Lecture 15. Flagellated Protozoa: American Trypanosomes 2

Question 1

What are not effective ways of controlling T. cruzi infections?

Answer 1

Vaccines - no vaccine exists
Chemotherapy cannot be relied upon to prevent spread of infection: no solution to chronic infections (asymptomatic carriers), not widely available and vector infestation leads to high levels of re-infection

Question 2

To control T. cruzi infections, what must be focussed on?

Answer 2

Prevention
Interrupting transmissions by controlling intra-domiciliary vectors in Latin America, screening bloodbanks for transfusions, screening organs for transplantation and screening for congenital infection

Question 3

What is indoor residual spraying (IRS)?

Answer 3

The spraying of houses with insecticide at a specific rate (delivery dose has to be at correct level to allow pesticide to last long and function)

Question 4

Besides IRS, what other control options are there to prevent T. cruzi?

Answer 4

Insecticide impregnated nets (ITNs)
House improvement (Environmental management) - plastering of walls to seal gaps
Social interventions (education and community participation), ‘acquiring good practices’ and engagement with community leaders
Integrated Vector Control (IVC) and political commitment

Question 5

What was the Southern Cone Initiative?

Answer 5

Launched in 1991
Domestic Vector Elimination
Serological screening blood
Housing improvement

Question 6

What age groups were examined in the Southern Cone Initiative and why?

Answer 6

Children to young adults, easier to identify Chagas than in adults and more cost effective

Question 7

Which country was able to interrupt transmission of T. cruzi in 1997?

Answer 7

Uruguay

Question 8

What vector did the Central American Initiative and the Andean Initiative try to eliminate?

Answer 8

Eliminate R. prolixus (and reduce domestic T. dimidiata

Question 9

How many countries have eliminated T. cruzi transmission by R. prolixus?

Answer 9

6 of the Central American countries
Majority of districts in 2 of the Andean countries (Peru and Columbia)

Question 10

What area is a hotspot for T. cruzi infection?

Answer 10

Gran Chaco region (spans over Argentina, Bolivia and Paraguay)
Indigenous communities with high transmission
House infestation levels at 82-100%
T. infestans infection with T.cruzi, 50%

Question 11

What are the issues in the Bolivian Chaco region that allow T. cruzi infections?

Answer 11

House infestation Index (proportion of houses infested): 18%-73%
Triatomine Infection prevalence with T. cruzi: 29%-82%
House construction (wall crevices) main risk factor for persistent infestation
20-30% infection prevalence in children <18yrs
86-97% infection prevalence in adults >30yrs
45% T. infestans populations variably resistant to IRS chemicals

Question 12

In which group of people is the prevalence of T. cruzi higher in Bolivian Chaco when compared to less endemic states?

Answer 12

Pregnant women
Repeat vector infection exposure in Camiri (rural) sustains inflammatory responses at a higher chronic levels, increasing cardiac morbidity, but possibly enabling exposed women to control parasitemia even in the face of pregnancy-induced Th2 polarisation
Babies also infected by congenital transmission

Question 13

Why is the household infection index so important?

Answer 13

If household infestation index falls below a certain threshold, trigger vector control by IRS
IRS “attack phase” conducted typically until reduced to <5% of houses infested; Infestation indicated by Timed Manual Capture (TMC): 0.5 person hours search for vectors in household buildings

Question 14

What is successful vector control required to trigger?

Answer 14

Human treatment
Treatment of childhood infection recommended only if area is under effective vector control commonly defined as: House infestation Index: <3% houses in community infested, and absence of infestation in case households

Question 15

What are the problems with IRS in practice?

Answer 15

Only 10% of filter papers reached acceptable target delivery dose

Question 16

What is Alpha-cypermerthrin?

Answer 16

Target delivery dose 50mg/m² +/-20% tolerance (regional control program reccomedation); WHO reccomends 25-30mg/m² against for mosquitos

Question 17

When can IRS only work?

Answer 17

If IRS is sustained and there is supervised, community-based vector control along with housing improvement

Question 18

What are wild T. investans populations associated with?

Answer 18

Wild guinea pigs in a sylvatic cycle

Question 19

What does higher T. infestans genetic diversity in Bolivian Andes suugest?

Answer 19

Region of origin and geographic dispersal
Andean and non-Andean T. infestans populations are essentially different
Still limited gene flow between sylvatic and domestic populations

Question 20

What is xenodiagnosis?

Answer 20

To see if an animal becomes a vector after eating an infected host (very unreliable)

Question 21

What insecticides are used to target Triatomides?

Answer 21

DDT is not effective against Triatomids
Organophosphates (Dieldrin); Organochlorines (benzene hexachloride (BHC)); Carbamates ( propoxur)
Synthetic pyrethroids (cypermethrin, cyfluthrin, deltamethrin, permethrin, lambdacyhalothrin) are used more commonly these days

Question 22

Where has most resistance to insecticides been observed in triatomines?

Answer 22

Highly endemic Gran Chaco region (45% of populations)
No apparent association with time or IRS pressure

Question 23

In the last 25 years, what effect has Chagas control had on Latin America?

Answer 23

Incidence declined from 700,000 to 40,000 cases per year
Mortality decline: 45,000 to 12,500 deaths
But still annual cost of US$8 billion
Elimination of T.infestans in majority of region but not all

Question 24

What are the WHO/PAHO goals for endemic and non-endemic countries in regards to T. cruzi?

Answer 24

1. Elimination as a public health problem (in 20 endemic countries + 75% access to treatment)
2. Interruption of intradomiciliary (vectorial) transmission (0% Infestation Index and 0% T. cruzi human infection incidence
3. Elimination of blood transfusion & tissue transplant transmission (100% of target countries)
4. Interruption of congenital transmission: 90% screening coverage and treatment of women, and when infected, screening of their new-borns and siblings