Laboratory Methods for Diagnosis of Parasitic Infections: Overview

Chapter 47


Laboratory Methods for Diagnosis of Parasitic Infections


Overview



The field of parasitology is often associated with tropical areas; however, many parasitic organisms that infect humans are worldwide in distribution and occur with some frequency in the temperate zones. Also, an increase in the number of compromised patients, particularly those who are immunodeficient or immunosuppressed, has led to increased interest in the field of parasitology. These individuals are greatly at risk for certain parasitic infections. Parasites of humans are classified into six major divisions:



Identification of parasitic organisms depends on morphologic criteria; accurate depiction of these criteria, in turn, depends on correct specimen collection and adequate fixation. Improperly submitted specimens may result in failure to detect or misidentification of the organisms. Tables 47-1 to 47-3 present information on the various groups of parasites, those that may be recovered from various body sites, the most frequently used specimen collection approaches, and appropriate processing methods.



TABLE 47-1


Description of the More Common Groups of Human Parasites












































































Parasite Group Description
Protozoa, Intestinal  
Amebae Single-celled organisms; pseudopodia (motility), trophozoite, and cyst stages in the life cycle.
Exceptions: Some have no identified cyst.
Fecal-oral transmission of the infective cyst.
Entamoeba histolytica causes amebiasis and is the most significant organism in this group.
Flagellates Protozoa with characteristic flagella.
Fecal-oral transmission.
Exceptions: Dientamoeba fragilis (internal flagella) and the genus Trichomonas have a trophozoite and no cyst stage. Reproduction by longitudinal binary fission.
Examples: Giardia lamblia and D. fragilis.
Ciliates Single-celled protozoa; cilia (motility), which beat in a coordinated, rhythmic pattern, moving the trophozoite in a spiral path.
Trophozoite and cyst stages in the life cycle; both stages show a large macronucleus and a micronucleus.
Fecal-oral transmission.
Balantidium coli is the single human pathogen in the group.
Coccidia Protozoa; asexual and sexual life cycles.
Fecal-oral transmission via contaminated food and/or water. Infective stage (oocyst) containing sporocysts and/or sporozoites.
Examples: Cryptosporidium spp., Cyclospora cayetanensis, Isospora belli, and Sarcocystis spp.
Microsporidia Small (1-2.5 µm) intestinal protozoa.
Transmission by ingestion, inhalation, or direct inoculation of spores.
Nine genera cause disease in humans; the two most important are Encephalitozoon and Enterocytozoon.
Protozoa, Other Sites  
Amebae Pathogenic, free-living organisms associated with warm freshwater environments.
Except for Entamoeba gingivalis (found in the mouth), they have been isolated from the central nervous system (CNS), eye, and other body sites.
Examples: Naegleria fowleri—acute CNS infection and death. Chronic CNS disease (Acanthamoeba spp., Balamuthia mandrillaris, and
Acanthamoeba spp. can also cause keratitis).
Flagellates Have flagella (long, proteinaceous organelles used for motility).
Sexual transmission.
Examples: Trichomonas vaginalis is located in the genitourinary system.
Trichomonas tenax can be identified in the mouth and is considered nonpathogenic.
Coccidia Obligate intracellular, spore forming.
Transmission is typically fecal-oral through ingestion of contaminated materials or food.
Examples: Cryptosporidium spp. and Toxoplasma gondii.
Microsporidia Small (1-2.5 µm) spore-forming protozoa.
Transmission is typically by ingestion of spores.
Life cycles vary considerably; some have an asexual life cycle, whereas others are complex and have both asexual and sexual life cycles and multiple hosts.
Examples: Encephalitozoon, Pleistophora, Trachipleistophora, and Brachiola spp.
Protozoa, Blood and Tissue  
Malaria, babesiosis Arthropod vector–borne protozoa.
Transmission via insect bite.
Examples: Plasmodium spp. includes parasites that undergo exoerythrocytic and pigment-producing erythrocytic schizogony in vertebrates and a sexual stage followed by sporogony in mosquitoes.
Babesia spp. are tick-borne and can cause severe disease in patients who have been splenectomized or otherwise immunologically compromised.
Flagellates
(leishmaniae)
Trypanosomatid protozoa; two morphologic forms—promastigotes (anterior flagellum) in the insect host and amastigote (no flagella) in the vertebrate host.
Transmission is through an insect vector.
Recovery and identification of the organisms are related to body site. Recovery of leishmanial amastigotes is limited to the site of the lesion in infections other than those caused by the Leishmania donovani complex (visceral leishmaniasis).
Flagellates
(trypanosomes)
Trypanosomatid protozoa; morphologic forms are identified based on the position, length, and attachment site of the flagella. At some time in their life cycle, these protozoa have the trypomastigote form with the typical undulating membrane and free flagellum at the anterior end.
Transmission is typically through an insect vector.
Some organisms cause African sleeping sickness (e.g., Trypanosoma brucei gambiense, T. b. rhodesiense). The etiologic agent of American trypanosomiasis is T. cruzi, which has amastigote and trypomastigote stages in the mammalian host and an epimastigote form in the arthropod host.
Nematodes, intestinal Helminthic parasites; roundworms.
Nematodes have separate sexes, are elongate-cylindrical and bilaterally symmetrical with a triradiate symmetry at the anterior end. Nematodes have an outer cuticle layer, no circular muscles, and a pseudocele that contains all systems (digestive, excretory, nervous, reproductive).
Transmission is by ingestion of eggs or by skin penetration of larval forms from the soil.
Examples: Ascaris, Enterobius, Trichuris, and Strongyloides spp. and hookworm.
Nematodes, tissue Helminthic parasites; roundworms.
Many of these organisms are rarely seen in the United States; however, some are important and are found worldwide. Diagnosis may be difficult if the only specimens are obtained through biopsy and/or autopsy, and interpretation must be based on examination of histologic preparations.
Examples: Trichinella spp., visceral larva migrans (VLM), ocular larva migrans (OLM), cutaneous larva migrans (CLM).
Nematodes, filarial Helminthic round worms.
Transmission is via arthropods.
Adult worms tend to live in the tissues or lymphatics of the vertebrate host. The diagnosis is made on the basis of recovery and identification of the larval worms (microfilariae) in the blood, other body fluids, or skin.
Examples: Wuchereria, Brugia, Loa, and Onchocerca spp.
Cestodes, intestinal Helminthic tapeworms. Adult tapeworm consists of a chain of egg-producing units called proglottids, which develop from the neck region of the attachment organ (scolex). Food is absorbed through the worm’s integument. The intermediate host contains the larval forms that are acquired through ingestion of the adult tapeworm eggs.
Transmission is through the ingestion of larval forms in poorly cooked or raw meat or freshwater fish.
Examples: Dipylidium caninum (infection is acquired by accidental ingestion of dog fleas).
Hymenolepis nana and H. diminuta are transmitted via ingestion of certain arthropods (fleas, beetles). Also, H. nana can be transmitted through egg ingestion (life cycle can bypass the intermediate beetle host).
Humans can serve as both the intermediate and definitive hosts in H. nana and Taenia solium infections.
Cestodes, tissue Tissue tapeworms.
Transmission is through ingestion of certain tapeworm eggs or accidental contact with certain larval forms, leading to tissue infection. Humans serve as the accidental intermediate host.
Examples: Taenia solium, Echinococcus granulosus, and several other species.
Trematodes, intestinal Flatworms that are exclusively parasitic. Except for the schistosomes (blood flukes), flukes are hermaphroditic. They may be flattened; most have oral and ventral suckers.
Transmission: Intestinal trematodes require a freshwater snail to serve as an intermediate host; these infections are food borne (freshwater fish, mollusks, or plants).
Example: Fasciolopsis buski, the giant intestinal fluke.
Trematodes, liver, lung Transmission: Liver and lung trematodes require a freshwater snail to serve as an intermediate host; these infections are food borne (freshwater fish, crayfish or crabs, or plants).
Examples: Public health concerns include cholangiocarcinoma associated with Clonorchis and Opisthorchis infections, severe liver disease associated with Fasciola infections, and misdiagnosis of tuberculosis in individuals infected with Paragonimus spp.
Trematodes, blood Schistosomes; sexes are separate. Males are characterized by an infolded body that forms the gynecophoral canal in which the female worm is held during copulation and oviposition.
Transmission: Infection is acquired by skin penetration by the cercarial forms that are released from freshwater snails. The adult worms reside in the blood vessels over the small intestine, large intestine, or bladder. Examples: Schistosoma mansoni, S. haematobium, and S. japonicum.


TABLE 47-2


Body Sites and Parasite Recovery (Trophozoites, Cysts, Oocysts, Spores, Adults, Larvae, Eggs, Amastigotes, Trypomastigotes)

















































Site Parasites
Blood  
Red cells Plasmodium spp.
Babesia spp.
White cells Leishmania spp.
Toxoplasma gondii
Whole blood/plasma Trypanosoma spp.
Microfilariae
Bone marrow Leishmania spp.
Trypanosoma cruzi
Plasmodium spp.
Central Nervous System  
Cutaneous ulcers Taenia solium (cysticerci)
Echinococcus spp.
Naegleria fowleri
Acanthamoeba spp.
Balamuthia mandrillaris
Sappinia diploidea
Toxoplasma gondii
Microsporidia
Trypanosoma spp.
Intestinal tract Leishmania spp.
Acanthamoeba spp.
Entamoeba histolytica
Entamoeba dispar
Entamoeba coli
Entamoeba hartmanni
Endolimax nana
Iodamoeba bütschlii
Blastocystis hominis
Giardia lamblia
Chilomastix mesnili
Dientamoeba fragilis
Pentatrichomonas hominis
Balantidium coli
Cryptosporidium spp.
Cyclospora cayetanensis
Isospora belli
Microsporidia
Ascaris lumbricoides
Enterobius vermicularis
Hookworm
Strongyloides stercoralis
Trichuris trichiura
Hymenolepis nana
Hymenolepis diminuta
Taenia saginata
Taenia solium
Diphyllobothrium latum
Clonorchis sinensis (Opisthorchis)
Paragonimus spp.
Schistosoma spp.
Fasciolopsis buski
Fasciola hepatica
Metagonimus yokogawai
Heterophyes heterophyes
Liver, spleen Echinococcus spp.
Entamoeba histolytica
Leishmania donovani
Microsporidia
Lung Cryptosporidium spp.*
Echinococcus spp.
Paragonimus spp.
Microsporidia
Muscle Taenia solium (cysticerci)
Trichinella spp.
Onchocerca volvulus (nodules)
Trypanosoma cruzi
Microsporidia
Skin Leishmania spp.
Onchocerca volvulus
Microfilariae
Urogenital system Trichomonas vaginalis
Schistosoma spp.
Microsporidia
Microfilariae
Eye Acanthamoeba spp.
Toxoplasma gondii
Loa loa
Microsporidia

Note: This table does not include every possible parasite that can be found in a particular body site; the most likely organisms have been listed


*Disseminated in severely immunosuppressed individuals.



TABLE 47-3


Specimens and/or Body Site: Specimen Options, Collection and Transport Methods, and Processing





















































































































































































Specimens and/or Body Site Specimen Options Collection and Transport Methods Specimen Processing Comments
Stool for ova and parasite (O&P) examination Fresh stool image pint waxed container; 30 min if liquid, 60 min if semi formed, 24 h if formed; delivery to laboratory Direct wet smear (not on formed specimen), concentration, permanent stained smear Stool specimens containing barium are unacceptable; intestinal protozoa may be undetectable for 5 to 10 days after barium use. Certain substances and medications also impede detection of intestinal protozoa: mineral oil, bismuth, antibiotics, antimalarial agents, and nonabsorbable antidiarrheal preparations. After administration of any of these compounds, parasitic organisms may not be recovered for a week to several weeks. Specimen collection should be delayed after barium or antibiotics are administered for 5 to 10 days or at least 2 weeks, respectively.
Preserved stool* 5% or 10% formalin, MIF, SAF, Schaudinn’s, PVA, modified PVA, single vial systems, universal fixative Concentration, permanent stained smear
Depending on specimen (fresh or preserved) and patient’s clinical history, immunoassays may also be performed.
Stool for culture of nematodes Fresh stool, entire stool specimen image pint waxed container; immediate delivery to laboratory Filter paper strip, Petri dish, agar plate, charcoal cultures are all available. Fresh stool (do not refrigerate) is required for these procedures.
Stool for recovery of tapeworm scolex Preserved stool, entire stool specimen 5% or 10% formalin (10% recommended) The stool is filtered with a series of mesh screens and examined for the very small tapeworm scolex (proof of therapy efficacy) and/or proglottids (uncommon procedure but an option). After treatment for tapeworm removal, the patient should be instructed to take a saline cathartic and to collect all stool material passed for the next 24 hr. The stool should be immediately placed in 10% formalin and thoroughly broken up and mixed with the preservative (1-gallon [3.8-liter] plastic jars are recommended, half full of 10% formalin).
Adult worms, worm segments Saline, 70% alcohol
Cellophane tape preparation for pinworms Surface sample from perianal skin; anal impression smear Cellulose (Scotch) tape preparation or commercial sampling paddle or swab Tape is lifted from a slide, a drop of xylene substitute is added, the tape is replaced, and the specimen is ready for examination under the microscope. Specimens should be collected late at night after the person has been asleep for several hours or first thing in the morning before going to the bathroom or taking a shower. At least 4 to 6 consecutive negative tapes are required to rule out the infection.
Sigmoid colon Sigmoidoscopy material, prepared as smears Fresh or PVA or Schaudinn’s smears; specimen is taken with a spatula rather than cotton-tipped swabs; transported as smears in preservative Direct wet smear, permanent stained smears Material from the mucosal surface should be aspirated or scraped; it should not be obtained with cotton-tipped swabs. At least six representative areas of the mucosa should be sampled and examined (six samples, six slides). A parasitology specimen tray (containing Schaudinn’s fixative, PVA, and 5% or 10% formalin) should be provided, or a trained technologist should be available at the time of sigmoidoscopy to prepare the slides. Examination of sigmoidoscopy specimens does not take the place of routine O&P examinations.
If the amount of material is limited, use of a fixative containing PVA is highly recommended.
Duodenum Duodenal contents Entero-Test or aspirates; string in Petri dish or tube; immediate transport to laboratory The specimen may be centrifuged (10 min at 500× g) and should be examined immediately as a wet mount for motile organisms. Iodine also can be used.
Direct wet smear of mucus; permanent stained smears can also be prepared.
A fresh specimen is required; the amount may vary from < 0.5 mL to several milliliters of fluid. If the specimen cannot be completely examined within 2 hr after it is taken, any remaining material should be preserved in 5% to 10% formalin.
Entero-Test capsule (string) Duodenal contents Entero-Test (string test) in Petri dish (fresh) or preserved in PVA Bile-stained mucus clinging to the yarn should be scraped off (mucus can also be removed by pulling the yarn between thumb and finger) and collected in a small Petri dish; disposable gloves are recommended. Usually 4 or 5 drops of material are obtained. The specimen should be examined immediately as a wet mount for motile organisms (iodine may be added later to facilitate identification of any organisms present). Organism motility is like that described previously for duodenal drainage. The pH of the terminal end of the yarn should be checked to ensure adequate passage into the duodenum (a very low pH means that it never left the stomach). The terminal end of the yarn should be yellow-green, indicating that it was in the duodenum (the bile duct drains into the intestine at this point).
Permanent stained smears can also be prepared.
If the specimen cannot be completely examined within 1 hr after removal of the yarn, the material should be preserved in 5% to 10% formalin or PVA-mucus smears should be prepared.
Urogenital tract Vaginal discharge
Urethral discharge
Prostatic secretions
Saline swab, transport swab (no charcoal), culture medium, plastic envelope culture,
air-dried smear for FA
Direct wet smear; fluorescence; urine must be centrifuged before examination. Fresh specimens are required; an air-dried smear may be an option for fluorescence. Do not refrigerate swabs and/or culture containers at any time, because motility and/or ability to grow will probably be lost.
  Urine Single unpreserved specimen, 24-hr unpreserved specimen, early morning
Nucleic acid-based testing media according to manufacturer’s instructions
Examination of urinary sediment may be indicated in certain filarial infections. Administration of the drug diethylcarbamazine (Hetrazan) has been reported to enhance the recovery of microfilariae from the urine. The triple-concentration technique is recommended for the recovery of microfilariae. The membrane filtration technique can also be used with urine for the recovery of microfilariae. A membrane filter technique for the recovery of Schistosoma haematobium eggs has also been useful.  
Sputum Sputum True sputum (not saliva) Direct wet smear; permanent stained smears; fluorescence also available (Calcofluor for microsporidia). Sputum is usually examined as a wet mount (saline or iodine), using low and high dry power (×100 and ×400). The specimen is not concentrated before preparation of the wet mount. If the sputum is thick, an equal amount of 3% sodium hydroxide (NaOH) (or undiluted chlorine bleach) can be added; the specimen is thoroughly mixed and then centrifuged. NaOH should not be used if the examiner is looking for Entamoeba spp. or Trichomonas tenax.
After centrifugation, the supernatant fluid is discarded, and the sediment can be examined as a wet mount with saline or iodine. If examination must be delayed for any reason, the sputum should be fixed in 5% or 10% formalin to preserve helminth eggs or larvae or in PVA fixative to be stained later for protozoa.
True sputum is required; all specimens, especially induced specimens and BAL, should be delivered immediately to the laboratory (do not refrigerate).
Induced sputum No preservative (10% formalin if time delay)
Bronchoalveolar lavage (BAL) Sterile; immediate delivery to laboratory
Aspirates Bone marrow Sterile; immediate delivery to laboratory Permanent stained smears; cultures can also be set (specifically designed for the recovery of blood parasites). All aspirates for culture must be collected using sterile conditions and containers; this is mandatory for culture isolation of leishmaniae and trypanosomes.
Cutaneous ulcers Sterile plus air-dried smears
Liver, spleen Sterile, collected in 4 separate aliquots (liver)
Lung
 Transbronchial aspirate Air-dried smears
 Tracheobronchial aspirate Air-dried smears
Central nervous system Cerebrospinal fluid (CSF) Sterile Direct wet smear, permanent stained smears; culture for free-living amebae (Naegleria, Acanthamoeba spp.). All specimens must be transported immediately to the laboratory (STAT procedure).
Biopsy Intestinal tract Routine histology Direct wet smears, permanent stained smears; specimens to histology for routine processing. The more material that is collected and tested, the more likely the organism is to be isolated and subsequently identified. Sterile collection is required for all specimens that will be cultured; bacterial and/or fungal contamination prevents isolation of parasites in culture.
Cutaneous ulcers Sterile, nonsterile to histopathology
(formalin acceptable)
Eye Sterile (in saline), nonsterile to histopathology
 Scrapings Sterile (in saline)
Cornea (scrapings) Collected by physician, placed directly on microscope slide Fixed using methyl alcohol and stained using Calcofluor white Helpful in diagnosis of Acanthamoeba keratitis
Liver, spleen Sterile, nonsterile to histopathology    
Lung    
 Brush biopsy Air-dried smears    
 Open lung biopsy Air-dried smears    
Muscle Fresh, squash preparation, nonsterile to histopathology    
Skin biopsy Nonsterile to histopathology
(formalin acceptable)
   
 Scrapings Sterile (in saline), nonsterile to histopathology    
Skin snip Aseptic, smear or vial
No preservative
   
Blood Smears of whole blood Fresh (first choice)
Thick and thin films; immediate delivery to laboratory
Thick and thin films, specialized concentrations and/or screening methods Examination of blood films (particularly for malaria) is considered a STAT procedure; immediate delivery to the laboratory is mandatory.
  Anticoagulated blood Anticoagulant (second choice)
EDTA* (first choice)
Heparin (second choice)
Thick and thin films, specialized concentrations and/or rapid methods
QBC Microhematocrit Centrifugation Method (Becton Dickinson, Tropical Disease Diagnostics, Sparks, Maryland)
Delivery to the laboratory within 30 min or less; if this time frame is not met, typical parasite morphology may not be seen in blood collected using anticoagulants.
Knott concentration procedure: Used primarily to detect microfilariae in the blood, especially when a light infection is suspected. The disadvantage of the procedure is that the microfilariae are killed by the formalin and therefore are not seen as motile organisms.
Membrane filtration technique: This technique, using Nuclepore filters, has proved highly efficient in demonstrating filarial infections when microfilaremias are of low density. It has also been successfully used in field surveys.


image


image


image


EDTA, Ethylenediaminetetraacetic acid; FA, fluorescent antibody; MIF, merthiolate-iodine-formalin; PVA, polyvinyl alcohol; SAF, sodium acetate–acetic acid–formalin.


*A number of new stool fixatives are available; some use a zinc sulfate base rather than mercuric chloride. Some collection vials can be used as a single-vial system; both the concentration and permanent stained smear can be performed from the preserved stool. However, not all single-vial systems (proprietary formulas) provide material that can be used for fecal immunoassay procedures. A universal fixative is now available (TOTAL-FIX), which contains no formalin, no mercury, and no PVA.


Modified from Garcia LS: Diagnostic medical parasitology, ed 5, Washington, DC, 2007, ASM Press.



Epidemiology


Parasites usually are restricted to specialized environments inside and outside their hosts. A zoonosis is a disease of wild or domestic animals that occurs in humans as a result of parasitic infection. Animals that are potential sources of infection for humans are called reservoir hosts. The host specificity of any particular parasite influences factors associated with transmission and control. When humans are the only host for a parasite or a stage of its development, control options are relatively easy to define. However, if an infection is a zoonosis, control measures can become complex because of the existence of one or more reservoir animals. Some organisms are free-living during stages of their life cycles and do not depend on the human host for survival. In some cases, the human becomes an accidental host.


Parasites are transmitted from host to host through sexual means (venereal transmission) (Trichomonas vaginalis), from ingestion of infective forms in food or water (Giardia lamblia, Cryptosporidium spp., Ascaris lumbricoides), through skin penetration of infective larvae (Strongyloides stercoralis, hookworm), or through the bites of various arthropods (Plasmodium, Trypanosoma, Leishmania) (Table 47-4).



TABLE 47-4


Epidemiology of the More Common Groups of Human Parasites



















































































































Parasite Group Habitat (Reservoir) Mode of Transmission Prevention
Protozoa, Intestinal      
Amebae Single-celled organisms generally found in humans. Although certain animals harbor some of these organisms, they are not considered important reservoir hosts. Humans acquire infections by ingesting food and water contaminated with fecal material containing the resistant, infective cyst stage of the protozoa. Various sexual practices have also been documented in transmission. Preventive measures include increased attention to personal hygiene and sanitation measures; elimination of sexual activities that may involve fecal-oral contact.
Flagellates The flagellates are generally found in humans. Although certain animals harbor some of these organisms, they are not considered important reservoir hosts; one exception may be animals, such as the beaver, that harbor Giardia lamblia. Contaminated water supplies are also a source. Humans acquire infections by ingesting food and water contaminated with fecal material containing the resistant, infective cyst stage of the protozoa; in some cases (Dientamoeba fragilis), no cyst stage has been identified; the trophozoite forms may be transmitted from person to person in certain helminth eggs. Preventive measures include increased attention to personal hygiene and sanitation measures; elimination of sexual activities that may involve fecal-oral contact; adequate water treatment (including filtration) is required; also awareness of environmental sources of infection.
Ciliates Balantidium coli is generally found in humans, but it is also found in pigs. In some areas of the world, pigs are considered important reservoir hosts. Humans acquire infections by ingesting food and water contaminated with fecal material containing the resistant, infective cyst stage of the protozoa. Preventive measures include increased attention to personal hygiene and sanitation measures, as well as elimination of sexual activities that may involve fecal-oral contact.
Coccidia Coccidia are found in humans. In some cases (e.g., cryptosporidiosis) animal reservoirs (cattle) can serve as important hosts. The muscle of various animals may contain sarcocysts that are infective for humans through the consumption of raw or poorly cooked meat. Numerous waterborne outbreaks with Cryptosporidium spp. have been reported throughout the world. Coccidian oocysts are extremely resistant to environmental conditions, particularly if they are kept moist. These protozoa are acquired through ingestion of various meats or by fecal-oral transmission through contaminated food and/or water. The infective forms are called oocysts (Cryptosporidium spp., Isospora (Cystoisospora) belli, Cyclospora cayetanensis) or sarcocysts (Sarcocystis spp.), which are contained in infected meat. Cryptosporidia have also been implicated in nosocomial infections. Preventive measures include increased attention to personal hygiene and sanitation measures; elimination of sexual activities that may involve fecal-oral contact. Adequate water treatment (including filtration) is mandatory; awareness of environmental sources of infection also is important.
Microsporidia Microsporidia can infect every living animal, some of which probably serve as reservoir hosts for human infection. However, host specificity has not been well defined to date. The spores are environmentally resistant and can survive years if kept moist. Infection with microsporidial spores usually occurs through ingestion; however, inhalation of spores and direct inoculation from the environment almost certainly occur. Preventive measures include increased attention to personal hygiene and sanitation measures; increased awareness of environmental exposure possibilities; and adequate water treatment.
Protozoa, Other Sites      
Amebae Free-living amebae are associated with warm, freshwater environments; they are also found in soil. Although humans can harbor these organisms, person-to-person transfer is thought to be rare. Environmental sources are the primary link to human infection. Contaminated eye care solutions have been linked to organisms that cause keratitis. Infection occurs through contact with contaminated water; organisms enter through the nasal mucosa and may travel via the olfactory nerve to the brain. Disease can be very severe and life-threatening; keratitis is also caused by these organisms, and infection can be linked to blindness or severe corneal damage. Eye infections can be linked to contaminated lens solutions or direct, accidental inoculation of the eye from environmental water and/or soil sources. Avoidance of contaminated environmental water and soil sources; adequate care of contact lens systems.
Flagellates Trichomonas vaginalis infection is found in a large percentage of humans; humans may present as symptomatic or asymptomatic. Person-to-person transfer is very common; reinfection is also common, particularly if sexual partners are not treated. T. vaginalis is found in the genitourinary system and is usually acquired by sexual transmission. Awareness of sexual transmission; treatment of all partners when infection is diagnosed in an individual patient.
Protozoa, Blood and Tissue      
Malaria, babesiosis Humans harbor the five species of malaria (Plasmodium vivax, P. ovale, P. malariae, P. knowlesi, and P. falciparum). Other animals can carry Babesia spp., and animal reservoir hosts play a large role in human transmission. These organisms are arthropod-borne, Plasmodium spp. by the female anopheline mosquito and Babesia spp. by one or more genera of ticks. These infections can also be transmitted transplacentally, via shared needles, through blood transfusions, and from organ transplants. Vector control; awareness of transmission through blood transfusions, shared drug needles, congenital infections, and organ transplants. Careful monitoring of the blood supply. Malaria prophylaxis if traveling to endemic areas.
Flagellates
(leishmaniae)
Some strains of leishmaniae have reservoir hosts (e.g., dogs for the Mediterranean strain of Leishmania donovani and wild rodents for the African strains of L. donovani.) L. tropica also has been linked to the same two animal reservoirs. Transmission is through the bite of infected sandflies. Infection can also occur from person to person (cutaneous lesions), from blood transfusion, shared needles, and organ transplants. Vector control; avoiding environmental sources (e.g., dogs, wild rodents); careful handling of all clinical specimens from infected patients.
Flagellates
(trypanosomes)
Humans are the only known hosts for Trypanosoma brucei gambiense (West African trypanosomiasis); Trypanosoma brucei rhodesiense (East African trypanosomiasis) infections are found in a number of antelope and other ungulates that act as reservoir hosts. Rodents and some mammals are reservoir hosts for Trypanosoma cruzi. Transmission is through the bite of the infected tsetse fly and through blood transfusion, shared needles, and organ transplants.
Transmission of T. cruzi is through the infected feces of the triatomid bug; the bug takes a blood meal, immediately defecates, and the human host scratches the infected feces into the bite site; bug saliva contains an irritant that stimulates scratching.
Vector control; awareness of potential exposure/infection from blood sources (transfusions, shared needles, organ transplants). Laboratory accidents while handling infected blood have been reported.
Nematodes, intestinal These roundworms generally do not have animal reservoirs relevant to human infection. One exception is the pig ascarid; human infections have been reported. These worms are found worldwide; Ascaris lumbricoides is probably the most common parasite of humans, although some would argue that Enterobius vermicularis is number one. Strongyloides stercoralis is particularly important as the causative agent of severe disease in the compromised host. A. lumbricoides and Trichuris trichiura eggs must undergo development in the soil before they are infective; thus children who play in the dirt are a particularly high-risk group. Ingestion of food and water contaminated with infective eggs is the primary route of infection.
Hookworm and S. stercoralis infections are initiated by larval penetration of the skin from contaminated soil.
Pinworm infection (E. vermicularis) is acquired through ingestion of infective eggs from the environment (hand-to-mouth).
Avoiding ingestion of contaminated soil and/or avoiding frequenting soil contaminated with hookworm eggs (pets, soil, water, warmth, warm weather); treatment for pinworm is recommended, but reinfection is common.
Nematodes, tissue Trichinella spp. have a number of animal reservoir hosts, including bears, walruses, pigs, rodents, and other animals. Dog and cat hookworms cause cutaneous larva migrans (CLM), and the dog and cat ascarid, Toxocara spp., causes visceral and ocular larva migrans (VLM, OLM). These infections can be serious and cause severe disease if not treated. Trichinella organisms are acquired by ingestion of raw or poorly cooked infected meat.
CLM is caused by skin penetration of infective larvae from the soil; children should avoid sandboxes where dogs and cats are known to defecate. Larval migration is limited to the skin.
VLM and OLM are caused by accidental ingestion of Toxocara spp. eggs from contaminated soil; larval migration occurs throughout the body, including the eyes.
Adequate cooking of infected meat; awareness of possibility of contaminated soils for dog and cat hookworms and/or ascarids; covering of all sandboxes where pets have access to defecation and children play.
Nematodes, filarial Wuchereria bancrofti, Loa loa, and Onchocerca volvulus have no animal reservoirs and are found only in humans, whereas Brugia spp. can also be found in cats and monkeys. Dracunculus medinensis can infect dogs, cats, and monkeys and also humans. Filarial nematodes are transmitted through the bite of a blood-sucking arthropod (midges, mosquitoes, flies). Dracunculus infections are acquired through ingestion of water contaminated with small crustaceans, Cyclops spp., which contain infective larvae. Vector control; protection of well-water sources.
Cestodes, intestinal The human serves as the definitive host for beef (Taenia saginata) and pork (Taenia solium) tapeworms; cows/camels and pigs serve as intermediate hosts, respectively. Humans also serve as the intermediate host for T. solium (cysticercosis). Diphyllobothrium latum adult tapeworms can be found in a number of wild animals, the most important being dogs, bears, seals, and walruses, which serve as reservoir hosts; humans are the definitive host. Hymenolepis nana (dwarf tapeworm) can occur in rodents; humans can serve as both intermediate and definitive hosts, with development from the egg to adult worm occurring in the human intestine. Human infection with the adult worm occurs through ingestion of raw or poorly cooked meat (beef, camel, pork) containing the intermediate forms, the cysticerci. Humans become the accidental intermediate host when eggs from an adult T. solium tapeworm are ingested. The cysticerci develop in the muscle and tissues of the human rather than the pig. Infection with the adult D. latum tapeworm occurs through ingestion of poorly cooked freshwater fish containing the sparganum or plerocercoid larval form. Infection with H. nana is primarily acquired through accidental ingestion of eggs from an adult tapeworm. Adequate cooking of infected meat; treatment of patients harboring adult tapeworms (accidental ingestion of eggs can lead to infection).
Cestodes, tissue Adult worms are found in a variety of animals; the human becomes the accidental intermediate host after ingestion of eggs from the adult worms. Reservoir hosts include dogs, cats, and rodents. Ingestion of certain tapeworm eggs or accidental contact with certain larval forms can lead to tissue infection with Taenia solium, Echinococcus spp., and several others. Preventive measures involve increased attention to personal hygiene and sanitation measures.
Trematodes, intestinal Fish-eating wild and domestic animals serve as reservoir hosts. The definitive host of Fasciolopsis buski is the pig. Ingestion of water chestnut and caltrop (raw, peeled with the teeth) is the source of infection; metacercariae are encysted on the plant material. Pig feces are used to fertilize various water plant crops. Avoiding eating raw water plants that may contain encysted larval forms of the flukes; adequate waste disposal of farm animal feces (pigs).
Trematodes, liver, lung Cats, dogs, and wild fish-eating mammals can serve as reservoir hosts for Opisthorchis spp., Clonorchis sinensis, and Paragonimus spp. Fasciola hepatica is normally a parasite of sheep, and F. gigantica is a parasite of cattle; humans are accidental hosts. Infection occurs through ingestion of raw or poorly cooked fish, crabs, crayfish, and certain plants in or on which metacercariae are encysted. Infection with Fasciola spp. is not easily acquired (the parasite is not that well adapted to the human host). Thorough cooking of potentially infected fish, crabs, crayfish; avoiding eating raw water plants that may contain encysted metacercariae.
Trematodes, blood Schistosoma mansoni and S. haematobium appear to be restricted to the human host; S. japonicum can be found in cattle, deer, dogs, and rodents. The worms mature in the blood vessels, and eggs make their way outside the body in stool and/or urine. The freshwater snail is a mandatory part of the life cycle (contains developmental forms of schistosome). Infection occurs through skin penetration by infected cercariae released from a freshwater snail containing the intermediate stages of the schistosome life cycle. Cercariae can be released from the snail intermediate host singly or in groups. Protection from potentially contaminated water sources; awareness of mode of transmission; proper handling of human waste containing eggs (continued infection of snail intermediate hosts).

Aug 25, 2016 | Posted by in MICROBIOLOGY | Comments Off on Laboratory Methods for Diagnosis of Parasitic Infections: Overview
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