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	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.
Stool for ova and parasite (O&P) examination	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	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).
Stool for recovery of tapeworm scolex	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.

^†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

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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.