Classification of Parasites Flashcards

What is a Parasite?

• Organism that lives in or on another organism, deriving nutrients at the host's expense.

Overview of Parasites

• Approximately 65,000 species of parasites exist.
• About 10,000 species affect plants and animals.
• 2,000 to 3,000 species are parasitic to humans and animals.
• 300 species are deemed medically important.
• Three out of seven diseases are considered eradicable by the CDC.

Host Types

• Definitive or final host: Harbors the adult or sexual stage of the parasite.
• Intermediate host: Host where larval or immature stages of helminths develop before infecting the definitive host.
• Paratenic or transport host: Host that does not allow further development of larval stages but may transmit them if ingested by another host.
• Aberrant host: Host in which the parasite fails to complete its development, often leading to mortality.

Life Cycles of Parasites

• Direct Life Cycle: Life cycle that does not require an intermediate host.
• Indirect Life Cycle: Life cycle that requires an intermediate host.

Major Groups of Parasites

• Protozoa: Single-cell eukaryotes, primitive life forms that often invade, multiply, and cause disease.
• Helminths: Include classes such as Platyhelminthes (flatworms) and Nemathelminthes (roundworms).
• Arthropods: Include subclasses of insects and arachnids.

Helminths

• Platyhelminthes (flatworms): Expand into Trematodes (flukes) and Cestodes (tapeworms).
  • Trematodes (e.g., Digenea) have various morphological features, largely hermaphroditic, and require one or more intermediate hosts for reproduction.
  • Cestodes are characterized by a scolex (head), neck, proglottids (body segments), and no gut.

Protozoan Characteristics

• Most protozoans reproduce asexually via binary fission.
• Apicomplexia: Involves asexual phase (schizogony) and sexual phase (gametogony).
• Infective stage for many protozoa is the sporozoite.

Helminth Life Cycle Generalities

• Typically complex, often needing intermediate hosts.
• Disease impact can be mild in definitive hosts but debilitating in intermediate or accidental hosts.

Trematode Characteristics

• Bilaterally symmetrical, dorsoventrally flattened, equipped with a modified skin (tegument).
• Possess a simplistic gut and respiratory system employing diffusion through the tegument.
• Reproductive strategies are primarily hermaphroditic.

Cestode Orders

• Cyclophyllidea: Requires only one intermediate host.
• Pseudophyllidea: Requires two intermediate hosts, the first generally being a crustacean.

Cestode Features

• Characterized by being dorsoventrally flattened, segmented, and hermaphroditic.
• Adults release proglottids into the environment, aiding in reproduction.

Important Cestode Anatomy

• The adult tapeworm includes the scolex at its head, a neck, and proglottids that store eggs, often excreted in the host's feces.