02.14 Parasites and Parasite Replication

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  1. Parasites and Parasite Replication
    1. Two Life cycle Categories of parasites
      1. Direct (monoxenous) Life Cycles
        1. Parasitic stage: spend most of their adult ives in one host
        2. Free living stage: Offspring transmitted from one host to another
        3. Lack intermediate stage (must leave host, survive outside host and find new host.)
          1. Obligate Parasites require parasitic stage
          2. Facultative Parasites can skip parasitic stage for multiple generations
        4. Ex. Nematodes, trypanosomatids, and cryptospridium
      2. Indirect (heteroxenous) Life Cycles
        1. Two host stages that require a definitive host and an intermediate host.
            1. The definitive host stage is required for reproduction and the adult life phase.
            2. The intermediate host is when parasite transmission to a host occurs and facilitate disease transmission (ex mosquitoes that pass immature parasites through their proboscis into bloodstream of definitive host.)
            3. Ex. filarial nematodes, plasmodium and Leishmania
    2. most common types of parasites that infect humans
      1. Tapeworm
      2. Roundworm
      3. Hookworm
      4. Cryptosporidium
      5. Plasmodium (Malaria)


Today we’re going to be talking about parasites and parasite replication.

In this lesson on parasites and parasite replication  we will cover the two life cycle categories of parasites and look at some common human parasites.


So the first life cycle I want to discuss is the Direct or monoxenous life cycle.  With monoxenous parasites it is not required for the parasite to o carry out different stages in different hosts. These parasites will spend most of their adult life in one host (parasitic stage) and then their offspring can be transmitted from one host to another (free living stage).  So these parasites lack the  need for an intermediate stage. In other words they do NOT need to leave the host, survive outside the host and find a new host to complete their life cycle. Obligate Parasites require parasitic stage as opposed to Facultative Parasites can skip parasitic stage for multiple generations.


So certain nematodes like this common one used in research C. elegans, Trypanosomatids like this one that causes African Sleeping sickness and lastly we have Cryptosporidium which forms have been known to infect drinking water and cause diarrhea.


The other parasite life cycle is called Hetereoxenous or the indirect life cycle. Here parasites require a definitive host stage to reproduce and mature to adulthood. But it relies on an intermediate host to transmit it to another host  which transmits the disease.And we are going to go to the the next slide for examples to understand this better.


So these are 3 examples of parasites with indirect life cycles. Filarial Nematodes are a Superfamily  of nematodes highly specific to vertebrates (except) fish. Depicted here is the classic heartworm nematode whose scientific name is Dirofilaria immitis that is known to parasitize in dogs and sometimes humans. Filarial nematodes can’t reproduce in the definitive host and can’t infect another definitive host directly, but must make its way through the host’s body to where an intermediate host (like a mosquito) that acts as a vector can swallow it. It must succeed in invading this vector (mosquito) quickly b/c unlike adult filarial worms, microfilariae only survive for a few months and they develop no further unless they are ingested by a suitable blood-feeding female insect. In the intermediate host the filarial nematode can develop further till the vector conveys it to another definitive host. In the new definitive host the microfilaria complete the final stage of development into sexual maturity; the process takes a few months. The mature filaria then must mate. This means that an invasion by one worm cannot produce an infection, as it really takes years of exposure to infections before a serious disease condition can develop in the human host.  Leishmania cause  leishmaniasis (which causes skin ulcers) and are spread by sandflies.


And here are 5 common human parasites: tapeworms, roundworms, hookworms, cryptosporidium and plasmodium. These are such diverse and prolific organisms that exist all over this planet.

And upon researching parastire I came across this quote that really spoke to me as to the volume of these parasites exist, especially the nematodes. Nathan Cobb, a nematologist described the ubiquity of nematodes on Earth as thus “ In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable since, for every massing of human beings, there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.”

So in review, parasites are known to have either monoxenous life cycle, that have a direct life cycle that exists in one host. Or heteroxenous life cycle that is indirect and requires an intermediate host to complete its life cycle.


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