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Giardia and Other Protozoa. Giardia duodenalis cyst and trophozoite. Most common human parasite in USA, especially day-care children

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Giardia and Other Protozoa

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giardia duodenalis cyst and trophozoite
Giardia duodenalis cyst and trophozoite
  • Most common human parasite in USA, especially day-care children
  • Most common cause of water-borne disease outbreaks; Ordinary water chlorination not sufficient; Use filtration methods or organic chlorine which kills in 2 minutes at 22oC
  • Zoonotic potential: Mountain stream controversy – ‘Beaver fever’; cat-human, dog-human or human-human?
  • Dozens of ‘species’ can be divided into morphological groups that do not cross-infect (G. duodenalis group in mammals; G. muris group in rodents, reptiles and birds; G. agilis group in amphibians)
  • Strain differences are important to pathogenicity, host preference, drug sensitivity, etc.


  • Cysts 10-12u; Trophozoites 15u
  • Prepatent period is 1-2 weeks
  • Crowding; young non-immune or immunodeficient animals
  • Upper 2/3 small intestine
  • Most are asymptomatic or have soft, mucoid, fetid stool +/- blood flecks
  • Pathogenesis: Damages glycocalyx and microvilli (50% shorter) of small intestine malabsorbtion syndrome with variable diarrhea that varies from soft stool to projectile, voluminous small bowel diarrhea; Vomiting common.
  • Strains are important. Immunity to one strain does not confer immunity to successive outbreaks from other strains
  • FOR DIAGNOSIS, REMEMBER- Giardia is an intermittent shedder; must do 3 examinations on different days call negative, even in clinical cases. Fecal ELISA coproantigen detection is more consistent
  • Giardia Pentatrichomonas Balantidium Entamoeba
  • Direct Smear
  • Saline + + + +
  • ‘Falling leaf’ Spiral, jerky Rapid, ciliated pseudopod trophozoitestrophozoites motility motility
  • Lugol’s + - + +/- Kills troph, Kills and Kills troph, Trichrome or cysts golden, distorts + cysts Iron Hema- eyes, axostyle toxylin best
  • ZnSO4 Float + + - + (cysts) - (Sp Gr 1.18) Most accurate ‘monkey face’
  • Formalin- + + + + ethyl acetate
  • Fecal ELISA ++ - - - most sensitive

Giardia Pentatrichomonas Balantidium Entamoeba

Metrinidazole + + ? + (Flagyl) 25 mg/kg BID, 5d

Quinacrine + (Atabrine) 6.6 mg/kg BID, 5d or 9mg/kg SID, 9d

Furazolidone + + 4mg/kg BID, 7d

Fenbendazole + 50mg/kg SID, 3-5d

Albendazole + 25mg/kg BID, 2d

Ipronidazole + (Entryl) 126mg/liter, 7d

control and prevention
Control and Prevention
  • Recurrent kennel/cattery outbreaks may occur; common pet shop,shelter pups/kittens infected often as early as 3 weeks after birth
  • 10% Clorox, Roccal, Lysol kill cysts in cages, runs
  • Quarantine,examine +/- treat newly introduced animals (new strains)
  • Question as to whether treatment really ‘cures’ or just contains infection
  • GiardiaVax, a killed vaccine (give twice 2-4 weeks apart to >8 week old dogs) was recently marketed for multiple strains with the label claim of reduced clinical disease, less cysts shed – Potential use in integrated control programs in high risk situations
  • ~ 8u, variable size, trophozoite is the only form, no cysts
  • Characteristic spiral, jerky motility
  • Dog, cat, human, other spp
  • Large intestine
  • Opportunistic pathogen?
  • Proliferates with fluid diarrhea from other causes
balantidium coli
Balantidium coli
  • 60u ciliated trophozoite, 50u cyst; Macronucleus, micronucleus stain well
  • Pig, human. Dog incidentally introduced to life cycle, contaminated environment
  • Large intestine, ‘Ulcerative colitis’
  • Metrinidazole ?

Ulcerative Colitis

entamoeba histolytica
Entamoeba histolytica
  • 30u trophozoite, 15u cyst
  • Human, primates; Dog and cat incidental infection, trophozoites can be found but no cysts are shed
  • Large intestine; Ulcerative colitis, diarrhea (‘amoebic dysentery’); in humans hepatic abcesses may occur
  • Metrinidazole