Ecological aspects of mosquitoes (Diptera: Culicidae) in the gallery forest of Brasília National Park, Brazil, with an emphasis on potential vectors of yellow fever

INTRODUCTION

Mosquitoes have been widely studied because of their role in the transmission of tropical diseases, such as malaria, dengue and yellow fever¹. According to Mathers et al.², infectious diseases transmitted by mosquitoes have a high impact on public health and are responsible for approximately 15% of all disability adjusted life years (DALYs) attributed to infectious and parasitic diseases throughout the world.

Yellow fever (YF) is an acute febrile illness that results from infection by an arbovirus of the genus Flavivirus ³ that remains endemic or enzootic in Africa and South and Central America^1,4,5. In the urban cycle of YF, humans are the only known vertebrate host of the virus, and it is transmitted by the vector Aedes (Stegomyia) aegypti Linnaeus, 1762⁶. In contrast, sylvatic YF is primarily associated with nonhuman primates (NHP), and its vectors are female mosquitoes of the genus Haemagogus Williston, 1876, Sabethes Robineau-Desvoidy 1827 and Aedes Meigen 1818 in South and Central America^7,8.

According to Tauil⁹, Brazil is the country with the largest endemic area of sylvatic YF in the world, at approximately 5 million km². Between the years 2000 and 2009, a total of 320 human cases of YF, including 152 deaths, were recorded in 15 states. Between 2007 and 2009, outbreaks of sylvatic YF and epidemics in the NHP population occurred in several Brazilian states, primarily in the extra-Amazonian region^10,11.

During the last outbreak (2007-2008) in the Federal District (FD), 90 human cases of YF were suspected, 15 of which were confirmed to be sylvatic YF: two were in 2007 (one was autochthonous) (SINAN-net), and 13 were in 2008 (six were autochthonous)¹². During the same period, the deaths of several NHP cases were reported, three of which were attributed to YF¹¹. Entomological investigations discovered mosquitoes related to the transmission of this arbovirus, and the YF virus was isolated from Haemagogus (Haemagogus) janthinomys Dyar, 1921 and Haemagogus (Conopostegus) leucocelaenus Dyar & Shannon, 1924¹³.

The Brasilia National Park (BNP) is a tourist destination that receives approximately 260,000 visitors a year. This preservation area contains populations of NHPs¹⁴ and mosquitoes of the genus Sabethes and Haemagogus^13,15. It is noteworthy that during the outbreak of sylvatic YF that occurred between 2007 and 2008 in the FD, multiple NHPs died in the BNP^13,15. Therefore, it is important to understand the ecological aspects of the mosquito species in the BNP, with an emphasis on the YF potential vectors. Moreover, the detection of flaviviruses in mosquitoes captured in this conservation unit is important information for the surveillance of this arbovirus in the FD.

In this context, the objectives of the present study were the following: I) to analyze the richness and abundance of the mosquitoes captured during different seasons in different strata of the gallery forest of the BNP, with an emphasis on potential YF vectors; and II) to determine the percentage of the captured mosquitoes that are infected with a flavivirus.

METHODS

RESULTS

Between September 2010 and August 2011, 2,677 female mosquitoes belonging to the Culicinae (n=2,607, 97.4%) and Anophelinae (n=70, 2.6%) subfamilies were captured in the BNP. The Culicinae fauna included specimens of 10 genera and 25 species (Table 1). In total, 1,848 (69%) mosquitoes were captured at ground level.

According to the species accumulation curve (Figure 1), the overall sampling effort (60 days) was satisfactory to represent the species richness, indicating a trend towards stabilization of the number of accumulated species from the 54^th day.

FIGURE 1 – Species richness curve of mosquitoes captured between September 2010 and August 2011 at Brasilia National Park, Federal District, Brazil. 

The most abundant species were Sabethes (Sabethes) albiprivus Theobald, 1903; Limatus durhamiiTheobald, 1901; Haemagogus (Conopostegus) leucocelaenus Dyar & Shannon, 1924; Haemagogus (Haemagogus) janthinomys Dyar, 1921; Aedes (Ochlerotatus) scapularis Rondani, 1848; Psorophora (Janthinosoma) ferox Von Humboldt, 1819; and Aedes (Ochlerotatus) serratus Theobald, 1901. Together, these seven species accounted for 86.4% of the total number of mosquitoes captured. The other species were represented by few individuals, and six species were represented by a single specimen (Figure 2).

FIGURE 2 – Abundance distribution curve of mosquitoes captured between September 2010 and August 2011 at Brasilia National Park, Federal District, Brazil. 

The seven most abundant species were also classified as constant. Five species were classified as accessories, and 17 others were classified as accidental. It was also observed that, in terms of dominance, only three species were eudominant (Sa. albiprivus, Li. durhamii and Hg. leucocelaenus). Another three were dominant (Hg. janthinomys, Ae. scapularis and Ps. ferox), four were subdominant (Ae. serratus, Cq. arribalzagae, Anopheles kompi and Sa. belisarioi), two were classified as eventual and all of the others were considered rare (Table 1).

*Dom: dominance; **Const: constancy; Rr: rare (D<1%); Ac: accidental (C<25%); D: dominant (D>5-10%); C: constant (C>50%); Sd: subdominant (D>2-5%); A: accessory (C>25-50%); Ev: eventual (D>1-2%); E: eudominant (D>10%).

With respect to the monthly distribution, the mosquito abundance was higher during the first and second quarter of rainfall. The largest number of mosquitoes at both the canopy and ground levels was captured during the first quarter of rainfall (Table 2).

The highest diversity (H’=0.961), evenness (J=0.671) and richness (N=27) of mosquito species were detected at ground level. The Berger-Parker dominance index was similar for the ground (D_bp=0.034) and the canopy (D_bp=0.035). For the different rainfall classes, we observed a greater richness during the second quarter of rainfall (N=21). The greatest evenness was observed during the intermediate period (J=0.820), and the highest level of dominance was observed during the dry season (D_bp=0.147).

The number of captured specimens differed statistically according to rainfall period for Li. durhamii(Kruskal-Wallis H 3,60=35.2, p<0.01), Hg. leucocelaenus (Kruskal-Wallis H_3.60=41.1, p<0.01), Hg. janthinomys (Kruskal-Wallis H_3.60=37.6, p<0.01), Sabethis albiprivus (Kruskal-Wallis H_3.60=37.4, p<0.01), Ps. ferox (Kruskal-Wallis H_3.60=18.6, p < 0.01) and Ae. serratus (Kruskal-Wallis H_3.60=17.5, p<0.01). These species were more abundant during the rainy quarters. During the dry season, the potential YF vectors exhibited very low frequency and abundance, with the exception of Ae. scapularis andAe. serratus.

Significant differences were detected between the occurrence of mosquitoes on the ground and in the canopy for the species Hg. janthinomys (χ²=4.9, p=0.02), Li. durhamii (χ²=4.0, p=0.04), Ps. ferox (χ²=3.9, p=0.04), Ae. scapularis (χ²=12.1, p<0.01) and Ae. serratus (χ²=24.4, p<0.01). Except for Hg. janthinomys, these species were more frequent on the ground. With respect to the monthly abundance of these mosquitoes, it was observed that Hg. janthinomys was more abundant in the canopy, in contrast to the other species (Figure 3).

FIGURE 3 Number of mosquitoes of species that are potential vectors of YF in Brasilia National Park, Federal District, Brazil, on the ground and in the canopy between September 2010 and August 2011. Only the most abundant species (>100 individuals captured) are listed. Hg.: Haemagogus; Ae.: Aedes ; Sa.: Sabethes; Ps.: Psorophora; YF: yellow fever. 

None of the 302 batches of mosquitoes (N=2,677) sent to the Virology Laboratory-LACEN/DF for arbovirus isolation tested positive for flaviviruses.

DISCUSSION

The present study shows that the abundance and species richness of mosquitoes in the gallery forest of the BNP are higher on the ground and during the rainy season. Even species with recognized acrodendrophilic habits, such as Hg. janthinomys, were captured on the ground; these results are similar to those of Alencar et al.²⁸ and Ramirez et al.²⁹. These results indicate that precipitation is a crucial factor explaining the richness and abundance of Culicidae fauna in the gallery forests of the Cerrado, as has already been observed in other Brazilian biomes^8,30,31.

Among the species identified in the BNP, 11 have been previously found to be naturally infected with the YF virus: Ae. aegypti, Ae. scapularis, Ae. serratus, Ps. albipes, Ps. ferox, Hg. leucocelaenus, Hg. janthinomys, Hg. tropicalis, Sa. chloropterus, Sa. soperi and Sa. albiprivus ^{13,20,32–36} . Moreover, the natural infection of Hg. janthinomys and Hg. leucocelaenus by the YF virus has been previously reported in the administrative region of São Sebastião, FD¹³. However, none of the 302 batches of 2,677 mosquitoes examined were infected with flaviviruses. This may be because of a low rate of infection among primates in the BNP, as this work was conducted during the interepidemic period. Furthermore, the infection of mosquitoes by the YF virus tends to be low, even in areas of YF outbreaks. The results of Cardoso et al.³⁴reinforce this hypothesis, showing that Hg. leucocelaenus and Ae. serratus in YF outbreak areas have minimum infection rates of 3.70 and 1.88, respectively.

Three specimens of Ae. aegypti were detected in the gallery forest of the BNP. The occurrence of this species in urban environments has been widely reported³⁷; however, the occurrence of Ae. aegypti in sylvatic environments is rare. Barbosa et al.³⁸ and Soares et al.³⁹ reported the presence of this species in a rural area in the states of Amazonas and Rio de Janeiro. Tauil⁶ highlights the difficulty of identifying whether some cases of YF are transmitted by sylvatic or urban vectors. Considering that the sampling areas were located near recreational areas in the BNP, it is possible that the specimens of Ae. aegypticaptured in the gallery forest were from artificial breeding sites. Future monitoring studies of mosquitoes in gallery forests may better clarify the occurrence of Ae. aegypti in sylvatic environments.

Environments that are still considered sylvatic are strongly influenced by the rapid urbanization process, in which population and housing growth generate anthropogenic changes that contribute to the establishment of mosquito species that are better adapted to the human environment⁴⁰. The FD has undergone considerable environmental changes caused by the expansion of its urban area, which can change the behavior of the mosquito species present in the BNP. The occurrence of species that are adapted to urban anthropogenic environments, such as Ae. aegypti and Culex (Culex) quinquefasciatus Say, 1823, could be attributed to ecological changes that occurred with the construction of the Setor Habitacional Noroeste.

Aedes albopictus was also found during the survey. The ability of this species to transmit arboviruses has already been demonstrated^41–43. It is noteworthy that Ae. albopictus has been documented as naturally infected with the virus that causes eastern equine encephalitis^44,45 and with dengue virus during an outbreak that occurred in Mexico⁴⁶. Some authors admit the possibility that Ae. albopictus may become a vector that connects sylvatic and urban cycles of YF in Brazil^20,39,47. Gomes et al.⁴¹ and Albuquerque et al.⁴⁸ point to the need to monitor this species in Brazil.

There are few studies related to the vertical distribution of mosquitoes in the Central-West Region of Brazil⁴⁹. In the present study, we detected a higher occurrence of Hg. janthinomys in the canopy, reinforcing the known acrodendrophilic behavior of this species^29,50. Specimens of Hg. leucocelaenus were very common on the ground. Forattini et al.⁵¹ in São Paulo and Pinto et al.⁵⁰ in Pará also reported the predominance of Hg. leucocelaenus on the ground. However, Guimarães et al.⁵² showed that 74% of Hg. leucocelaenus specimens were captured in the canopy at the National Park of the Serra dos Orgãos, Rio de Janeiro. Ae. scapularis and Ae. serratus showed significant differences in habitat use and were more abundant on the ground. The same preference for these species to occur at ground level has also been observed by Fe et al.⁵³, Forattini et al.⁵¹, Guimaraes et al.⁵² and Julião et al.³⁰. Moreover, the highest occurrence of Ps. ferox at ground level was also observed by Forattini et al.⁵¹.

There is evidence that vertical distribution differences between and within species depends upon the type of vegetation cover. Forattini et al.⁵¹ suggested that a vertical distribution difference is most evident in tropical rain forests, where the trees are tall and dense. In less dense forests and in places with long dry seasons, these differences tend to decrease, as observed in the present study.

Other important factors influencing the vertical distribution of mosquitoes are vertical mobility and daily activity. According to Service⁵⁴, mosquitoes can be attracted to a feeding source at a distance of seven to 30 meters. Forattini et al.⁵¹ and Vasconcelos et al.⁵⁵ showed that even species that were predominantly active in the higher levels of the canopy, such as Hg. janthinomys, often move to ground level when they detect the presence of a feeding source. Moreover, Guimarães et al.⁵² showed differences in the vertical distribution of An. cruzii captured at different periods of the day, with the highest number of specimens found in the canopy at night. Therefore, the vertical movement of mosquitoes can weaken the evidence of vertical stratification of species, and it obviously influences the dynamics of pathogen transmission³⁰.

The effects of rainfall on the total abundance of mosquitoes captured were evident. There was a strong influence of rainfall on the annual cycle of the species of Sabethes and Haemagogus, which have eggs that are very resistant to desiccation and sometimes require repeated contact with water to hatch²⁰. Similar situations related to these genera have been described in Mato Grosso³¹ and Tocantins⁵⁶. Studies in southern Brazil10 and in Trinidad⁵⁷ also indicated a higher occurrence of Hg. leucocelaenus during the rainy months. The highest occurrence of A. scapularis, A. serratus and Ps. ferox in the rainy season is also in agreement with the literature²⁰.

The present study increases the information available regarding the ecology of important YF virus vectors in Central Brazil. Although seroepidemiological surveys indicate that YF control efforts have achieved good results⁵⁸, entomological monitoring is recommended in the BNP and in other areas that are susceptible to the transmission of the YF virus. We emphasize that the BNP is located 10km from the center of Brasília and 2km from the Setor Habitacional Noroeste, where many apartments are being constructed. Furthermore, there are houses located at the edge of a roadway that is very close to the park. Because the flying range of YF vectors can reach 500m⁵⁹, there is the possibility of sylvatic YF transmission to people who reside close to the BNP and to park visitors. This transmission would be more likely during periods of NHP epidemics, when the natural infection rate of mosquitoes tends to be higher³⁴. Although no viral isolates were obtained from the specimens captured in this study, studies related to the ongoing natural infection of mosquitoes is of paramount importance as a means of predicting events related to YF and other arboviruses. Such information will contribute to a better understanding of the true role of the vector species that occur within Central Brazil. It is also recommended that the relevant agencies give special attention to the deaths of NHPs and to the vaccination programs against YF for visitors to the BNP.