Medical Parasitology

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Medical Parasitology

• Karen L. Bennett, PhD.
• M609 Medical Sciences Bldg.
• March 9, 2004

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Introduction to medically relevant
parasites Parasites are eucaryotic organisms,
like us, and contain nuclei, cell organelles and,
for the helminths, tissues like our own. They
are not endogenous to humans. Therefore exposure
and the means of entry are important to the
disease process. Exposure severity of
illness often related to the infective dose, with
additional organisms acquired over time.
Parasitic infections are often chronic. Entry
enter by ingestion or direct penetration of the
skin. Knowing the means of entry for the
organisms discussed is important for disease
prevention.
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Introduction medically relevant
parasites Adherence parasites adhere to
specific host tissues, ie. malaria and red blood
cells or hookworms to the intestinal villi.
Identifying the methods and recognition molecules
are useful subjects of research, in order to
prevent parasite adherence, but will not be
considered today. Replication most protozoans
multiply in the host only a few worms do.
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Introduction basic clinical parasitology Cell
and tissue damage. Parasites dont usually
produce toxins tissue damage can be due to
mechanical blockage and inflammatory host
response. Toxic products are often released when
the parasites die. The host often produces
immunopathologic responses. Parasites have
developed many elaborate means to avoid the
hosts immune response.
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Anti-parasitic agents Why are there so few
effective anti-parasitic drugs? Only 3 new
anti-malarial drugs in past 20 years! (over 1,200
new drugs approved by FDA in that timeframe).
Is there a problem with resistance? Yes! What
are the current best means for prevention? Good
hygiene, simple techniques like bed nets for
mosquitoes. How do the major anti-parasitic
drugs work? Many block critical metabolic
pathways best are parasite specific.
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Laboratory Diagnosis of Parasitic
Diseases Diagnosis often difficult. Traditional
methods are by microscopic exam. Newer means,
including western blots, ELISAs and PCR are
expensive. A few, unusual, low-tech methods will
be mentioned. The medical practitioner must be
aware of the possibility of parasites.
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Drug of choice for Trichomonas (and for Giardia,
another flagellate) is Flagyl.
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CRYPTOSPORIDIUM

• TALES FROM THE CRYPTO

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CASE 19

• In Milwaukee, WI water contamination from a
  sewage treatment plant killed 100 people and
  affected more than 400,000 in 1993. The parasite
  that caused the disease was determined to be
  cryptosporidium.

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SYMPTOMS

• Immunocompetent
• Mild self-limiting enterocolitis (watery
  bloodless diarrhea, abdominal pain, nausea,
  vomiting, and fever)
• Spontaneous remission is common (usually within
  10 days)

• Immunocompromised
• 50 or more stools per day
• Dehydration (fatigue, abdominal cramping, and
  nausea)
• Lasts months or years
• Common in AIDS patients

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LAB DIAGNOSIS

• Microscopic exam
• Acid fast stain of stool sample
• Endoscopic biopsy of small intestine

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Cryptosporidium oocysts with acid-fast stain
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LAB DIAGNOSIS

• Immunodiagnosis
• Immunofluorescence assay (IFA)
• Enzyme linked immunoabsorbant assay (ELISA)
• Polymerase Chain Reaction (PCR)
• Test of choice

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LAB DIAGNOSIS
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LIFE CYCLE

• Cryptosporidium lives and grows in variety of
  animals geese to snakes to cows, sheep and pigs
  to humans.
• Cryptosproridium completes its cycle in a single
  host.
• The Species of Crypto known to infect humans is
  Cryptosporidium Parvum.

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LIFE CYCLE

• Infectious agents are the OOCYSTS
• In immunocompromised patients ID50 is about 10 to
  30 oocysts
• Autoinfection takes place in 2 ways-
• gt Merozoites attach to nearby epithelial
  cells and spread infection
• gt thin walled oocysts excyst and continue
  to spread infection within the body

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A scanning electron micrograph of Cryptosporidium
lining the intestinal tract. (From Gardiner et
al., 1988, An Atlas of Protozoan Parasites in
Animal Tissues, USDA Agriculture Handbook No.
651.)
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A scanning electron micrograph of a broken meront
of Cryptosporidium showing the merozoites
within.  (From Gardiner et al., 1988, An Atlas
of Protozon Parasites in Animal Tissues, USDA
Agriculture Handbook No. 651.)
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LIFE CYCLE
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TRANSMISSION AND EPIDEMIOLOGY

• Person to person (fecal-oral)
• Animal to human
• Contamination of water supplies (result of waste
  runoff)
• WATER-BORNE MOST COMMON

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TREATMENT

• Immunocompetent
• Self-limiting
• Usually symptoms subside within 10 days

• Immunocompromised
• Cocktail therapy -used to treat symptoms but NOT
  THE DISEASE
• Drugs include letrazuril, azithromycin,
  paramycin, and hyperimmune bovine colostral
  immunoglobulin

The only immunity is previous exposure and
extent of this immunity is not known.
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PREVENTION

• Wash hands
• Wash fruits and vegetables
• Avoid untreated water
• Treat contaminated water
• MAINTAIN PROPER HYGIENE!!

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WATER PREVENTION

• Ozone
• UV light
• Inexpensive tests to eliminate water borne
  pathogens
• Resistance to filtration
• Resistance to chlorine
• Kills all spores after one minute rapid boiling
• Chlorine not effective against crypto!!

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INTERESTING FACT

• In Uganda, the human population shares habitat
  with free ranging gorillas. These people are
  infected with animal-adapted genotype of
  Cryptosporidium parvum.

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INTERESTING FACT

• There were 6 outbreaks between 1984 and 1994 in
  the US.
• In MO there were 26 confirmed cases from motel
  pools in 1994.

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INTERESTING FACT

• Cattle alone produce about 4.57 tons of
  Cryptosporidium oocysts per year in the US
• BEWARE !

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FUN FACT

• Crypto-nite
• Cryptosporidium is the main reactive agent
  causing Superman Sickness

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Microsporidium Encephalitozooon cuniculi
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Microsporidia spores Gram positive in biopsy
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Nature 2001 Vol 414450
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Nature 2001 Vol. 414401
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Multiple rings of P. falciparum trophozoites can
be found in a single red blood cell
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The of red blood cells showing parasites is
often low, making diagnosis difficult.
This slide shows the advanced ring stage of P.
vivax
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Seen here is a red blood cell filled with
multiple schizonts ready to burst free.
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Nature 2001 Vol. 415670
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