There are hundreds, maybe thousands of organisms which can cause waterborne diseases. One useful way to classify these pathogens is by biological type. In this module we'll briefly talk about four different classes of pathogens. The most important ones are the protozoa, bacteria and viruses, but we'll also give an example of one helminth or parasitic worm which is transmitted through drinking water. The first thing to note about pathogens is that they are small. A human hair is about 0.1 millimeters or a hundred microns in diameter. Protozoa, the largest class of pathogens, are much smaller - usually about 10 to 50 microns in size. Protozoa are one-celled eukaryotes. That means that their cell has a nucleus, unlike prokaryotic cells, such as bacteria. Some protozoa can form an egg-like cyst to survive harsh conditions, such as exposure to extreme temperature, chemicals, or long periods without water or food. Many protozoa are parasites and cause diseases like malaria and giardia. Here you see entamoeba histolytica and cryptosporidium parvum, both of which can cause severe diarrhea. Entamoeba is the cause of amoebic dysentery or bloody diarrhea. This disease affects adults more than children and only about 10 to 20 percent of those infected with entamoeba develop symptoms. It is estimated to cost more than 100,000 deaths per year. Recent studies have shown that Cryptosporidium commonly called Crypto, is one of the leading causes of diarrheal disease among children. A large and important research project, the Global Enteric Multicenter Study or GEMS has found it to be the second most commonly isolated pathogen among children with diarrhea after rotavirus. Cryptosporidium forms cysts which are about 5 microns in diameter and are highly resistant to treatment. The first human cases of Crypto were reported in 1976 and the disease became infamous in 1993 when an outbreak occurred in the northern US city of Milwaukee. Cryptosporidium overwhelmed the treatment plant's filtration system and chlorine has no effect on Crypto. Over two weeks, 400,000 people became ill and over 100 deaths were caused; mainly in people with compromised immune systems. Cryptosporidium is a major cause of death among people with HIV/AIDS. The infectious dose for cryptosporidium and other protozoa is low. Even a single cyst can cause disease. Bacteria are organisms a few microns in size and of varying shapes, such as spheres, rods, or spirals. They are prokaryotes, which means that unlike the protozoa, they do not contain a nucleus. Bacteria are everywhere; growing in soil, in the deep ocean and even in acidic hot springs. A gram of soil or a milliliter of water can contain several million bacterial cells. Although the vast majority of bacteria are harmless or even beneficial to humans, a few can cause diseases such as cholera, trachoma, or salmonella. Here you see the E. coli and Vibrio cholera bacteria. They, like most bacteria, are about one or two microns in size. E.coli is universally found in human and animal feces, but is normally not pathogenic. In fact, E.coli is the preferred indicator bacteria used to demonstrate fecal contamination. However, some strains of E. coli do cause disease. These are called pathogenic E. coli and one group of these pathogens is the enterotoxigenic E. coli or ETEC. The GEMS Project found ETEC to be among the top five pathogens isolated from children with diarrhea and affects particularly small infants. The other bacterial pathogen most commonly found by GEMS was shigella, a distant relative of E.coli. Together, Shigella and E.coli are thought to cause up to one-third of the diarrheal disease mortality globally. Vibrio cholera causes the terrifying disease cholera. It can spread very rapidly under the right conditions and can cause severe diarrhea and death within hours if not treated. Cholera thrives in conditions where there is crowding and poor sanitation and hygiene. Outbreaks can occur after natural disasters, in refugee camps and in peri-urban slums. There are an estimated three to five million cholera cases and 100,000 to 120,000 deaths due to cholera every year. The smallest of the pathogens are the viruses, which are typically 0.1 microns - that's the same as 100 nanometers - or even smaller in diameter. Viruses don't have an independent metabolism. They can only grow or reproduce within a living host cell and cannot multiply within the environment. Many viruses are host-specific, causing disease in humans or specific animals only. Rotaviruses and Hepatitis A and E viruses are the most widespread waterborne viruses affecting humans. Here you see rotavirus at about 75 nanometers and Hepatitis E, which is quite small, at about 30 nanometers. You could line up thousands of hepatitis E viruses and still be smaller than a human hair. Rotavirus is one of the main pathogens causing diarrheal disease. The GEMS project found rotavirus to be the single most commonly isolated pathogen among children with diarrheal disease. It is particularly prevalent among the youngest children. Infected people shed large numbers of the virus in their feces; approximately 10 to the 11 viruses per gram of fecal matter. That's 100,000 of millions. The viruses are highly infective; only 10 to 100 virus particles are needed to cause infection. Rotavirus was estimated to cause over 500,000 deaths each year in 2004, with more than 85 percent of these deaths occurring in low-income countries in Africa and Asia. Hepatitis A is spread through food, water, or direct contact with infected people; Hepatitis E is mainly spread through drinking water. Both diseases are widespread in the developing world, in countries with poor sanitation and hygiene; nearly all children are infected before the age of 10. Every year there are over three million acute cases of Hepatitis E and over 50,000 deaths related to Hepatitis E. Finally, although it is nearly eradicated, polio remains an important waterborne virus. There were only 223 cases reported in 2012, down from some 350,000 cases in 1988. Only three countries remain polio-endemic - Afghanistan, Nigeria and Pakistan. But as long as a single child remains infected, children in all countries are at risk. Water, sanitation and hygiene play an important role in eliminating polio. From the smallest of pathogens to the largest. The helminths - or parasitic worms - which can invade human intestines or other tissues. Most helminth transmission is due to poor sanitation and hygiene, but one particularly nasty helminth is mainly transmitted through drinking water. This is dracunculiasis, or Guinea worm. Its larvae live in freshwater and are eaten by small aquatic insects called copepods. The larvae are tiny, but the copepods are big - typically about one millimeter - and visible to the naked eye. Once a person drinks water containing the copepods, hydrochloric acid in the stomach kills the insect and releases the larvae, which mature and grow into adult worms. These can reach a length of nearly a meter and be as wide as a spaghetti noodle. The worms migrate through the body and emerge through the skin, often near the feet. The worm has to be removed slowly, winding it up and pulling it out of the body, bit by bit, over weeks. This is extremely painful. It creates sores and blisters and people often soak their water - in water - feet in water to relieve the pain. The worm then discharges hundreds of thousands of larvae into the water, completing the transmission cycle. Major progress has been made towards eliminating guinea worm. Since 1986, a global eradication campaign has been led by the Carter Center in close collaboration with WHO, the CDC and UNICEF. The number of reported cases has dropped from three-and-a-half million in 1986 to just 542 in 2012. As of 2012, it is found only in a few hundred villages in remote rural areas, mostly in South Sudan, with a handful of cases in Ethiopia, Mali and Chad. Once Guinea worm is eradicated, it will be the only - only the second human diseaseever eradicated after smallpox; unless, of course, polio is eradicated first, which seems unlikely. And unlike smallpox, guinea worm will be beaten without vaccines or drugs through community scale interventions, which involve protection and treatment of drinking water. Because the copepod hosts are large, they can be readily removed from water using simple cloth or sand filters. So in summary, we've covered four main types of pathogens today. We've talked a little bit about helminths, with the example of dracunculiasis or Guinea worm disease, which is transmitted through these very large copepods - around a millimeter in diameter. The next biggest are the protozoa, such as entamoeba, cryptosporidium and gerardia, which are 10 to 50 microns in diameter and can form very resistant cysts. Then we've got the bacteria, such as E. coli, shekela, viviocholera, which are one or two or three microns in size and can cause serious diarrhea. And then finally, the smallest class is the viruses - rotavirus, hepatitis, poliovirus - which are even smaller than a micron, as small as 30 nanometers in size.
