Thursday, December 15, 2011

The Helminths



This week at Infection Landscapes I am beginning a new series on infections with helminths, which are parasitic worms. There are a staggering number of helminths that infect humans, which employ even more varied, and often mind boggling, strategies to complete their life cycles. Some helminths are transmitted fecal-orally, some are ingested in the muscle tissue of various animals, some are vector-borne, and some are transmitted by directly penetrating the surface of the skin. Almost all of these helminths require additional animal hosts to complete their life cycle. These organisms present some of the most fascinating disease ecologies and diverse landscape epidemiology in the world of infectious diseases. Unfortunately, many of these infections exact a high cost on the populations most affected. These make up the majority of the neglected tropical diseases, and together they comprise the largest overall disease burden to human beings on the planet. 

Helminths are eukaryotic multicellular endoparasites, so they must live within their hosts.

There are two very broad over-arching categories of helminths: the roundworms and the flatworms. The roundworms are known collectively as nematodes, and the flatworms are divided into the cestodes (tapeworms) and trematodes (flukes).

Nematodes vary greatly in size from microscopic micro-filariae to the massive Ascaris worms, but they are all cylindrical in shape and thus their designation as “round”. They have an alimentary tract, a nervous system, and they develop a cuticle as their outer covering.

Ascaris lumbricoides

Some of the nematode infections we'll cover are ascariasis, trichuriasis, hookworm, and toxocariasis. The first three are often referred to as the soil-transmitted helminths and together are perhaps the biggest contributor to overall morbidity in the developing world. Approximately 3 billion people are chronically infected with these worms across the globe. The last infection is a relatively common helminth infection in the developed world in poor populations in rural and inner-city communities, and may be a contributor to childhood asthma in the United States in socioeconomically disadvantaged populations.

Cestodes also vary in size but they are all planar in shape, thus their inclusion in the flatworms, and they are readily distinguished by their segmented bodies. These helminths have no digestive tract, but they do have a nervous system, and they develop a tegument as their outer covering

Taenia solium (pork tapeworm)

Some of the cestode infections we'll cover are taeniasis, caused by both the pork and beef tapeworms, and diphyllobothriasis, caused by the fish tapeworm.

Trematodes are, again, of variable size, but most trematode forms tend to be very small. Trematodes are also planar in shape, which is why they too are considered flatworms, however they are not segmented. These helminths do have an alimentary tract but they lack an anus. Trematodes also possess a nervous system and develop a tegument as their outer covering.

Schistosoma mansoni (blood fluke)

We'll cover two major trematode infections, schistosomiasis, and the infections caused by the incredibly varied liver flukes, which are comprised of several distinct genera and species of trematode all with highly varied disease ecology and landscape epidemiology.

The landscapes of infection that these helminths delineate are wildly varied. In part, this is because the modes of transmission are quite different across the helminths. For example, those that can be transmitted by the fecal-oral route may be highly influenced by the level of sanitation and water infrastructure in certain geographies. While, on the other hand, those helminths that are transmitted by a mosquito vector may be strongly influenced by fluctuations in climate and the specific ecology of the vector and not at all influenced by municipal sanitation.

Another critical aspect that defines the microgeographies of helminth infection is the fact that these parasites require multiple hosts, both definitive and intermediate, to complete their life cycles. As such, these organisms exist at the interface between the ecologies of different animals, including humans, which can make blocking infection extraordinarily difficult.



17 comments:

  1. This blog is a great source of information which is very useful for me. Thank you very much.

    BEST SOLUTION ON BLOATING.

    ReplyDelete
  2. Glad information for me.......

    ---------------------------------------------
    market report | research report
    ---------------------------------------------

    ReplyDelete
  3. One of the more interesting aspects of helminthology is the methodology of treatment for helminthic infections. You wrote about the bizzare lifestyles of many of these helminths, and it is, of course, this set of lifestyles that presents unique challenges to pharmaceutical organicists. Part of the difficulty stems from the fact that helminths are closer to mammals on the evolutionary timeline than are, for example, bacteria (I won't consider viruses, fungi, etc. because viruses, etc. present their own annoyingly unique pharmaceutical challenges). Because of this, it is more difficult to perceive biological targets that are not shared by the helminth and the mammal, whereas bacteria generally present a number of unique targets for safe drug implementation. In fact, early antihelminthics were toxic to the mammal. It is my understanding that the two major modern strategies for ridding helminthic infections are: starvation of the helminth and paralyzation of the helminth. Benzimidazoles are drugs that work in the former manner, and Macrolytic Lactones are drugs that work in the latter manner. These days, antihelminthics are much safer to the receiving mammal. I've even heard of drugs that target subtle, unique elements of architecture in the portions of the helminth that allow for internal attachment to the mammal. Did I hear this correctly?

    ReplyDelete
    Replies
    1. Hi John,
      There are many different mechanisms of anthelminitc drugs. Levamisole, pyrantel and morantel act on AcH receptors of nematode’s muscles and cause paralysis. Piperazine is a GABA agonist at receptors and cause again paralysis. It’s one type of selective drugs you referred to and selectively targets an isoform of GABA receptor unique to helminthes that they use for forgaing http://www.wormbook.org/chapters/www_gaba/gaba.html . Praziquantel is another specific drug that effects the tegument of trematodes by increasing permeability of Ca. Lastly, another major class are the Benzimidazoles bind to beta-tublins of nematodes, cestodes and flukes and inhibits microtubule formations. A majority of the anthelminitcs are broad and there are side effects to anthelminitc drugs for humans. Levamisole, for example has a major serious side effect of agrunlocytosis that depletes neutrophils and white blood cells. ("Levamisole". DEA)

      Delete
    2. Hi John,
      Your comments on treatments for helminthic infections seem very factually based and accurate. Another method of treatment I would like to point out, although albeit significantly more uncommon, is alternative medicine or "herbal treatments" for helminthes. My grandmother mentioned to me once while having a diner discussion that in old time Europe they would use a herbal cocktail to treat internal parasites that included wormwood, gentian, ginger and wild indigo. I’m not familiar with these particular plants and roots but it would be interesting to have their effectiveness studied. Little money is given to study alternative medicine as pharmaceutical companies fund and lobby research that will generate the most profit for them. I was told that Germany funds herbal medicine research but have yet to discover it.

      Delete
  4. I forgot to ask my major question: how have antihelminthics been implemented for widespread treatment, if they have? How expensive would such an implementation be? I ask because I am assuming that the developed nations that produce antihelminthics do not suffer to the same degree from helminthic infections as do underdeveloped nations?

    ReplyDelete
    Replies
    1. @John, I was not able to find out exactly how anthelminthic drugs have been implemented for widespread treatment. However, in a 2012 report by Prichard and colleagues community and individual interventions were methods used for implementing anthelmintic drug treatment in countries heavily burden by these parasites. Here is the link: http://www.plosntds.org/article/info%3Adoi%2F10.1371%2Fjournal.pntd.0001549. Also, one thing that should be considered when planning widespread drug distribution is whether or not these organisms that were once killed by a drug will become resistant to it. It may not necessarily be the expense that halts some of these treatments, but rather trying to prevent the emergence of “super-helminths” resistant to anthelminthic drugs (similar to “super-bugs” that are resistant to antibiotics).

      Delete
    2. There is a common theme in the discussion of most infectious diseases 1) they cause an unrivaled burden of disease in developing countries, and 2) they are almost always associated with poor sanitation and standing water. While there have been health improvements in many areas with the help of Western aid, it is going to take much more than these interventions in order to have large-scale, sustainable change. People need clean water. Contaminated water is at the root of many infectious diseases, especially the helminthic ones. I don’t mean to politicize my post so much, but it seems that most interventions focus on temporary solutions to ongoing issues (for example, distributing insecticide treated nets for malaria prevention, but not dealing with the open sewage problem in most of these countries.) Public health workers are doing great work with these programs, but it concerns me that most of the leaders of these countries are not making enough progress regarding infrastructure for water treatment. Antihelminthics provide crucial, life-saving support for people suffering with these infections. However, by ignoring the bigger problem, country leaders are essentially putting a Band-Aid on a stab wound.

      Delete
    3. Well said! Studying these infections makes it clear that common source for the ongoing burden is the poor sanitary conditions and poor water sources in underdeveloped countries. I did not realize the staggering burden of intestinal worms in the world. With intestinal worms infecting over 2 billion people across the world, mostly children, in areas with poor sanitation there should be more effective interventions for youth in high risk areas. Currently, many regions have implemented effective surveillance methods through the education system. Schools currently provide a good entry point for deworming activities, since they provide an easy provision of the health and hygiene education component, such as promotion of handwashing and improved sanitation.
      In 2013, over 368 million schoolchildren were treated with anthelminthic medicines in endemic countries, corresponding to 42% of all children who are at risk.

      Reference:
      http://www.who.int/mediacentre/factsheets/fs366/en/

      Delete
    4. I honestly did not really know about antihelminthics, so I looked into the current status of research and implementation. One of the especially interesting things I learned was that there is evidence that one of these medicines may be able to reduce malaria transmission. This especially interesting given the issue that John raised regarding developing vs. developed countries: "The villages are located in an area that experiences a high burden of both malaria and worm diseases, raising the possibility of addressing several health problems at once."

      Source: https://www.sciencedaily.com/releases/2015/10/151027132830.htm

      Delete
  5. When I used to work as a veterinary assistant, helminths were prevalent in puppies and kittens. It was interesting to read that these same helminths could also infect people.

    ReplyDelete
    Replies
    1. Patricia, it is interesting that you mention heminths were prevalent in puppies and kittens, is this true of adult dogs and cats? This makes me wonder why youunger animals are more vulnerable. Could it be the fact that many are transmited fecal-orally?

      Delete
  6. It was not immediately clear to me how helminthes require multiple hosts to complete their life cycle. As I understand it, the eggs mature after they enter the body of the host. Flukes, for example, first mature in freshwater snails, then after leaving the snails they enter the body of humans who swim or bathe in water in which they are active (http://www.medterms.com).

    ReplyDelete
    Replies
    1. Desrene, this is a generic introductory post. The life cycle for any specific helminth depends on the organism and its ecology, which are given full consideration for all of the major helminths on this website

      Delete
  7. S.Wallace- The disease burden of soil-transmitted helminths seems to exceed some well-known conditions such TB and Malaria. I am quite surprised that there are health disparities of neglected parasitic diseases such as Toxocariasis among poor communities in developed countries such as the United States. I also have to wonder or question how much attention this health problem is receiving especially if toxocariasis has some relation to asthma.

    ReplyDelete
    Replies
    1. While I am somewhat familiar with the first three nematode infections mentioned in the blog post: ascariasis, trichuriasis, and hookworm, I don’t think I’ve ever heard of toxocariasis. I did not know that toxocariasis is a common helminth infection among poor populations in rural and inner-city communities, which I also find surprising especially after reading that it may be a “contributor to childhood asthma in the United States in socioeconomically disadvantaged populations.” This lead me to reading more about the zoonotic ascarid Toxocara and found that there have been several studies such as a clinic based case control study, which studied the environmental exposure to toxocara as a possible risk factor for asthma. The study confirmed that there is a “significant association between asthma and risk factors and between Toxocara infection and risk factors” (Sharghi et. al 2001). Another interesting discovery of this study is that it found high prevalence of Toxocara infection among Hispanic children of Puerto Rican descent. Studies such a this one, depict the need to enforce methods of control and preventive measures against toxocariasis such as public education about the sources of infection and means of reducing it (i.e. prevention of soil contamination by dog and cat feces in areas where children play).

      Works Cited
      Sharghi, Neda, Schantz, Peter M., Caramico, Lisa, Ballas, Kathleen, Teague, Barbara A., and Hotez, Peter J. . Environmental Exposure to Toxocara as a Possible Risk Factor for Asthma: A Clinic-Based Case-Control Study. Clin Infect Dis. (2001) 32 (7): e111-e116 doi:10.1086/319593

      Delete
  8. "They comprise the largest overall disease burden to human beings on the planet."
    "ascariasis, trichuriasis, hookworm...are perhaps the biggest contributor to overall morbidity in the developing world."

    These were striking facts to read! Given the variety in transmission methods, I understand why prevention or treatment approaches would be complicated. One reason why these parasites are especially burdensome in certain areas. Warm, resource-poor geographies with poor sanitation systems might create a "perfect storm" for a group of diseases spread by mosquitos and fecal-oral routes.
    I would have been interested to read more about if/how helmniths contribute to morbidity and/or mortality in people with other concurrant conditions, particularly in areas with less health infrastructure.

    ReplyDelete

Note: Only a member of this blog may post a comment.