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.[1] 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
[7]
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.[5]
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.[4]
An example for a parasite that does
not follow the postulates is the plasmodium that causes malaria.
[9]
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.[1] 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]
[8]
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.
1) http://www.cdc.gov/parasites/
Works Cited
1) http://www.cdc.gov/parasites/
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.
ReplyDeleteI 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.
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