This study analyzed the relationship between ecological factors and human plague cases in Vietnam from 1997-2006. It found that plague cases occurred most often from February to April, which corresponds to periods of high flea index and rodent density. Poisson regression modeling revealed that higher flea indices and rodent densities were independent risk factors associated with increased plague occurrence after controlling for other climatic variables like temperature, rainfall, and humidity. The results provide insights into the ecology of plague in Vietnam that can help target public health interventions.

1. Human Plague:
An Ecologic Study in Vietnam
Hau V. Pham, Dat T. Dang,
Nguyen N. N. Tran, and Tuan V. Nguyen
Institute of Hygiene and Epidemiology of Tay Nguyen, Vietnam
Vietnam Field Epidemiology Training Programme Office,
Garvan Medical Research Institute, Sydney, Australia

2. Plague in the world
 Plague – a disease of major public health
importance in endemic countries.
 Re-emergence in developing countries.
 Ecologic factors: climate, rodent density,
presence of fleas.

3. Research Objectives
 Description of incidence of plague in
Vietnam.
 Analysis of ecologic factors and its
association with plague occurrence.

4. Setting and Time
 Time: 1997 – 2006
 Location: the Central
Highlands, Vietnam
Map of Vietnam and the Central Highlands region

5. Study Design and Methods
 Case identification using standard protocol
– From network of community health centers
– Confirmed by Center of Preventive Medicine
 Rodent density = No. of rodents caught / No.
trapping periods x 100.
 Total flea index = Flea index was derived as the
ratio of total of fleas collected over the number of
rodents.
 Ecology -- Temperature, number of sunshine hours,
humidity and rainfall were recorded by local
meteorological stations.

6. Data Analysis
 Outcome: actual number of plague cases.
 Risk factors: rodent density, flea index, and climate
data.
 Model – Poisson regression model was used to model
the relationship between the potential risk factors and
plague.
 Analysis was conducted with the program.

7. Geographic Characteristics
Characteristics Dak Lak Gia Lai
Average elevation (m) 500 850
Temperature (oC) 23.2 1.8 22.1  1.7
Average sunshine / year (hour) 2305  239 2369  181
Average rainfall / year (mm) 1674  171 2092  290
Relative humidity (%) 83.1  6.0 84.9  6.7

8. Distribution of Plague Cases
1997-2006

9. Incidence of Plague and
Mortality by Month
1 2 3 4 5 6 7 8 9 10 11 12
Month
Number
of
cases
0
20
40
60
80
100
120
140
Number of cases
Number of deaths
1 2 3 2 5 1 5 2 3 0 0 0

10. Rainfall and Humidity
1 2 3 4 5 6 7 8 9 10 11 12
Month
Rainfall
(mm)
0
100
200
300
400
1 2 3 4 5 6 7 8 9 10 11 12
Month
Relative
humidity
(%) 0
20
40
60
80
100
Rainfall Relative humidity

11. Distribution of Flea Index
1 2 3 4 5 6 7 8 9 10 11 12
Month
Flea
index
0.0
0.5
1.0
1.5
2.0
1 2 3 4 5 6 7 8 9 10 11 12
Month
Number
of
cases 0
20
40
60
80
100
120
140
Number of cases
Number of deaths
1 2 3 2 5 1 5 2 3 0 0 0
Flea index Incidence of plague

12. Plague Cases and Flea
Index
[0.0,0.5) [0.5,0.8) [0.8,1.3) [1.3,5.3]
Flea index (quartiles)
Number
of
cases
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5

13. Association between Ecologic
Factors and Risk of Plague
Occurrence
0.2 0.6 1.0 1.4 1.8 2.2
Risk ratio
Density of rodent pop. (+4)
Flea index (+1)
Average air temp. (+20C)
Sunshiness (+60 hrs)
Rainfall (+160 mm)
Relative humidity (+7%)

14. Summary and
Conclusions
 Cases of plague occurred more often during dry
season with a peak in February – April.
 The February – April period: high flea index and
high rodent dendity.
 Flea index and rodent density: independent risk
factors of plague occurrence.

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