1. Chapter 21

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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

9. 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

10. 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.

11. 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

12. Life Cycle of Malarial Parasites

13. 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.

14. 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.

15. 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

16. 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

17. 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.

18. 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

19. 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

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