Friday, December 23, 2011

Ascariasis



This time at Infection Landscapes I cover the first in the series on helminth infections. This week we will discuss ascariasis, which is one of the single biggest contributors to overall global morbidity and disability-adjusted life years.

The Worm. Ascariasis is caused by Ascaris lumbricoides, also known as the giant intestinal roundworm, which is a nematode:


A. lumbricoides worms undergo several unique developmental stages within their human host. An infection begins when the Ascaris eggs are consumed in contaminated water or food. The larvae emerge from the infective eggs in the small intestine, where the larvae then target the mucous membrane of the epithelium. The larvae penetrate the epithelium and make their way to the lungs by way of the blood and lymphatic circulation. The larvae will spend approximately two weeks in the lungs, where they continue their growth and development. Following this period of maturation, the larvae penetrate the alveoli and migrate up the bronchii and trachea until they reach the throat. Here they will be swallowed again and, after reaching the small intestine for the second time in their life cycle, develop into adult worms. If they are not agitated, the adult worms then remain in the small intestine where they obtain their nutrients from the ingested meals of the host. Female worms produce tens to hundreds of thousands of eggs per day, which are passed in the stool of the infected host. If the eggs are fertilized by an adult male in the lumen of the gut, then they will embryonate and become infective after three or more weeks outside the host in the environment. Eggs that are not fertilized will not become infective. Infective eggs that are ingested by a new host will emerge as larvae in the small intestine and the process begins again. The process within the human host, from the infection with embryonated eggs to the emergence of adult worms, typically requires approximately two to three months to complete. The adult worms will typically live up to 2 years within the gut lumen of their host. Below is a nice graph by the Centers for Disease Control and Prevention that depicts the complex life cycle of Ascaris lumbricoides:


And here is another nice graph by MetaPathogen.com that nicely contextualizes the life cycle within transmission:


The Disease. Ascariasis is most often an asymptomatic infection. However, given the invasive nature of these infections, particularly during the stages of larval development, clinical disease can manifest in a variety of organ systems. Hepatosplenomegaly and peritonitis are typical symptoms when clinical disease is apparent. Given the extended developmental period in the lungs, pneumonia is another common complication associated with ascariasis. In very high volume infections with large numbers of adult worms, the parasites can actually block the intestine, inhibiting digestion, as well as directly competing with the infected host for nutrients from food. The picture below shows a high volume infection, with many worms taken from a dissected intestinal section:


Agitation of the Ascaris worms is another potential problem during active infections. The most common causes of agitation are chemotoxicity, most often due to the effects of drugs used to treat other infections (a common occurrence in ascariasis-endemic areas), and fever, also a common occurrence among individuals co-infected with other pathogens. When agitated, the worms attempt to exit the host either by burrowing through tissue or by migrating up and out the gastrointestinal tract:


The former can cause severe tissue damage to various organ systems, while the latter can cause suffocation if the infection consists of a large number of worms. Overall, these infections are important contributors to morbidity because high volume or repeated infections involve worms that are competing with the host for food nutrients. As such, ongoing infections with large numbers of worms can lead to chronic malnutrition, and this is most pronounced in children.

The Epidemiology and the Landscape. Approximately one and a quarter billion people are infected with Ascaris lumbricoides. In some highly endemic areas of the Americas and Africa, infection prevalence can approach 50% to 95%, respectively. The sheer volume of infection is a major factor in why this neglected tropical disease is such an important contributor to global morbidity. 


Transmission is primarily via ingestion of water or food contaminated with the embryonated, infective Ascaris eggs. However, this is does not directly follow the fecal-oral route because, as described above, the eggs require an extended period outside the host in the environment to become infectious. As such, direct person to person transmission does not occur. Nevertheless, the transmission is still indirectly fecal-oral as the eggs are passed in the feces of an infected host and will ultimately infect a new susceptible host following ingestion of these eggs in a contaminated common vehicle, such as water or food.

Control and PreventionControl and prevention of ascariasis begins by following the usual guidelines: improving sanitation in resource poor areas and maintaining vigilance in personal hygiene. In most settings in the world where ascariasis is a significant contributor to morbidity, improved infrastructure that can maintain adequate water resources is a first priority in its prevention.


Secondarily, personal hygiene at the individual level, especially in the context of food preparation, can also be very important in preventing ascariasis: consistent hand washing, boiling water, and thoroughly cooking food are all important in stopping the chain of transmission. The implementation of these strategies would go far toward eliminating the morbidity attributable to malnutrition in children in much of the world. 

21 comments:

  1. Can deworming an entire endemic population simultaneously with improving personal hygiene and sanitation/ water structures get rid of this disease?

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  2. In response to Yousra:
    I do not think that such an approach would be successful. Numerous research articles which I found on this subject suggested that Ascariasis can also infect pig hosts. As well, Ascariasis can live in soil and vegetation, such that these would need to be sanitized as well, in addition to water sources. Finally, on a practical level, most infections with Ascariasis are asymptomatic. Therefore, I think it may be hard to convince a majority of infected people to seek treatment when they feel fine. This also presents another challenge. Even if one could convince the population of endemic regions to undergo treatment, travelers to any of these regions could also become infected. Therefore, such tourists would also have to be monitored, requiring international efforts.

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    1. Knowing that it has other non-human hosts and ideal environments, I agree that getting rid of the disease is more difficult. I think this disease may benefit more form proper cleaning and cooking technique. Most countries with a lot of poverty don't have the proper tools to ensure safe drinking water, or clean food. So this becomes a hard one to handle.

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  3. I wasn't aware that ascariasis worm causes pneumonia. The fact that while the infection occurs asymptomatically is scary - considering the morbidity and mortality it causes. In developed countries where there is clean water, sanitary conditions and the appropriate hygienic infrastructure the transmission of this disease should be minimal. As stated above, each person can further take personnel control by taking charge of their personnel hygiene. In undeveloped, third world countries it would be almost impossible to contain this disease since the manner and route of infection occurs via various routes and is easily transmitted.

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  4. It is interesting to learn that ascariasis is the biggest contributor to global morbidity and I have never heard about it until reading this post. In third world countries, the risk of ingesting contaminated food and water is higher and therefore transmission of infection can occur at a faster rate than anywhere else. It was also interesting to learn that in the beginning the larvae grow and develop in the lungs before being swallowed again. I'm wondering why the larvae are not entering the small intestine to grow and instead opt for the lungs. Perhaps the acidic nature of the gastrointestinal tract makes it difficult for the larvae to develop into adults? Overall this seems to be a very dangerous infection since most of the development stage occurs without any symptoms and the host is unaware of infection until it progressively gets worse.

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  5. This infection seems quite life threatening, especially to children. I wish I knew some of the symptoms that do appear, though I would assume that malnutrition is one of the later obvious symptoms. The fact that agitation can cause such severe reaction from the worms, is disturbing. it also shows the sensitivity of the worms. They clearly need very particular environments to live. Is there anything that can kill these parasites, even when getting to a point of agitation that people do not have to suffer such long-term effects or death? I've never heard of this prior so it was an interesting read!

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  6. One of the most interesting aspects of Ascaris lumbricoides is that it feeds on the digestion products of its hosts. This can cause a host of deficiencies ranging from caloric, protein and vitamin A deficiency to name a few. As a result this leads to many of the symptoms associated with infection such as retarded growth and also increased susceptibility to diseases like malaria (1). Moreover, the worm burden also varies due to host factors, importantly age and geophagy and immunity (2). Lastly, worms don’t multiply in the hosts but they can persist for about 2years at max.

    1. Le Hesran JY, Akiana J, Ndiaye el HM, et al. Severe malaria attack is associated with high prevalence of Ascaris lumbricoides infection among children in rural Senegal. Trans R Soc Trop Med Hyg. Jul 2004;98(7):397-9. [Medline].
    2. Geissler PW, Mwaniki D, Thiong F, et al. Geophagy as a risk factor for geohelminth infections: a longitudinal study of Kenyan primary schoolchildren. Trans R Soc Trop Med Hyg. Jan-Feb 1998;92(1):7-11. [Medline].


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  7. This was an extremely eye-opening post. Academically, I have been in the public health and medical field for the past six years and this is the first time I am hearing of this infection. Considering it is "one of the single biggest contributors to overall global morbidity and disability-adjusted life years," I would think a bigger deal would have been made about it.

    If students in the academic field are unaware of this issue (Lincy V seems to be in the same boat), how can we expect people in affected countries to be knowledgeable of it?

    I believe that even a basic educational intervention of what it is, how to help prevent it, and contributing risk factors could go a long way in decreasing its prevalence.

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    1. I have never heard off the disease but growing up in Jamaica , West Indies, parents regularly gave children "worm medicine" to try to expel worms from the body. So,I think many people were aware of the effects that worms have on the body. Malnutrition was one effect I heard mentioned regularly growing up. They may not have known the diseases by name but they had enough knowledge to try to prevent them.

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    2. Ascariasis is an infection I have also never heard about, even though being in the health field for most of my education. I think it is great that in Jamaica the parents were aware of some type of illness that affects the body, especially children, is from worms. What would be even better is it they were aware what exactly the symptoms were so even if their original plan of prevention failed, they would recognize the signs right away and be able to stop transmission from other family/community members.

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    3. Nicole I agree when health care providers or public health professional are practicing in endemic areas they need to maintain a very high index of suspicion, even if the disease is asymptomatic you need to think about what could be?

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  8. This article is very enlightening for me,but I wonder, is there any way of diagnosing the disease early before it leads to morbidity and mortality? This may be difficult seeing that symptoms may be manifested in other diseases,for example, pneumonia. The patient may be treated for the secondary illness without the primary cause being actually known. The article did not mention what treatments are available for this disease. Is it more difficult to treat than other diseases involving worms, seeing that it involves a giant intestinal roundworm?

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  9. After reading this article, I am not surprised that ascariasis is one of the single biggest contributors to overall global morbidity and disability-adjusted life years. The part that clarified this for me is the fact that the larvae penetrate the epithelium and make their way to the lungs. As stated, given the extended developmental period in the lungs, pneumonia is a complication associated with ascariasis. According to WHO, “pneumonia is the leading infectious cause of death in children worldwide, accounting for 15% of all deaths of children under 5 years old.” (http://www.who.int/mediacentre/factsheets/fs331/en/). The fact that pneumonia is a complication due to ascariasis makes since in terms of its high global morbidity.

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  10. In developing countries farmers and other people some times defecate in bush or vegetable areas leading to contamination of the vegetables and fruits with the ascaris eggs, beside manure fertilization is very common in Africa for example because its readily available, improper washing of vegetables and fruits make it even more complicated, because people buy the vegetables and take it for granted and only rinse it rather than thoroughly washing it, also because the disease is largely asymptomatic its hard to detect who can potentially transmit the disease.

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    1. The entry informs us about improved infrastructure to maintain adequate water resources as a first priority in ascariasis prevention. Khalid mentioned conditions of developing countries which explain why the prevalence of the infection can reach to 95% in some areas in Africa. As stated in the entry personal, hygiene and adequate food preparation is essential for possible treatment and prevention. I am curious to know which areas in America, rural or urban, are most likely to have an abundance of Ascariasis cases. America has steady water infrastructure also, restaurants and other food related companies are inspected regularly to prevent such infections. Therefore, I would also like to know particular factors that contribute to infection prevalence approaching 50% in areas of America.

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    2. Many comments before me had mentioned that they have never heard of acscariasis before, and to be honest neither have I. But judging from the map displayed in the article, it looks like cases of acscariasis dont seem to be common, although a separate article that I came across mentioned that acscariasis is most common in rural areas in America. In fact it also mentioned it is the third most prevalant helminth infection right behind hookworms and whipworms. Although most infections turn out to be asymptomatic and not a threat, the absurdly high 95% prevalance rate makes it a huge threat. We may have never heard of it, but in places where it is endemic, people often get re-infected and it leads to the scenarios depicted in the article. Fixing the sewage infrastructure is a huge step towards reducing the burden of acscariasis and many other diseases.

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  11. I’ve never really read about Ascariasis before but it is definitely scary to know that many people infected with this horrible disease have no symptoms; however, some infestations can cause various symptoms, depending on which part of your body is affected. According to the MayoClinic, tests that can be used to diagnose this infection include: stool tests, blood tests, and imaging tests (i.e. x-rays, ultrasound, CT, MRIs). Given that transmission is primarily via ingestion of water or food contaminated with the embryonated, infective Ascaris eggs, it is essential that sanitation in resource poor areas is improved and that people maintain personal hygiene. Awareness is of vital importance given that simple actions, such as those as mentioned in the post: consistent hand washing, boiling water, and thoroughly cooking food are all important in stopping the chain of transmission.
    Works Cited: http://www.mayoclinic.org/diseases-conditions/ascariasis/basics/tests-diagnosis/con-20027084

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  12. I found this post of Ascariasis caused by Ascaris lumbricoides to be eye opening snap shot of a contributor to malnutrition issues being that the parasite impedes digestion by blocking the intestine and absorbs nutrition from what the host eats. Although I am aware of the parasite roundworm because I actually picked up such when traveling in Spain, I have never thought of it in regards to malnutrition and had no idea of it's impact to global morbidity and disability adjusted life years thus I did not consider it's public health significance.

    Being that food and water, the most common vehicles of transmission and are necessities to live there absolutely needs to be education and reminders of healthful food hygiene as mentioned above as well as personal hygiene, looking at defection practices, sanitation infrastructure, agricultural practices for long term control.

    It seems as though there is a higher prevalence of Ascariasis in areas with poor socioeconomic conditions; however, food and water are essential to life and therefore I think it important that all populations (regardless of inhabiting a developed, developing or underdeveloped country) to be educated and reminded of important food hygiene practices especially since high levels of international traveling are so common today.

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  13. Unlike may previous commenters, I have been lucky/unlucky enough to see Ascaris in person in the stool of a friend who acquired it in India. It is gross but also pretty amazing to realize something that big is living inside people.

    The fact that Ascaris eggs must spend at least three weeks outside a host to become infective means that a few (not necessarily easy or cheap) interventions could drastically reduce infections. Places where sewage water is subsequently used in agriculture provide exactly the environment for transmission. Some form of treatment for water to be used for agriculture would mean no infective eggs on produce.

    A second clear intervention opportunity would be to have (again treated or otherwise clean) water available for handwashing after defecation, so that eggs from hands cannot make their way to fomites where they can hang out until they become infective.

    Luckily these intervention carry with them the added benefit of protecting against many other infections.

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  14. I was surprised to find that Ascariasis leads to such vast global morbidity and disability-adjusted life years since I had never really learned or read/heard in the media much about this parasitic worm, much less parasitic worms in general. After reading this post, it makes sense that, given its prevalence across impoverished communities around the world, a disease that thrives in unsanitary environments and impairs individual's body's normal behaviors over an extended period of time would indeed wreak this much havoc.

    I do wonder about the specificities in Ascaris lumbricoides movements throughout its life cycle. For instance, how does the worm successfully travel from location to location within the body or escape through the organs or throat? What directs its movement?

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  15. That's an insight thought Max T! I found an article online that answers your question of how the worm navigates within the body from organ to organ, (http://www.antimicrobe.org/new/b17.asp) which, of course, is based on many different factors such as: size, timing of larvae reproduction and the internal environment of the host. There are gaps, it seems, in specifying what propels the worm during voyage and how it manages to survive in the target organs. It is interesting to note the different therapies used in treating Ascariasis such as mass chemotherapy and group-targeted chemotherapy.

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