Parasites are organisms that inhabit and get food from a host and often cause diseases. Once a parasite gets into a host, it can go unnoticed, or it can cause severe infections. Usually when a microorganism is in a host, it can be found, isolated, and associated with a particular disease to then be treated. That is not always the case for parasites, which go against the criteria for identifying the relation between disease and microorganism.
Koch and His Postulates
1) The organism/pathogen must be present with every case of the disease and absent in healthy individuals
2) The pathogen must be grown in a pure culture when isolated from the diseased host
3) Then, that isolated pathogen should cause the same disease when re inoculated into a healthy test subject/host.
4) The pathogen must then be re isolated from the new host and cause the same disease in a pure culture as the initial one.
Parasites Disregard the Postulates
When the postulates were first introduced, they helped clarify questions in the scientific community about diseases related to pathogens. . The postulates could not be used for studying diseases caused by things such as parasites. Parasites are not the only ones that do not follow the postulates though; there were limitations to Koch’s findings when diseases such as Cholera and some viruses were studied. For example, Vibrio cholerae, the bacterium that causes cholera, can also be found in healthy hosts, invalidating the first postulate.
An example for a parasite that does not follow the postulates is the plasmodium that causes malaria.
The problems found in the postulates for studying parasites were:
1) Many parasites are asymptomatic and so we cannot tell when the parasite is present or not present. Also, the parasite could be hidden or dormant for its own benefit. A female mosquito carries and transfers the plasmodium but it could be present in a healthy host in low levels and therefore go unnoticed, voiding the first postulate. [4,2]
2) The second postulate states that the microorganism must be grown in a pure culture and parasites are not easily grown in pure cultures. Parasites need adequate conditions to grow in and so the second postulate was not useful in finding a parasite causing a disease. The plasmodium of malaria cannot be grown in a pure culture outside of a host, defying the second postulate. [4,2]
3) The third postulate states that the pathogen must infect another animal host and produce the same outcome. This cannot be tested because there may not be adequate animal hosts that would cause the same outcome as in humans. For malaria, humans cannot be tested on due to ethical issues. It would not be ethical to use humans as test subjects and perform experiments on. Some parasites only cause the disease when in humans and not in other animals. Also, some animals may not carry the parasite in the same way as humans. For another example, Kuchenmeister, a scientist, studied parasites that were seen as bladder worms in pigs but tapeworms causing Taeniasis in humans. This showed that there are different stages of a parasite’s life cycle expressed in different hosts making it difficult to say that the same parasite causes the same disease. There can be many intermediate hosts of parasites before reaching a definitive or final host.
4) The dilemma with the fourth postulate is that since the life stages of the parasites could be different in different hosts, it would be difficult to re-isolate the microorganism and grow it in a pure culture to produce identical results. Again, the plasmodium cannot be isolated and re-grown in a pure culture due to the nature of the parasite and the environment it needs to grow. [4,2]
The Postulates Now
Today, Koch’s postulates are still used in determining the links between microbes and the disease they cause but are not very useful in finding the relationship between certain microorganisms such as viruses and parasites and diseases they cause.
Like in most fields of science, the basic discovery of an idea can be changed or manipulated in a way to better relate to present day issues. As we learned in class, there are three main reasons as to why parasites escape Koch's Postulates. All of these reasons were well explained in your post. As science has continued to develop, there are other ways in which the cause of a disease can be identified. Although the postulates cannot be used in every case of a disease, it has allowed scientists to identify the cause of many diseases and have helped millions of humans.ReplyDelete
I agree with Nadja. I find that Koch's postulates have become a bit outdated, especially in regards to parasites as your post explains. There are even some types of viruses, such as HIV, that its is unethical to inject back into a human host, and others that cannot be grown in a pure culture. I think that because science has become more ethical since Koch's time, his postulates have become less practical on the newer diseases. However, I do believe that they can still provide a good basis for the study of how microorganisms cause disease, and can be used in combination with newer techniques to help establish causation between microorganisms and diseases.ReplyDelete