Guide to Assignment one:
One of the guiding principles of parasitology is that parasites are everywhere. And old verse, attributed by various to Jonathan Swift or Ogden Nash, goes like this.
So, naturalists observe, a flea
Hath smaller fleas that on him prey;
And these have smaller still to bite 'em,
And so proceed ad infinitum.
It implies that every organism has numerous parasites and these themselves also have parasites. In fact, most estimates today include parasitism as comprising over 50% of the described symbiotic relationships, or relationships in which animals live in close physical contact for a “significant portion” of their lives. The other symbiotic relationships are mutualism and commensalism. Only in parasitic relationships is harm done to one of the species involved in the relationship.
Although most organisms, including humans, harbor parasites, many hosts are asymptomatic and unaware of the existence of their parasites. It is only on the rare occasion when they cause us harm that we notice them.
Parasites of the world
Below is a list of the major parasitic diseases of humans. They in general belong to three big groups. The first four listed are nematodes. Single cell eukaryotic organisms that used to be placed in one big group, commonly known as protozoans, cause others, such as Malaria and Amoebiasis. Flukes and tapeworms belong to the major clade, Platyhelminthes, the same group that contains planarians. We will look at many of these relationships more closely later.| Ascaris (round worm) | 1.3 billion |
| Hookworms | 1.3 billion |
| Whipworms | 1 billion |
| Filarial worms | 657 million |
| Malaria (protozoan) | 500 million |
| Schistosomes (fluke) | 270 million |
| Amoebiasis (protozoan) | 50 million |
| Taeniad tapeworms | 50 million |
| Clonorchis (fluke) | 20 million |
| Chagas’ Disease (protozoan) | 15 million |
Do not worry about memorizing numbers and names right now, but do note that millions of individuals are infected by parasites and at a level that we consider their presence a serious disease. Also note that this may be an underestimate of the impact of parasitic disease since there are several diseases caused by parasites that are considered neglected, because their symptoms are too general and mimic other diseases. There are also parasitic diseases that are not recognized because they do not result in mortality or severe disabilities for years after infection.Answer question one on Assignment one. Assignment one can be downloaded from the schedule.
Numbers do not tell the whole story.
Malaria is a disease caused by a trypanosome, an unicellular organism. Although the numbers attributed to malaria are relatively small (see table above), it is estimated that a child dies every minute from malaria. Why? Parasitic diseases are also the diseases more likely to be borne by the poor. Children with more actively developing bodies than adults, and so in need of more nutrients, often suffer the most from parasitic infection. This is especially true among the poor where parents do not make enough to ensure proper living conditions for their children.
Go to the following web site and examine the links for Chagas disease and Cysticercosis. Pay particular attention to epidemiology and risk factors.
http://www.cdc.gov/globalhealth/ntd/diseases/index.html
Answer question two and three. Question three is a homework question to be handed in.
Parasites of the US
You may be surprised to note that parasites, even those that cause considerable harm, are alive and well in the United States.
Go to the following website
http://www.cdcmosis .gov/parasites/npi.html and visit the links (Click on the disease or appropriate topic) for Toxocariasis, Toxoplasmosis and Trichomoniasis. Also examine the fact sheet on NPIs in the United States.
Answer question four. As you fill out your first table, you are being introduced to the "meat" of most parasitology courses or the who (host), when (stage of life cycle) transmission occurs and if parasite involved is considered a true (i. e., Are humans the primary or definitive hosts?) "human" parasite, .
Zoonoses caused by parasites.
As you read about parasites of the US, you probably noted how many individuals were infected and not symptomatic. Many adults in regions where malaria is prevalent develop a resistance to the disease if they live through an attack as a child. Many diseases such as cysticercosis are due to parasites that find themselves in hosts other than their intended species, or at least, at a different stage in the life cycle than expected in a unintended host. In some sense these are mistakes that the parasite made in its search for the proper host at the proper time in its own life cycle. So if humans eat the cysts in under cooked pork, they end up with larvae that turn into fairly benign adults. You have to host a lot of tapeworms in your intestine to even begin to notice their presence. But if you eat the eggs, you find yourself with newly hatched larvae looking for a place to encyst, so they can be transmitted, when you are eaten by a new host. Just a case of bad timing for the parasite that also means you end up with a number of larvae cysts in your muscles, organs and brains, essentially turning the areas around the cysts into mush.
In general then, we expect parasites that have evolved with a host for some time and in their right host to cause less severe disease. The logic behind this is that the parasite itself lives and reproduces only as long as the host does. The assumption then is that if the relationship is a new one, hosts will exhibit more symptoms, than if the relationship is an old one. Keep this in mind as we look at a nematode new to human hosts and examine the symptoms of toxoplasmosis in more detail.
We will explore more about this generalization later, paying particular attention to when it is in the parasites’ interest not to reduce but increase virulence.
I would also like you to begin to appreciate that as humans encroach on wildlife, they risk encounters with new species of parasites. When these parasites try on humans as hosts, the results can be disastrous. You will find throughout this course both CDC and WHO websites warning about the danger of new zoonoses.
The term zoonosis (zoonoses, pl.) has been used in a variety of ways. If you look up a definition of the term, it will simply be defined as a disease which can be transmitted to humans from animals. This definition, however, could include all parasitic diseases. In practice, it is usually a term used to indicate a disease that has recently jumped from a non-human animals to humans. The Baylisascariasis story is one of a new such zoonosis. No parasitologist would refer to taeniasis or any parasitic infection that has been found in humans for centuries, and for which humans are considered the primary or definitive host, as a zoonosis. We will use this more restrictive definition of this term throughout this course.
Please view the video on this website. (You may have to close your browser after playing this video to stop the playing of the next video. It also takes a while for the video to open, you may want simply to copy and paste it into your browser and not click on the active link.)
https://www.youtube.com/watch?v=DxL2qHBetvI
Another newpaper story link about the rising concern of this zoonosis
More information in form of a summary pdf on Baylisascaris to examine.
http://www.cfsph.iastate.edu/Factsheets/pdfs/baylisascariasis.pdf
Please answer questions 5. This is a Homework question to be handed in.
Toxoplasmosis has often been described as a zoonosis although it really doesn't fit our restricted definiton of such. We will however use it as an example of how problemtically it can be to access the "real" effects of parasitic diseases Toxoplasmosis has been with humans for centuries. It is, as in the Cysticercosis story, a problem of a parasite causing a problem in a novel secondary or intermediary hosts.
First examine these websites that give more information on Toxoplasmosis, including routes of transmission to humans. You should firmly establish in your mind, what are the definite (primary), intermediary (seoncdary) hosts, and common modes of transmission to each.
https://en.wikipedia.org/wiki/Toxoplasmosis
https://www.cdc.gov/parasites/toxoplasmosis/index.html
Please visit these websites which examine the effects of toxoplasmosis on a more typical host, the rat.
https://www.smithsonianmag.com/science-nature/the-parasite-that-makes-a-rat-love-a-cat-86515093/
A more professional treatment: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4882154/
Physicians have been advising pregnant women about the dangers of toxoplasmosis infection from cats for years. To asses this risk, revisit your list of possible modes of transmission for this disease.
Here is cdc official statement on preventing toxoplasmosis transmission from mother to fetus.
. http://www.cdc.gov/mmwr/preview/mmwrhtml/rr4902a5.htm
Notice that it mentions other more common modes of transmission routes than through accidental tranmission through contact with feces from infected cats. This is also true for other previous references. https://www.cdc.gov/parasites/toxoplasmosis/index.html
The youtube presentation below I feel is a bit too "cute" for me, but his explanation behind the science of transmission of this disease and so the risks associated with cats, versus other modes of transmission, is clear and accurate. In fact, so far, the best and clearest presentation I have found to date on this. So if you can,please ignore the silliness of anvils killing cats, cowboy parasites and immune cells, and other images. Focus on when cats can be infectious and so pose a risk to pregnant females. https://www.youtube.com/watch?v=U9MU-FxsKRg
Answer question 6. This is a Homework question to be handed in.
Impact
It is difficult to access the true impact of a parasitic disease even on humans. Parasitic diseases do not claim as many lives as infections such as AIDS. Yet many people would argue that to put a true price on parasite diseases, you would have to consider the overall cost to society and so the work effort compromised and the monies needed to detect and treat these diseases. A number of researchers are trying to determine true cost in terms of non-mortality effects.
Read Supplement one that contains pages from a recently released study and then visit the website below.
https://www.youtube.com/watch?v=8yKJPKzplEE
You can imagine the problems scientists will encounter as they try to access the impact of this disease on humans.
Below is paper that questions even the effects of toxoplasmosis on rodents and in some way points to problems with accessing sublethal effects of any parasitic infection. https://journals.asm.org/doi/10.1128/mBio.02164-19
Then answer question 7 on the assignment sheet.
As expected, the money spent on a particular disease is not always correlated to the effects of the disease or the number of individuals affected. Malaria, for example, has more money spend on it than the total of funds spent on diagnosis, treatment and research into more effective cures for diseases caused by nematodes or Platyhelminthes.
Part of this concern is justified. Malaria is believed to kill one child every minute.
Part of the reason is political. Drug companies are prone to supporting research that results in new effective drugs and vaccines. In most common helminthic or “worm” infections, current drugs in use, are fairly effective. Companies would be less interested in these diseases than malaria or other infections that are more apt to evolve new resistances more quickly or have proven more difficult to control.
Our country spends millions of dollars on malaria research, (as you will learn from the text), because malaria was such a problem for our soldiers in past wars, deployed in areas where malaria was endemic.
Malaria was also once endemic to the southern United States, including North Carolina.
Go to the following web site. and view the video clip there. I have already mentioned that the CDC, whose web site you have used for this assignment, was initiated as part of effort to control malaria. This is one of their early attempts to educate the public about malaria and its transmission
(Just click on the play button, do not attempt to link to listen to story.)
Today malaria is still a disease found in the United States but now more often travelers and immigrants bring in the parasite. The agencies once involved exclusively in malaria research have become agencies with more extensive interests. In the meantime, the malarian parasites have become the model system worldwide for research in this area, essentially becoming the mouse or Drosophila of the parasitologist’s and immunologist’s world. So history is important in directing research. It also tells us, if nothing else, what does not work with regard to controlling a particular parasitic disease. Answer question 8.