Lecture 6. Flagellated Protozoa: Africa Trypanosomes 3

Question 1

Why is it important to diagnose HAT in Uganda?

Answer 1

The two HAT subspecies overlap in Uganda, with the different subtypes needing different treatments

Question 2

How many major epidemics of sleeping sickness did Uganda face in the 20th century?

Answer 2

3: 1900s, 1940s and 1980s

Question 3

What happened in the sleeping sickness epidemic in the 1900s?

Answer 3

1908: 1/3 population dead (300,000)
1909: survivors evacuated Uganda
From Congo

Question 4

What happened in the sleeping sickness epidemic in the 1940s?

Answer 4

1942: 2,432 cases, 274 dead
From Zambia via Tanzania

Question 5

What happened in the sleeping sickness epidemic in the 1980s?

Answer 5

1980: 9,000 cases
1987: 7,000 cases

Question 6

What did modelling sleeping sickness in South-East Uganda show us?

Answer 6

Human sleeping sickness (SRA analysis) 0.6% (6 /1000)
Fly biting preference: human 9%, cattle 23%
Cattle prevalence: T. b. brucei 45%, T. b. rhodesiensis 18%
Cattle 234x more likely to be source of human T. b. rhodesiensis than other humans (cows main reservoir)

Question 7

How many cattle would have to be treated to achieve an R₀ < 1 in SE Uganda?

Answer 7

20%

Question 7

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

Answer 7

Public-private partnership launched in Uganda, 2006
Response to possible convergence of T. b. rhodesiense and T. b. gambiense in N. Uganda

Question 8

Why were the early stage disease 1st line drugs used in the SOS initiative?

Answer 8

Suramin efficacious against T. b. rhodesiense
Pentamidine efficacious against T. b. gambiense
Pentamidine is not effective against early stage T. b. rhodesiense

Question 9

Why were the late stage disease 1st line drugs used in the SOS initiative?

Answer 9

Melarsoprol efficacious against both
Late stage treatment failures of T. b. gambiense increasing
An alternative is Eflornithine but is not effective against T. b. rhodesiense

Question 10

Why can we not just use insecticide on cattle constantly to kill the tsetse flies?

Answer 10

Because we need to consider the effects on tick-borne diseases (TBDs) of livestock

Question 11

What are five examples of tick-borne diseases that infect cattle and what do they cause?

Answer 11

Anaplasma marginale (anaplasmosis, gall sickness)
Babesia bigemina and B. bovis (babesiosis, redwater fever)
Cowdria ruminantium (cowdriosis, heartwater)
Theileria parva (theileriosis, East Coast Fever, Corridor disease)

Question 12

What is the incubation time for babesiosis in cattle?

Answer 12

3-21 days

Question 13

What does babesiosis cause in cattle?

Answer 13

High fever, anorexia, seek shade, weight loss, abortion, poor milk production

Question 14

What percentage of erythrocytes (RBC) are destroyed by babesiosis?

Answer 14

75%, causes the red blood cells to enter the urine

Question 15

What is the mortality rate of babesiosis?

Answer 15

Most survive but mortality up to >50% known, slow recovery if cattle do survive

Question 16

If a cow survives having babesiosis, what may it become?

Answer 16

A carrier for many years

Question 17

How does Babesia end up in the blood stream of cattle?

Answer 17

Babesia pass into the ovaries/eggs – vertical transmission
Migrate to the salivary glands to reproduce in larvae
Larvae await on grass stalks to attach
Babesia from salivary glands injected into the mammal’s bloodstream

Question 18

What is the life cycle of a host tick?

Answer 18

Larvae hatch from eggs and climb grass
Larvae climb off gras and attach to host
Larval stage → numphal stage → adult stage
Engorged adult female drops off, laying thousands of eggs and dies

Question 19

Why is treating cattle with insecticide complex?

Answer 19

Insecticides kill both ticks + tsetse
Need to maintain tick exposure while preventing tsetse bites to allow for acquired immunity
Insecticide costs

Question 20

What does endemic stability describe?

Answer 20

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

Question 20

Why is it good if cattle are infected with tick-borne diseases at a young age?

Answer 20

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

Question 21

What diseases follow endemic stability?

Answer 21

Babesiosis, cowdriosis and anaplasmosis
Uncertain for Theileria

Question 22

What diseases are not age related to severity and no conferred immunity?

Answer 22

Nagana (T. b. brucei, T. b. congolense, T. vivax)

Question 23

What does age of first exposure to tick fever determine?

Answer 23

Disease severity. Severity increases with age

Question 24

At what age should a calf be exposed to tick fever to have solid, long-lasting immunity?

Answer 24

Before 9 months

Question 25

What are two measures to maintain endemic stability within cattle?

Answer 25

Tsetse more susceptible to pyrethroid-treated cattle than are ticks- increasing intervention interval lowers impact on ticks
Tsetse preferentially feed on different sites of body, particularly legs- avoid tick attachment sites

Question 26

How does tsetse biting vary with individual cattle and herd sizes?

Answer 26

Tsetse flies prefer biting larger and older cows (more blood meals)
Larger herd sizes reduce the risk of infection

Question 27

What solutions do farmers use to achieve sustainable control of tsetse flies?

Answer 27

Tsetse preferentially feed on older & larger herd members so leave young cattle untreated to become exposed to ticks
Treating half the herd, applying insecticide to only the larger individuals, would be only slightly less effective than treating the entire herd but would reduce the amount of insecticide used
Selective treatment of hosts with Restricted Application Protocol (RAP) reduce insecticide costs by 90%, compared with the current regime of treating the entire herd, and the impact on non-target organisms

Question 28

What are the achievements of SOS so far?

Answer 28

During 8 wks initial emergency intervention, veterinary students (Makerere University) treated 250,000 cattle in 5 districts in
N. Uganda which reduced the prevalence of T. b. rhodesiense in cattle by 70%

Question 29

What lessons have we learned from SOS?

Answer 29

It works
Cost-effective: RAP is as effective as conventional whole-body spraying. US$0.06 per head/month “self-help” or US$50 per
km² of organised mass-treatment.

Question 30

Who pays for SOS?

Answer 30

Livestock owners: important “levers to purchase” control is the visibly fewer flies (incl nuisance flies), and ticks. Decentralised
“Bottom-up” approach

Question 31

How are vectors controlled in the absence of cattle?

Answer 31

It is predicted that minimum densities of 10 cattle/km² are necessary for cattle to be an important part of the tsetse blood source depending on availability of alternative zoonotic hosts
This precludes the use of RAP in many Gambian HAT foci
Traditional biconical traps and deployment is expensive costing an estimated US$556/km² per year
One recent development is the use of “tiny insecticide impregnated targets” against Gambian HAT transmission

Question 32

What are “tiny targets” that are used to control G. f. fuscipes?

Answer 32

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

Question 33

What do the results from a pilot study on the use of tiny target traps show us?

Answer 33

The observed declines indicated an imposed daily mortality of ~4% per day (Manga: 10 targets/km= placed at 100m intervals). And approaching ~6% per day (Big Chamaunga: targets 20/km= placed at 50m intervals).

Question 34

What impact would the use of tiny target traps have on HAT transmission?

Answer 34

A 72% reduction in tsetse numbers will stop HAT transmission (R₀<1)
The pilot study approach routinely achieves ~90% reductions whilst elsewhere ~80% reductions in tsetse populations have impacted on HAT transmission

Question 35

What are the general conclusions on the use of tiny targets?

Answer 35

Deployment of tiny traps could significantly reduce 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 or their numbers are no longer an issue
The model is quite sensitive to alterations in operational procedures. For example, reduced control (relaxation in effort) resulting in lower induced mortality rates results in rapid rebounds
This could results from waning operational sustainability
Cost-effective: reduces cost of vector control by >80% to US$85/km² per year from US$556/km² using traditional biconical traps, supports, teams and transport.

Question 36

What could jeopardise the progress towards elimination of T. b. gambiense in Uganda?

Answer 36

Possible spillover from conflict in South Sudan