Introduction to Parasitology - University of Pittsburgh Internal ...

Edsel Salvana, MD, DTM&H 

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What’s in a name? 

• Parasitology 

• Tropical Medicine and Hygiene 

• International Infectious Diseases 

• Global Health 

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Parasitology 

• Parasite – organism that lives at the expense of 

another without giving anything in return 

• Parasitology – study not just of the parasite, but the 

complex interactions between the parasite, its host 

and the environment 

• Classification often difficult and arbitrary – 

sometimes hard to distinguish between parasitism, 

commensalism and symbiosis 

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Tropical Medicine and Hygiene 

• Traditionally deals with those diseases that are found 

in tropical countries 

• Since most of these are infectious in nature, hygiene 

and sanitation has been classically been associated 

with this branch of medicine 

• Artificial/ arbitrary since a lot of these diseases are 

not just confined to tropical countries 

• With advent of widespread travel and immigration, 

some believe that this branch is an anachronism

International Infectious Diseases 

• More modern interpretation of tropical medicine, 

recognizing that infectious diseases are not just 

confined to tropical countries 

• Still limited in scope (infectious diseases only), 

includes travel medicine 

• Criticism is that it still uses developed countries as a 

frame of reference (what is not “international?”)

Global Health 

• All encompassing, preferred to tropical medicine and 

hygiene 

• Looks at diseases and health from a global perspective 

• “Politically correct” 

• Criticism is that it is still a de facto “Western‐centric” 

term – otherwise, Global Health would include all 

diseases and conditions affecting the health of 

everyone

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Classification schemes for parasites 

et al. 

• Inherently problematic since division at best will 

likely be artificial and arbitrary 

• Genomic classification likely has best hard evidence, 

but may not be practical or useful in the field 

• Multiple schemes can be used, suits different 

purposes for different workers (epidemiologist, basic 

scientist, clinician)

5‐Kingdom based (Obsolete) 

• Animal: worms and arthropods, toxins and zoonoses 

• Plant: algae, poisonous plants 

• Protozoan: amoebae, apicomplexans, ciliates, 

coccidians 

• Fungus: Penicillum, Paracoccidiomycosis 

• Bacteria: anthrax, meloidosis, leptospirosis, plague 

• (Virus): Dengue, Chikungunya, other arboviruses, 

Hantavirus, Ebola, SARS

I WORMS 

• Flat worms (Platyhelminthes): include trematodes 

(flukes), cestodes (tapeworms) 

• Round worms (Nematodes): include soil‐transmitted 

helminths (Ascaris, hookworms, whipworms, 

pinworm, Strongyloides), filarial parasites (blood 

filaria e.g. Wuchereria, Brugia, Mansonella; tissue 

filaria: Onchocerca, Loa), other nematodes 

(Dracunculus, Trichinella) 

• Annelids: leeches 


I WORMS 

• Generally have complicated life cycles 

• Definitive host is where sexual stage occurs 

• May have multiple intermediate hosts 

• Man may be an intermediate or definitive host, or 

may be an accidental host 

• More likely to occure in warmer climates where there 

is an abundance of intermediate vectors

Arthropods 

• Ectoparasites such as mites, ticks, fleas 

• Larval forms can invade deeper: botfly, miasis 

• Can be vectors of diseases as well: arboviruses, 

cholera, malaria, trypanosomes, Leishmania , typhus, 

Lyme disease, rickettsial diseases 


Other animals, toxins 

and zoonoses 

• Vampire bats 

• Jellyfish, fire coral, stone fish 

• Snake, scorpion, spider and insect bites 

• Other animal bites 

http://en.wikipedia.org/wiki/File:Jelly_Monterey.jpg 

• Poisonous animals when ingested: venomous frogs, 

blowfish, pufferfish, Scombroid 

• Zoonoses – diseases from animal vectors: e.g. rabies, 

leptospirosis, cat‐scratch disease, Q‐fever, anthrax

Plant 

• Rare algae can cause disease: Prototheca 

• Poisonous plants can cause discomfort to death: 

poison ivy, deadly nightshade, belladona 

• Toxin secreting algae: algal blooms, paralytic shellfish 

poisoning, Ciguatera, neurotoxic shellfish poisoning 

etc. 

corp2365, NOAA Corps Collection

Protozoans 

• Parasitic and free living amoebae (Entamoeba, 

Acanthamoeba, Naegleria) 

• Ciliates (Balantidium), Coccidians (Isospora, 

Cyclospora, Microspora), Apicomplexans 

(Plasmodium, Babesia, Cryptosporidium, 

Toxoplasmosis) 


Bacteria and Viruses 

• You get the picture

Diagnosis 

• If you see the bug, you probably have it 

• Microscopy remains the cheapest method with basic 

equipment requirements 

• Serology more sensitive, but may not be able to 

distinguish between active and old infection 

• Molecular diagnostics such as antigen detection and 

PCR are the most accurate, but are more expensive 

and may require specialized equipment 

• Ideal test is simple to use, highly accurate and cheap

Public Health Image Library #1342 


Microscopy 

• May not be the most sensitive, but very specific 

• Minimal equipment requirements 

• Operator dependent, more sensitive in experienced 

hands 

• False positives may occur due to artefacts, minimized 

by highly trained personnel 

• Low sensitivity may be improved by using multiple 

specimens 

• Most effective for heavy infections (e.g., high worm 

burden leads to more eggs per gram of stool)

Fecalysis 

• Used to diagnose and identify enteric parasites 

• Can be qualitative or quantitative 

• Kato‐Katz: quantitative method, uses fixed amount of 

stool and allows examination of a larger amount 

compared to simple smears, leading to higher 

sensitivity 

• Concentration methods may be used to increase yield 

such as centrifugation and use of “floatation” 

techniques


• Specialized stool tests to increase yield include 

hatching tests for schistosomes, agar culture for 

strongyloides, special stains for coccidians and other 

protozoa 

• Sometimes, microscopy is the only method that 

allows identification of specific organisms, as well as 

faster identification of mixed infection

Blood smears 

• Allows for identification of blood borne parasites 

• Thin smears usually allows for species identification, 

thick smears for detection since a larger sample is 

used 

• Less sensitive in light infections 

• E.g malaria smears, trypanosomes, lymphatic filariasis 


Serology 

• Relies on the presence of antibodies to parasite 

antigens 

• Uses ELISA format, colorimetric methods, or more 

portable assays such as lateral flow 

• Most useful for demonstrating exposure, and may 

provide confirmation for clinical findings and other 

ancillary tests 

• Less useful in endemic areas, or in persons with 

previous exposure since this does not distinguish 

between active and passive infection

• Paired acute and convalescent sera most accurate, but 

usually is a retrospective diagnosis 

• E.g. dengue IgG/IgM, typhidot, other arboviruses, 

serologies for cysticercus, echinococcus

Antigen detection 

• Very sensitive 

• Useful in following treatment response, as only active 

infection leads to antigen secretion 

• Some specificity issues in cross‐reactivity between 

species 

• Point‐of‐care tests have been developed which allow 

antigen detection in the field – still relatively 

expensive but has been very successful when 

combined with mass treatment programs (e.g. ICT 

card test for microfilaria)

http://www.filariasis.org/resources/howdiagnosed.htm

PCR methods 

• Very sensitive 

• Highly technical, requires specialized equipment and 

training, expensive 

• Contamination is an issue 

• Allows for very specific identification of organisms in 

minute quantities, can provide genetic epidemiologic 

data 

• Isothermic methods being developed for field 

application (e.g. LAMP)

Other tests 

• Radiology –useful in specific disease entities such as 

Schistosoma japonicum (network/lacelike pattern of 

the liver), lymphatic filariaisis (filarial “dance” sign), 

amoebic liver abscess, echinococcus etc., CT imaging/ 

MRI (cysticercosis) 

• Xenodiagnosis – e.g. Chagas disease 

• Biopsy – schistosoma (rectal or cystoscopy), 

Trichinella, cysticercosis 

• Ancillary tests: eosinophilia, hematuria, CSF studies

Sample case 

• 28/M recently returned from Southeast Asia after 

spending 1 month in different countries there, presents to 

your clinic with fever and chills. He did not receive any 

travel specific vaccinations other than his routine 

childhood vaccinations and an occasional flu shot. He 

went to rural areas and did not use any mosquito 

repellant, nor did he take any malaria prophylaxis. He did 

not boil or purify his water, and he ate uncooked and 

partially cooked exotic foods. He does not know when his 

last PPD was, or if he ever got one. He did have 

unprotected sex with multiple partners, both male and 

female. He was bitten by different insects and fleas, and 

was also bitten by a feral dog 2 weeks ago. What does he 

have?

• Most common cause of fever in returning travelers is 

still malaria followed by dengue 

• Multiple exposures require extensive workup 

• May have more than one process going on 

• May have potentially lethal consequences

Summary 

• Global Health affects us all 

• Parasitology is more complicated than it seems 

• Classification systems are still in flux, and different 

schema have their advantages and disadvantages 

• No all‐encompassing or “more correct way” of doing 

things 

• Multiple methods for parasitologic evaluation, each with 

advantages and disadvantages 

• Travelers are at risk for multiple diseases, knowing these 

and having a high index of suspicion is key to correct 

diagnosis

Introduction to Parasitology - University of Pittsburgh Internal ...

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