Parasites are organisms that live on or in a host organism and get their food from or at the expense of the host. There are three main classes of parasites that can cause problems for humans: protozoa, helminths, and ectoparasites.
Protozoa
Protozoa are single-celled parasites that can live and multiply within the human body. Some of the most common protozoan parasites that infect humans include:
- Plasmodium species – cause malaria
- Entamoeba histolytica – causes amoebic dysentery
- Giardia intestinalis – causes giardiasis
- Cryptosporidium – causes cryptosporidiosis
- Toxoplasma gondii – causes toxoplasmosis
- Trichomonas vaginalis – causes trichomoniasis
Protozoan parasites are transmitted through contaminated food or water, contact with infected feces, or through the bite of an infected insect like a mosquito. They can cause acute symptoms like diarrhea, abdominal pain, fever, chills, nausea, and vomiting. Some protozoa can cause chronic infections and more serious complications like:
- Liver abscesses
- Loss of vision
- Seizures
- Anemia
- Birth defects
- Respiratory failure
Malaria
Malaria deserves special mention as one of the most severe diseases caused by a protozoan parasite. It is estimated that in 2019 there were 229 million cases of malaria worldwide resulting in 409,000 deaths. The parasites Plasmodium falciparum and Plasmodium vivax cause the most infections worldwide. Symptoms of malaria include:
- Fever
- Chills
- Headache
- Nausea and vomiting
- Muscle pain
- Fatigue
If malaria is not promptly treated, it can progress to severe illness and death. Complications include:
- Cerebral malaria – parasite infection of the brain characterized by altered mental status, seizures, coma
- Pulmonary edema – fluid buildup in the lungs
- Kidney failure
- Liver failure
- Anemia – due to destruction of red blood cells
- Low blood sugar
The recurrent fevers caused by malaria parasites also take an economic toll in endemic areas by reducing productivity and absenteeism. Cerebral malaria has a mortality rate of 15-25% even with treatment.
Helminths
Helminths are parasitic worms that live inside the body and can grow to be visible to the naked eye. The major groups of helminths that are human parasites include:
- Roundworms (nematodes) – Ascaris lumbricoides, hookworms, Strongyloides stercoralis
- Tapeworms (cestodes) – Taenia saginata, Taenia solium
- Flukes (trematodes) – Schistosoma species, liver flukes
Like protozoa, transmission occurs through contaminated food and water or skin penetration. Helminths drain nutrients from the body leading to weakness, weight loss, and impairment of physical and cognitive development in children. Other problems can include:
- Iron-deficiency anemia
- Abdominal pain
- Diarrhea
- Skin rashes
- Cough and wheezing from larval migration through the lungs
- Bladder and kidney damage
- Enlargement of organs like the liver or spleen
Schistosomiasis
Schistosomiasis deserves special mention as one of the most prevalent helminthic parasites of humans worldwide. It is estimated that more than 200 million people are infected. The disease is caused by several species of Schistosoma flukes transmitted in freshwater snails. Acute symptoms include:
- Fever
- Chills
- Cough
- Muscle aches
- Abdominal pain
- Diarrhea
Chronic schistosomiasis can lead to:
- Damage to liver, intestines, spleen, lungs, and bladder
- Cancer of the bladder
- Cognitive impairment in children
- Increased susceptibility to HIV/AIDS
| Helminth Species | Number Infected (millions) |
|---|---|
| Ascaris lumbricoides (roundworm) | 807 |
| Trichuris trichiura (whipworm) | 604 |
| Necator americanus and Ancylostoma duodenale (hookworms) | 576 |
| Schistosoma mansoni and S. haematobium (blood flukes) | 207 |
Table 1. Estimated number of people infected worldwide with the most common helminth parasites (from CDC).
Ectoparasites
Ectoparasites live on the external surface of the host’s body. Major groups include:
- Lice – Pediculus humanus capitis, Pthirus pubis, P. humanus corporis
- Bed bugs – Cimex lectularius, Cimex hemipterus
- Fleas – Pulex irritans, Ctenocephalides felis
- Mites – Sarcoptes scabiei
- Ticks – Ixodes, Dermacentor, Rhipicephalus, others
Although they reside externally, ectoparasites can still cause a number of clinical manifestations by their feeding, including:
- Allergic reactions manifested as rashes, itching, inflammation
- Skin irritation and discomfort
- Secondary infections of skin or wounds
- Anemia from blood loss
- Transmission of other infections like typhus, Rocky Mountain spotted fever, Lyme disease, relapsing fever
Scabies
Scabies mites burrow into the skin and cause intense pruritis and a pimple-like rash. Complications include:
- Impetigo – bacterial skin infection
- Abscesses
- Sepsis
- Glomerulonephritis – inflammation of kidneys
It is estimated that 200 million cases occur per year. Crusted scabies in immunocompromised persons is especially severe.
| Ectoparasite | Diseases Transmitted |
|---|---|
| Lice | Epidemic typhus, trench fever, relapsing fever |
| Fleas | Plague, murine typhus |
| Mites | Scrub typhus, rickettsialpox |
| Ticks | Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, babesiosis, Crimean-Congo hemorrhagic fever, tick paralysis |
Table 2. Examples of diseases transmitted by ectoparasites.
Diagnosis
Diagnosis of parasitic infections relies on:
- Microscopic detection of eggs or parasites in stool, tissue, blood, or other specimens
- Immunologic tests for antibodies or antigens
- Molecular techniques like PCR
- Imaging studies like CT scans to detect organ enlargement or calcification
- Endoscopy to directly visualize parasites attaching to mucosa
Often multiple stool specimens over several days are needed, as shedding of parasites can be intermittent.
Treatment
Treatment involves antiparasitic drugs that target the specific parasite:
- Protozoa: Metronidazole, tinidazole, nitazoxanide, atovaquone, paromomycin, quinine, chloroquine, others
- Helminths: Albendazole, mebendazole, ivermectin, praziquantel, niclosamide, pyrantel pamoate
- Ectoparasites: Permethrin, ivermectin, malathion, benzyl benzoate, spinosad, others
Duration of treatment depends on the parasite but may be 1-2 weeks or longer. Supportive therapy like blood transfusions and IV fluids may be needed with severe infections.
Prevention
Preventive measures against parasites include:
- Improved sanitation infrastructure in endemic areas
- Handwashing with soap and clean water
- Cooking food thoroughly and preventing cross-contamination
- Avoiding contamination of water sources with feces
- Wearing shoes in areas where parasite eggs are common in soil
- Clearing snail habitats to control schistosomiasis transmission
- Use of bed nets and insect repellants
- Iron supplementation and deworming in endemic regions
- Mass drug administration in high-risk communities
Developing vaccines against malaria, hookworm, and schistosomiasis could provide substantial public health benefits but these are still in early experimental stages.
Conclusion
Parasitic diseases remain a significant cause of morbidity and mortality around the world. Protozoa like malaria parasites and helminths like intestinal worms and schistosomes are the source of greatest disease burden. Ectoparasites can also transmit deadly infections like plague. While effective treatments exist, control is challenging due to transmission through contaminated environments. Increased public health efforts focusing on water sanitation, vector control, mass drug administration, and vaccines will be key to reducing the impact of parasitic diseases on vulnerable populations.
