Flagellated Protozoa: African Trypanosomes (4-6)

Question 1

What are protozoa?

Answer 1

Single-celled eukaryotes
→ micro-parasites

Question 2

What are kinetoplastids?

Answer 2

A group of flagellated protozoa
→ characterised by kinetoplast - a large DNA-containing structure (high conc DNA)

Question 3

What are the morphological forms of hemoflagellates?

Answer 3

Amastigotes → sequesters in cells
Promastigotes (1 flagella), Epimastigote (2 flagellum) → highly motile pulled by flagellum
Trypomastigote → infective form - metacyclic

Question 4

What disease does Trypanosoma brucei gambiense cause?

Answer 4

Sleeping sickness (chronic) in humans
→ Central and West Africa
→ Anthroponosis transmission (human to human via tsetse fly)
→ Vector Glossina palpalis

Question 5

What disease does Trypanosoma brucei rhodesiense cause?

Answer 5

Sleeping sickness (acute) in humans
→ East and South Africa
→ Zoonotic transmission
→ Vector Glossina morsitans

Question 6

What 3 animal trypanosomes cause Nagana in cattle?

Answer 6

Trypanosoma brucei brucei → chronic nagana
Trypanosoma congolense → chronic nagana
Trypanosoma vivax → acute nagana

clinical outcomes = anaemia, intermittent fever, diarrhoea, rapid loss of condition, severe emaciation and often death

Question 7

Why is cattle nagana significant?

Answer 7

Can’t raise cattle in tsetse-infected areas
→ 1.6million DALYs (disability adjusted life years)
→ agricultural production losses worth US4.75 billion/year

Question 8

Where is Trypanosoma congolese distributed?

Answer 8

Causes bovine trypanosomiasis (Nagana - chronic)

Central and South Africa

Question 9

Where is Trypanosoma vivax distributed?

Answer 9

Central and South Africa

South America

Question 10

Where is Trypanosoma evansi distributed?

Answer 10

Causes cattle Sura

North Africa
South America
Asia
Russia

Question 11

Who does Trypanosoma brucei spp. affect?

Answer 11

Causes HAT - Human African trypanosomiasis
→ 60 million at risk
→ endemic in 36 countries
→ 500,000 cases p.a.
→ deaths: 50,000 p.a.

Question 12

What are the two parasite species causing HAT?

Answer 12

HAT (Human African trypanosomiasis)

Trypanosoma brucei gambiense → chronic disease, lasts for several years

Trypanosoma brucei rhodesiense → acute disease, lasts for months

Question 13

How does Trypanosoma brucei gambiense cause disease?

Answer 13

Causes HAT - >90% cases of sleeping sickness worldwide
→ chronic disease, lasts for several years
→ asymptomatic for a long time

Anthroponotic transmission
→ human reservoir, vector-borne
→ “Riverine” tsetse vector
→ fatal if untreated

Question 14

How does Trypanosoma brucei rhodesiense cause disease?

Answer 14

Causes HAT - <10% cases of sleeping sickness worldwide
→ acute disease, lasts for months

Zoonosis
→ domestic and wildlife reservoir (can reinfect humans), vector-borne
→ “Savannah” tsetse vector
→ fatal if untreated

Question 15

What parasite species causes acute HAT?

Answer 15

Trypanosoma brucei rhodesiense
→ acute virulent infection, incubation 1-4 weeks, quickly detectable

Question 16

What is the vector for African Trypanosomes?

Answer 16

Tsetse flies (Glossina spp)
→ life cycle unique as they get pregnant and give birth to larvae

Question 17

How are African Trypanosomes transmitted?

Answer 17

Infected tsetse fly takes blood meal (injects metacyclic trypmastigote)
→ transform into bloodstream trypomastigote
→ multiply by binary fission

Tsetse fly takes blood meal (bloodstream typomastigotes are ingested)
→ transform into pro cyclic trypomastigote in fly mid gut
→ multiply by binary fission
→ transform into epimastigotes
→ in salivary gland: multiply and transform into metacylic trypomastigote

Question 18

What are the two phases of HAT?

Answer 18

Early stage → haemolymphatic (stage 1)

Late stage → encephalitic (stage 2) ‘sleeping sickness’

Question 19

What happens in the early stage of HAT?

Answer 19

Chancre arises at the site of bite
Parasite spreads through lymphatic system and invades the bloodstream

Symptoms → enlarged lymph glands/spleen, local oedema (swelling), cardiac abnormalities, headaches
(non-specific)

Question 20

What happens in the late stage of HAT?

Answer 20

Parasite invades the internal organs including the CNS

Symptoms → severe headaches, personality changes, sleeping pattern affected, weight loss, coma and death

Neuropathy: Acute haemorrhagic leucoencephalopathy (AHL)
→ widespread fibrinoid necrosis in the walls of small blood vessels in affected regions of the brain

Question 21

How does the pathology compare between T. b. gambiense vs T. b. rhodesiense?

Answer 21

T. b. gambiense (C & WA) → long asymptomatic period (months to years), emergent disease advanced, necessitates early diagnosis
T. b. rhodesiense ( E & AS) → acute virulent infection, incubation 1-4 weeks, quickly detectable

both → no quire immunity, no vaccine, fatal without treatment

Question 22

How does HAT use its variant surface glycoprotein (VSG) as a decoy antigen?

Answer 22

VSG covers the entire parasite surface including the flagellum → 10^6 produced per cell
→ molecule is highly immunogenic - elicits string antibody response from infected host
Has&raquo_space;200 VSG genes under transcriptional control → is able to switch expression - constantly changing indignity, immune system can’t keep up
→ results in undulating fever which can last for months
→ cyclical pattern of fevers (sign of trypanosome infection) - high no. of parasites = fever, each peak in parasites are antigenically distinct

Question 23

How is HAT diagnosed?

Answer 23

Symptomatic, systemic and passive screening:
→ serological test: card agglutination test (CATT) - for T. b. gambiense only, has low parasitaemia - harder to see on microscope, detects antibodies in patients blood serum
→ rapid serological test
→ microscopy
Clinical or serological positive evidence:
→ diagnosis of stage of disease is a necessary step to identify the appropriate treatment
→ inspection of CSF obtained by lumber puncture to define disease stage
→ trypanosomes (or high WBC count) demonstrates in CSF indicates 2nd stage chronic disease

Question 24

How is HAT treated?

Answer 24

Chemotherapy is the primary treatment for HAT - type depends on stage of the disease

Suramin (1920) → early stage T. b. rhodesiense, $10
Pentamidine (1940) → early stage T. b. gambiense, $20
Melarsoprol (1949) → late stage both, $50
→ only drug effective against both late stage HATs, toxic (arsenic based - kills 5-10% people treated), resistance observed
Eflornithine (1981) → late stage T. b. gambiense, $250
Nifurtimox (2009) → late stage T. b. gambiense - combination drug
Fexinidazole (2019) → both stage T. b. gambiense
→ new oral therapy, rapid approval given in DRC where 85% of HAT is found

Question 25

What control measures can be used to reduce transmission of HAT?

Answer 25

Parasites → resistant breeds (cattle), chemotherapy
Tsetse flies (vector) → traps, insecticides, sterile insect technique (e.g. Zanzibar)

Question 26

How does the approach to control HAT differ between T. b. gambiense and T. b. rhodesiense?

Answer 26

T. b. gambiense → to reduce person to person transmission
→ active or passive case detection and treatment (major component of control)
→ vector plays little to no part of control (low cost-effectiveness)
→ large scale epidemics in 20th century controlled by 1960s, by active case detection and treatment programmes

T. b. rhodesiense → to reduce transmission from zoonotic reservoirs
→ vector control is central - treating humans doesn’t interrupt transmission cycle
→ cattle treatment becoming more common (e.g. Uganda)
→ case screening conducted for humanitarian reasons

(vector control prevents initial infection, case treatment reduces circulation of parasites - both should reduce disease)

Question 27

What is the rationale behind vector control?

Answer 27

→ low density (1 per 1.4m bush)
→ low reproductive potential (~8 offspring x 10d cycle)
→ 4% sustained mortality of females per day should cause extinction

Question 28

What are some insecticide application used for vector control?

Answer 28

traps/targets e.g. tiny target traps
Drips or ‘pour-on’ topical treatment e.g. RAP
Ultra-low volume (ULV) spraying
Sterile insect technique

Question 29

What are some standard tsetse traps?

Answer 29

1. Electrified black target with flanking net
2. Biconical trap with flanking electrified grid → blue attracts

Question 30

What is the sterile insect technique?

Answer 30

Used to manage insect populations
→ involves mass rearing target, radiation to cause sterilisation, released to wild, compete with wild insect, no viable offspring produced, population decline, continued release

e.g. in Zanzibar $ 8 million
1988 → tsetse population suppressed using insecticide-treated cattle and traps
1994-1997 → >8.5 million sterilised males released by aircraft, serial release of 60,000 per week by aircraft in 1-2km flight lines
1995 → sterile:indigenous male ratio 100:1, 70% barren females
1996-7 → tsetse population crash, Glossina austeni eradicated

Question 31

Could sterile insect release work everywhere?

Answer 31

Reviewed by Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC)
→ 22 Glossina species over 8.5-9 million km, up to 4 different species coexist
→ cost of US$6mil per 1600km, total cost: US$67bil
→ requires 350,000 years to eradicate 22 species

Question 32

What progress has been made towards HAT elimination?

Answer 32

1999 - 2010 → new cases of T. b. gambiense fell 75% from 27,862 to 6,984, T. b. rhodesiense* fell 75% from 619 to 155
→ >40% of endemic countries report fewer than 20 cases per year

2010-2015 → reduced number of reported cases <3,000 in 2015, 10,000 in 2009
→ enhances support for surveillance now defined by accurate maps
→ lowest reported incidence for 75 years achieved by: active surveillance of defined high-risk foci, donation of pentamidine and nifurtimox, donation of eflonithine against chronic HAT

Question 33

What are the WHO elimination initiatives against HAT?

Answer 33

Current elimination targets (2012 London Declaration) → eliminate HAT by 2030, in selected countries of low incidence by 2020

→ scale up tiny insecticide impregnated targets - easy to distribute
→ new improved diagnostic tests
→ utilise new drug fexnidazole
→ chemotherapy/RAP for cattle against acute disease in Uganda

→ control seems to be working in many places, activate foci being generally well known and reducing in size
→ political instability in some regions, reduced monitoring = may hide scale of problem
→ most important areas id disease burden in DRC with 85% of transmission
→ outstanding questions: can we eliminate/eradicate? if so, at what cost to other vector-borne Diseases?

Model projections indicate elimination goal may not be met until later this century (2059-2092) under current intervention strategy

Question 34

What is the Stamp Out Sleeping Sickness (SOS) initiative?

Answer 34

Public-private partnership launched in Uganda 2006 → response to possible convergence of T.b.rhodesiense and T.b.gambiense in N Uganda
→ treatment where human infections are mixed is more difficult

Early stage disease 1st line drugs
→ suramin, pentaminidine,

Late stage disease 1st line drugs
→ melarsoprol
→ late stage treatment failures of T.b.gambiense increasing
→ an alternative is eflornithine

Question 35

What is restricted application protocol?

Answer 35

Cattle dunked in insecticide periodically
→ requires a lot of insecticide - costly
→ kills everything on the cow - need to maintain tick exposure while preventing tsetse bites

RAP is refined treatment that involves spraying at dip concentration only to the tsetse predilection feedings sites on cattle (legs and belly) rather than entire animal - benefits go beyond reducing incidence in livestock

Question 36

What is the prevalence of sleeping sickness in SE Uganda?

Answer 36

→ human sleeping sickness 0.6% (6/1000)
→ fly biting preference: human 9%, cattle 23%
→ cattle prevalence: T.b.brucei 45%, T.b.rhodesiense* 18%
→ cattle 234x more likely to be source of human T.b.rhodesiense than humans
→ predicated that 20% of cattle needed treatment to achieve R0<1 - stop spread

Question 37

How is cattle babesiosis spread?

Answer 37

Via tick vector - Boophilus spp
→ Babesia pass into the ovaries/eggs - vertical transmission
→ migrate to the salivary glands to reproduce larvae
→ larvae await on grass stalks to attach to passing host
→ Babesia from salivary glands infected into the mammal’s blood stream

Question 38

What is babesiosis in cattle?

Answer 38

Cattle disease spread via tick vector
→ incubation 3-21 days
→ symptoms: high fever, anorexia, seek shake, weight loss, abortion, poor milk production
→ extensive erythrocytic lysis: 75% erythrocytes destroyed in few days
→ most survive but mortality up to >50% known, slow recovery
→ carrier status (low parasitaemia) for years

about 300million cattle in tropical and subtropical regions are at risk of infection with Babesia bovis and Babesia bigemina

Question 39

What is tick-borne disease endemic/enzootic stability?

Answer 39

Endemic stability → a dynamic epidemiological state in which clinical disease is rare in spite of a high incidence of infection within a population

1. Disease is more likely or more severe in older than younger susceptible
2. Following infection, the probability that subsequent infection results in disease is reduced

Calf exposure to infected ticks is key to immunity → the age of first exposure to tick fever determines disease severity, severity increases with age
→ if exposed by 9 months, solid long-lasting immunityW

Question 40

What measures are used in restricted application protocol (RAP) to maintain endemic stability?

Answer 40

1. Tsetse more susceptible to parathyroid-treated cattle than ticks - increasing intervention interval lowers impact on ticks
2. Tsetse preferentially feed on different sites go body, particularly legs - avoid tick attachment sites

Question 41

What are the methods for sustainable control of trypanosomiasis?

Answer 41

Tsetse preferentially feed on older and larger herd members → leave young cattle untreated with insecticide to become exposed to ticks
Treating half the herd → only on the larger individuals, less effective but reduced amount of insecticide used
Selective treatment of hosts with restricted application protocol (RAP) → reduce insecticide costs by 90% compared to treating entire herd, reduce impact on non-target organisms

Question 42

What is the SOS initiative integrated control of T.b.rhodesiense source?

Answer 42

Aims → cleaning cattle through treatment, preventing tsetse re-infection by regular spraying RAP
Achievements → during 8 weeks emergency intervention vet students treated 250,000 cattle in 5 districts in N Uganda reducing prevalence of rhod in cattle by 70%
→ it works, cost-effective
→ payed by livestock owners ‘levers to purchase’ control is the visibly fewer flies and ticks - decentralised ‘bottom-up’ approach

Question 43

What are ‘tiny insecticide impregnated targets’ used against Gambian HAT transmission?

Answer 43

Densities of 10cattle/km2 necessary for cattle to be important part of tsetse blood source → precludes the use of RAP in may Gambian HAT foci
→ traditional biconical traps is expensive US$56/km2 per year
Thus this was developed

Pthalogen blue polyester cloth (25x25cm) attached to fine (150 denier) block polyethylene mosquito netting (25x25cm) impregnated with deltamethrin (300mg/m2) → deployed with the bottom edge ~10cm above soil surface to catch the flies

Question 44

What is the rationale behind using ‘tiny insecticide impregnated targets’ for Gambian HAT?

Answer 44

→ current low levels of HAT is the opportunity to push for its elimination
→ case detection and treatment doesn’t prevent infection - and coverage is only 75%
→ low sensitivity of diagnostic tests
→ residual levels of infections remain in communities to sustain transmission
→ animal reservoirs may play larger role than previously thought
→ traditional vector-control seen as to expensive in resource-poor settings
→ availability of these is cost-effective
→ Riverine tsetse concentrate near lakes or rivers

Question 45

How could tiny traps affect HAT transmission?

Answer 45

Deployment of tiny traps could significantly recuse the tsetse population to cease HAT transmission
→ combined with case detection and treatment more modest reductions in tsetse population could hold the population below the transmission threshold
→ elimination of tsetse is not required to halt HAT transmission: once T.b. gambiense ceases to circulate in a foci then tsetse flies numbers aren’t an issue
→ model is sensitive to alterations in operational procedures - waning operational sustainability
→ cost-effective: reduces costs of vector control by >80% to US$85/km2 from 556 using traditional biconical traps, supports, teams and transport

Question 46

What progress has been made towards elimination of T.b. gambiense in Uganda?

Answer 46

On verge of elimination → only 4 cases in 2016 and 2017 each
→ in jeopardy from conflict in South Sudan
→ effort in surveillance, treatment and transmission reduction must be maintained