Pathogens are micro-organisms that can cause disease. Pathogens of particular concern include bacteria, viruses and protozoa. Many of the pathogens that can be spread through drinking water are also spread through other means, including direct contact with people who are sick, contact with animals, contaminated food, and swimming in contaminated pools.
The image above shows different kinds of bacteria that can occur in food and/or water.
Pathogenic bacteria can occur in surface water in large numbers, either being excreted in faeces or occurring naturally in the environment. Bacteria typically range in size between 0.5 and 2 micrometres. Disease-causing bacteria that can be transmitted by water include Vibrio cholerae, Salmonella sp, Campylobacter sp, Shigella sp, and Staphylococcus aureus.
Viruses vary widely in size and shape but are the smallest of all pathogens. Typically they range in size from 0.03 to 0.1 micrometres. There are more than 100 known types of human and animal enteric viruses that may be transmissible through water. Viral diseases that can be transmitted by water include rotavirus, enterovirus, norovirus and hepatitis A.
Protozoa are single celled eukaryotes. Members of a number of groups of protozoans are waterborne pathogens, including the amoebae Naeglaria fowleri and Entamoeba hystolitica, and Giardia and Cryptosporidium. There is a greater range in size among the protozoa.
Enteric protozoa such as Cryptosporidium parvum and Giardia lamblia are considered the most important protozoa pathogens in water quality testing. They form thick walled oocysts and cysts that survive for long periods in the environment and are resistant to disinfectants. Cryptosporidium oocysts are small, in the range of four to six micrometres, while Giardia cysts are larger, ranging in size from 10 to 15 micrometres.
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