Abstracts

Studies on mosquitoes (Diptera: Culicidae) and anthropic environment. 1- Parity of blood seeking Anopheles (Kerteszia) in South-Eastern Brazil^* * Supported by Grant of "Fundação de Amparo à Pesquisa do Estado de SãoPaulo" (FAPESP). Processo n o90/3371-6.

Estudos sobre mosquitos (Diptera: Culicidae) e ambiente antrópico. 1- Paridade de Anopheles (Kerteszia) em atividade hematófaga, na região sudeste do Brasil

Oswaldo Paulo Forattini^I; Iná Kakitani^I; Eduardo Massad^II; Almério de Castro Gomes^I

^IDepartment of Epidemiology, School of Public Health, University of S. Paulo. Taxonomic and Systematic Research in Medical Entomology Unit of the University of S. Paulo - S. Paulo, Brazil

^IIDepartment of Pathology, School of Medicine, University of S. Paulo - S. Paulo, Brazil

ABSTRACT

Populations of Anopheles (Kerteszia) were sampled fortnightly over a one-year period (August 1991 to July 1992) at Ribeira Valley, S. Paulo State, Brazil. Indoor and outdoor collections were made on human bait at evening crepuscular period. The Polovodova technique for age grading was applied to 3,501 females of Anopheles cruzii and to 416 females of An. bellator. That sample represented 34.4% of the total number of mosquitoes collected. The most abundant species found was An. cruzii. However, An. bellator showed an endophagy that was almost three times greater than that of An. cruzii. The overall parous rate was 25.4% and uniparity was practically dominant one. A proportion of 26.9% of An. cruzii and 12.0% of An. bellator were found to be uniparous. Only three outdoor females of the former species (0.1%) showed biparity. Parity of An. cruzii was higher in females caught outdoors than in those caught indoors. Nevertheless, 497 nulliparous females examined (417 cruzii and 80 bellator) had ovaries that had advanced to Christophers and Mer stages III to V. These results imply that these females had already practised hematophagy. Relating these results to those from the parous females, a high statistical significance was found, leading to the conclusion that gonothophic discordance is a common pattern among these anophelines. Further, these results obtained with human bait catches strongly suggest that nearly 38.0% of these host-seeking females had already taken at least one previous blood-meal. So it is possible that enough time could thus be available for the plasmodian development in the vectors.

Keywords:Anopheles, physiology. Parity. Endophagy. Domiciliation.

RESUMO

Relata-se os resultados obtidos em coletas regulares de Anopheles cruzii e An. bellator, mediante o emprego de isca humana e por ocasião do crepúsculo vespertino. Objetivou-se, precipuamente, conhecer a paridade de populações dessas espécies, quando em plena tentativa hematófaga, tanto no ambiente intra como peridomiciliar. As coletas foram levadas a efeito no ecossistema primitivo da Mata Atlântica meridional do Brasil, em região do Vale do Ribeira, sudeste do Estado de São Paulo, Brasil, durante o período de agosto de 1991 a julho de 1992. Precedeu-se ao exame de amostra, correspondente a 34,4% do total de mosquitos coletados, mediante o emprego da técnica de Polovodova para a observação de dilatações ovariolares. Ao lado disso, o desenvolvimento dos ovários foi classificado de acordo com as fases de Christophers e Mer.

Os dados obtidos permitiram constatar a dominância de An. cruzii. No entanto, pôde-se detectar maior atividade endofágica por parte de An. bellator, a qual se mostrou três vezes maior do que a apresentada por aquele outro anofelíneo. No que concerne à paridade, houve franco predomínio da nuliparidade (74,6%), com o restante constituído, praticamente, por uniparidade (25,4%). A presença de maior número de dilatações ovariolares limitou-se a poucos casos de biparidade em An. cruzii. A paridade apresentada por este revelou-se maior no peridomicílio, reforçando a feição exófila de seu comportamento, ao passo que para An. bellator mostrou-se uniforme em relação dos dois locais de coleta. Observou-se que 17,0% de fêmeas nulíparas revelaram ovários cujo desenvolvimento tinha atingido as fases III a V de Christophers e Mer, o que evidenciou o exercício de hematofagia previamente à primeira oviposição. Dessa maneira, pôde-se considerar como sendo de 38,1% o percentual da amostra total examinada que, pelo menos, já realizara um repasto sangüíneo ao tentar repeti-lo na isca humana. Esses valores foram de 34,8% para An. cruzii e de 8,7% para An. bellator. Diante disso, conclui-se pela provável existência de discordância gonotrófica, o que permite levantar a hipótese de, entre outros fatores, haver viabilidade de desenvolvimento plasmodiano no organismo desses vetores.

Descritores:Anopheles, fisiologia. Paridade. Endofagia. Domiciliação.

Introduction

Anopheline (Kerteszia) species are regarded as malaria vectors in the Atlantic Rain Forest System of southern and south-eastern Brazil, where that infection presents a low endemic level. Several interacting factors are recognized as determinants of the anopheline vectorial capacity. Combining parasitological and entomological parameters it is possible to attempt an estimation of the malaria reproductive ratio (Garrett-Jones^11,12, 1964; Dye⁵, 1986). As a critical parameter, vector survivorship may be mentioned because it determines the longevity, or expected duration, of vector infective life. The most commonly used measure is the parity, or parous rate, that is the age-composition of mosquitoes populations evaluated in terms of the female's gonotrophic cycles count. For that, the Polovodova technique represents the most widely used method (Detinova^3,4,1962,1968).

Indeed, just parity determination gives only a crude estimative of mosquito age-composition, mainly as regards the Anopheles species in which blood meals and eggs laying may occur soon after the emergence. However, parity knowledge allows the differentiation of the young female section of the population from the older one. In that sense it has been the most frequent index actually used for the determination of the vector's age-composition.

Through monitoring local anopheline populations, both in natural and anthropic environments, it should be possible to evaluate the regional receptivity to malaria endemicity. With this in view, the Ribeira Project was planned to carry out research in the so-called Valley region, in the south-eastern region of S. Paulo State, Brazil. In this paper, the results of observations on the age-composition of blood-seeking Anopheles (Kerteszia) cruzii and An. (Ker.) bellator, with indoor and outdoor human bait, are reported.

Study Site

Field works took place in an area near the village of Itapitangui, Cananéia County, known as the "Fonte" (Spring) region because it contains (Andrade Farm), neighbouring on the "Sítio Itapoã" (Itapoan Farm), a former observation site, the results from which have already been published, including a more detailed regional description (Fig.1) (Forattini et al.⁷,1986).

The primitive natural environment is represented by the "Mata Atlântica" (Atlantic Rain Forest) covering the slopes of the "Serra do Mar" (Coastal reservoir for water supply. There are several localities in that region, including the one that was chosen for these observations, called "Sitio Andrade" Mountain Range). Research took place at the local anthropic environment, approximately one hundred meters away, in a straight line, from that forest. Figures 2, 3 and 4 show landscape photographs of this study site.

Material and Method

To sample the biting mosquito population, the collectors caught females as they came to feed from them on their exposed legs. The captures were performed indoors and outdoor during the evening crepuscular period (Forattini et al.⁷, 1986), for two hours uninterruptedly, with a regular fortnightly rhythm over one year, from August 1991 to July 1992.

After identification, the dissections were performed and the state of the ovaries and the condition of the follicular stalks were noted. The number of females dissected varied in accordance with the number of mosquitoes caught. As a whole, the percentage of specimens examined were 64.4 and 88.0 of the total An. cruzii and An. bellator indoor capture, while 30.8 and 40.0, respectively, were of each of those species caught outdoors. With the purpose of detecting blood present before egg-laying, ovarian follicle stages were identified according to the Christophers and Mer classification (Forattini⁶,1962).

Results

Twenty-five sampling catches were made in all and 11,373 females of Anopheles (Kerteszia) were obtained (Table 1). Of that number, 92.3% were An. cruzii and 7.7% were An. bellator. Nevertheless, regarding the two collection sites, An. bellator indoor presence (22.6%) was higher than that of An. cruzii (7.4%) as a proportion of the total number of the anophelines obtained. Thus comparing the mean, both indoor and outdoor, of these mosquitoes, it varied significantly (x²=9.81; p=0.001737). Thus, An. bellator has a probability of 3.71 higher than An. cruzii of being caught biting a man inside his house.

In view of the fact that in catches aimed at blood-seeking mosquitoes, the number caught corresponds, at least theoretically, to the number of bites that would be received by the catcher, that is, to the term ma of the equation of the malaria reproductive rate (Reisen¹⁶,1989), it was possible to estimate the endophagy index (Ribeiro and Janz¹⁶,1990) at 0.22 for An. bellator and 0.08 for An. cruzii.

Of the total obtained, 3,917 (34.4%) specimens were dissected, 3,501 (89.4%) of which were An. cruzii and 416 (10.6%) An. bellator (Tables 2 and 3). The overall parous rate was 25.4% which included 27.0% of the former and 12.0% of the latter species. With regard to the catching sites, indoor parity rates were 21.9% for An. cruzii and 11.6% for An. bellator, while the outdoor ones were 27.9% and 12.2%, respectively. That parous distribution was significant only for the former species (x²=7.44; p=0.0063678). Regarding the seasonal distribution, it was not possible to reach any consistent conclusion, although data suggested some parity increase related to the February-April period.

The uniparous condition, corresponding to one follicular stalk dilation, was practically the only one found. Nevertheless, ovarian development stages, according to Christophers and Mer, were found, as shown by the Tables 4 and 5. Of the nulliparous females examined, 497 (17.0%) had ovaries that had reached stage III or above. Of that total, 417 (83.7%) were An. cruzii and 80 (16.1%) An. bellator. Of the parous females, 60 (6.0%) had ovaries at stage III, the greater number of them belonging to An. cruzii. Comparing those nulliparous and parous rates, highly significant differences were found, both for An. cruzii (x²=597.26; p<0.00000001) and An. bellator (x²=11.04; p=0.0008933).

Discussion

The dominance of the Kerteszia mosquito in the primitive Atlantic Rain Forest ecosystem of South and South-Eastern Brazil (Forattini et al.⁸, 1986) is well known. Despite the fact that these anophelines, particularly An. cruzii and An. bellator, are regular frequenters of the peridomiciliar environments situated near the forests, it seems that after their blood meal they have no tendency to rest inside human dwellings. For this reason they are considered as vectors with low domiciliarity, at least in the Ribeira Valley and neighbouring regions (Consolim et al.², 1979; Forattini et al.⁹, 1990). Nevertheless, this exophilic behavior, commonly recorded for other species of the same subgenus, is a recognized feature assigned to these anophelines as malaria vectors (Forattini⁶, 1962; Quiñones et al.¹⁵, 1984; Carvajal et al.¹,1989). In this case the indoor presence must be considered as characterizing endophagy, which seems to be higher for An. bellator than for An. cruzii (Table 1). Generally speaking, the endophagic habit of the former was about three times more frequent than that of the latter. To this behavior, it must be added to An. bellator very regular presence along the catching year, showing smaller seasonal variations at the higher density period (February-March) than was shown by An. cruzii over the same period. Thus it was possible to verify that the monthly catches of the latter species practically established the incidence profile (Fig.5). Therefore, it seems reasonable to suppose that, at least in the study area, An. bellator presented some level of domiciliar tendency, even if limited to some degree of endophagy.

Regarding parity of anopheline populations, there is general agreement that the most suggestive data are those obtained through the examination of blood-seeking female samples (Garrett-Jones and Grab¹¹, 1964). The obvious reason is that if they are females searching for hosts it would be interesting to investigate the possibility of previously-taken blood meals. Usually catches are simultaneously performed both indoors and outdoors. As for Kerteszia in the environment here studied data so far available concern An. cruzii. At Paraná State, biogeographically analogous to our region, indoor parous rates of 17.0% and outdoor ones of 15.0%, with as statistically significant difference, have been recorded (Luz et al.¹³, 1979). Nevertheless, the results here reported show outdoor parous rates of An. cruzii significantly higher than the indoor ones (Table 2). These data agree with the absolute values found and so point to the high exophily of this anopheline (Table 1). With respect to An. bellator, the similarity of the results obtained from the catches at the two sites can be interpreted as a result of the more uniform behaviour of this mosquito, in accordance with its endophagic habit.

As for age composition, parity was practically limited to the uniparous condition, that is, the finding of only one dilation of the follicular stalk. The biparous condition appeared rarely, in no more than 0.1% of An. cruzii blood-seeking females, agreeing with the results, mentioned above, obtained in Paraná. These aspects contrasted notably with those reported in Colombia for An. neivai, a species of the same subgenus, which showed up to ten follicular stalk dilations (Murillo et al.¹⁴, 1989). It may be possible to suppose a higher level of domiciliar activity for this anopheline due to more intense local human presence than at the site of the present studies, where Kerteszia mosquitoes seem to preserve a higher degree of sylvatic behaviour.

It is recognized that under normal circumstances nulliparous anophelines will not develop ovaries beyond the Christophers and Mer Stage II without a blood meal. So the discovery of Stages III or above in nulliparous individuals means that these females had already taken at least one blood meal. As shown in Tables 4 and 5, of the total nulliparous and parous females examined, 557 (14.2%) had ovaries that had advanced to Stage III or above. Of that total, 476 (12.1%) were An. cruzii and 81 (2.1%) An. bellator. The significant difference found between nulliparous and parous allows one to consider the presence of Stages III to V in nulliparous females elevated, higher than in the parous ones. Thus it seems quite consistent to admit the hypothesis that these mosquitoes present a significant proportion of females with gonotrophic discordance, starting hematophagy early, before their first egg-laying. As it is possible to infer from the results obtained here, information has been obtained that might be useful in the assessment of south Brazilian Kerteszia vector capacity. Concerning longevity, the data yield knowledge of the age composition of the human-blood-seeking females. This initial phase of research only obtained information about general parity conditions and parameters of endophagy. Endophagic An. bellator, in spite of its relatively smaller density on human bait, shows evident endophilic behavior. Regarding the condition of completely dominant nulliparity, the non-negligible rates of nulliparous females with ovaries developed beyond Stage II of Christophers and Mer called our attention. So, the addition of these mosquitoes to the parous ones, leads to the conclusion that 1,493 (38.1%) of blood-seeking anophelines caught on human bait had taken at least one previous blood meal. By species, the rates were 34.8% for An. cruzii and 8.7% for An. bellator. These data strongly suggest that a significant part of that mosquito population need multiple blood feeding to develop a first clutch of eggs. This hypothesis leads to the supposition of an increased probability of malaria parasite infection and development in these vectors.

Recebido para publicação em 30.9.1992

Aprovado para publicação em 11.12.1992

Reprints: O.P. Forattini - Av. Dr. Arnaldo, 715 - 01246-904 -S. Paulo, SP -Brazil.

Edição subvencionada pela FAPESP. Processo Medicina 93/ 0208-5.

This article was received correction in agreement with the CORRIGENDA.

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