Correlation of Aedes aegypti infestation indices in the urban area of Merida, Mexico

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1 Correlation of Aedes aegypti infestation indices in the urban area of Merida, Mexico Pablo Manrique Saide P. Coleman & C. Davies UADY-FMVZ-Departamento de Zoología London School of Hygiene & Tropical Medicine

2 Introduction This work is part of a project focused on (1) describing the patterns of dengue fever (DF) and dengue hemorrhagic fever (DHF) in Mexico (2) identifying the determinants of the observed patterns at different spatial scales within Mexico (3) describe and explain the patterns of Aedes aegypti distribution in a typical Mexican city identifying what needs to be measured and how best to carry out measurements for entomological risk (methods and at what spatial scale) (4) determine how to use this information to inform control. This is the preliminary report of one of the objectives: to quantify the relationship between different Aedes aegypti infestation indices (eggs, larval, pupal and adult)

Study site The study took place in the urban area of the municipality of Merida located in the Peninsula of Yucatan South East Mexico Merida has 662,530 inhabitants that live in about 160,000

3 Study site The study took place in the urban area of the municipality of Merida located in the Peninsula of Yucatan South East Mexico Merida has 662,530 inhabitants that live in about 160,000 households Intra-urban division through Basic Geo-statistical Areas (302 AGEBs) Gulf of Mexico This study was conducted in 29 of the total number of AGEBs (20 households per AGEB, 580 premises) Two surveys (early and late rainy season): June 23 rd -July 7 th 2003 August 25- September 8 th 2003 Guatemala Merida Yucatan Belize 100 Km N

4 4 N

6 Entomological survey 1. Adult female mosquitoes: Adult house density index (AHDI, Number of adult female Ae. aegypti mosquitoes collected in each house for 15 minutes). 2. Mosquito larvae and pupae: House index (HI); Container index (CI); Breteau index (BI); Larval density index (LDI) and Pupae density index (PDI).

7 Entomological survey 3. Weekly ovitrap surveys Ovitrap positivity index (OPI) Ovitrap density index (ODI) 4. These ovitrap results were compared with Ae. aegypti larval, pupal, and adult indices.

8 Preliminary Results Results indicated that the House Index (HI) was the only traditionally used entomological index that was a good predictor of the average number of female Ae. aegypti per house (AHDI), at least at the resolution of the AGEB. Neither larvae or pupae density indicators appeared to be good predictors of the average number of female Ae. aegypti per house (AHDI). Similarly Container index and Breteau Index showed poor correlations with ADHI.

9 Preliminary Results In contrast, oviposition measures showed statistically significant associations with AHDI

10 ln(ahdi + 0.1) 0-1 ln(ahdi + 0.1) ln(opi + 1) ln(odi + 1) Number of female Ae. aegypti (AHDI) obtained from the houses in Merida as a function of (OPI): Number of positive ovitraps/ovitraps examined 100 (left) and (ODI): Number of Ae. aegypti eggs in a week/premise (right).

11 Preliminary Results This indicates that ovitraps could represent a reliable method for Ae. aegypti infestation risk (adults) Information obtained from the use of ovitraps may be more sensitive than the indices traditionally used.

12 Preliminary Results Given the very low proportion of total containers with water (8%) estimation of the HI was very time consuming. In mosquito low-abundance situations, visual inspection of breeding sites can also result in low larval-pupae records, underestimating mosquito populations. This is not the case with ovitraps, which allow sensitive detection even in of areas infested with Ae. aegypti at low densities. Operationally, the use of ovitraps is both less intrusive and more economically than traditional entomological measures. However, despite this study, there is relatively little available information on the relation between ovitraps data and adult infestation risks.

13 Future work These results are currently being used to characterize the spatial distribution of Ae. aegypti populations within the AGEBs of Merida. From samples of households in all the AGEBs surveyed in two entomological surveys of 2003 we are examining the (1) underlying spatial structure of Ae. aegypti infestations (larvae, pupae and adult) (2) temporal stability of that structure comparing first and second surveys, and (3) correlation between clusters at different life stages of the mosquito. A geographical information system (GIS) in under construction to store all data relative to the study, including geographical coordinates of the households and AGEBS and data of incidence of Ae. aegypti, to represent and to analyse the spatial patterns.

14 Future work Explore the relation between entomological-indices and other household level variables, such as the degree of sanitation Analyse the spatial clustering of Ae. aegypti infestation within Merida. Analysis of the relationship between entomological indices, as well as household level surrogate indicators of Ae. aegypti infestation, and dengue fever incidence will also be conducted. In addition, we are carrying out the evaluation of current Aedes control activities using changes in ovitraps index measures

15 Spatial depiction of relevant entomological indices in AGEBs sampled in the Mexican city of Merida HOUSE INDEX 1-5 >5-10 >10-15 >15-20 >20 OVITRAP DENSITY INDEX ADULT HOUSE DENSITY INDEX

16 HOUSE QUALITY INDEX Spatial depiction of relevant house related indices in AGEBs sampled in the Mexican city of Merida >50 %HOUSES WITH NO WATER INSIDE THE HOUSES

Acknowledgments Inhabitants of Merida for their collaboration UADY-PRIORI: Project Caracterización y distribución espacial de la comunidad de mosquitos domésticos y peridomésticos de Mérida, Yucatán,

17 Acknowledgments Inhabitants of Merida for their collaboration UADY-PRIORI: Project Caracterización y distribución espacial de la comunidad de mosquitos domésticos y peridomésticos de Mérida, Yucatán, México PRIORI-FMVZ Consejo Nacional de Ciencia y Tecnología London School of Hygiene & Tropical Medicine Department of Infectious and Tropical Diseases Disease Control & Vector Biology Unit Dr. Clive Davies, Dr. Paul Coleman, Dr. Diarmid Campbell-Lendrum, Dr. Jo Lines

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