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The common parasites (including various worms) that infect us


All of us are probably aware of the word “parasite”, but how many of us actually know what a parasite is?

Or how they enter our body, what diseases they cause and how to treat such an infection?

Basically, a parasite is an organism that is dependent on a host for its survival.

A host is an organism that harbors the parasite, providing it with nourishment and shelter, and is usually larger in size than the parasite.

Parasites can be of different types, e.g. the single-celled protozoa, flukes, tapeworms, roundworms and so on.

Some parasites can stay in just one host throughout its lifetime, while others require two hosts (one intermediate and one final) in order to complete its growth and development.

There are many ways human beings can get parasitic infections.

Some of the common ways are from mother to child in the womb or during delivery (congenital infection); direct or indirect contact, including during sex; inhalation; bite of a vector; the fecal-oral route; blood transfusion or contamination; and through injury of the skin and mucous membranes.

But the most common mode of infection is via the fecal-oral route.

This occurs through the consumption of raw or undercooked food like fish, meat and vegetables, or water, which is contaminated with feces containing the infective stages of a parasite.

Here are some of the common and medically important parasites that are prevalent in our region.

> Plasmodium

Plasmodium is a genus of single-cell DNA-based organisms that are obligate parasites of vertebrates and insects.

An obligate parasite is one that cannot survive without a host.

Plasmodium parasites cause malaria, which affects over 40% of the world’s population and remains the world’s most devastating human parasitic infection.

It is the most important cause of fever and illness in the tropical world.

It is also probably one of the oldest diseases known to mankind.

There are four species of this parasite: Plasmodium vivax, P. ovale, P. malariae and P. falciparum.

The most dangerous of the four is P. falciparum, which accounts for 90% of malarial deaths.

Plasmodium is transmitted to humans via the bites of infected female Anopheles mosquitoes.

The attack begins with fever, chills and rigors, with the patient’s temperature rising to as high as 40ºC and falling again over a period of several hours.

Other symptoms include a poor general condition, headache, diarrhea, nausea and vomiting.

Chloroquine is usually used for treatment.

Entamoeba histolytica

One of the most important and pathogenic parasites in humans, E. histolytica is primarily a human parasite.

It is transmitted by fecal contamination of drinking water and food by direct contact with dirty hands or objects, as well as through sexual contact.

It is the cause of amoebic dysentery (bloody diarrhea) and colonic ulcerations.

The symptoms are often quite mild and may include diarrhea, stomach pain and stomach cramps.

More severe infection may cause fever, profuse diarrhea, abdominal pain, jaundice, anorexia and weight loss.

In rare cases, E. histolytica invades the liver and forms an abscess.

Metronidazole or tinidazole is usually used for treatment.

> Giardia lamblia

This is a flagellated parasitic microorganism that colonies and reproduces in the small intestine, causing the diarrheal illness known as giardiasis.

Its infection occurs through the consumption of cysts in contaminated water or food, or via the fecal-oral route (through poor hygiene practices).

Giardiasis affects people of all ages.

Common symptoms include diarrhea, low grade fever, gas, stomach or abdominal cramps, nausea, dehydration and various neurologic symptoms.

Metronidazole is usually used for treatment.

> Enterobius vermicularis

This parasite is also known as pinworm, seatworm or threadworm.

Its natural habitat is the caecum, appendix and the adjacent ascending colon in humans.

It causes enterobiasis, which commonly occurs in children.

Pinworm infection usually occurs through the ingestion of infectious eggs on contaminated hands, food, water, clothing and dust, which are transferred directly via the fecal-oral route by one’s fingers.

This is facilitated by a perianal itch (pruritis ani) caused by the presence of pinworm eggs in the perianal folds.

Transfer of the eggs commonly occurs when scratching the itch is followed by nail-biting, poor hygiene or inadequate hand-washing.

Common symptoms include itching of the anal or vaginal area, insomnia, irritability and restlessness, intermittent abdominal pain, and nausea.

Albendazole or mebendazole is usually used for treatment.

Ascaris lumbricoides

Also known as roundworm, this parasite can grow to a length of 35cm inside a human host.

It is the most common and largest parasitic worm in humans.

Infection occurs after accidentally ingesting the eggs, either due to poor hygiene after being in contact with soil contaminated by human feces that contain the eggs, or eating uncooked food contaminated by soil that contains the eggs.

Common symptoms include abdominal pain, diarrhea, cough, blood-tinged sputum, shortness of breath, asthma, fever (from migrating parasites that invade the lungs) and muscle pains.

Severe infections can cause intestinal blockage and impair growth in children.

Albendazole or mebendazole is usually used for treatment.

> Hookworms

There are two main types of hookworms: Ancylostoma duodenale and Necator americanus.

Hookworm infection is a leading cause of maternal and child death in the developing countries of the tropics and subtropics.

It usually infects humans as larvae, which develop from eggs that are excreted in human feces onto soil.

These larvae will then penetrate the skin of a person walking barefooted on the contaminated soil.

A. duodenale can also cause infection through the ingestions of its larvae.

The danger of hookworms is that they feed voraciously on human blood as adults living in the small intestine.

Common symptoms of infection include low grade fever, iron deficiency anaemia, nausea, vomiting, diarrhea, abdominal discomfort, cramps, anorexia and weight loss.

Symptoms involving the skin include intense itching, oedema, redness of the skin (erythema) and rash.

Severe infection in children can affect their physical and mental growth and development.

Albendazole or mebendazole is usually used for treatment.

> Trichuris trichiura

This parasite is commonly known as a whipworm because it resembles a whip.

Humans become infected with the parasite via ingestion of infective eggs from contact with hands or food contaminated with soil that contain the eggs.

Common symptoms include bloody diarrhea, painful or frequent defecation, abdominal pain, nausea, vomiting, headaches, fecal incontinence, and sudden and unexpected weight loss.

Albendazole or mebendazole is usually used for treatment.

> Toxoplasma gondii

This is one of the most common human parasites in developed countries and causes the disease toxoplasmosis.

It is transmitted through eating raw or undercooked beef, lamb or pork contaminated with the parasite’s tissue cysts; consuming soil, milk, water or vegetables contaminated with its oocytes; and inhalation of its oocysts.

Congenital toxoplasmosis can occur if the mother is infected during pregnancy.

Toxoplasmosis usually causes no obvious symptoms in adults.

Some people may occasionally experience a few weeks or months of mild, flu-like illness, including muscle aches and tender lymph nodes.

In a small number of people, eye problems such as blurred vision, floaters, pain, red eye, metamorphopsia (when straight lines look rounded or curvy) and photophobia (extreme sensitivity to light) may develop.

Pyrimethamine trimethoprim/sulphamethoxazole, sulphadiazine, clindamycin and prednisone are usually used for treatment.

> Fasciola hepatica

Also known as the common liver fluke or sheep liver fluke, F. hepatica is a parasitic flatworm that infects the livers of various mammals, including humans.

It causes the disease called fascioliasis, which is classified as a neglected tropical disease.

Transmission occurs through the consumption of raw, freshwater vegetation on which the flukes in their metacercariae form are encysted.

Humans are often infected by eating raw watercress.

Liver flukes infect the liver, gallbladder and bile duct in humans.

Common symptoms include abdominal pain, anaemia, hepatic tenderness, an enlarged liver (hepatomegaly) caused by oedema, intermittent fever, jaundice, lethargy, nausea, prolonged high fever, vomiting and secondary infections.

Bithionol or triclabendazole is usually used for treatment.

Diagnosis and prevention

There are many more parasites, aside from those mentioned in this article, that can cause infection in humans.

Generally, parasitic infections are diagnosed in the laboratory through blood tests, stool examination to detect parasitic eggs, and microscopy.

Parasitic infections can sometimes be very serious, but early detection and treatment can provide good outcomes.

There are several steps we can take to lower the risk of contracting a parasitic infection, including:

·         Washing our hands regularly, especially after handling uncooked food or feces

·         Cooking food to its recommended internal temperature

·         Washing vegetables and fruits properly before cooking or eating

·         Practicing safe sex

·         Drinking clean water, e.g. bottled water, while travelling

·         Avoiding swallowing water from lakes, streams or ponds.

In other words, we must maintain personal hygiene at all times.

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