Lecture 16: Parasitic Pathogens - Malaria

General features of protozoa

Unicellular eukaryotic organisms (single cells)

Combine all vital functions in one cell: metabolism, uptake/excretion, reproduction, motility, stimulation

Short generation times & high rates of reproduction within host → overwhelms host in disease

Infections are short & long term; tends to induce immunity in hosts that survive initial onslaught

Free-living & parasitic

Protozoa specifics

Taxonomy difficult due to lack of genome sequences

Extracellular & intracellular parasites & stages

Adapted through evolution to conditions in host ie. low O2 → anaerobic; often specialised to point of obligate parasitism

2 stages of multiplication in many species
Sexual in mature forms & asexual in larval stages

Important in human diseases esp. developing countries

No vaccine - controlled by drugs & blocking transmission

Major classes of medically important protozoa

1 Kinetoplastids - African & American trypanosomes

2 Anaerobic protozoa - Giardia spp.

3 Apicomplexan protozoa - Plasmodium spp.

Kinetoplastid diseases in humans

Chagas disease - South American trypanosomiasis

Sleeping sickness - African trypanosomiasis

Leishmaniasis

Kinetoplast

Dense kDNA containing granule found within cell’s single mitochondrion

Found near basal body located at the base of the flagellum

Previously believed to be associated w/ cell movement

Distinct region of the mitochondria

Kinetoplastid structure

Kinetoplast within mitochondria

Nucleus w/ nucleolus

Flagellum

African trypanosomiasis (African sleeping sickness)

Patchy distribution in equatorial Africa

60 million people in 36 nations are at risk of infection

Incidences increased from 1960s to end of 1990s (300-500k cases in 1998)
Less than 10,000 in 2009, 3796 in 2014, 2804 in 2015

T. brucei rhodesiense

Causes East African sleeping sickness

Zimbabwe, Malawi, Uganda

T. brucei brucei

Infects animals & livestock

T. brucei gambiense

Causes West & Central African sleeping sickness

Trypanosome transmission

Mostly transmitted to humans via tsetse fly bite (insect vector)

Mother to child - trypanosome crosses placenta & infects the foetus

Mechanical transmission via other bloodsucking insects is possible

Accidental infections in labs via contaminated needles

Transmission via sexual contact has been documented

Trypanosome stages in humans

All stages are extracellular, trypanosome transmitted via bite

Metacyclic trypomastigotes are injected into skin tissue, enter lymphatic system, transform into bloodstream trypomastigotes, pass into bloodstream & spread in host body fluids

Trypomastigotes replicate via binary fission in body fluids ie. blood, lymph, spinal fluid

Trypanosome stages in tsetse fly

Tsetse fly ingests trypomastigotes as part of blood meal

Transforms into procyclic trypomastigotes in midgut & multiplies via binary fission

Procyclic trypomastigotes leave midgut & transform into epimastigotes

Epimastigotes multiply in salivary gland & transform into metacyclic trypomastigotes which are injected during feeding

T. brucei rhodesiense disease & symptoms

T. b. rhodesiense = rapidly progressing disease, death within 1 year

Much higher parasite count

Asymptomatic carriers are rare

Neurological manifestations within weeks after infection

Antelope → tsetse fly → human

T. brucei gambiense disease & symptoms

T. b. gambiense = slow progressing disease, death within 2-3 years

98% of all reported cases

Low parasite counts in blood

Disease progression characterised by invasion of lymphatics in T. b. gambiense infection

Asymptomatic carriers are common

Human → tsetse fly → human

Neurological symptoms apparent 6-12 months after infection

African trypanosomiasis (sleeping sickness) disease progression

Blood → lymphatics → CNS → death

Trypanosomal chancre = local inflammation nodule during 1-2 week asymptomatic period

Blood
Acute blood stage infection = irregular episodes of fever & headache

Lymphatics
Lymphadenopathy (enlarged lymph nodes), weight loss, weakness, rash, itching & intermittent febrile attacks (fever, headache)

CNS
Invades CNS & causes nervous system impairment; apathy, confusion, motor changes, extreme fatigue during day & extreme agitation at night

If untreated CNS stage progresses to convulsions or coma → death

African trypanosomiasis diagnosis

Parasite detection in blood via microscopy & Giemsa stained blood smears

Mini-anion exchange centrifugation technique (mAECT): blood cells are more negatively charged than trypanosomes & are retained on column

Trypanosomes are more likely to be detected during symptomatic periods

Treatment differs depending on CNS involvement - tests for parasites or elevated white blood cells in cerebral spinal fluid

African trypanosomiasis treatment

Pentamidine = early stage T. b. gambiense

Suramin = early stage T. b. gambiense & rhodesiense

Melarsoprol = late stage T. b. gambiense & rhodesiense (CNS involved)

Eflornithine = late stage T. b. gambiense involving CNS

Nifurtimox & Eflornithine = 1st line treatment for T. b. gambiense

No vaccines

American trypanosomiasis (chagas disease)

T. cruzi

Endemic to South & Central America - 5.7m infections in 2010

Triatomine bug (kissing/assassin bug) = insect vector

Intracellular stages

T. cruzi transmission

Mostly via contaminating triatomine bug bite wound w/ bug feces or mucosal membranes

Scratching by host increases likelihood of infection

Cimex lectularius (common bed bug) observed as insect vector

T. cruzi life cycle

In triatomine bug epimastigotes replicate in midgut & transform into metacyclic trypomastigotes in hindgut

Metacyclic trypomastigotes transmitted via bug feces into wound/membrane

Metacyclic trypomastigotes penetrate cells at bite wound & inside cells they transform into amastigotes
ie. liver, spleen, lymph nodes & CNS cells

Amastigotes replicate via binary fission inside cells of infected tissues - up to 500 per cell

Intracellular amastigotes transform into trypomastigotes & burst out of cell & enter bloodstream

Trypomastigotes can infect other cells & transform into amastigotes again

Trypomastigotes are then ingested by bloodsucking triatomine bug

American trypanosomiasis diagnosis/treatment

Diagnosis via blood screening for trypomastigotes

40% are asymptomatic - silent spreaders

Chronic chagas disease = 45% cardiomyopathy
Leads to sudden death due to accumulative damage to heart cells

80+% cure for acute diseases
Less effective in chronic

No vaccine available

Anaerobic protozoa

Adapted to low O2 in gut lumen

No mitochondria, cytochrome-mediated electron transport or oxidative phosphorylation

Energy = glucose fermentation in cytosol

Generally aerotolerant