Saturday, April 19, 2014

Intermediate Host: The Delivery Boy of the Parasitic World

Why use an intermediate host?
Parasites have often been known to take a skewed path to their ideal host. What is the point of an intermediate host? What makes the longer path the better one? It seems more time and energy consuming, and overall inefficient. Well, sometimes it is. But more often than not, an intermediate host is beneficial, even necessary, to a parasite’s life cycle. Intermediate hosts can increase the probability of infection the definitive host[1], provide transport to the definitive host, or even supply nutrients for the parasite’s development until a suitable host comes along[2].  

Types of Transmission
Before we talk about all the different instances in which using an intermediate/ host is helpful to a parasite, it’s important to know that there are 2 kinds of transport through a host: passive and active. Passive transmission is essentially the host’s accidental ingestion/inhalation/etc of an infective source. For example, Dracunculiasis, or more commonly referred to as Guinea Worm Disease, is the result of passive transport. The nematode responsible for the disease, Dracunculus medinensis, is ingested by a copepod in its L1 stage. The parasite develops into the L3 phase while in the copepod, which can be swallowed by a human (the definitive host) when clean water is unavailable[3]. This parasite did not manipulate its host’s behavior in attempt to reach another host, but rather just let things happen as they did. Active transmission, on the other hand, is displayed when a parasite manipulates the paratenic host’s behavior or deliberately performs some kind of action to open the encounter filter and increase the probability of reaching its definitive host. The human Bot Fly, or Dermatobia hominis, is a good example of active transport. The fly lays its eggs on the abdomen of a mosquito (the intermediate host) that will be stimulated and released when the mosquito comes in contact, or takes a bloodmeal from, a human [4].

Compilation of Boy Fly removals. [8]

Infective vs. Infestive
Trichinella's Life Cycle [6]
Both passive and active transmission can be divided further into categories; active transmission, for example, can be attained by either an infected source or an infested source. An infested source essentially implies that the parasite is free-living, while infective sources utilize a living organism as an intermediate host [4]. Pigs and cows are both infective sources/intermediate hosts for certain Trichinella species. T. spiralisis the most common, and therefore most devastating, species in the Trichinella family. The T. spiralis life cycle is direct, meaning that it can be completed within a single host, and therefore does not need an intermediate host. However,  there often are multiple hosts that T. spiralis will go through during its life. T. spiralis larvae encase themselves in cysts inside the muscle of its current host (usually a pig), which affects the mobility of the infected muscle. If the host’s muscles are rigid and immobile, it will be more likely to get caught, and therefore eaten, which will pass the nematode onto another host. The parasite increases its chances of continuing its life cycle by shortening that of its current host.

Mind Control of Mice?
Several parasites exhibit mind control
 over their hosts.[7]
Another example of a parasite using an intermediate host to increase its chances of ingestion is Toxoplasma gondii. T. gondii can only reproduce in cats, but it is often found in rats and other rodents. Why on earth would a parasite be found in both rodents and cats? Because the protozoa controls its host’s (the rodent) mind to decrease fear of cats. Usually, when a rodent/rat smells cat urine, it turns and runs as fast as it can. But when infected with T. gondii, the rodent is intrigued by the smell, and it sticks around to find the source of the smell leading to its imminent consumption[5]. The effects of T. gondii on human brains is currently being studied, and so far it looks like humans are also susceptible to parasitic mind control. Apparently, humans infected with T. gondii are more likely to take on dangerous tasks and get into car accidents. Science North talks about T. gondii more in the video below.

          More info about T. gondii. [9]

1. Art of Being  A Parasite, by Claude Combes


  1. It's interesting to know that T. gondii can also infect humans and cause them to perform dangerous tasks or get into car accidents. If T. gondii, however, is infected in a pregnant women, it can be transmitted to the fetus and cause mental retardation, blindness, epilepsy, and possibly death. Although the lady in the video did not mention how T. gongii can occur in humans, I found out that there are actually two other ways aside from acquiring it through the placenta. Ingesting oocyst from soil, or the more common way to get a parasite, eating raw or undercooked meat that happened to contain the T.gondii tissue cysts. The easiest way to avoid getting T. gongii then would be to not consume raw meat since it is one of the highest pathogens to contribute to acquired foodborne illnesses that result in hospitalization.

    Article where I got the info from:

  2. Another interesting example of passive transmission includes the amoeba parasite Entamoeba histolytica and its human host. This disease, also known as Amebiasis, is spread through the fecal-oral route. As Emily stated, the human host unknowingly approaches this parasite through contaminated food and/or water. On the other side, the parasite is lazily waiting for its host to come and find it. Humans ingest the infective cyst form of the parasite. The cysts then become trophozoites that move to the colon and reproduce asexually. The trophozoites produce cysts, which are released through feces. Passive transmission of this disease is extremely common in developing nations that have poor sanitation measures. It can be argued that it is easier for the host to find its awaiting parasite in such areas.

  3. The paths that parasites take to their "final destinations", as it were, are fascinating. The journeys that you describe T. gondii, and Trichinella taking through different hosts to reach its definitive hosts reminds me of the journey that the Hookworm takes. Although hookworms do not necessarily travel through different hosts, the route they take to get to their nutritive stage of adult growth is diverse. Instead of simply being digested by one animal and then passed to another, or entering into one host and making its way to the organ where it can feed and grow by the most direct route, ingestion, hookworms maintain a tedious path to reach the human intestines. First a human walks through an area where hookworms are found in the soil (normally where they have been deposited through human feces). Then, they make their way to the lungs and heart through the circulatory system, only to be coughed up in order to eventually make it to the intestines where they live and grow into their adult stage. This long journey, although effectively depositing the worm where it needs to be, seems roundabout at best.

  4. I find the study of intermediate host to be particularly interesting because on one hand some parasites need them in order to reach the definitive host and other do not. For example, plasmodium and other microscopic parasites would probably never reach the definitive host with out the help of an intermediate host, usually enabling it to have a route directly into the body. On the other hand you have parasites like hookworm that do not need an intermediate host and are capable of making it to their definitive host on with out he aid of another spices. I would also be interested to see the success rates of parities that use intermediate host and those that done, and if there is an sort of increased fecundity in those who do no, to compensate for the fact that it may be more difficult to interact with its definitive host.

  5. So, I must begin by saying I never want to get bit by an infected bot fly after watching that video, and yes I watched it in its entirety. Taylor's idea of testing whether or not there is increased fecundity between those who use an intermediate host and those who do not is very interesting and I would be curious to see the results. We have talked many times in class about how different parasites use different intermediate hosts for transport and growth purposes so I definitely see how having intermediate hosts is beneficial to the parasite. However, I suppose you could look at the cons of having intermediate hosts such as you mentioned, more energy expenditure and longer routes. Parasites often seem to have a convoluted way of doing things, however, it seems to work for them. By using intermediate hosts as a way to get nutrients and grow they are probably more virulent by the time they reach their definitive host.

  6. I found it interesting that there is indeed a parasite that can perform mind control in humans as well. T. gondii and the effects on pregnant woman are extremely graphic and way more than what I would expect. I also found the bot fly video very interesting because it really does show what can happen when a human is "infected" by a bot fly. Overall I found this article very informative and exciting to read.

  7. It is interesting and scary that parasites can use "mind control" on their intermediate hosts to increase its chance of getting to the definite or final host. This reminds me of the flatworm Leucochloridium paradoxum that uses a gastropod as its intermediate hosts. As we learned in class, these flatworms travel to the "zombie" snails antennae to resemble big juicy caterpillars to birds which are the final host. The parasites alter the snails ability of light perception causing it to come out in the sunlight to be easily seen by birds. It seems as if parasites are willing to do almost anything to reach the next stage in their life cycle. Intermediate hosts are helpful to continue the life cycle of a parasite if the parasite can manipulate its intermediate host to pass on the parasite to the final host.

  8. I, like Brittany, watched the entire bot fly video. I don't know what made me do that, maybe it was T. gondii. Anywho, you do bring up a really interesting point early in your post: why do parasites use intermediates to infect their final hosts? It is important to keep in mind that its not a conscience decision that parasites make when infecting or being transmitted, along the lines of :"oh yeah! cat gut! I can totally live here!" It is more like something in the cat gut, some chemical, interacts chemically with some protein on the surface of the cyst the entice it into emerging. Moreover, it wasn't as if T. gondii intentionally sought after mice, cognizant of the food chain. It was more likely that those parasites that had a certain genetic make up, that allowed them to survive in a mice, were selected for. All by chance. So not only did a mouse have to accidentally have to ingest an infective T gondii, but also that T. gondii had to survive, thrive, and then be consumed by a cat, over and over again.

  9. Very well-written article! The advantages and disadvantage of intermediate hosts in a parasites' life cycles are fascinating to me. In recent years, some thought another advantage to using a second intermediate host is to increase the chances of intermixing between different clones, thus reducing the dangers of mating only between genetically identical clones after transmission into into the definitive host (Raush et al., 2005). This is a theory that I had not heard before, so I found it especially intriguing. If you would like to read the abstract and full article for yourself, here's the link:

  10. Toxoplasma gondii to me is one of the most fascinating parasites that we have learned this semester. The research that has been done to see how it affected humans’ behavior is just very interesting! It really amazes me how it causes males to be more jealous, how a shorter attention span, and more prone to be involved in risky behavior and causes females to be more “loving” and open. Research also suggests that infected females are more attractive to males but females find uninfected males are attractive, which I find weird. I also did some further research and found that mothers that are infected with this parasite are actually more likely to birth sons. I also found that a person with a Rh – negative blood with the parasite are actually 2.5 times likely to be involved in a car accident. Apparently in France, only 12% of the population are not carriers of T. gondii, I guess this could be the reason why France is one of the most romantic countries..

  11. The mind controlling parasite reminds me of the parasitic insect called Gordius aquaticus. This parasite usually lives in water and as an egg it waits to be eaten by the hosts such as wart, grasshopper and camel cricket (Rhaphidophoridae). When it gets into the host's body, it grows inside, exploiting host's nutrition. When it grows large enough and reaches host's brain, it starts to manipulate host to jump into the water so that it can be released from the host's body and lay eggs and start the new cycle. This parasite is dreadful since it basically makes the host to commit suicide by drowning itself!

  12. As most have stated, the concept of a mind controlling parasite is definitely very interesting. Can you imagine the repercussions regarding our legal system if someone could claim that they committed a crime not of their own will but because of a parasite. Even worse, what about the various individuals who would abuse this in order to avoid crime. That alone is a frightening fact that we may face as research continues. Back to the subject, I find it interesting that those parasites who utilize "roundabout" methods for replication are still ecologically successful. I wonder if this is true only because they did not have to compete with the host. for instance, if two parasites had to compete for the same host, would the slower parasite who required an intermediate instead of directly infecting the definitive host be eventually wiped out via natural selection?

  13. "Good thing I'm a dog person," is what I sad upon finishing this. The existence of a parasite that can infect and "control" humans is a humbling realization for a species that considers itself the "master of all." Snails, fish, and other "lower" animals were ones that are usually susceptible to dangers like this, but to find that we are sharing a boat with them, is very eye opening. Luckily, T. gondii is a dead end parasite unless we were to be eaten by a lion, tiger, or other big cat; although it makes one rethink any ideas of owning big cats. And upon doing some deeper research into this topic, I found that parasites such as T. gondii are thought to be the reason behind ancient Egypt's worshiping of cats.

  14. Interesting,
    Best regards!

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