Parasites

last authored: Feb 2010, David LaPierre
last reviewed: March 2010, Rosemary Drisdelle

 

Introduction

Parasites are organisms that live in, or on other organisms (hosts) and obtain everything they need from the host. They include various organisms, including protozoa, helminths, and arthropods. In medicine, organisms like viruses and bacteria are not considered to be parasites, even though they often fit the definition.

 

Parasites are extremely prevalent and cause disease and death on a massive scale, particularly in the developing world. They are often considered tropical diseases, and less attention and money is spent on their prevention, diagnosis, and treatment compared with diseases such as cancer or heart disease. Accordingly, many of them are termed 'neglected diseases', even though their impact is so profound.

 

 

 

Types of Parasites

  • protozoa
  • helminths
  • arthropods

Protozoa

Protozoa are single-celled eukaryotic organisms that reproduce by binary fission. They usually require an aquatic environment and most cannot withstand dry conditions, with transmission sometimes accomplished through arthropod vectors. Other protozoa have a lifecycle which includes dormant cysts and active trophozoites. Numerically, relatively few parasites are usually required for infection, though disease results from large numbers. Protozoans tend to cause intestinal or blood infections, though there are exceptions.

Important protozoa include:

Protozoa come with various lifecyles and forms:

  • amoebae move by extending pseudopods; parasitic amoebae are usually intestinal parasites that form cysts
  • flagellates are primarily intestinal and blood parasites which use flagella for movement; most form cysts for survival in the environment if they infect the gut, and are transmitted by vectors if they infect blood
  • sporozoa (apicomplexans) use gliding motility
  • ciliates
  • microsporidia (now classified as a fungus)

Helminths

Helminths are worm-like, multicellular organisms that are much larger than protozoa. There are over three billion intestinal helminth infections worldwide, causing significant malnourishment, growth impairment, and disease.

 

Cestodes (tapeworms)

 

Trematodes (flukes)

  • Fasciola hepatica
  • Fasciolopsis buski
  • Opistorchis viverrini
  • Schistosoma spp
  • Chlonorchis sinensis
  • Paragonimus spp.

 

Nematodes (roundworms)

 

 

Most nematodes reproduce sexually. Cestodes and trematodes – with the exception of Shistosoma spp.- are hermaphrodites, meaning they have both male and female organs.

Arthropods

Arthropods are invertebrates such as insects and ticks. Many arthropods act as vectors, transmitting parasites or other pathogens. Other arthropods appear not to transmit disease, but act as ectoparasites by simply feeding on humans.

 

parasite vectors include:

  • mosquitoes (malaria, microfilariae)
  • tse-tse flies (trypanosomes)
  • black flies (river blindness)
  • reduviid bugs (Chagas disease)
  • ticks (babesiosis)

vectors of non-parasite pathogens include:

  • mosquitoes (yellow fever, dengue fever, West Nile virus, other encephalitis viruses)
  • ticks (encephalitis viruses, Lyme disease)
  • fleas (epidemic typhus)

 

ectoparasites which do not appear to transmit disease include:

  • bedbugs
  • head lice
  • pubic lice
  • scabies

 

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Parasites and Disease

Many parasitic infections cause no disease in healthy people, though immunosuppressive states, such as AIDS or following organ transplantation, can lead to their emergence.

Disease is otherwise often associated with prolonged or repeated infection. However, some parasitic infections can be serious and rapidly fatal - malaria (Plasmodium species) being the best example.

 

Several parasitic infections are zoonoses, meaning they cross to humans from animals, such as mammals, birds, and reptiles. A complete life cycle depends on infection of animals and humans. An example of this is Schistosoma, which develops asexually in snails before infecting humans. Other zoonoses infect humans only accidentally, and other Schistosomal infections can lead to skin invasion and irritation, followed by parasite death and disease resolution.

 

Parasites adapt to their existence by developing attachment mechanisms (ie hooks), defence mechanisms (ie nematode resistance to digestion), or increased reproductive capacity (ie Ascaris). Some parasites require a human host for at least part of their life cycles; others can survive in reservoir hosts in the environment.

 

Reservoirs can be the animals that can cause zoonotic infections, other humans, or the environment. The immune response to parasites occurs with varying degrees of success.

 

Pathways of Infection

Parasites can infect humans in many ways. These include the fecal-oral route, ingestion of raw or undercooked foods, via sexual intercourse, direct skin penetration, by vectors, and through blood donation. Arthopods typically spread through close contact with an infested person. Proper sanitation, and disposal of human wastes, is critical for control of intestinal parasite spread.

 

Damage Caused by Parasites

Parasite infection can cause tissue damage through direct parasitic action, or via the impact of the immune system. Amoebae cause direct cell destruction, while schistosomiasis and filariasis cause chronic inflammation. In these conditions, it is not the adults, but rather their offspring that mediate the immune response.

 

The immune system often deals poorly with parasitic infection. Eosinophils are the cells most responsible. Giardia attachment is blocked by IgA.

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Parasite Survival

Parasites have many mechanisms to avoid death, mediated by antibodies and leukocytes. Schistosomes coat themselves with human proteins to avoid detection, trypanosomes change their surface antigens to avoid antibody binding, and Leishmania live within macrophages and produce superoxide dismutase to inactivate toxic superoxide.

 

Intestinal parasites are primarily diagnosed by direct examination of stool samples following staining techniques. Blood-borne parasites can be identified on a peripheral smear. The immune response to many pathogens can also be determined by serological testing. 

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Resources and References