2-34 Introduction to Parasitic Diseases

Question 1

What are some types of parasitic relationships?

Answer 1

• Obligate or facultative
• Accidental: affects an unusual host (e.g., Toxocara canis, a dog parasite, in humans)
• Temporary or permanent parasite
• Opportunistic: capable of producing disease in an immunodeficient host (e.g., AIDS and cancer patients) where it is either found in a latent form or causes a self-limiting disease (e.g., Toxoplasma gondii)
• Zoonotic: primarily infects animals but is transmittable to humans (e.g., Cryptosporidium parvum)

Question 2

What are the most common parasitic infections in the U.S.?

Answer 2

• Trichomoniasis (STI)
• Giardiasis
• Cryptosporidosis
• Toxoplasmosis

Question 3

What are ectoparasites and endoparasites?

Answer 3

Ectoparasites live on the outside of the host, generally attaching temporarily to the skin to feed. They are arthropods that either cause diseases or act as vectors transmitting other parasites. Include fleas, lice, mosquitoes, bugs, mites, ticks, etc.

Endoparasites live within the body of the host. They are classified by the body tissue they inhabit, such as intestinal or atrial. Include helminthes (worms of various types), protozoa, or sometimes larval stages of arthropods (insects, mites, etc.).

Question 4

What are the two kinds of hosts, with regard to parasitism?

Answer 4

A definitive host (DH) harbors the adult or sexually mature stages of the parasite, OR is the one in whom sexual reproduction occurs. Transmission from host to host can occur directly.

An intermediate host (IH) harbors larval or sexually immature stages of the parasite, OR is the one in whom asexual reproduction occurs. The transmission to the definitive host requires a vector (which may be the ingestion of the intermediate host which is infected with the parasite).

Question 5

What are the 5 ways that parasitic infections can be transmitted to hosts?

Answer 5

1. Direct ingestion of infective larvae, eggs or cysts: typical of many nematodes
2. Eating the intermediate host: ex) Diphyllobothrium latum (the fish tapeworm), where a human ingests raw or undercooked fish infected w/ the larva
3. Parasite actively penetrates the principal host: ex) On contact w/ the human host, the larvae penetrate the skin (usu. the bottom of the foot) and are carried through the blood vessels → heart → lungs → pulmonary alveoli → bronchial tree → pharynx → small intestine, where they reside and mature into adults
4. Parasite is maternally transmitted: ex) Toxoplasma gondii. Infected pregnant women can pass the infection to their unborn babies. Fetal damage is often more severe the earlier in pregnancy the transmission occurs
5. Vector-borne transmission: Usu. an arthropod or insect, via a bite while the arthropod is obtaining a blood meal. ex) Malaria (mosquito), Lyme disease (tick), and Rocky Mountain Spotted Fever (tick)

Question 6

What is the pathogenesis of Toxoplasma gondii?

Answer 6

Cats become infected by eating infected rodents, birds, or other small animals. The parasite is then passed in the cat’s feces as oocysts. Pregnant women who ingest (inhale) the oocyst can pass the infection to their unborn babies. Fetal damage is often more severe the earlier in pregnancy the transmission occurs

Question 7

What are some considerations for the human immune response to parasitic infections?

Answer 7

• Heterogeneity w/ respect to lifecycles and antigenic expression is a key feature of parasitic agents
• Many parasitic infxns are chronic in nature
• Mechanisms of evasion differ substantially from bacterial infxns
• Many parasites undergo significant genetic and antigenic variation w/in a relatively short time
• Innate immunity in natural hosts may be genetically determined
• Humans, as well as animals, differ widely in their ability to handle complex antigens, such as those found in parasites

Question 8

What immune response characterizes parasitic infections?

Answer 8

Eosinophilia and high levels of IgE.

• IgE binds to mast cells and basophils
• When specific antigen-Ab combinations occur, mast cells degranulate and release chemotactic factors, histamine, prostaglandins, and other mediators
• One such mediator is eosinophil chemo. factor, which attracts eosinophils to the infected area to destroy parasites via degranulation or Ab-dependent cellular cytotoxicity
• It is believed that the ability to produce IgE evolved mainly for the purpose of dealing with parasitic infections

Question 9

Which parasites are capable of manipulating the immune system?

Answer 9

Helminth parasites, which often induce a long-lasting, asymptomatic form of infection associated with downmodulated immune responsiveness.

Epidemiologic studies suggest that people who carry helminths have less immune-mediated disease.

Question 10

What are some parasitic strategies for survival in the host?

Answer 10

The most important survival strategy that parasites have is their complicated lifecycle. A host may be harboring several different stages in the lifecycle, thus making an effective immune response difficult.

Antigenic mimicry: Parasite expresses epitopes similar, if not identical, to host molecules, which may suppress the immune system.

Antigenic shedding: Similar to behavior of bacteria that shed capsular material into the host environment to evade the immune system.

Antigenic variation: Parasites generate novel antigens by random mutation, potentially w/ a freq. sufficient to evade the immune system on an ongoing basis. ex) Malaria parasite, via single nucleotide point replacement

Antigenic concealment: Parasite conceals its antigens from the host by sequestering itself inside of the host’s cells w/out displaying its antigens. Can be an intracellular phase in lifecycle. Ex) Plasmodium species, Trypanosoma cruzi, Leishmania, and Cryptosporidium parvum

Question 11

What are the two major classes of parasites?

Answer 11

Parasitic protozoa (unicellular): amoeba, flagellates (Trichomonas, Giardia, Trypanosoma), ciliates, sporozoa (Plasmodium, Cryptosporidium)

Parasitic protozoa all inhabit the GI tract, except Trichomonas vaginalis.

Parasitic helminths (multicellular): nematodes (roundworms), cestodes (flatworms/tapeworms), trematodes (flukes)

Question 12

How are parasitic infections diagnosed?

Answer 12

Microscopy:

• O&P, blood films, arthropod ID, fluids/tissues, cultures
• Requires visual review, morphological assessment
• Organism vs. artifact, size, external/internal morphology
• Possibility of infection (geographic area, collection/processing options)

Other:

• Immunoassays, serologies (expertise, reagents, interpretation)
• DFA, ELISA, cartridge: setups easy, but reading more difficult
• Most diagnostic methods categorized as high complexity (training, judgment, interpretation)