Chapter 21

Definitions Parasitology is a branch of microbiology; it is the scientific study of parasites. Parasitism is a symbiotic relationship that is of benefit to one party or symbiont (the parasite) and usually detrimental to the other party (the host). Parasites are organisms that live on or in other living organisms (hosts), at whose expense they gain some advantage. Parasites that live on the outside of the host’s body are called ectoparasites. Parasites that live inside the host are called endoparasites.

Definitions (cont.) The life cycle of a parasite may involve one or more hosts. If more than one host is involved, the definitive host is the host that harbors the adult or sexual stage of the parasite. The host that harbors the larval or asexual stage is the intermediate host. An accidental host is one that can serve as a host, but is not the usual host in the parasite’s life cycle. A dead-end host is one in which the parasite cannot continue its life cycle.

Definitions (cont.) A facultative parasite is an organism that can be parasitic, but does not have to live as a parasite; it is capable of an independent life. Example: the free-living amoeba Naegleria fowleri, which causes primary amebic meningoencephalitis An obligate parasite has no choice; it must inhabit a host(s). Most parasites that infect humans are obligate parasites.

How Parasites Cause Disease The manner in which parasites cause damage to their host varies from one species of parasite to another, and often depends on the number of parasites that are present. Some parasites produce toxins; some produce harmful enzymes; some invasive and migratory parasites cause physical damage to tissues and organs; some cause the destruction of individual cells; and some cause occlusion of blood vessels and other tubular structures. Sometimes, the host immune response to the parasites causes more injury than do the parasites themselves.

Parasitic Protozoa Most protozoa are unicellular. They are sometimes classified taxonomically by their mode of locomotion; amebae move by means of pseudopodia (false feet); flagellates move by means of flagella; ciliates move by means of cilia; sporozoans do not move. Not all protozoa are parasitic (e.g., Paramecium spp.). Protozoal infections are most often diagnosed by microscopic examination of body fluids, tissue specimens, or feces— specimens are examined for motile trophozoites and dormant cyst stages.

Protozoal Infections of the Skin Leishmaniasis Caused by various species of flagellated protozoa in the genus Leishmania Usually transmitted via the bite of an infected sand fly There are three forms of the disease: cutaneous, mucocutaneous, visceral. Cutaneous and mucocutaneous leishmaniasis can cause severe tissue damage and disfigurement. Visceral leishmaniasis can lead to death.

Protozoal Infections of the Eyes Amebic conjunctivitis and keratoconjunctivitis Caused by several species of amebae in the genus Acanthamoeba (facultative parasites) Can lead to loss of vision or enucleation Toxoplasmosis Caused by Toxoplasma gondii, an intracellular sporozoan Can involve the CNS, lungs, muscles, or heart, as well as the eyes Acquired by ingesting cysts or oocysts

Protozoal Infections of the Gastrointestinal Tract Disease Etiologic agent Mode of Transmission Amebiasis Entamoeba histolytica Humans; fecally contaminated food or water Balantidiasis Balantidium coli Pigs; food or water contaminated with pig feces Cryptosporidiosis Cryptosporidium parvum Infected humans, cattle, and other domesticated animals Cyclosporiasis Cyclospora cayetanensis Fecally contaminated water and produce Giardiasis Giardia lamblia Humans and infected animals

Protozoal Infections of the Genitourinary Tract Trichomoniasis Caused by Trichomonas vaginalis, a flagellate A sexually transmitted disease of men and women Usually symptomatic in women and asymptomatic in men T. vaginalis causes about 1/3 of the cases of vaginitis in the United States. T. vaginalis vaginitis is usually diagnosed by observation of motile trophozoites in a saline wet mount of vaginal discharge material.

Protozoal Infections of the Circulatory System Disease Etiologic agent Mode of Transmission African Trypanosomiasis 2 different subspecies of Trypanosoma brucei Bite of infected tsetse fly American trypanosomiasis (Chagas Disease) T. cruzi Reduvid bug (also known as a triatome, kissing, or cone-nosed bug) Babesiosis Babesia microti and other Babesia spp. Tick bite Malaria 4 different species of Plasmodium Bite of infected female Anopheles mosquito

Life Cycle of Malarial Parasites

Protozoal Infections of the CNS Primary amebic meningoencephalitis Caused by Naegleria fowleri, an ameboflagellate Acanthamoeba and Balamuthia spp. can cause similar conditions. Persons often become infected by swimming and/or diving in ameba-contaminated water (e.g., “the old swimming hole”). After the amebae colonize nasal tissues, they invade the brain and meninges by traveling along the olfactory nerves; the disease is often fatal.

Helminths The word helminth means parasitic worm Helminths are multicellular, eukaryotic organisms; two major divisions: (1) round worms or nematodes, and (2) flatworms or platyhelminthes. The flatworms are further divided into tapeworms (cestodes) and flukes (trematodes). The helminth life cycle has three stages: egg, larva, and the adult worm. Helminth infections are primarily acquired by ingesting the larval stage; in some helminth diseases, larvae enter by penetration of the skin.

Helminth Infections of Humans Location Helminth disease Cause Skin Onchocerciasis Onchocerca volvulus Muscle/Subcutaneous tissue Trichinosis Dracunculiasis Trichinella spiralis Dracunculus medinensis Eyes Loiasis O. volvulus Loa loa Respiratory system Paragonimiasis Paragonimus westermani Circulatory system Filariasis Schistosomiasis Wuchereria bancrofti/Brugia malayi Schistosoma spp. Central nervous system Cysticercosis Hydatid cyst disease Taenia solium (cysts) Echinococcus granulosis or E. multilocularis

Helminth Infections of the GI Tract Ascariasis – Ascaris lumbricoides Hookworm infection – Ancylostoma duodenale or Necator americanus Pinworm – Enterobius vermicularis Whipworm – Trichuris trichiura Strongyloidiasis – Strongyloides stercoralis Beef tapeworm – Taenia saginata Dog tapeworm – Dipylidium caninum Dwarf tapeworm – Hymenolepis nana Fish tapeworm – Diphyllobothrium latum Pork tapeworm – Taenia solium Rat tapeworm – Hymenolepis diminuta Fasciolopsiasis – Fasciolopsis buski Fascioliasis – Fasciola hepatica Clonorchiasis – Clonorchis sinensis

Medically Important Arthropods There ae three classes of arthropods studied in parasitology courses: Insects (e.g., lice, fleas, mosquitoes) Arachnids (e.g., mites and ticks) Crustaceans (e.g., crabs, crayfish, and certain Cyclops species) Arthropods serve as mechanical or biologic vectors in the transmission of certain infectious diseases. Mechanical vectors pick up a parasite at point A and drop it off at point B. Biological vectors harbor the parasite in their body, where the parasite matures and/or multiplies.

Ways in Which Arthropods May Be Involved in Human Diseases Type of Involvement Example(s) The arthropod may actually be the cause of the disease. Scabies, a disease in which microscopic mites live in subcutaneous tunnels and cause intense itching The arthropod may serve as the intermediate host in the life cycle of a parasite. Flea in the life cycle of the dog tapeworm; beetle in the life cycle of the rat tapeworm The arthropod may serve as the definitive host in the life cycles of a parasite Female Anopheles mosquito in the life cycle of malarial parasites The arthropod may serve as a vector in the transmission of an infectious disease. Oriental rat flea in the transmission of plague; tick in the transmission of Rocky Mountain spotted fever and Lyme disease

Medically Important Arthropods (cont.) Dermacentor andersoni, wood tick, one of the tick vectors of spotted fever rickettsiosis B. Xenopsylla cheopis, oriental rat flea, vector of plague and endemic typhus

Medically Important Arthropods (cont.) C. Pediculus humanus, human body louse (a vector of epidemic typhus) D. Phthirus pubis, the pubic or crab louse