Joumal of the American Mosquito Control Association, 19(3):199-205,2003Copyright @ 2OO3 by the American Mosquito Control Association, Inc.

SURVEILLANCE FOR THE DENGUE VECTOR AEDES AEGYPTI INTOBAGO. WEST INDIES

DAVE D. CHADEE

Insect Vector Control Division, 3 Queen Street, St. Joseph, Trinidad, West Indies

ABSTRACT. An island-wide house survey was conducted in January 2OO2 to determine the geographic

distribution, container profile, and population density of the Aedes aegypti in Tobago, West Indies. The results

showed the Ae. aegypti infestation levels were significantly different (P > 0.01) among the 4 districts, with

greater infestation levels (P > 0.01) observed in the Northem and Windward districts than in the Central and

Leeward disricts. From the 50 towns in Tobago, houses were found positive in Delaford (21), Argyle (18), and

Goodwood (14), representing 42.l%o of the total number of positive houses in the Windward district (3,971

houses); Parlatuvier (15), Whim (14), Castara (12), and Bloody Bay (12), representing62.3Vo of the total number

of positive houses in the Northern district (3,087 houses); Calderhall (12), Mason Hall (11), and Govemment

House (10), representing 46.5Vo of the total number of positive houses in the Central district (4,706 houses);

and Lambeau (10), Bucco (6), and Bethel (6), representing 53.7Va of the total number of positive houses in the

Leeward district (3,175 houses). The majority (66 or 63.5Vo) of dengue cases occurred in the Central district

where the Breteau indices ranged from 7.1 to 44.0 (mean = 16.6). These results suggest that a more systematic

and sustained vector control program that uses both biological and chemical control methods should be adoptedto reduce Ae. aegypti populations to below dengue transmission tbresholds.

KEY WORDS Dengue, Aedes aegypti, Tobago

INTRODUCTION

In 1960, Trinidad and Tobago were declared freeof Aedes aegypti (L.) mosquitoes, but Trinidad soonbecame reinfested in the period 196l-62 (Chadee1989). However, Tobago remained virtually free ofthe vector until 1981 (Chadee et al. 1984, Chadee1990) when 2 foci were discovered in Roxboroughand were quickly eradicated (Chadee 1990). In1982, the Insect Vector Control Division (IVCD)detected Ae. aegypti at the Scarborough Wharvesand at Lambeau, a town at the periphery of thecapital, Scarborough. These foci of infestationswere quickly extirpated using an ultra-low volumeadulticide (malathion), residual spraying (fenthion),and focal treatment (temephos).

During the period 1983-88, Tobago again be-came reinfested with Ae. aegypti mosquitoes, whichstarted with the lst focus being detected in waterdrums that were transported to Tobago from Trini-dad in January 1983; this was followed by furthershipments in April and June 1984 (Chadee 1990).Thereafter, the geographic spread of Ae. aegyptieggs was assisted by the distribution and sale ofwater drums to various parts of the southern halfof the island. Chadee (1990) suggested methods fortreating drums prior to shipment to Tobago, but nomethodology was implemented. Consequently, af-ter 1988 the Ae. aegypti mosquito invasion contin-ued unabated throughout the island for the rext 12years with potentially disastrous consequences.

By November 2OOl, cases of dengue and denguehemorrhagic fever were detected on the island forthe lst time, and staff members of IVCD, Tlinidad,were invited to assist in the Ae. aegypti eradication/control program in Tobago. At this point it was dis-covered that the Ae. aegypti program in Tobago was

not properly coordinated, and no systematicallyconducted inspection and treatment cycles ofhouseholds had been done since 1998. The reasonsfor the Tobago program failure have been summa-ized by Chadee (2OO2) and include inadequatetransportation, insufficient supply of insecticides,lack of suitable equipment, lack of training, andpoor morale among workers.

The purpose of this study was to determine Ae.aegypti infestation levels in every town and villagein Tobago by using evaluators from Tiinidad andTobago Insect Vector Control Units.

MATERIALS AND METHODS

On January 6, 2OO2, the Trinidad IVCD team of8 men with 4 evaluators from the Tobago VectorControl Unit conducted an evaluation of the entireisland of Tobago, which is divided into 4 vectorcontrol districts: Windward, Northern, Cenral, andLeeward. These districts are used for planning andadministration of the program. T}ne Ae. aegypti geo-graphic distribution and density of infestation werestudied, starting from Cambelton in the noftheast toPlymouth in the southwest of the island (Fig. l).The distribution of Ae. aegypti was assessed usinga 1:10 ratio for house inspections. All containerswere examined and immature mosquitoes collectedaccording to the Pan American Health Organization(PAHO) (1968) guidelines. A1l data were docu-mented with standard IVCD forms that indicatedthe householder's name. address. number of con-tainers inspected and positive, and number of Ae.aegypti foci discovered.

All immature mosquitoes found were collectedwith strainers and dippers, placed into vials, labeledaccording to container types and locality, and sent

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to the IVCD laboratory, Trinidad, for rearing andidentification.

Data were analyzed by transformation into con-tingency tables and subjected to a G-test to com-pare the distribution of dengue cases and Ae. ae-gypti infestatiton levels at the district levels (Petrieand Sabin 2000). The Breteau index (number ofpositive containers per 100 houses inspected) andHouse index (number of positive houses per num-ber of houses inspected) were calculated to estab-lish a relation between positive containers and pos-itive houses, which is considered to be mostinformative measure (PAHO 1994).

Lehr's method (Petrie and Sabin 2000) was usedto determine the sample size that would representthe population of Tobago at the 5Vo significancelevel. A sample size of 256 houses from each dis-trict was calculated to obtain a 9O7o chance of de-tecting differences in the number of positive housesand containers in each districtl the 95Vo confidenceinterval around sample estimates would be about+9Vo (Petrie and Sabin 2000).

RESULTS

Results of the Ae. aegypti evaluation programconducted in January, 2OO2, at the town and districtlevels in Tobago are summarized in Thbles 1-4.When the Ae. aegypti infestation levels from the 4districts were compared, significant differences (G =

32.O, dt : O.3, P > 0.01) were observed among the

. F. { .1

dcrse-FourmBloody-

4 districts but especially among the northem andsouthern halves of the island. In fact, when theNorthern and Windward districts were combined andcompared with the combined Central and Leewarddistricts, significant differences (G : 16.0, df : O.2,P > 0.01) were observed. These results suggest thattlrle Ae. aegypti infestations were greater in theNorthern and Windward districts than in the south-em part of the island, which comprised the Centraland Leeward districts. In the 4 districts, the mini-mum sample size of 256 houses per district was ex-ceeded by at least lO0%, making the data collectedrepresentative at the 9Vo significance level.

Windward district

Table I shows a total of 3,971 houses in theWindward district, of which 865 (2l%o) were in-spected. One hundred twenty-six (l4.6Vo) of thehouses were positive for Ae. aegypti. Of the 18towns in the Windward district, the largest numberof positive houses was found in Delaford (21), Ar-gyle (18), and Goodwood (14), which represented42.l%o of the total number of positive houses. Thetotal number of containers inspected was I1,697, ofwhich 182 were positive for Ae. aegypti breeding.The container type, which represented the largestnumber of containers, belonged to the small mis-cellaneous container category found outdoors(6,020). The main breeding foci of Ae. aegypti were

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drums (69) and small miscellaneous containers(55), followed by tubs and basins (41) and tires (8).

Northern district

Table 2 shows a total of 3,087 houses in theNorthern district, of which 635 (2l%o) were inspect-ed. One hundred six (19.7Vo) of the houses werepositive for Ae. aegyptL Of the 12 towns in theNorthern district, the largest number of positivehouses was found in Parlatuvier (15), Whim (14),Les Couteaux (13), Castara (12), and Bloody Bay(12), which represented 62.3Vo of the total numberof positive houses. The total number of containersinspected was 6,305, of which 143 were positivefor Ae. aegypti breeding. The container type, whichrepresented the largest number of containers, be-longed to the small miscellaneous container cate-gory found outdoors (3,113). The main foci of Ae.aegypti were drums (56) and small miscellaneouscontainers (42), followed by tubs and basins (31),tires (8), indoor containers (4), and water tanks (2).

Central district

Table 3 shows a total of 4,706 houses in the Cen-tral district, of which 646 (13.7Vo) were inspected.Seventy-one (llVo) of the houses were positive forAe. aegypti. Of the 1l towns in the Central district,the largest number of positive houses was found inCalderhall (12), Government House (10), and Ma-son Hall (11), which represented 46.5Vo of the totalnumber of positive houses. The total number ofcontainers inspected was 10,053, of which 107were positive for Ae. aegypti breeding. The con-tainer type, which represented the largest numberof containers, belonged to the small miscellaneouscontainer category found outdoors (5,039). Themain foci of Ae. aegypti were small miscellaneouscontainers (37) found outdoors, followed by tubsand basins (29), drums (26), tires (9), and indoorcontainers (2).

Leeward district

Table 4 shows a total of 3.175 houses in the Lee-ward district, of which 645 (20Vo) were inspected.Forty-one (6.4Vo) of the houses were positive forAe. aegypti. Of the 9 towns in the Leeward district,the largest number of positive houses was found inLambeau (lO), Bucco (6), and Bethel (6), whichrepresented 53.7Vo of the total number of positivehouses. The total number of containers inspectedwas 10,609, of which 69 were positive for Ae. ae-gypti breeding. The container type, which repre-sented the largest number of containers, belongedto the small miscellaneous container categoryfound outdoors (5,631). The main breeding foci ofAe. aegypti were drums (25) and small miscella-neous containerc (22), followed by tubs and basins(13), tires (7), and water tanks (2).

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Breteau indices and Dengue cases in Tobago

Table 5 shows a summary of the number of den-gue cases and Ae. aegypti Breteau indices. In all 4districts, the Breteau indices exceeded 5, which isconsidered the threshold level for disease transmis-sion (Macdonald 1956). The majority (66, or63.57o) of dengue cases occuned in the Central dis-trict, where the Breteau indices ranged from 7.1 to44.0 (mean : 16.6). Mason Hall (15), Scarborough(13), Signal Hill (7), and Lambeau (6) were themain towns with a large number of dengue cases.These 4 townships accounted for over TOVo of thedengue cases reported from the Central and Lee-ward districts and 39.5Vo of all dengue cases re-ported in Tobago during 2001.

DISCUSSION

This survey represents the 1st island-wide eval-uation ofAe. aegypti in Tobago since 1998 and pro-vides ample evidence of the wide geographic dis-tribution of this vector species in Tobago (Fig.l).The Ae. aegypti population density throughout theisland was high, exceeding the Breteau index of 5,which is regarded as the disease transmissionthresholC (Macdonald 1956) (Table 5). The occur-rence of dengue fever cases in the Central elistrictwas significantly correlated with the Ae. aegyptipopulation, although the Ae. aegypti populationdensity was lower in the Central district than thatobserved in other districts (Table 5). These resultssuggest that people living close to the capital andmajor tourist centers located in the Central and Lee-ward districts were at greater risk of contracting{engue by virtue of being in contact with Ae. ae-gypti, which mav have taken blood meals from vi-remic tourists and local travelers fio-m both Trini-dad and Tobago, The large number of dengrre casesin Scarborough, Mason Hall, Lambeau, and SfunalHill supports this hypothesis.

The Ae. aegypti population infestation levels inthe Northern and Windward districts were sisnifi-cantly different (P > 0.Ol) from those observ-ed inthe Central and Leeward districts. These variableinfestation levels were possibly due to differencesin operational procedures and management practic-es used by vector control managers and staff. Forexample, Chadee (2002) showed that the districrsthat were far from the Central and Leeward districtadministrative centers were neglected (with little orno supervision, no equipment and supplies, notransportation, and lack of political will), whichpossibly resulted in low morale among all levels ofstaff-

Focks and Chadee (1997), working on the epi-derniological significance of the Ae. aegypti pupalindlex, showed that counties close to Port of Spain,the capital city of Trinidad, generally have a morereliable water supply and better sanitation with few-er mosquitoes than do outlying counties. The pres-

204 JounNel or rue Avsmcat Mosquno CoNTRoL AssocrlrroN VoL. 19, No. 3

Table 5. Summary of house inspection and treatment evaluations conducted in all 4 districts in Tobago(Januarv 2002).

DistrictHousesinspected Dengue

(population) casesHouses Wet Containersposit ive containers posit ive

Breteau indices House(range) indices

WindwardNorthernCentralLeewardTotal

865 (5,208)63s (4,445)646 (3,876)64s (3,22s)

2,791 (16,754)

9t 266l 7

104

1261067 14 1

344

11,6976,30s

10,05310,60938,664

182143t0769

50r

2t.s (4.6-48.6) 14.621.7 (11-8-42.9) 16.716.6 (7.r-44.O) 10.9l 1.0 (4.7-r 9.1 ) 6.317.9 (4.6-48.6) L2.l

ent results were consistent with these observationswith a much larger number of Ae. aegypti breedingfoci, water drums, tubs, and basins observed in theNorthern and Windward districts as opposed to theCentral district, where Scarborough is located.

The impact of recent infiastructural develop-ments in the Central and Leeward districts mayhave resulted in improved water storage practices,with an increase in the number of water storagetanks and a decrease in the number of water drums.For example, in the Leeward district, 900 storagetanks were observed as opposed to 529 drums.Conversely, in the Northern district, 683 drumswere observed as opposed to 480 tanks (Tables 2and 4). In addition, the present results indicate aswitch in the major breeding focus of Ae. aegyptifrom drums to small miscellaneous containers, es-pecially in the Central district (Tables l-4).

These results suggest that the water storage con-tainer types used by householders and Ae. aegyptiin Tobago need to be closely examined before plan-ning control programs. For example, although tanksreplaced water drums in the Central district, thenumber positive for Ae. aegypti breeding was quitesmall (Tables l-4). Similar results were sbtainedfbr indoor containers, such as vases and water re-ceptacles, in the Northern and Central districts.

It is noteworthy that the frequency of containerusage by Ae. aegypti suggests that a source reduc-tion program for small miscellaneous containerscould potentially eliminate more than 307o of thefoci of breeding. The inspection and treatment ofindoor containers (vases and potted plants) and wa-ter tanks should be reexamined because they rep-resent only a modicum of the Ae. aegypti breedingsites (Tables l-4). Focks and Chadee (1997) re-ported similar results, suggesting that these indoorcontainers are inconsequential because of theirsmall numbers and lack of appreciable Ae. aegyptiand can be best controlled by educating household-ers rather than by centralized control measures,which are difficult to execute because of house-holders' objections and "closed houses" lChadeel 988) .

In a previous study, Chadee (1990) reported thatdrums were the containers most frequently used byAe. aegypti (35.4Vo) in Tobago, and 12 years laterduring the present study similar levels of drum in-festation were observed (367o). Therefore, it is

clear that a systematic approach should be adoptedto prevent the proliferation of Ae. aegypti in waterdrums. For example, the provision of house con-nections from a central water supply, a long termobjective, would not solve the water storage prob-1em unless it is reliable, as demonstrated in Vene-zuela and Trinidad (Barrera et al. 1993, Chadee andRahaman 2000). Alternatively, the physical exclu-sion of mosquitoes by providing protective coversfor drums is another option; howeveq this methodwould be effective only if the covers are easy touse (PAHO 1994). The introduction of larvivorousfish such as Poecilla reticulata (L.) or other bio-Iogical control agents such as copepods, bacteria,"la untadita" method of combining detergent andbleach to destroy Ae. aegypti eggs (Sherman et al.1998), or use of appropriate insecticides like te-mephos are avai lable options.

In conclusion, the use of the 1-in-10 house in-spection and treatment sampling methodology pro-vides a snapshot of the incidence and geographicdistribution of Ae. aegyprl in Tobago. In addition,this study also provides some of the risk factors foran epidemic outbreak of both classical dengue anddengue hemorrhagic fever, given Breteau indices inthe order of 4.6 to 48.6 throughout the island.Therefore, a more systematic and sustained vectorcontrol approach should be adopted to stem thewave of dengue transmission and reduce vectorpopulations to below epidemic transmission thresh-olds.

ACKNOWLEDGMENTS

I wish to thank C. Alfred, Secretary for Health,Tobago House of Assembly for her enthusiasdcsupport and C.C. Tilluckdharry, Specialist MedicalOfficer, Insect Vector Control Division, Ttinidad,for his support for the Tobago program. In addition,I thank V. George, PHI 4 (Trinidad), C. Arendell,PHI 4 (Tobago), W. Mohammed, PHI 3 (Ttinidad),K. Maharaj PHI 3 (Tiinidad), and Lynch-Benjamin,PHI 2 (Tobago) for dedicated service during thecourse of the field program in Tobago. I also thankS. Deonarine (HCO l); A. Brosco (HCO 1), L. Pun-nette, S. Cooke, L. James, D. Lalla, B. Persad, K.Ram, and S. Lochan from Trinidad and T Brawth-waite (HCO 3), I. Daly (HCO 2) and M. Boyce, M.Renwick, E. Collette, and H. Gordon from Tobago

SppreMsen 2003 AEDEy AEGyp'il SunvBrr-LlNcs rN Toelco 205

for working long hours to achieve the coverage ofTobago within 10 days.

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