S H O R T C O M M U N I C A T I O N

Bloodmeal size and nutritional status of Triatoma infestans under natural climatic conditions

s I Lv I A Universidad de Ccirdoba, Argentina

c A T A LA Citedra de Biologia Celular y Centro de lnvestigaciones Entoniol6gicas, FCEFN,

Key words. Tricrzomrr infc.srans, bloodmeal size, nutrition, defaecation time, Argentina.

7 ~ m m r i ; u irtjiwui;s (Klug) (Hemiptcra: Reduvidae) is an important domestic \'ector o f Trypartosoma c w z i (Chagas) in the seven southernmost countries of Latin America. It5 ability to transmit this posterior station parasite, causative agent of Chaga\ disease, depends largely on biting fre- yuency and bloodmeal size which, in turn. affect the host contact rate. rejection volunie and timing of defaecation (C'atali. 1991; Trumper & Gorla, 1991). Most data on these factors are based on laboratory studies. often with partially starved insects feeding on a restrained host o r artificial membrane, resulting in a high proportion of replete bloodmeals and defaecation occurring while the insect remains on the host (e.g. Cuba et ul., 1978: Piesman s( Sherlock, 1983; Szumlewics. 1976). In contrast. labora- tory records o f feeding from an unrestrained host - with the effect of host irritability on feeding success, o r when bloodmeals are offered daily, indicates that significantly less blood i \ ingested (Schofield. 1982: Asin. 1992: Lazzari. unpuhl. data).

In studies of 7:~~If~sruiz.s maintained with access to chicken hosts tinder natural climatic conditions in the chaco region of i ' qxnt ina ( l o r details o f methods see Gorla. 1991: Ciorla CQ Schofielcl. 1989). we established that T.itz,#hm.\

ry 2 ~ - 3 days during summer, but virtually no feeding ackivity occurs during the cold \\inter (Catald. 1091 ). Another study at optimal temperatures revealed a blood consumption rate of 9% per day (Catala et al.. 1092), indicating that T.irzfestarzs rarely takes a complete Rloodmeal. The effects o f ingesting partial bloodmeals on clefaccation frequency could markedly affect the rate o f tranrmission of 7 ' . m t : i (Trumper & Gorla. 1991 ).

this pattern o f activity. we measured the bloodmeiil \ire o f ' / ' , / t ! f c . \ t ~ 4 J l . S nymphs and adults among

Corrcspondeiice: Dr Silvia C'atala. Catedra de Biologia Celular. Facult; iJ dc <'icncia\ Exactas. Fisicas ! Naturalcs. Universidad !Y;icioii;il ilc ('Ordoha. Cii\ilia 3'15. Veler Sarsticld ?YO. Z O W <'ordotx i . Argcritinu

populations maintained (for 3 years) in chicken-houses at our field site in the Argentina chaco. Two chicken-horses were selected for the study, one (nest L) with a low-density bug population (108 nymphs and adults) and one (nest H) with a high-density bug population ( 1 152 nymphs and adults). Both had four chicken hosts available to the hugs daily froni 22.00 to 07.00 hours.

Both chicken-houses (made o f loose adobe blocks) were dismantled: all adults and fourth and fifth instar nymphs from nest L were individually weighed and marked with acrylic paint. together with a large random sample o f the same stages from nest H. The chicken-houses were then reassembled and the bugs released back where they came from. The following morning, each chicken-house was dismantled again and all marked bugs werc reweighed. Weight increase above 1 mg was attributed to blood inges- tion. Feeding frequency (interval in days between suc- cessive feeds) was estimated by assessment of colourless urine, as proposed by Catali (1991). During the 24h of this experiment, the mean temperature registered by therniohygrograph at the site was 27 rt 3°C.

Since the fresh weight of bugs prior to the experiment showed n o significant differences between the two popu- lations. data could be pooled for each stage of nymphs and sex o f adults. Similarly, nieasurements o f bloodmeal weights and estimates of feeding frequencies showed no significant differences between the two populations, again allowing data to be pooled.

As expected (cf. Schofield, IWOb), initial nymphal weights showed a bimodal frequency distribution for each stage. while adult weight showed a near-normal distri- bution for each sex. Existing marks on adult hugs \from the monthly population studies using these experimental populations (Gorla cYL Schofield, I9X9)l allowccl older female bugs. i.e. >30 days since adult emergence, t o be distinguished from younger females. The formcr weighed significantly more at the start o f the experiment f428.O -+ 35.8 nig) than the younger females (290.') ? 48.1 mg).

Although bloodmeal size was not correlated with fresh weight before the experiment, feeding frequency of fifth- stage nymphs was significantly correlated with initial weight ( r = -0.82; t i = 94; P = 0.006): the lighter the insects the higher the proportion of them that fed. Among females, however, the opposite occurred, such that a higher pro- portion of older heavier females had fed, compared t o the younger and lighter females.

Average bloodmeal size amongst those insects deemed to have fed (Table 1) was significantly less, for all stages, than for T.irzfe.fe.c.tuns under laboratory conditions (Szumlewicz, 1976) (Fig. 1). This effect was particularly marked for fifth- stage nymphs, which are often considered the most epi- demiologically significant members of domestic T. irifestuns populations, because of their relative density (Schofield, 1980a), high biting rate (Catalfi, 1991, and this work, Table 1) and high infection rates (Giojalas et uf., 1990). This finding is very relevant to the transmission of T.cruzi because there is a marked negative relationship between bloodmeal size and defaecation time in both Rhodrzius prolixus Stake (Kirk & Schofield, 1987) and T.infestans (Trumper & Gorla, 1991). For example, using the relation- ship established by Trumper & Gorla (1991), the average bloodmeal size of bugs reported by Szumlewicz (1976) would imply an average time from interruption of bite to first defaecation of 5.6 min (range 4.6-6.8), whereas this would increase to 15.3 min (range 12.8- 19.6) on the data given here. This suggests that the fifth-instar nymphs observed in the present work would have commenced defaecation after they had left the host, effectively dim- inishing the likelihood of T.cruzi transmission in a similar domestic environment.

Quantitative variables and processes often differ between

400-/

350 -,'

300.'

250 -

200

150-'

100-

50

' Q 2 - "

-'

v F M STAGE

Fig. 1. Triurorna ~nfe.stuns bloodmeal size from laboratory studies ( 0 ) (Szumlewicz, 1976) comparcd with field observations (this paper) represented by striped histogram columns.

Table 1. Initial weight, bloodmeal size and feeding frequency (days) of Triatoma infesrans in chicken-houses under natural climatic conditions in the Argentina chaco.

Stage of bug

Fourth Fifth

Initial weight mg 87.1 252.5 SD 36.1 96.1 n 29 94

Bloodmeal size mg 42.3 78.6 SD 24.5 40.5 n 5 25

Feeding frequency Days 8.88 3.68 n 29 94

~

FF MM

310.2 237.9 90.0 46.6 29 1s

63.6 37.5 37.0 20.0 1s 2

1.93 9 29 I8

laboratory and field conditions, as shown by these results, raising the relevance of experimental field studies in clari- fying the transmission dynamics of parasites. Moreover, these data support the idea that the size of bloodmeals tends to decrease in established bug populations, with a corresponding increase in defaecation time and decrease in transmission likelihood. The implication is that estah- lished domestic populations of T. itzfestuns would represent relatively less risk for T.cruzi transmission, compared to newly founded and expanding populations of bugs in newly colonized houses or those reinfested after control interventions. Once again, the need for continuous epi- demiological vigilance in Chagas disease vector control must be emphasized (Dias, 1987).

Acknowledgments

This work received financial support from CONICOR and CONICET. 1 thank Dr C. J . Schofield for critical reading of the text.

References

Asin, S. (1992) Efecto de la temperatura sobre la competencia vectorial de Triatoma infestans. Thesis, FCEFN, Universidad Nacional de Cordoba. Argentina.

Catala, S.S. (1991) Biting rate in Triatorna infesfans under natu- ral climatic conditions in Argentina. Medical and Veteritzury Entomology. 5 , 325 -333.

Catala, S.S. , Giojalas, L. & Crocco. L.C. (1992) Temperature effects upon blood consumption in Triutoma infesruns. Mernririus d o lnstituto Oswddo Cruz, 87. 473-476.

Cuba, C.C., Alvarenga, N.J., Barreto, A.C.. Marsden. P.D. & Chiarini, C. (1978) Nuevos estudios comparativos entre Diperalogasrer rnurirnus y 'Friarornu infesfuns en acnodiagnciskico dc infeccion chagasica cronica hurnana. Revista do lrzsfitirro dr medicinu Tropical, Sao I'aulo, 20. 135-151.

Dias, J.C.P. (19x7) Control of Chagas diseasc in Brazil. Parusit-

Giojalas. L., Catali. S.. Asin. S. &r Gorla. D. (19YO) Seasonal changes in infectivity of domestic populations of Triarorna infestuns. Transacriott.s of rlre Roval Society of Tropical .Medicine and tfvgiene, 84, 439-442.

Gorla. I>. (1991) Recovery of Triatoma infesttrm populations after insecticide application: an expcrimental field study. .Mdicul and Vrrerinarv Enromology. 5 . 31 1-324.

Gorla, D. & Schofield. C.J. (1989) Population dynamics of Triarorw infestairs under natural climatic conditions in Argentina chaco. )Medical and I’ererinary Entomology. 3. 179 - 194.

Kirk. M.L. & Schoficld. C.J. (1987) Density-dependent timing of defaecation by Rhodriui.\ prolixio and its implications for the transmission o f Tryminosornu cni;i. Trurisuctions of tlre Row1 Societv of Tropical Medicine mid Hygiene. 81. 348-340.

Piesman. J . & Sherloek. I . (1983) Factors controlling the volume of feces produced by triatominae vectors of Chagas‘ discasc. AWI Tropicu. 30. 351-358.

dog? Today. 3. 336-341. Schofield. C.J. (1980a) Dcnsity regulation of domestic populations

of Tricrrorna infestatis i n Brazil. Transactions of the Royul Sociery of Tropical Medicine arid Hygiene, 74, 761-769.

Schotield. C.J. (1980b) Nutritional status of domestic populations of Triutotnic infesratis. Transactions of /he Roy1 Society of T I Y I ~ J I ~ U I .\ledicine cind Hygiene, 74, 770-778.

Schoficld. C.J. (1982) the role of blood intake in density regu- lation of populations of Triaroriia infestans (Klug) (Hemiptcra, Reduviidac). Bulletin of’ Entornologicul Research, 72, 617-629.

Szumlewicz. A . P . (1976) Laboratory colonies of Triatominac, hiology and population dynamics. New Approuches in American Try~Ja,io.sor,ziasb Keseurch, pp. 63-82. PAHO Publ. 318, M’ashington. D.C.

Trumper. E. & Gorla, D. (1991) Dcnsity dependent timing of dcfaecation by Tritrtomu infestatis. Trunsuctions of the Roval Sociery of Tropicul Medicine arid Hvgiene, 85, 800-802.

Acccptcd 8 August 1993

Book Review MOSQUITO ECOLOGY FIELD SAMPLING METHODS, 2nd Edn. By M. W. Service. Elsevier Applied Science, London. 1993. xii + 988pp. ISBN 1 85166 798 9. Hb f9S.

This rnugnum opus of loo0 pages remains true to the style and qualities of the first edition published in 1976, giving 40% more copy for 68% higher price. It serves the needs of anyone wanting to investigate the ecology of mosquitoes (Culicidae) and other haematophagous Diptera. But it conveys only a limited amount of actual ecological infor- mation. The niche for a lucid textbook of mosquito ecology remains occupied, but not necessarily filled, by Marston Bates (1949) The Nuturul History of Mosquitoes.

Mike Service has, however, provided us with a welcome update of his field guide to Culicidology. Most of what we know about mosquito field biology has been inferred from these often idiosyncratic sampling methods giving insights equivalent to ‘seeing in the dark’. The best way to really appreciate this book, with its overwhelming amount of information. is to need to use it for practical purposes, especially for a beginner. Chapters deal systematically with sampling of mosquito eggs, larvae and adults by variouh types o f trapping, estimates of population size, dispersal, mortality-rates and species diversity. The book comprehensively cites references (up to 1991) to sampling methods for understanding synecology, techniques for examining mosquito physiology to interpret autecology.

and is replete with appropriate statistical procedures. Summaries of subjects resemble entries from an abstracting journal, specifying who did something as well as what they found. This eponymous approach to ecology is recorded like a judicial enquiry, whereby evidence is always at- tributed to those accountable for it. In reviewing the methods more than what they show, Service judiciously indicates his own preferences, as in emphasizing certain merits of indirect rather than direct ELISA for bloodmeal identification.

Whatever the technical value of this hook, it gains much from the superlative indexes to authors, species and other subjects, totalling 63 pages with over 6000 keywords, compiled by Wendy Service. Almost everything in the book can thus be traced easily, although the booming subject of attractants (e.g. C 0 2 , octenol, lactic acid) is not indexed as such. References are given as full title citations at the end of each Chapter, involving some repetition, using World List abbreviations of journal titles. Many simple line drawings and photographs illustrate the designs of traps, and the glossy book cover bears an exquisite colour portrait of Aedes getriculutus.