Medical Parasitology Karen L. Bennett, PhD.M609 Medical Sciences Bldg. March 9, 2004
Introduction to medically relevant parasitesParasites are eucaryotic organisms, like us, andcontain nuclei, cell organelles and, for the helminths,tissues like our own. They are not endogenous tohumans. Therefore exposure and the means of entryare important to the disease process.Exposure: severity of illness often related to theinfective dose, with additional organisms acquiredover time. Parasitic infections are often chronic.Entry: enter by ingestion or direct penetration of theskin. Knowing the means of entry for the organismsdiscussed is important for disease prevention.
Introduction: medically relevant parasitesAdherence: parasites adhere to specific host tissues, ie. malariaand red blood cells or hookworms to the intestinal villi. Identifyingthe methods and recognition molecules are useful subjects ofresearch, in order to prevent parasite adherence, but will not beconsidered today. Replication: most protozoans multiply in the host; only a fewworms do.
Introduction: basic clinical parasitologyCell and tissue damage. Parasites don’t usually producetoxins; tissue damage can be due to mechanical blockage andinflammatory host response. Toxic products are often releasedwhen the parasites die. The host often producesimmunopathologic responses. Parasites have developed manyelaborate means to avoid the host’s immune response.
Anti-parasitic agentsWhy are there so few effective anti-parasitic drugs? Only 3 new anti-malarialdrugs in past 20 years! (over 1,200 new drugsapproved by FDA in that timeframe).Is there a problem with resistance?Yes!What are the current best means forprevention? Good hygiene, simpletechniques like bed nets for mosquitoes.How do the major anti-parasitic drugswork? Many block critical metabolicpathways; best are parasite specific.
Laboratory Diagnosis of Parasitic DiseasesDiagnosis 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 ofparasites.
Drug of choice for Trichomonas (and for Giardia, another flagellate) is Flagyl.
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• Immunocompetent • Immunocompromised – Mild self-limiting – 50 or more stools per enterocolitis (watery day bloodless diarrhea, – Dehydration (fatigue, abdominal pain, abdominal cramping, nausea, vomiting, and and nausea) fever) – Lasts months or years – Spontaneous remission – Common in AIDS is common (usually patients within 10 days)
LAB DIAGNOSIS • Microscopic exam – Acid fast stain of stool sample – Endoscopic biopsy of small intestine
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.
LIFE CYCLE• Infectious agents are the OOCYSTS• 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 Cryptosporidiumshowing the merozoites within. (From: Gardiner et al., 1988, An Atlas of Protozon Parasites in Animal Tissues, USDA Agriculture Handbook No. 651.)
TRANSMISSION AND• EPIDEMIOLOGY Person to person (fecal- oral)• Animal to human• Contamination of water supplies (result of waste runoff)• *WATER-BORNE MOST COMMON* COMMON
TREATMENT• Immunocompetent • Immunocompromised – Self-limiting – Cocktail therapy -used to treat symptoms but NOT THE DISEASE – Usually symptoms subside within 10 days – Drugs include: letrazuril, azithromycin, paramycin, and hyperimmune bovine colostral immunoglobulin *The only immunity is previous exposure and extent of this immunity is not known.*
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!!”
INTERESTING FACT• In Uganda, the human population shares habitat with free ranging gorillas. These people are infected with animal- adapted genotype of Crypto spo ridium 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 !
FUN FACT • “Crypto”-nite – Cryptosporidium is the main reactive agent causing Superman Sickness