Protozoan Cell Biology and Disease

What causes visceral parasitic infections?

amoebae, ciliates and coccidia

Give examples of flagellate coccidia

Giardia and Trichomonas

Give examples of apicomplexa coccidia

cryptosporidium and toxoplasma

What is Entamoeba histolytica?

anaerobic protist which causes amoebiasis

How is E. histolytica transmitted to humans?

through contaminated water and food

cysts produced by infective form of E. histolytica

• resistant to gastric acids

• excystation produces 8 trophozoites per cyst in small intestine

amoebiasis symptoms

asymptomatic or diarrhoea and/or dysentery

What happens if amoebiasis is left untreated?

E. histolytica may invade liver and in 10% of cases, the lungs and brain

how is amoebiasis diagnosed?

cysts in stool observed by microscopy

amoebiasis treatment

Azoles, particularly metronidazole. Paromomycin or iodoquinol used for luminal amoebiasis

How does metronidazole work?

Pyruvate:ferredoxin oxidoreductase (POR) reduces metronidazole to its active form, producing covalent protein and DNA adducts → parasite death

reduction of metronidazole nitro groups

produces nitroso- intermediates which covalently bind DNA, disrupting helical structure, inducing strand breaks and inhibiting nucleic acid synthesis → cell death

iodoquinol

iron chelator not taken up by intestinal cells

What is Balantidium coli?

Ciliated intestinal parasite of humans and swine

How is B. coli transmitted to humans?

via faecally contaminated water. Infection caused by cysts

diagnosis of B. coli

occurs from microscopy of stools and colon tissue

B. coli treatment

tetracycline, metronidazole and iodoquinol

Why is tetracycline used to treat B. coli?

it may inhibit protein synthesis by binding mitochondrial ribosomes

What happens when B. coli cysts are ingested?

The cysts form trophozoites which multiply. Some invade the colon wall. Cysts and trophozoites are then excreted

Giardia intestinalis

Flagellated anaerobic parasite which produces highly resistant cysts and causes giardiasis

Giardia intestinalis contains mitosomes. What are mitosomes?

mitochondrial remnant organelles not reliant on the electron transport chain.

giardiasis symptoms

explosive foul-smelling diarrhoea, intestinal cramps, nausea, weight loss and malaise. Some people are asymptomatic carriers

How is giardiasis diagnosed?

via cysts and trophozoites in faeces

How is giardiasis treated?

metronidazole and tinidazole

What happens after infective G. intestinalis cysts are ingested?

They remain in the intestine and form trophozoites. Cysts and trophozoites are excreted but trophozoites don’t survive the environment

What is Trichomonas vaginalis?

Sexually transmitted, flagellated anaerobic parasite. Survives on moist surfaces so can be transmitted by toilet seats, sauna benches and towels

T. vaginalis symptoms

asymptomatic in males. Causes purulent vaginal discharge, itching and burning in women

T. vaginalis diagnosis

microscopy and cell culture from patient secretions

T. vaginalis treatment

metronidazole

T. vaginalis trophozoites

in vaginal and prostatic secretions as well as urine. Multiply by binary fission and colonise vagina/urethra orifice

Cryptosporidium parvum

Apicomplexan parasite of warm-blooded animals which produces thick-walled cells known as oocysts

How is C. parvum transmitted?

oocysts are shed in infected faeces and transmitted in faecally contaminated water

How are C. parvum oocysts removed from infected water?

Oocysts resistant to chlorine and filtration so sedimentation and filtration methods most effective

Where does the C. parvum parasite develop?

just under epithelial cell membrane in an intracellular, extracytoplasmic position

C. parvum diagnosis

direct fluorescence antibody assay of a stool sample

C. parvum treatment

infection self limited but can be treated with nitazoxanide (interferes with anaerobic energy metabolism)

C. parvum sporozoites

sporozoites develop into trophozoites inside oocysyts

asexual C. parvum trophozoite cycle

trophozoite → type I meront → merozoite → trophozoite

What happens when C. parvum merozoites leave the asexual cycle?

develop into type II meronts which then form merozoites in a sexual cycle

undifferentiated gamonts in the C. parvum cycle

form micro and macrogamonts, allowing zygotes to form which produce thin and thick walled oocysts

thick walled C. parvum oocysts

exit by excretion to infect someone else

thin walled C. parvum oocysts

remain in the intestine

Toxoplasma gondii

Apicomplexan protist parasite of warm blooded animals with similar life cycle to C. parvum

transmission of T. gondii

cats and undercooked meat

toxoplasma symptoms

asymptomatic but causes eye, brain and organ damage in immunocompromised individuals

What happens if a pregnant mother is infected with Toxoplasma?

may cause birth defects eg hydrocephalus

Toxoplasma treatment

Sulphadiazine and Pyrimethamine

Naegleria fowleri

Free-living amoeba found in soil and water

N. fowleri transmission

swimming in warm, soil-contaminated water eg hot springs or lakes. Enters via nose and burrows directly into the brain

N. fowleri symptoms

extensive brain haemorrhage and damage (meningoencephalitis). Usually fatal

N. fowleri diagnosis

from cerebrospinal fluid

N. fowleri treatment (only effective if diagnosed early)

Azoles, amphotericin B, rifampicin and miltefosine

Acanthamoeba

eg castellani, polyphage and culburtsoni. Found in fresh and salt water

How do Acanthamoeba cause infection?

via contaminated contact lenses or contact lens cases. Can also infect broken skin

Acanthamoeba keratitis (AK)

eye disease caused by Acanthamoeba

cutaneous Amoebiasis

skin disease caused by Acanthamoeba

Acanthamoeba diagnosis

microscopic examination of tissue samples

two forms of Acanthamoeba

• favourable conditions = active trophozoite

• non-favourable conditions = non-dividing, resistant cysts

Acanthamoeba keratitis treatment

Not always successful, usually needs corneal transplant although reactivation can happen

Plasmodium life cycle in mosquitoes

sporogenic

Plasmodium life cycle in humans

• liver = exo-erythrocytic

• blood = erythrocytic

How is malaria diagnosed?

microscope identification of Plasmodium-infected erythrocytes in blood smears. May appear as trophozoite rings, gametocytes or schizonts

prevention and treatment of malaria using drugs

chloroquine and mefloquine (Lariam), pyrimethamine, doxycycline and clindamycin

Plasmodium

Apicomplexan parasite causing malaria. Uses Anopholes mosquitoes as vectors

What are the 5 species of Plasmodium able to infect humans?

P. malariae, P. vivax, P. ovale, P. falciparum and P. knowlesi

Trypanosomatids

Cause blood and tissue parasitic infections

What is leishmania?

flagellated protozoan

How is Leishmania transmitted and what cells does it infect?

transmitted by sandfly bites. Infects and grows in macrophages

How is leishmaniasis diagnosed?

tissue microscopy, PCR and serology

Leishmania life cycle in sandflies

• sandfly ingests amastigote-infected macrophages

• amastigote → promastigote in midgut

• promastigotes divide and migrate to proboscis

• sandfly bites human

Leishmania life cycle in humans

• promastigotes phagocytosed

• pro → amastigotes in macrophages

• amastigotes multiply in various tissues inc macrophages

Leishmania mexicana

causes cutaneous leishmaniasis → skin nodules and ulcers

leishmaniasis treatment

pentavalent antimonial compounds (toxic), amphotericin B (expensive) and miltefosine

Leishmania braziliensis

mucocutaneous leishmaniasis → destruction of mouth, nose and throat mucosa and cartilage

Leishmania donovani

visceral leishmaniasis → liver, spleen and bone marrow damage

post-kala-azar dermal leishmanoid (PKDL)

VL relapse due to incomplete destruction of L. donovani

subspecies of Trypanosoma brucei

T. brucei gambiense and T. brucei rhodesiense - latter more severe

Trypanosoma brucei

causes African sleeping sickness

how is African sleeping sickness transmitted?

tsetse fly bite

stage I African sleeping sickness

intermittent fever

Stage II sleeping sickness

parasites cross blood-brain barrier → brain tissue inflammation and necrosis

stage II sleeping sickness diagnosis

spinal puncture and microscopy of cerebrospinal fluid

The meningoencephalitic stage of sleeping sickness is where the CNS is invaded. What are the symptoms?

headaches, somnolence, abnormal behaviour, lethargy, loss of consciousness and coma

African sleeping sickness treatment

suramin, melarsoprol, pentamidine and eflornithine

T. brucei life cycle in the Tsetse fly

• fly ingests trypomastigotes

• trypomastigotes → procyclic in midgut, divide by binary fission

• procylic trypomastigotes → epimastigotes

• epi multiply in salivary gland and become metacyclic

• fly bites human

T. brucei life cycle in humans

• trypomastigotes transformed and carried to other sites via blood

• multiply by binary fission in blood, lymph and spinal fluid

Trypanosoma cruzi

causes Chagas’ disease aka American trypanosomiasis

transmission of T. cruzi

bite and defecation of the Triatomine bug

Chagas diagnosis

microscopy/serology

Chagas treatment

nifurtimox (Lampit) and benznidazole (Radanil)

What organs does T. cruzi affect?

heart, GI tract and CNS

T. cruzi life cycle in triatomine bug

• bug ingests trypomastigotes

• trypomastigotes → epimastigotes in midgut

• transform to metacyclic trypomastigotes in hindgut

• bites human and passes metacyclic trypomastigotes in faeces which enter the wound

T. cruzi life cycle in humans

• trypomastigotes penetrate cells at bite wound

• transform → amastigotes inside cells

• amastigotes multiply by binary fission

• transform → trypomastigotes, burst from cell and enter bloodstream

• infect other cells and cycle continues