introduction to parasitology

• symbiosis - relationship where unlike organisms exist together
• three types of symbiotic relationship

  1. commensalism - one benefits from another without harming (human body and normal flora)
  2. mutualism - mutually benefit each other
  3. parasitism - one benefits to the detriment of the other (parasite and host)
• parasitism based on habitat

  • ectoparasite - parasite lives outsite the host's body (insects), invasion is called infestation
  • endoparasite - parasite lives inside the host's body (worms), invasion is called infection
• parasitism based on ability to live independently of the host

  • facultative parasite - independent parasites
  • obligate parasite - parasite must live inside the host
• parasitism based on mode of living

  • permanent parasites - remains in host from early to maturity
  • intermitten parasite - only visits the host during feeding
  • incidental parasite - occurs in unusual host
  • transitory parasite - larva develops in host and becomes free-living after adult stage
  • erratic parasites - seen in unsual organ
  • aberrant parasites - wandering parasite, host cannot develop further
• types of host

  • definitive host - harbor the adult stage of parasite (sexual stage occurs)
  • intermediate host - harbor the larval stage of parasite (asexual stage occurs)
  • reservoir host - vertebrate hosts that harbor parasite, acts as additional source of infection
  • paratenic host - mean of transport for parasite
  • accidental host - serves as host in a parasite's life cycle, not a usual host ; zoonosis (human) and dead-end (parasite cannot complete life cycle)
• most common source of exposure to infection is soil contaminated or polluted with human feces
• most common mode of transmission of most intestinal parasite is ingestion of contaminated food and water (fecal-oral)

  • contaminated water - intestinal protozoa (cyst)
  • contaminated food - contains mature larval stage (enterobius vermicularis)
• mode of transmission

  • skin transmission - hookworms and storngyloides
  • direct transmission (sexual intercourse) - trichomonal vaginalis
  • transmammary (milk) - strongyloides and ancylostoma
  • transplacental / congenital infection - toxoplasma gondii and malarial parasite
  • inhalation of airbore - eggs of enterobius vermicularis and pinworm
  • vector transmission (blood-sucking insects) - mosquito: malaria, leishmaniasis, trypanosomiasis, filariasis
• portal of exit

  • anus - most common (eggs eggs of roundworms are excreted together with human fece, and contaminate soil and water)
  • urine - trichomonas vaginalis, strongyloides stercorais, schistosoma haemotobium
  • sputum - lung fluke (paragonimus westermani), roundworm (ascaris lumbricoides - larval stage)
• pathogenesis - development of disease and chain of event leading to that disease
• inapparent infection - no symptoms and remains inapparent continuosly for long period of time (malaria)
• mechanism of disease production (traumatic damage)

  • due to direct physical damage
  • entry of inefective larvae of hookworms or blood flukes produces slight physical damage
  • large number of worms may produce acute intestinal obstruction
• mechanism of disease production (lytic nectrosis)

  • substances (enzymes) produced by parasites may cause harm to the host tissues
  • entamoeba histolytica - releases enzyme that lyse tissue for nutrient needs
• mechanism of disease production (stimulation of host tissue reaction)

  • majority of anuman parasite provoke host tissue reactions
  • increase in eosinophils - caused by helminths
  • destruction of red blood cell - hookworm infection or malaria
• mechanism of disease production (stimulation of cancer growth)

  • neoplastic growth
  • schistosoma japonicum (blood fluke) - cancer of the liver
  • chonorchis sinensis (liver fluke) - cancer of the biliary ducts
• mechanism of disease production (toxic and allergic phenoma)

  • proteins produced by parasite may lead to hypersensitivity reacrion
  • enterobius vermicularis - occurs in the anus (pruritis ani)
• presence of parasite and damage they produce may favor the entry and proliferation of other organisms (ulceration and erosion)
• life cycle of parasite

  • mode of transmission > infective stage > pathogenic stage > diagnostic stage > parasite enters outside environment > source of infection (soil or water) > emerge from water, food, soild > mode of transmission
• two major groups of parasites: protozoa and helminths