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The Top 10 Deadliest Parasites in the World

Larry Slawson received his Masters Degree at UNC Charlotte. He specializes in Russian and Ukrainian History.

Up-close view of a Pinworm infection.

Up-close view of a Pinworm infection.

10. Pinworms

Pinworms, also known as the “threadworm,” is a parasitic disease known to cause enterobiasis (or "pinworm infection"). Found worldwide, the Pinworm is one of the most common parasitic infections in North America, Western Europe, and Oceania. The parasite itself is a thin, white worm known scientifically as the Enterobius vermicularis. The worm is usually the length of a staple and is known to infect the colon and rectum of humans. Although all individuals (regardless of age, sex, or race) are susceptible to the disease, researchers have discovered that children are at the highest risk of infection. Because Pinworms are spread via the fecal-oral route, children (particularly those in childcare centers) are at a much higher risk of infection as finger-sucking and nail-biting are more common in this age group. Common symptoms of the parasite include severe itching around the anal/rectum region (particularly at night), abdominal pain, irritability, nausea, weight loss, and the presence of small worms around the rectal area. In more severe cases, pinworms can even enter into the urinary tract causing infections and difficulties with urination.

Diagnosis of Pinworms is relatively easy for doctors, and includes a “tape test” to collect samples from the anal area. Sometimes, simple visual inspection with a flashlight is enough to detect the parasite, particularly in severe cases. Although rarely deadly, rapid treatment of a Pinworm infection is needed to prevent further spread of the parasite. Treatment options include EMVERM (mebendazole) which is approximately 95-percent effective against the infection.

Toxoplasmosis infection.

Toxoplasmosis infection.

9. Toxoplasma Gondii

Toxoplasma Gondii is a highly infectious parasite known to cause toxoplasmosis. The disease is common in uncooked food, but is found predominantly in cat feces. The disease can be spread to adults and children who come into contact with fecal matter, with pregnant women being among the most susceptible to infection. For this reason, pregnant women are often encouraged to avoid cats during their pregnancy, as toxoplasmosis can be given to unborn babies causing severe complications (known as congenital toxoplasmosis). It is estimated that nearly half of the world’s population is infected with the parasite at some point in their life, with approximately 11-percent of the American population affected annually (most unaware of their infection). There are also 200,000 new cases of congenital toxoplasmosis diagnosed each year.

After being infected, diagnosis of the parasite is difficult to obtain as the disease possesses no obvious symptoms aside from a mild flu-like illness a few weeks after infection. Infection is known to have three separate stages, including “Acute Toxoplasmosis,” “Latent Toxoplasmosis,” and “Cutaneous Toxoplasmosis.” However, in healthy adults, symptoms rarely present themselves as the body’s immune system is usually able to suppress the parasite enough to prevent sudden illness. Left untreated, the parasite can have severe consequences, particularly for expecting mothers. This includes miscarriages, and an increased risk of blindness, epilepsy, or severe mental abnormalities for infants. In the United States alone, nearly 750 people die of the parasite each year, with half of these cases being from contaminated meat. For this reason, toxoplasmosis is considered the “third leading cause of foodborne deaths in the country” ( Experts agree that cleanliness and proper food preparation are the best remedies for toxoplasmosis ( For cat owners, the CDC also recommends that litter boxes should be changed daily to prevent a buildup of the parasite in your home, and to avoid feeding cats raw/uncooked meat. Although difficult to treat, it should also be noted that multiple treatment options involving a combination of pyrimethamine, sulfadiazine, and folinic acid have proven effective in eliminating the parasite.

Filariasis infection (cycle).

Filariasis infection (cycle).

8. Filarial Worms

Filariasis is a parasitic disease caused by several species of filarial worms (also known as roundworms). The worms are commonly spread via mosquito bites, where they enter the blood stream of their new host and actively attack the lymphatic system. Although many individuals experience no symptoms upon initial infection, others develop severe swelling of the arms, breasts, legs, and genitals known as elephantiasis. Although rarely fatal, the worm’s impairment of the lymphatic system causes severe disabilities causing extreme pain and often times social exclusion throughout third-world countries (as the individuals are usually stigmatized by local communities). The parasite is believed to infect over 120 million people very year, leaving approximately 1/3 of these individuals (40 million) disfigured and permanently incapacitated ( Due to the fact that filarial worms are spread via mosquito transmission, the World Health Organization estimates that nearly 886 million people are at risk of contracting the infection in a total of 52 different countries, worldwide. Although a treatable condition (particularly when infection first occurs), long-term infection is known to also cause secondary bacterial infections, and substantially weaken the immune system of the parasite’s host.

Numerous treatments exist to combat filarial worms, including albendazole, ivermectin, and diethylcarbamazine citrate. Regular use of these medicines have proven highly effective in eradicating microfilariae from the blood stream within weeks. Mosquito control remains the best option for eradicating the spread of this parasite, along with preliminary treatment of populations susceptible to the disease.

Tapeworm (up close).

Tapeworm (up close).

7. Tapeworms

The Tapeworm is a flat, segmented worm known to reside in the intestinal tract of animals, and is believed to infect livestock after the consumption of contaminated water. These parasites are then spread to humans after consuming raw or uncooked meat from infected animals (including pigs, fish, and cows). Although easily treated, long-term infection can cause life-threatening conditions for humans as the worm continues to grow; blocking intestinal ducts, causing seizures, muscle and eye damage, as well severe damage to vital organs within the body. In more advanced cases, the tapeworm is also capable of entering other parts of the body, causing severe damage to an individual’s heart, liver, and brain. The Tapeworm thrives in the human body by absorbing key nutrients through their skin. Although most tapeworms are relatively small (1/250 of an inch), some have reached astounding lengths of nearly 50 feet (15.23 meters). Although deaths are relatively rare in the United States, the World Health Organization (WHO) estimates that nearly 50,000 people die from tapeworm infections (and their complications) each year, worldwide.

The lack of symptoms caused by a tapeworm infection make diagnosis difficult. Occasionally, the only symptom present with tapeworms is the passing of worm segments during bowel movements. For the most part, however, symptoms are vague and include general nausea, abdominal pain, hunger (or loss of appetite), diarrhea, lethargy, weight loss, and vitamin deficiencies. If a tapeworm infection is suspected, doctors often use a combination of tests including CT scans or MRIs, blood tests, and stool samples to confirm the parasite’s presence. If diagnosis of a tapeworm infection is confirmed, oral-based medicine – praziquantel (Biltricide) – is administered to kill the worm. Praziquantel is highly effective (95-percent cure rate), and is known to paralyze the tapeworm, causing them to release themselves from the intestine, dissolve, and pass during bowel movements ( Experts agree that the most effective way to prevent tapeworm infections is to practice good hygiene, and proper food-preparation techniques (cooking food to appropriate temperatures).

Hookworms (up close).

Hookworms (up close).

6. Hookworms

Hookworms are parasitic worms that are known to infect the small intestine of humans. Currently, there are two species of the parasite known to infect humans, including the Ancylostoma duodenale and Necator americanus. The parasite is primarily found in the soil of warm and moist climates; particularly in areas where sanitation levels are below standard. For this reason, walking barefoot on contaminated ground remains the primary transmitter for this disease, as the larvae attaches itself to the body and penetrates through the skin. After infection, common symptoms include itching and localized rashes. However, these symptoms are usually short-lived, as light infections rarely produce additional complications. In cases of serious infections, individuals are known to experience non-specific symptoms including abdominal pain, diarrhea, weight loss, lethargy, loss of appetite, and anemia. Advanced symptoms also include extreme weight loss, cardiac failure, and indigestion, vomiting, and the development of protein deficiencies. Infections in children are most problematic, as the physical and cognitive development of the child can be greatly affected by the presence of hookworms in their bodies. Although rarely fatal, the Center for Disease Control (CDC) estimates that approximately 576 to 740 million people are infected by hookworms each year, worldwide. Death rates vary significantly for the parasite, with numbers being as high as 62,000 deaths per year ( Of these fatalities, extreme blood loss and anemia-related issues appear to be the primary cause of death, as the hookworm is known to cause damage to various blood vessels throughout the body.

Stool samples remain the best test for confirming the presence of hookworms in humans. After a successful diagnosis, Anthelminthic medications such as mebendazole and albendazole are often used for a period of 1 to 3 days, with a high-degree of efficacy. Iron supplements are also prescribed for individuals suffering from anemia-related complications from the parasite. Experts agree that the best form of prevention for hookworms is good hygiene, and using proper footwear when outdoors.

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Whipworm infection.

Whipworm infection.

5. Whipworms (Trichuriasis)

Whipworms are a soil-based parasite known to infect approximately 604 to 795 million people per year, worldwide. Similar to hookworms, whipworms are primarily transmitted to humans via contaminated soil. Vegetables, fruits, and dirty hands/fingers are often the primary transmitters of the parasite’s eggs, which enter humans through the mouth if proper hygiene is not followed regularly. Upon entering the body, whipworms take up residence in the large intestine causing either light or heavy infections over time. In light infections, individuals are usually asymptomatic; however, in cases of heavy infection, the most common symptom of whipworms includes painful bowel movements that contain a mixture of mucus, blood, and water ( In more serious infections, bloody diarrhea, extreme nausea, vomiting, headaches, and severe weight loss are also common, along with rectal prolapse, and appendicitis. In children, the effects of whipworms are often the most serious, and can impede cognitive growth and development, as well as anemia, and nutritional deficiency. Although the number of deaths caused from the parasite are currently unknown, it is estimated that whipworm-related deaths are around the same number as hookworms, approximately 62,000 per year, worldwide.

Similar to hookworms, diagnosis of a whipworm infection is obtained by routine stool sample tests. Upon successfully diagnosing an individual with the parasite, both albendazole and mebendazole are the primary drugs used for treatment, along with iron supplements for those suffering anemia-related illnesses from the worms. Treatment is highly effective, with infections clearing after only three days. Experts agree that good hygiene and proper food preparation are the best methods to prevent infection.

Blood fluke.

Blood fluke.

4. Schistosoma (Blood Flukes)

The Schistosoma, also know as “Blood Flukes,” are parasitic flatworms from the genus trematodes. Blood Flukes derive from freshwater snails, and are commonly found in ponds, lakes, and standing bodies of water. Humans commonly contract the parasite after wading into infected water. Once in, the flukes attach themselves to the skin, and literally drill their way into the body; entering blood vessels until they make their way to the intestinal tract of their host. From there, the Blood Fluke begins producing large numbers of eggs, which are flushed out of the body (via stool), and hatch later in fresh water. Although the eggs are incapable of hatching inside their human host, on occasion the eggs get swept towards the liver where they cause severe inflammation and swelling. Symptoms are relatively rare for Blood Fluke infections, making diagnosis difficult. Depending on the species of the Blood Fluke, humans can develop a wide range of issues, however, including fever, cough, abdominal cramps/pain, diarrhea, hepatosplenomegaly (swelling of the liver and spleen, as well as eosinophilia (high-levels of white blood cells). It is important to note that these symptoms derive from the body’s reaction to the eggs, rather than the worms themselves. Left untreated, Blood Flukes are also capable of causing anemia, malnutrition, learning difficulties/neurological disorders, and severe damage to the liver, intestine, spleen, bladder, and lungs that can be fatal over time. With the ability to thrive in humans for decades (upwards of 30+ years), the Blood Fluke is also capable of inflicting a large array of diseases and disorders in the human body, including bladder cancer, paralysis, as well as brain and spinal cord inflammation ( Approximately 200-million people are known to be infected by the parasite annually, with reactions to the Blood Fluke’s eggs occurring 1 to 2 months after infection. Some individuals, however, have reported rashes and itchy skin within a few days of infection. Although death rates vary widely, the World Health Organization estimates that nearly 200,000 people die each year, with an additional 20-million individuals suffering severe complications from the parasite.

Treatment options for the Blood Fluke are available, including the prescription medication known as Praziquantel. Only 1-2 days of medication are needed (on average) to eradicate the worms. Experts agree that the best way to combat Blood Flukes is to avoid swimming/wading in stagnant water, and to avoid drinking water that is potentially contaminated or polluted (

Acanthamoeba infection.

Acanthamoeba infection.

3. Acanthamoeba

Acanthamoeba is an extremely dangerous amoeba parasite found in soil, air, ponds, lakes, and oceans worldwide. The parasite is known to cause Acanthamoeba Keratitis (which is a rare but serious infection of the eye that can result in permanent blindness), as well as GAE - Granulomatous Amebic Encephalitis (Brain and Spinal Cord Infection), or “Disseminated Infection.” Left untreated, the amoeba is capable of affecting the skin and central nervous systems of its host with deadly consequences. Although most people are exposed to the Acanthamoeba during their lifetimes, only a very few people will become ill after their exposure. Although little is known about the parasite, researchers believe that it infects humans through the nasal passageways, eyes, or ulcerated/broken skin. Depending on the infection, diagnosis of the parasite is often quite difficult, and is usually not noticed until the infection reaches advanced stages. In individuals with GAE, swelling and inflammation of the lungs, sinuses, and brain are common, along with loss of coordination, mental duress, fever, muscle weakness, paralysis, double vision, and a host of various neurological problems. In Acanthamoeba Keratitis, symptoms vary significantly, but generally involve severe eye pain, eye redness, blurry vision, sensitivity to light, and the sensation of something being in the eye ( Early diagnosis is critical for successful treatment, and usually involves a combination of brain scans, spinal taps, and biopsies to confirm the infection.

Although prescription-strength medications are available for patients suffering from Keratitis, treatment is often long and difficult. Likewise, skin infections caused by the parasite usually involve extensive creams and medications to prevent its spread. In advanced stages involving infection of the central nervous system, brain, and spinal cord, the amoeba is often fatal. Prevention of the parasite includes swimming in properly maintained pools, not wearing contacts while swimming, avoiding pools or bodies of water if you have cuts, and regularly cleaning contact lenses.

Malaria infection in the bloodstream.

Malaria infection in the bloodstream.

Malaria Quote

"There are more people dying of malaria than any specific cancer."

— Bill Gates

2. Malaria

Malaria is a mosquito-based disease caused by four different parasites known as Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malaria. The parasites are known to actively infect mosquitos which, in turn, transmit the disease to humans when they are bitten. Although less common, malaria may also be spread via blood transfusions, organ transplants, or through the sharing of contaminated needles. Following initial infection, an incubation period of approximately seven to thirty days follows before the first symptoms begin. In other species of malaria, the first symptoms may not appear for several months (upwards of a year). As a result, diagnosing malaria, particularly in its inchoate phases, is relatively difficult for doctors. Currently two variants of the disease are recognized: uncomplicated and severe. In uncomplicated cases, symptoms last only six to ten hours and include fever, chills, headache, nausea, vomiting, malaise, and severe sweating. In severe malaria, however, the disease is known to attack the body’s vital organs and blood stream, causing severe anemia (due to the destruction of red blood cells), loss of consciousness, seizures, coma, and neurological abnormalities. Other issues include hypoglycemia, metabolic acidosis, severe respiratory distress (ARDS), hemoglobinuria (presence of hemoglobin in urine), cerebral malaria, as well as acute kidney failure. Although antimalarial medications exist to combat the parasite, rapid treatment is necessary to prevent further spread of the disease throughout the body. Blood tests are currently the best diagnostic tool available to doctors, allowing health care providers the ability to confirm the presence of malarial parasites by microscopic analysis of the blood.

According to UNICEF, nearly 600 million people are infected by malaria each year (worldwide), with nearly 1-million of these individuals dying from the parasite ( Children, pregnant women, the elderly, and those with compromised immune systems are particularly susceptible to malaria, as their bodies are incapable of fighting infections. It is estimated that nearly 3,000 children die per year from the parasite, while those who survive are often left with physical and mental impairments ( Currently, mosquito control remains the only effective option for preventing malaria around the world. Due to the high costs associated with this, however, many third-world countries are simply unable to combat the parasite in an effective manner.

Naegleria Fowleri in its three stages of development (Left to right).

Naegleria Fowleri in its three stages of development (Left to right).

1. Naegleria Fowleri (Brain-Eating Amoeba)

Naegleria is an amoeba (a single-celled organism) found in the soil and warm bodies of water, including lakes, ponds, rivers, swimming pools, and hot springs. Although several species of this amoeba exist, only one is known to actively infect humans: Naegleria Fowleri (Brain-Eating Amoeba). The amoeba is considered extremely dangerous to humans, and often enters the body through the nose. This commonly occurs when individuals are swimming or diving in warm bodies of water, as the sudden entry of water into the nose allows the amoeba to travel deep into the nasal cavity, where it begins to actively move towards the brain. Although infections for this parasite are extremely rare (affecting only 34 people in the United States during the last ten years), experts agree that infections appear to be on the rise, worldwide, with the summer months being the primary time of infection (due to the presence of warmer waters).

Symptoms of an infection usually being within five days of exposure. As the amoeba enters the brain area, the parasite is known to cause “Primary Amebic Meningoencephalitis” (PAM) – a brain infection that rapidly destroys brain tissue. Initial symptoms are very similar to bacterial meningitis, and include severe headache, nausea, vomiting, and fevers. As the infection progresses, advanced symptoms include stiff neck, lack of coordination, confusion, seizures, and hallucinations as the infection begins to cause severe brain swelling. After the first signs of the parasite begin, death usually follows within five days. The parasite is considered extremely deadly, even when caught early. Out of 145 infections in the United States between 1962 to 2018, the Center for Disease Control states that only four people have survived; a fatality rate of over 97-percent. Although new treatments are being developed to counteract the parasite (including the drug miltefosine, which has had promising results in laboratory experiments), the disease is still poorly understood and poses a significant risk to those who are infected. Fortunately, infections from the amoeba are extremely rare, despite the presence of these parasites in many locations, worldwide. Researchers agree that the best way to prevent infection is to properly maintain pools, and to swim using nose-plugs (or similar devices) to prevent the parasite from entering the naval cavity.


Works Cited:

Articles / Books:

  • “Acanthamoeba - Granulomatous Amebic Encephalitis (GAE); Keratitis | Acanthamoeba | Parasites | CDC.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, n.d.
  • “CDC - Hookworm.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, n.d.
  • “CDC - Trichuriasis.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, n.d.
  • “CDC - Malaria - About Malaria.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, n.d.
  • Naegleria Fowleri. Centers for Disease Control and Prevention. CDC, n.d.
  • “Pinworms.” MedlinePlus. U.S. National Library of Medicine, March 6, 2018.
  • “Tapeworms in Humans: Causes, Symptoms, and Treatments.” WebMD. WebMD, n.d.
  • “Toxoplasmosis.” Mayo Clinic. Mayo Foundation for Medical Education and Research, October 3, 2017.

This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

© 2019 Larry Slawson


Hammad Ur Rehman Bajwa on April 30, 2020:

Such an informative article, By the way i am working currently on Toxoplasma gondii molecular genetics and Filarial worms especially that are found in dogs

Larry Slawson (author) from North Carolina on September 01, 2019:

Thank you clovisj! So glad you enjoyed!

Clovis from Texas on August 31, 2019:

This is so informative! I didn't even know about some of these!

Larry Slawson (author) from North Carolina on August 27, 2019:

@John Thank you so much! So glad you enjoyed!

John Wilsdon on August 25, 2019:

This is a great article; so informative. I am forwarding this to both of my daughters.

Larry Slawson (author) from North Carolina on August 23, 2019:

I was actually wondering about that myself, Sam. From what I can tell though, a lot of the 600 million infections are people "reinfected" by the parasite each year. A lot of people are known to also suffer relapses of the disease only a few months after treatment since it remains in their bloodstream (apparently, this happens in a lot of cases). The CDC commented on this a few years ago, and they believe a lot of it has to do with the huge mosquito problem that exists in these countries (in particular, countries in Southeast Asia and Africa). People get better from the disease, only to be bitten by an infected mosquito months or years later and fall ill again. Pretty tragic, when you think about it. Over time, people develop a partial immunity to malaria; however, its only effective to a certain degree.

Sam Shepards from Europe on August 23, 2019:

One remark: According to UNICEF, nearly 600 million people are infected by malaria each year (worldwide) ???

Do they count all infected people not only that year, because if you have 600 million new infections you would have the entire world population in 12 years?

Or do they mean certain people keep on being infected by new bites, but are immune ?

Sam Shepards from Europe on August 23, 2019:

Those symptoms brrrrr. Always something to keep in mind when traveling. I don't like to take Maleron pills, but if you do jungle treks it's the only way. Some famous athlete from my country was in Congo for a couple of days publicity... Mararia. Coma and now amputating toes. Heavy stuff...

Larry Slawson (author) from North Carolina on August 23, 2019:

Thank you my friend! I agree. I'm actually planning on doing some smaller articles on each one of these parasites in the very near future. I had originally intended for this to be a short article, but there was so much information about each parasite that it quickly went over 3,000+ words haha!

Eric Dierker from Spring Valley, CA. U.S.A. on August 22, 2019:

Very interesting. I had to break it up not reasonable minutes. I wonder if maybe you do 3 smaller ones. Help us also to see serious differences.

I try to teach so hard the lesson that we have some baddas stuff in our bodies to fight such. Take care of them.

Really interesting my boy and I will do a drive by on this.

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