Helminths | A brief Introduction to Parasites

a microscopic photo of a helminth
Instagram: @microbes_and_pathogens

Helminths are invertebrate, parasitic worms which inhabit human intestinal and ventral sites. They are classified into 4 phylum: Nematoda, Annelida, Platyhelminths and Acanthocephala. Whipworms, hookworms and pinworms are found within the Nematode phylum alongside parasitic worms causing strongyloidiasis and trichinosis. Annelida are leeches and are the only true segmented worms. Cestodes also known as tapeworms and Trematodes or flukes are common classes within the platyhelminth phylum. Schistosomiasis is caused by a type of worm within the Strigeiformes order of platyhelminths. Acanthocephala are thorny-head worms with only two subclasses. 

microscopic picture of Steinernema Feltiae - Entomopathenogenic Nematodes one of the most common helminths
Nematode. Instagram: @mitemunchers

Several of the diseases listed in the World Health Organisations eradication campaign are caused by Helminths, such as lymphatic filariasis caused by a roundworm from the Nematoda phylum.

Epidemiology of Helminth Infection

Over 2 billion of the world population are infested by soil-transmitted Helminths. Africa, South Asia and South America have the highest prevalence and over 900 million children live in these high risk areas. 800 million people are infected worldwide by roundworms and over 450 million and 430 million are infected with whipworm and hookworm respectively. Infection with pinworm is thought to be as high as 50% in some the the Eastern Balkan states. Whilst infestation rarely leads to death or significant morbidity, long term affects can be significant.

Transmission of Helminths

Each groups has evolved with different strategies for entering the human host. These methods vary from direct oral ingestion from contaminated food to being transmitted by mosquitos and other arthropods. Hookworms can penetrate human skin, making walking bare foot a risk factor for infestation. Trematodes, for example, are carried by snails and fish, they are either directly ingested or enter the skin through hair shafts. Cestodes reside in fish, beef and fork, infecting hosts when inadequately cooked and ingested. Nematode eggs have been isolated from paper money in many endemic countries.

Global map showing coloured areas related to the national burden of infection with helminths
Worldwide national burden of Helminth Infestation Source: A Critical Appraisal of Control Strategies for Soil-Transmitted Helminths

Each species goes through several stages inside the human host. Eggs may pass through the lymphatics, lungs and liver before eventually residing in the portal circulation or intestine. Species such as ascaris lumbricoides, a type of nematode roundworm cause physical obstruction of the lymphatics. Other species may produce peptides, down regulating the hosts immunological response. In people who are co-infected with HIV this can make them even more susceptible to opportunistic infections.

The ability of worms to survive in such inhospitable environments remains somewhat a mystery. Some would think that the human bloodstream would exert a constant attack of host immune cells yet some species of worm can live here for several years.

Signs and Symptoms of Helminth Infection

Childhood infestation with worms leads to malnutrition, stunted growth and impaired intellectual development. Hookworm can specifically cause anaemia in children so Hb should always be checked if there is clinical suspicion. Other common symptoms include bloating, non-specific abdominal pain and a long history of mild diarrhoea, however, in the majority of cases, hosts will be asymptomatic.

Bloody loose stools are specifically associated with whipworm infestation and severe infection can lead to rectal prolapse. Pinworm classically causes perianal itching whilst roundworm infection is associated with respiratory infection in the early stages. 

Cysticercosis is a tissue infection which can be caused by a cestode tapeworm transmitted in pork. It is a major cause of seizures in the developing world and should always be considered in patients presenting with new-onset of seizures.

MRI showing cerebral cysticercosis
Cerebral Cysticercosis. Instagram: nyonyamahameru

Treatment

Treat all cases of roundworm, hookworm and whipworm with an initial stat dose of 400mg dose of Albendazole without food, reduced to 200mg for children between 1 and 2 years old. If there is suspicion of pinworm the treatment should continue for three days. The dose can be repeated on day 8 in cases of severe infection and iron supplements given to any patient with co-existing anaemia.

If you suspect neurocystocercosis from CT or MRI scan and serological tests, you should treat hydrocephalus or raised intra-cranial pressure prior to the commencement of antiparasitics. This can be done with corticosteroids prior to a 14 day course of Albendazole. Thereby reducing the inflammation associated with degenerating cysts.

Health Education is extremely important in endemic areas. Re-infection is incredibly common and it is therefore advisable to treat whole households simultaneously to reduce this risk. Frequent hand washing and bathing can help reduce transmission rates and washing fruits and vegetables with purified water is another protective measure. Additionally, we can reduce Hookworm transmission by encouraging people to wear shoes.

Helminth Health Burden in Bocas Del Toro

Escapemedic is currently working with US based organisation Floating Doctors. The organisation holds mobile clinics in the indigenous Ngöbe communities in the Bocas Del Toro province of Panama. Research conducted by Hannah Elsevier found that Helminth infestation accounted for 38% of the organisations disease burden since 2011. Over 35% of patients infected with Helminths sourced their water from rivers. As a result of these findings, the organisation has been working hard over recent years to reduce this health burden. We encourage communities to source their water from wells or rain tanks where possible and give Albendazole to every patient who attends clinic every three months.

several doctors on a cayuco in the beautiful turquoise seas of bocas del toro
Floating Doctors on their way to clinic. Instagram: @escapemedic

Floating doctors partners with Vitamin Angels, an organisation committed to improving the nutritional status of children worldwide. If you work for or run your own Non-governmental Organisation and are interested in applying for a partnership with them, head over to their website for more information.

Sources:

  1. Medical Microbiology. Baron, S. 1996. Chapter 86.
  2. Prevalence and distribution of soil-transmitted helminth infections in India. Salam, N. Azam, S. 2017.
  3. World health Organisation: https://www.who.int/intestinal_worms/epidemiology/en/
  4. Corticosteroid Use in neurocysticercosis. Nash, T. 2011
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Helminths are invertebrate, parasitic worms which inhabit human intestinal and ventral sites. They are classified into 4 phylum: Nematoda, Annelida, Platyhelminths and Acanthocephala. Whipworms,...
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