Parasitology

• Medical Parasitology deals with parasites that cause human infections and the diseases they produce
• Medical Parasitology is broadly divided into two parts:

  • Protozoology/Protistology (Study of Protists)
  • Helminthology (Study of Parasitic worms "Helminths")
• Key Figures in Parasitology:

  • Antonie Philips van Leeuwenhoek:
  • Father of Microbiology
  • First to introduce the use of single lens microscope (1681)
  • Discovered protists and bacteria
  • Coined the term "Animalcules"
  • Observed Giardia in his own stool
  • Louis Pasteur:
  • Renowned for discoveries in vaccination, microbial fermentation, and pasteurization
  • First published scientific study on a protozoal disease during investigation of an epidemic silk worm disease in South Europe (1870)
  • Patrick Manson:
  • Father of Tropical Medicine
• • Made important discoveries in Parasitology
  • Made a seminal discovery about the role of mosquitoes in filariasis, the first evidence of vector transmission (1878)
  • Charles Louise Alphonse Laveran:
  • French military physician
  • Discovered the pathogenic agent for malaria (1880)
  • Won the 1907 Nobel Prize in Physiology or Medicine
  • Ronald Ross:
  • Military medical officer in India
  • Famous for discovering the mosquito transmission of malaria (1897)
  • Started the identification of a large number of vector-borne diseases
• • First Briton to be awarded the Nobel Prize for Physiology or Medicine in 1902
• Parasites are living organisms that depend on a living host for their nourishment and survival. They multiply or undergo development in the host. The term "parasite" is usually applied to Protozoa (unicellular organisms) and Helminths (multicellular organisms)
• Types of Parasites (Based on location):

  • Ectoparasite (Infestation): inhabit only the body surface of the host without penetrating the tissue
  • Free-living parasite (Independent): nonparasitic stages of active existence, which live independent of the host
  • Endoparasite (Infection): a parasite that lives within the body of the host and causes an infection
• Types of Endoparasites:

  • Obligate parasite: parasite which cannot exist without a host
  • Facultative parasite: organism may either live as a parasite or as a free-living form
  • Accidental parasite: parasite that infects an unusual host (accidental host); also called incidental parasite
  • Aberrant parasite: parasite that infects a host where they cannot develop further; also known as wandering or erratic parasite; parasite that wanders into an organ in which it is not usually found
• Host is defined as an organism that harbors the parasite and provides nourishment and shelter to the latter and is relatively larger than the parasite
• Types of Host:

  • Definitive: The host in which the adult parasite lives and undergoes sexual reproduction
  • Intermediate: The host in which the larval stage of the parasite lives or asexual multiplication takes place
  • Paratenic: A host in which the larval stage of the parasite remains viable without further development and transmits the infection to another host
  • Reservoir: In an endemic area, a parasitic infection is continuously kept up by the presence of a host, which harbors the parasite and acts as an important source of infection to other susceptible hosts
• • Accidental: The host in which the parasite is not usually found
• Zoonosis:

  • The word zoonosis was introduced by Rudolf Virchow in 1880 to include the diseases shared in nature by man and animals
  • Defined by the World Health Organization (WHO) as diseases and infections naturally transmitted between vertebrate animals and man
• Types of Zoonosis:

  • Anthropozoonoses: Infections transmitted to man from lower vertebrate animals
  • Zooanthroponoses: Infections transmitted from man to lower vertebrate animals
  • Protozoal Zoonoses
  • Helminthic Zoonoses
• Types of Relationship:

  • Symbiosis: Both host and parasite are dependent upon each other without suffering harm from the association
  • Commensalism: Only the parasite derives benefit from the association without causing any injury to the host
  • Parasitism: The parasite derives benefits and the host is always harmed due to the association; the parasite cannot have an independent life
• Types of Life Cycle:

  • Direct life cycle: When a parasite requires only a single host to complete its development
  • Indirect life cycle: When a parasite requires two or more species of host to complete its development
• Soil polluted with embryonated eggs (roundworm, whipworm) may be ingested or infected larvae in soil may penetrate exposed skin (hookworm)
• Infective forms of parasites present in water may be ingested (cyst of ameba and Giardia)
• Water containing the intermediate host may be swallowed (cyclops containing guinea worm larva)
• Infected larvae in water may enter by penetrating exposed skin (cercariae of schistosomes)
• Ingestion of contaminated food or vegetables containing infective stage of parasite (amebic cysts, Toxoplasma oocysts, Echinococcus eggs)
• Ingestion of raw or undercooked meat harboring infective larvae (measly pork containing cysticercus cellulosae, the larval stage of Taenia solium)
• Biological vectors assist in the transfer of parasites and the parasites undergo development or multiplication in their body as well
• Mechanical vectors assist in the transfer of parasitic form between hosts but are not essential in the life cycle of the parasite
• A person infected with a parasite without any clinical or subclinical disease is known as a carrier and can transmit the parasite to others
• Infected individual becomes his own direct source of infection (Autoinfection)
• Infected individual is further infected with the same species leading to massive infection (Hyperinfection or Superinfection)
• The most common method of transmission is through oral route by contaminated food, water, soiled fingers, or fomites
• Entry through skin is another important mode of transmission. Hookworm infection is acquired when the larvae enter the skin of persons walking barefooted on contaminated soil
• Many parasitic diseases are transmitted by insect bite (Vector transmission)
• Parasitic infection may be transmitted by person-to-person contact (Direct transmission)
• Mother to fetus transmission may take place in malaria and toxoplasmosis (Vertical transmission)
• Transmission seen in transfusion malaria and toxoplasmosis after organ transplantation (Iatrogenic transmission)
• Parasitic infections may remain inapparent or give rise to clinical disease with various forms: acute, subacute, chronic, latent, or recurrent
• Enzymes produced by some parasites can cause lytic necrosis (Lytic Necrosis)
• Attachment of hookworms on jejunal mucosa leads to traumatic damage of villi and bleeding at the site of attachment (Trauma)
• Clinical illness may be caused by host immune response to parasitic infection (Allergic manifestations)
• Masses of roundworm cause intestinal obstruction; Plasmodium falciparum malaria may produce blockage of brain capillaries in cerebral malaria (Physical obstruction)
• Clinical illness may be caused by inflammatory changes and consequent fibrosis; Some parasites produce cystic lesion that may compress the surrounding tissue or organ (Inflammatory reaction, Neoplasia, Space occupying lesions)
• Parasites elicit immunoresponses in the host, both humoral as well as cellular, but immunological protection against parasitic infections is less efficient than against bacterial or viral infections
• Several factors contribute to the less efficient immunity against parasitic infections, such as parasites being larger or more complex structurally and antigenically, some parasites living intracellularly or inside body cavities