and inorganic chemicals, including chlorine and iodine compounds, and most heavy metals, thereby improving odor, taste, and safety. GAC filters trap, but do not kill, microorganisms, and they are generally not rated for microbe removal.
In resource-limited international settings, communities and households might use filters made from ceramic clay or simple sand and gravel (slow sand or biosand). When no other means of disinfection is available in remote or austere situations, travelers and wilderness visitors can improvise an emergency gravel and sand filter using a 20-liter (≈5.5 gallon) bucket (see Figure 2-02).
Figure 2-01. Water contaminants: particle sizes & filtration methods
Table 2-11 Waterborne pathogens (average sizes) & filter pore size needed to achieve disinfection
|WATERBORNE PATHOGEN||AVERAGE SIZE (µm)||FILTER PORE SIZE NEEDED (μM)||FILTER CLASS|
|Viruses||0.03||Not specified (optimally ≤0.01)||Ultrafilter|
|Enteric bacteria (e.g., Escherichia coli)||0.5 ×2–8||≤0.2–0.4||Microfilter|
|Giardia cysts||8 × 19||≤3.0–5.0||Microfilter|
|Helminth eggs||30 × 60||Not specified||Any|
|Schistosome larvae||50 × 100||Not specified||Any|
Figure 2-02. Emergency gravel and sand filter
Gravel and sand filters are constructed by forming layers of aggregate increasing from very fine sand at the top to large gravel at the bottom near the exit port. An emergency sand filter can be made in a 20 L (≈5 gal) bucket, composed of a 10-centimeter (≈4 inch) layer of gravel beneath a 25-centimeter (≈10 inch) layer of sand; a layer of cotton cloth, sandwiched between two layers of wire mesh, separates the sand and gravel layers.
Chlorine Compounds & Iodine
Chemical disinfectants for drinking water treatment, including chlorine compounds, iodine, and chlorine dioxide, commonly are available as commercial products. Sodium hypochlorite, the active ingredient in common household bleach, has been used for over a century and is the primary disinfectant promoted by CDC and the World Health Organization (WHO). Other chlorine-containing compounds, widely available in granular or tablet formulations (e.g., calcium hypochlorite and sodium dichloroisocyanurate), are equally effective for water treatment.
An advantage of chemical water disinfection products is flexible dosing that enables their use by individual travelers, small or large groups, or communities. In emergency situations, or when other commercial chemical disinfection water treatment products are not available, household bleach can be used with flexible dosing based on water volume and clarity. Refer to CDC recommendations.
Given adequate concentrations and length of exposure (contact time), chlorine and iodine have similar activity and are effective against bacteria and viruses (see Effect of Chlorination on Inactivating Selected Pathogens. Although Giardia cysts are more resistant than other bacteria and viruses to chemical disinfection, field-level concentrations of chlorine and iodine are effective against this parasite when longer contact times are used. For this reason, dosing and concentrations of chemical disinfection products are generally targeted at Giardia cysts.
Another common protozoan parasite, Cryptosporidium, is poorly inactivated by chlorine- or iodine-based disinfection at practical concentrations, even with extended contact times. Chemical disinfection can be supplemented with filtration to remove these resistant oocysts from drinking water.
Cloudy water contains disinfectant-neutralizing substances and requires higher concentrations or contact times with chemical disinfectants. Advise travelers to clarify cloudy water using settling, coagulation/flocculation, or filtration (described above) before adding the disinfectant.
Because iodine has physiologic activity, WHO recommends limiting drinking iodine-disinfected water to a few weeks. People with unstable thyroid disease or known iodine allergy should not use iodine for chemical disinfection. In addition, pregnant people should not use iodine to disinfect water for prolonged periods of time because of potential adverse effects on the fetal thyroid. Advise pregnant travelers to use an alternative method of water disinfection (e.g., heat, chlorination, filtration).
Taste preference for iodine over chlorine is individual; neither is particularly palatable in doses recommended for field use. The taste of halogen-treated water can be improved by running the water through a filter containing GAC, by adding a pinch of powdered ascorbic acid (vitamin C), or by adding 5–10 drops of 3% hydrogen peroxide per quart (32 oz; ≈1 L) of water, then stirring or shaking, which can be repeated until the taste of chlorine or iodine is gone.
Chlorine dioxide (ClO2) kills most waterborne pathogens, including Cryptosporidium oocysts, at practical doses and contact times. Several commercial ClO2 products are available in liquid or tablet form, but relatively few data are available on testing of these products for different water conditions.
Salt (Sodium Chloride) Electrolysis
Electrolytic water purifiers generate a mixture of oxidants, including hypochlorite, by passing an electrical current through a simple brine salt solution. Commercially available small units use salt, water, and a 12-volt DC (automobile) battery to quickly create a chlorine solution that can be used treat ≤200 liters of water.
Silver & Other Products
Silver ion has bactericidal effects in low doses; attractive features include lack of color, taste, and odor, and the ability of a thin coating on a container to maintain a steady, low concentration in water. Silver ion concentration in water can be strongly affected by adsorption onto the surface of the container, and limited testing on viruses and cysts has been performed. Silver is widely used by European travelers as a drinking water disinfectant, but in the United States, silver is approved only for maintaining microbiologic quality of stored water. Silver is available alone or in combination with chlorine in tablet formulation. Several other common products, including hydrogen peroxide, citrus juice, and potassium permanganate, have antibacterial effects in water and are marketed in commercial products for travelers. However, none has sufficient data to recommend them for water disinfection at low doses in the field.
Ultraviolet radiation (UVR) kills bacteria, viruses, and Cryptosporidium oocysts in water; efficacy depends on dose and exposure time. Moreover, because suspended particles can shield microorganisms from UVR, UVR units have limited effectiveness in disinfecting water with high levels of suspended solids and turbidity. In the field, portable battery-operated units capable of delivering a metered, timed dose of UVR are an effective way to disinfect 1–2 liters of clear water at a time. Larger units with greater outputs are available for use in places where a power source is available.
Using sunlight to irradiate water (solar disinfection or SODIS) can improve the microbiologic quality of water and can be used in austere emergency situations. Because UVR is blocked by particles, travelers should clarify highly turbid water first. The optimal procedure is to use transparent bottles (e.g., clear plastic beverage bottles) laid on their side and exposed to sunlight for a minimum of 6 hours with intermittent agitation. Under cloudy weather conditions, water must be placed in the sun for 2 consecutive days. The Swiss Federal Institute of Aquatic Sciences and Technology provides more details on SODIS.
Choosing a Disinfection Technique
Table 2-12 summarizes advantages and disadvantages of field water disinfection techniques and their microbicidal efficacy. Travelers can use a UVR-generating device or liquid bleach (1–2 drops of per quart [liter] of water) to disinfect tap water. Trekkers or campers might prefer to use filters rated to remove viruses. Advise travelers to practice disinfection methods before leaving for their destination.
Table 2-12 Field water disinfection techniques: effectiveness against waterborne pathogens
|TECHNIQUE||BACTERIA||VIRUSES||PROTOZOAN CYSTS (e.g., GIARDIA, AMEBAS)||CRYPTOSPORIDIUM||HELMINTHS & SCHISTOSOMES|
1Many filters make no claims for viruses. Hollow- fiber filters with ultrafiltration pore size of 0.01 μm and reverse osmosis are effective.
2Higher concentrations and longer contact time are required to disinfect waterborne protozoan cysts than bacteria or viruses.
3Helminth eggs are not very susceptible to chlorine or iodine, but risk for waterborne transmission is very low.
The following authors contributed to the previous version of this chapter: Howard D. Backer, Vincent Hill
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