PARA Lecture 16 10/28/2026

Example of a parasite that causes tissue damage

Entamoeba histolytica

What percent of Entamoeba histolytica infected individuals develop amoebic dysentery

10%

How does amoebic dysentery work

entamoeba histolytica penetrates and feeds on the intestinal epithelium leading to frequent, bloody bowel movements

What can happen as Entamoeba histolytica feed on intestinal epithelium

they may penetrate the serous membrane and enter muscle, leading to a perforated bowel and secondary bacterial infection of the peritoneal cavity

Virulence factors

molecules produced by parasites (and other microorganisms), such as toxins and enzymes, that allow them to colonize a host, evade immune defenses, and cause disease

What are the virulence factors of E. histolytica

• overexpression of various digestive enzymes (cysteine proteases)

• overabundance of surface proteins (lectins) for binding host cells

• caspase 3

• phagocytosis and trogocytosis of host cells

• secretion of immunomodulatory proteins

What can parasitic infections do to host cells

alter host-cell growth patterns

Hypertrophy

an increase in the size of an organ or tissue resulting from an enlargement of its component cells

Hyperplasia

an increase in cell number

Metaplasia

cells of one type are replaced by cells of another type (usually reversible)

What do leishmania donovani do to monocytes

Monocytes infected with the parasites become hypertrophic to accommodate their replication

What do trypanosoma cruzi do to chagas patients

cardiac muscle cells undergo hypertrophy due to increased expression of contractile proteins

What does merocytsis kathae do in scallops

merogony progresses in adductor muscle cells- development leads to hypertrophy, cell rupture and eventual death

Example of hyperplasic response beneficial to parasite

Fasciola hepatica-

adults mature in bile ducts leading to proliferation of duct epithelial cells → flukes feed on these newly produced cells

Example of hyperplasic response in water

Glochida, larval stages of certain freshwater bivalves, which attach to gill filaments of fish leading to hyperplasia of gill filaments reducing oxygen intake

How can Glochida induced hyperplasia benefit the host

In coho salmon, hyperplasia of gill filaments is advantageous as they slough off and bivalve larvae are shed with them

What is another example of parasite-induced hyperplasia benefiting the host

weep and sweep response to helminths

Parasite-induced metaplasia example

Trichinella spiralis- muscle fiber cells become nurse cells

How does trichinella spiralis make muscle cells into nurse cells

• Adults inhabit cytoplasm of several adjacent mucosal epithelial cells in intestine

• Larve travel thru circulatory system until they reach skeletal muscle

• Penetration of muscle fiber cell leads to alteration of gene expression

• Larvae encysted inside nurse cell within skeletal muscle

• Stimulate formation of new blood vessels around nurse cells to provide themselves nourishment

What alterations in gene expression happen when Trichinella spiralis turns muscle cells into nurse cells

• Expression of contractile proteins drops

• mitochondria degenerate

• smooth ER rises

• secretion of collagen that forms protective layer around nematode

What usually happens to a cell if the offending stimulus causing hyperplasia or metaplasia is terminated

Cell growth patterns usually return to normal

What happens if cell growth patterns do not return to normal after the offending stimulus is terminated

Neoplasia

Neoplasia

abnormal proliferation of cells in response to an external stimulus that continues even after cessation of the stimulus

What are neoplasms that do not invade adjacent tissues considered

benign

What are neoplasms that do invade adjacent tissues or spread to other parts of the body considered

malignant cancers

Example of parasite-induced neoplasia in people

Opisthorichis viverrini-

causes cancer of bile duct after coinfection with Helicobacter pylori

Additional damage from release of bacterial toxins triggers initiation of cholangiocarcinoma

Parasite-induced neoplasia example in dogs

Spriocerca lupi (nematode, esophegeal worm) of dogs,

• Larvae penetrate dog’s stomach

• Enter circulatory system

• Penetrate aorta

• Migrate to esophagus

• Esophageal cysts containing adult worms can interfere with swallowing and cause emaciation

• Cyst become neoplasms leading to esophageal sarcomas

How can parasites affect host nutrition

• divert nutrients to themselves

• intestinal helminths can lead to nutrient depletion and/or intestinal bleeding (most issues in children or immunocompromised individuals)

Example of parasite affecting host nutrition

Diphyllobothrium latum- broad fish tapeworm where most humans remain asymptomatic or have mild symptoms

Some have severe vitamin b12 deficiency

What can vitamin B12 deficiency from Diphyllobothrium latum cause

• Decreases RBC synthesis leading to megaloblastic anemia

• Impacts lipid synthesis leading to chronic infections that can lead to demyelination of neuron, causing neurological symptoms

Two ways Giardia Lamblia infection can lead to host nutrient deprivation****

• Appetite loss due to GI discomfort leading to further nutrient depletion

• Induces intestinal cells to secrete chloride ions into intestinal lumen leading to further loss of water and electrolytes

What deficienty can Plasmodium infections result in

Iron deficiency

How can Plasmodium infections result in host iron deficiency (RBCs)

Red blood cell lysis

How can Plasmodium infections result in host iron deficiency (competition)

Can result from competition between strains-

• Once certain parasitemia hit, replicating merozoites release iron regulatory hormone hepcidin to prevent newly inoculated sporozoites from replicating in liver

• Increased hepcidin = decreased iron concentration in hepatocytes