Medical Parasitology

Karen L. Bennett, PhD.M609 Medical Sciences Bldg.

March 9, 2004

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.

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.

Introduction: basic clinical parasitology

Cell and tissue damage. Parasites don’t 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 host’s immune response.

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.

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.

Drug of choice for Trichomonas (and for Giardia, another flagellate) is Flagyl.

CRYPTOSPORIDIUMCRYPTOSPORIDIUMTALES FROM THE “CRYPTO”

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.

SYMPTOMS

• ImmunocompetentImmunocompetent– Mild self-limiting

enterocolitis (watery bloodless diarrhea, abdominal pain, nausea, vomiting, and fever)

– Spontaneous remission is common (usually within 10 days)

• Immunocompromised Immunocompromised – 50 or more stools per

day

– Dehydration (fatigue, abdominal cramping, and nausea)

– Lasts months or years

– Common in AIDS patients

LAB DIAGNOSIS

• Microscopic examMicroscopic exam– Acid fast stain of

stool sample– Endoscopic biopsy

of small intestine

Cryptosporidium oocysts with acid-fast stain

LAB DIAGNOSIS

• ImmunodiagnosisImmunodiagnosis– Immunofluorescence

assay (IFA)– Enzyme linked

immunoabsorbant assay (ELISA)

• Polymerase Chain Reaction (PCR)– Test of choice

LAB DIAGNOSIS

LIFE CYCLELIFE 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.Cryptosporidium Parvum.

LIFE CYCLELIFE CYCLE• Infectious agents are the OOCYSTSOOCYSTS• In immunocompromised patients ID50 is about

10 to 30 oocysts• Autoinfection takes place in 2 ways- > Merozoites attach to nearby epithelial

cells and spread infection > thin walled oocysts excyst and continue to

spread infection within the body

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.)

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.)

LIFE CYCLELIFE CYCLE

TRANSMISSION AND TRANSMISSION AND EPIDEMIOLOGYEPIDEMIOLOGY

• Person to person (fecal-oral)

• Animal to human• Contamination of water supplies (result of waste runoff)

• *WATER-BORNE MOST *WATER-BORNE MOST COMMONCOMMON*

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.*

PREVENTION• Wash hands• Wash fruits and

vegetables • Avoid untreated

water• Treat contaminated

water• MAINTAIN PROPER

HYGIENE!!

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 Chlorine not effective against crypto!!”against crypto!!”

INTERESTING FACT

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

         

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.

INTERESTING FACT

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

BEWARE !

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FUN FACTFUN FACT

• “Crypto”-nite– Cryptosporidium is

the main reactive agent causing Superman Sickness

Microsporidium: Encephalitozooon cuniculi

Microsporidia spores:Gram positive in biopsy

Nature 2001 Vol 414:450

Nature 2001 Vol. 414:401

Multiple rings of P. falciparum trophozoites can be found in a single red blood cell

The % of red blood cells showing parasites is often low, making diagnosis difficult.

This slide shows the advanced ring stage of P. vivax

Seen here is a red blood cell filled with multiple schizonts ready to burst free.

Nature 2001 Vol. 415:670