Some algae produce toxins. These toxins can enter the food chain through shellfish or fish. Eating the toxins can cause a variety of severe conditions in humans resulting, in some cases, in paralysis and even death. The risk of illness is highest when algal blooms form in sea or fresh water.
Cyanobacteria (formerly called blue-green algae) also form extensive blooms in fresh water in the right climatic conditions and can kill any animals that drink it. The toxins produced by these microbes are also linked with gastroenteritis in humans.
Picture 5.7a A slide of brain tissue that has been infected with prions.
Mad cow disease (BSE) and the similar human vCreutzfeldt-Jakob-Disease (CJD) are fatal diseases. In each case, the brain tissue breaks down. Scientists believe that rogue proteins called prions are to blame.
What are prions?
These are not microbes in the usual sense because they are not alive, but the illness they cause can be transmitted from one animal to another. It seems that people eating meat from BSE-infected cattle can sometimes develop a new and frightening form of CJD years later. The beef in these cases was probably contaminated with affected brain and spinal cord tissue.
What can be done?
The government has introduced measures in slaughterhouses and butchers to minimise the risk to beef consumers. The diseases caused an economic crisis in the meat industry world-wide. No-one knows what the real threat from BSE is to people or how many cases of vCJD there will be in the future.
Extensive research into the subject is underway, but it is a slow and difficult process because prions cannot be cultured in the laboratory like bacteria or fungi.