The present study showed that the occurrence of synanthropic triatomines in the state of Bahia has changed after 44 years of systematized control actions were initiated in 1975. P. megistus was the predominant species between 1957 and 1971. After the certification of interruption of Chagas disease transmission by T. infestans in 2006, T. sordida and T. pseudomaculata became the most frequent species in Bahia.
Bahia was the last Brazilian state to receive PAHO’s certification, probably due to specific T. infestans identification errors and appearance of new records of this species in the study area . Nine species biogeographically unlikely to be found within households in Bahia [25, 33] were recorded by the surveillance service: Belminus laportei, Eratyrus mucronatus, Microtriatoma trinidadensis, Panstrongylus tupynambai, P. lignarius, Rhodnius prolixus, Triatoma maculata, T. circummaculata, and T. rubrovaria. To reduce taxonomic identification errors, a guide was developed  and, more recently, a guidance manual to the surveillance service of triatomines in Bahia, with identification keys, diagnosis, and distribution of vector species .
In general, there was a reduction in the occurrence of the main T. cruzi vector species in the 1960s and 1970s. In period A, T. infestans were found in 9% of the municipalities and represented 3.7% of collected triatomines while P. megistus was found in 42% of the municipalities, with a relative abundance of 62% of collected triatomines . In period B, T. infestans were only identified in seven municipalities among residual colonies [34, 35], representing 0.4% of collected triatomines. The occurrence of P. megistus has greatly reduced in the state of Bahia, especially in the region of Recôncavo Baiano and the metropolitan region of Salvador. Its occurrence has also reduced in other Brazilian states [11, 36-38]. Three hypotheses could explain this reduction in the metropolitan region of Salvador: (a) Chemical control was successfully performed over four decades resulting in the elimination of domestic populations of P. megistus; (b) Intense urbanization on these municipalities resulted in deforestation and fragmentation of Atlantic forest areas, which is the natural habitat of the species ; (c) Housing improvement, with progressive depletion of adobe houses [39,40], a favorable environment for P. megistus colonization. Elimination of T. infestans in several municipalities in Western and Central Bahia can be explained by the T. infestans elimination plan, which was intensified in 2004 with actions of household spraying of insecticides, followed by research and capture of triatomines, completed in about 500,000 homes . However, residual colonies of the species were detected in other municipalities [28, 29] which requires constant monitoring to definitively eliminate T. infestans from the state of Bahia. Thus, we suggest that surveillance and triatomine control activities should be prioritized in these regions.
In addition to the chemical control performed in Bahia since 1975, intensified in 1991, which aimed to eliminate T. infestans [11, 15], other social actions were implemented by the Federal Government such as the growth acceleration program intended to improve housing quality, with the replacement of mud-houses by brick-houses. It may also have influenced the reduction of household colonization by T. infestans and by other household species [39, 40].
Despite the success of actions to control triatomines, several other native species were recorded in a larger number of municipalities. They were captured in the household and frequently colonized peridomestic structures. Among them, some were infected and many were being fed by domestic animals and even human blood . In period A, T. sordida was recorded in 26% of the municipalities, representing 23% of collected triatomines, while T. pseudomaculata was recorded in 14% of the municipalities with a relative abundance of 2.5% of collected triatomines . In period B, T. sordida was recorded in 38% of municipalities, representing about 82% of triatomines. T. sordida has been the most common species among different regions of the state of Bahia. In a similar situation, T. pseudomaculata was the species that were identified in the largest number of state municipalities in period B (42%). The systematic vector-control actions had a very low impact on the spatial distribution of T. sordida and T. pseudomaculata in the state of Bahia. There was a higher occurrence of T. sordida in Western and South Central Bahia, where there are areas of Cerrado, which is the origin biome of this species’ natural populations [1,2,25]. The highest occurrence of T. pseudomaculata in Western and Central Bahia is in accordance with the occurrence predictions of the species based on environmental variables . The number of specimens of T. sordida and T. pseudomaculata together exceeded 90% of all triatomines collected in the state between 2006 and 2019.
It was observed a higher occurrence of T. brasiliensis and records of similar species in Bahia, expanding observations of Ribeiro-Jr et al. . Before systematized control actions, T. brasiliensis was registered in 6% of the municipalities of the state of Bahia, and between 2006 and 2019 at least 93 municipalities (22%) registered this species. In the last few years, other species of the T. brasiliensis complex have been described in the state of Bahia, such as T. juazeirensis, T. melanica, T. sherlocki, and T. petrocchiae . Among these species, we highlight T. juazeirensis since it was collected predominantly inside the household units. Other species such as T. tibiamaculata, P. geniculatus, T. melanocephala, and P. lutzi were detected in the households, mainly the adults.
There was an important difference in the occurrence of T. tibiamaculata between the two periods. In period A, this species was recorded in about 1% of the municipalities with a relative abundance of 0.01%  and in period B it was recorded in 4% of the municipalities with a relative abundance of 0.55%. In recent decades, T. tibiamaculata, which is naturally found in the nests of marsupials, rodents, and epiphytes in forests [1,22], has been recorded in peridomestic palms and inside houses, offering risk of T. cruzi transmission in the city of Salvador .
The present study has some limitations. The use of reference databases does not allow a broad analysis of the data; it was not possible to obtain data on the occurrence of triatomines within habitats (intra and peridomestic) in period A (1957-1971). Moreover, not all 417 municipalities collect data regularly in period B; the health surveillance service classifies municipalities in high, medium and low risk of transmission, emphasizing that there is no obligation to conduct regular entomological research in low-risk municipalities, based on the classification presented by Brazil’s Ministry of Health . Future studies reassessing risk classification at a municipal level are urgent for the redefinition of areas and risk of transmission. Further studies should analyze the situation of these silent areas to explain whether the absence of triatomines is due to the functioning of the service or biogeographic issues related to triatomines. In period A, species of T. brasiliensis complex were considered as one species - stricto sensu - while in period B T. brasiliensis complex was classified into six species: T. brasiliensis, T. juazeirensis, T. melanica, T. lenti, T. bahiensis, and T. petrocchiae .
This study concludes that there was a change in the occurrence of triatomines between the analyzed periods, 40 years after the implementation of the vector-control program in the state of Bahia. We observed a reduction of the occurrence of P. megistus and T. infestans and an increase of relative abundance and geographic distribution of T. sordida and T. pseudomaculata. Changes in the occurrence of T. cruzi main vectors between the studied periods indicate the need for continuity of entomological surveillance actions of Chagas disease. Also, it highlights the importance of promoting and the strengthening of surveillance, community participation, and health education actions of Chagas disease, in addition to new strategies to control triatomines.