It came in the water and caused disease

The outbreak

Cryptosporidium parvum stained with a fluorescent antibody. Image by the Environmental Protection Agency

In the spring of 1993, Milwaukee, a city in the US state Wisconsin, was facing a mysterious outbreak of severe diarrhoea. In the course of the events at least 400,000 people became ill and 4000 were hospitalized. The situation was so bad that some hospital laboratories even ran out of culture medium for their tests. However none of the undertaken tests was positive for bacteria nor viruses. Only by chance a laboratory technician noticed “voids” in one of her stool samples and could stain them with fluorescent antibodies against

Crptosporidium. What she saw were the nearly invisible oocysts, of the parasite Cryptosporidium. These characteristic oocysts, which are the equivalent of eggs, are also the origin of its name which translates into “hidden seed”.

This protozoan parasite is a member of the phylum, apicomplexa which also includes the malaria parasite. It was considered unimportant after its discovery in 1907 because it did not seem to cause any problems in humans. When it was found to cause disease in immunocompromised patients especially HIV patients, this view changed. Currently there are ~20 different species known to science, of those C. parvumcauses the most human infections.

Cryptosporidium oocysts in the faces of a immunocompromised patient. The black bar indicates 5µm to compare the size. This image comes from the CDC.

Remarkably this parasite can complete all its stages within the human host and releases its oocysts into the faeces. Those oocysts are extremely resistant to most disinfectants including chlorine used in drinking water preparation.

The cause of the outbreak

After the organism was confirmed the data recorded by the two drinking water plants were examined. It showed that the turbidity, describing the amount of particles in the water, was drastically increased. However it never breached the upper limits given by the authorities. A telephone survey showed that people receiving water from one of the two treatment plants were twice as likely to become ill. The reason for the high turbidity might have been a defective streaming-plant monitor, a device for adjusting coagulant added to the water which should remove any particles making the water cloudy.

How the oocysts could get into lake Michigan, from where the water is drawn cannot be proven. One reason might have been that the very rainy spring, combined with melting snow caused the storm sewers to overflow. So the rivers feeding the lake probably have been contaminated with oocysts from cattle manure, or human sewage. Further the plume at the river mouth tends to flow southwards from where the affected plant draws its water. A DNA analysis suggested that human faeces were the most likely source of the oocysts.

Lessons Milwaukee taught us

Considering that this was not the first such outbreak the burning questions are; did we learn anything from it and did we adopt it?

It showed that a neglected pathogen such as Cryptosporidium can cause disease in healthy individuals on a  very large scale.  Further it illustarted drastically that turbidity is not merely an aesthetic factor but can indicate the presence of oocysts. As a consequence the limits were reduced and are now monitored continuously with automated alarms. Through a new filter system and the revised coagulation system the Milwaukee treatment plant is now able to keep its mean turbidity at 0.01 nephelometric turbidity units (NTU) which is considerably below the levels during the outbreak.

Studies showed that Cryptosporidium oocysts can be effectively inactivated through ozone or UV radiation because these destroy the cell wall and make chlorine effective. This technique is now more commonly used. Other measures outside the treatment plant was the extension of the water suction pipe further into the lake and the construction of an extensive tunnel system below the city. These tunnels are able to store more excess water in case of heavy rainfalls, to prevent sewage getting into the river.

All the above approaches are centralized and aim to prevent a contamination in the treatment process. However maybe a more secure way is the additional installation of so called point of use filters in each household. These protected their users during the outbreak if they had pores of 1µm or less in diameter.

This outbreak, as many others showed how vulnerable our drinking water supply is to contamination and how essential it is to learn from cases such as the Milwaukee outbreak how to protect it.


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