There are hundreds of parasites that prefer humans as their host of choice, but only a few that can cause serious parasitic diseases. Human parasites fall into three categories: protozoa, helminths (or worms), and ectoparasites. Although ectoparasites can be a carrier for disease, they rarely cause diseases.
Definition & Facts
Parasitic disease is defined as a disorder of structure or function caused by a parasite. Many parasitic diseases are easy to treat; some are not. Untreated, some parasitic diseases can cause death.
The most consequential parasitic disease in the world is malaria. According to the Centers for Disease Control and Prevention (CDC) malaria kills 660,000 people per year on average. Over 200 million cases of malaria were reported in 2015.
Other parasitic diseases that cause over a million infections per year include dysentery, cryptosporidiosis, giardiasis, trichomoniasis, Chagas disease, African sleeping sickness, leishmaniasis, filariasis and amebiasis. These parasitic diseases are most common in the moist, warm climates of the tropics and the subtropics and thrive in poor communities with inadequate health care infrastructure.
Symptoms & Complaints
Because there are a multitude of parasitic diseases, there are a corresponding multitude of symptoms. Common symptoms of parasitic diseases caused by protozoa include fever, vomiting, diarrhea, dehydration, muscle aches, and unexplained weight loss.
The symptoms of malaria include moderate to severe shaking and chills, sweating, fever (often very high), vomiting, headache, and diarrhea. Malaria is indicated if these symptoms occur after visiting, or living in, a high-risk malaria region.
Both bacterial and amoebic dysentery cause severe diarrhea, often with blood or mucus, and stomach cramps. Amoebic dysentery, which is more common in the tropics, tends to have more severe symptoms and can include fever, chills, nausea, and fatigue.
Symptoms of parasitic diseases caused by helminths vary considerably depending on the type of worm, the number and volume of the worms, where the worms are in the body and how an individual’s immune system responds to the infestation. Some minor infestations do not cause any symptoms.
Ascariasis, or roundworm, is the most common of the helminths. It affects over one billion people per year. The symptoms of a roundworm infestation include worms in the stool, fever and vomiting, abdominal swelling and abdominal pain, coughing, and wheezing (worms can sometimes be coughed up). Anemia and intestinal blockages can occur in severe cases.
While it is correct to say that parasites cause parasitic diseases, the underlying cause of the spread of these diseases is often poor sanitation and lack of public health infrastructure. Malaria, for example, has been largely eradicated from the United States through extensive mosquito control efforts.
Parasites can be found in water, soil, food, waste, and blood. Some parasites can be transmitted through sexual contact and are thus sexually transmitted diseases. Waterborne parasites are common worldwide with the most impactful to human health being schistosomiasis, amebiasis, cryptosporidiosis, giardiasis, and Guinea worm disease. Schistosomiasis, caused by parasitic worms, is second only to malaria in devastating health impact. According to the CDC, nearly 200 million people are infected with schistosomiasis each year.
Inadequate sanitation and poor hygiene (mostly due to the absence of clean water) are common causes of the spread of parasitic diseases. The proper disposal of fecal matter is of particular concern. In addition to physical contact with feces, parasite eggs in dried feces can become airborne. Many types of helminths lay eggs in the digestive tracts of humans which can survive the digestive process and end up in feces. Protozoan parasites are also capable of surviving the digestive process and are often ingested via the fecal-oral route.
Diagnosis & Tests
The diagnosis of most parasitic diseases can be complicated because the symptoms are often not specific to any particular disease. In addition to physical symptoms, clinicians look at a patient’s travel history, geographic location, and clinical medical history. Traditional tests for diagnosing parasitic diseases include blood tests, examination of stool samples, and endoscopy or colonoscopy.
In extreme cases, clinicians may use imaging studies like computed tomography (CT) scans, X-rays, or magnetic resonance imaging (MRI) scans to check for internal damage or lesions caused by parasites.
Serology-based diagnostic tools are now being used by both the CDC and by the National Reference Centre for Parasitology (NRCP) at McGill University to more precisely diagnose parasitic diseases. Serology looks at the blood’s immune response to pathogens. These tests look for the presence of antigens and antibodies and can extract valuable information about the level and history of a parasitic infestation amongst other information. Mass spectrometry, proteomics, and molecular-based approaches are also being studied for their use in parasitic disease diagnosis.
Treatment & Therapy
Depending on the severity of the disease, treatment for parasitic diseases can be as simple as taking a single pill to completing a regimen of medications both intravenously and orally. The medication metronidazole has been shown to be an effective treatment for dysentery, giardiasis and trichomoniasis.
Prevention & Prophylaxis
Because parasites are often present in feces, correctly disposing of fecal matter is vital. This could entail anything from building an outhouse to burying fecal waste. Filtering and sanitizing drinking water can also eliminate parasites. Mosquitos, tsetse flies and other blood-sucking insects can be greatly reduced with investment in aggressive control measures.