An updated review of the invasive Aedes albopictus in the Americas; the minimum infection rate suggests that is more efficient in the vertical than horizontal transmission of arboviruses

The aim of the work was to update the distribution range of Aedes (Stegomyia) albopictus Skuse in the Americas, review the blood feeding patterns and compare the minimum infection rate (MIR) between studies of vertical and horizontal transmission of arboviruses. The current distribution of Ae. albopictus encompasses 21 countries in the Americas. In eleven published papers on the blood feeding pattern of Ae. albopictus, DNA from 16 species of mammals and five species of avian species was found. The most common host is humans, and dogs. We found 24 published papers on the identification of arboviruses in wild populations of Ae. albopictus with the potential to infect humans and animals. Eight arboviruses have been isolated in different studies carried out in Brazil, USA, Mexico, Colombia, and Costa Rica. Fifty-eight percent (14/24) of the publications reported vertical transmission of arbovirus. Positive pools were higher in vertical (8.45%) compared to horizontal transmission (0.97%). This was supported by the MIR, which was 3 times greater in vertical (MIR=3.21) than horizontal transmission (MIR= 1.08). In conclusion, Ae. albopictus is an invasive mosquito with wide phenotypic plasticity to adapt to broad and new areas, high vectorial competence to transmit several arboviruses mainly by transovarial transmission, it can participate in the endemic transmission, and serve as a bridge vector for emerging arboviruses between sylvan, rural, and urban areas.


Introduction
Aedes (Stegomyia) albopictus Skuse is a mosquito native to Southeast Asia, colloquially known as the Asian tiger mosquito or Asian mosquito. The mosquito was described by Skuse (1894) in the city of Calcutta, India [1,2]. At the beginning of 2000´s, its importance as a vector of arboviruses was restricted to Asian and African countries [1]. Currently, Ae. albopictus is present on all continents except Antarctica [3]. It has been observed that once established in new geographic areas, it is capable of becoming involved in the natural cycles of arbovirus transmission. For example, in Europe it has colonized several countries and was involved in dengue outbreaks in France, Italy, and Spain [4][5][6]. In Italy, the genome of the chikungunya virus was identified in Ae. albopictus and it was incriminated as the vector that caused the local outbreaks of chikungunya fever [7]. Likewise, autochthonous cases of Zika fever occurred in France and Ae. albopictus was suspected as the transmitter of the virus [8]. Recently, in Brazil, Yellow Fever virus (YFV) was detected in females mosquitoes in Rio do Janeiro State [9]. Based on the background, the mosquito is considered a species with the potential to increase the risk of arbovirus both in urban and sylvan cycles transmission in the Americas.
Arboviruses of medical and veterinary importance have been isolated in wild populations of Ae. albopictus [10][11][12][13][14][15][16][17][18][19]. Notably, the Asian mosquito has a great capacity to acquire arboviruses and transmit them to its offspring. The findings of transovarian transmission have been consistent and very frequent [21,22]. Studies carried out in North and South America have found the dengue (all serotypes), Zika and La Crosse viruses in larvae and males of Ae. albopictus [20,[23][24][25][26][27][28][29][30][31][32][33]. Evidence suggests that the mosquito may have a reservoir role for the dengue virus by keeping it silent in nature [21]. In Brazil, the detection of DENV-3 in males of Ae. albopictus was carried out in years in which no autochthonous human cases with this serotype were recorded, suggesting that the silent circulation of DENV-3 occurs by a vertical transmission mechanism [33]. Additionally, also in Rio do Janeiro State, YFV was isolated in Ae. albopictus females, which could imply that it could be acting as an additional jungle or rural vector causing a possible transmission bridge to the urban area [9].
Since 1985, when it became known that Ae. albopictus had colonized the state of Texas, EE. UU., 36 years have passed since its introduction in America. Despite the importance as a vector of arboviruses, few studies have evaluated the vectorial capacity (e.g., gonotrophic cycle length, daily survival probability, parity index and the proportion of bites made by females on humans). Studies on the blood feeding pattern of Ae. albopictus have been carried out in the Unites States of America and Brazil [14,[34][35][36][37][38][39][40][41][42][43]. The results indicate that it is an opportunistic mosquito. DNA from humans and a diverse range of wild and domestic animals have been identified in the blood meal of the mosquito [36,37,39,43].
Here, we update the distribution range of Ae. albopictus in the Americas, review the blood feeding patterns, and compare the minimum infection rate (MIR) between studies of vertical and horizontal transmission of arboviruses.

Selection criteria and search strategy
The analysis only included works carried out in the Americas (north, south, central and the Caribbean), with topics focused on the first report of Ae. albopictus from each American country, blood feeding patterns and reports of natural infection with arbovirus.
Databases of Google Scholar, PubMed Health (National Center for Biotechnology Information at the National Library of Medicine), SciELO (Scientific Electronic Library Online), and Web of Science (Thompson Reuters) were used for the literature review. The search was done with combination of keywords including "Aedes albopictus" AND "first report", "first record", "new records", "blood meal", "feeding pattern", "arbovirus" " dengue", "Zika", "chikungunya", "America". Additional references were facilitated by colleagues.
Importation of references and removal of duplicate references were done using the bibliographical software package, Mendeley version 1.19.8 (Elsevier, Amsterdam, Netherlands). All titles, abstracts and selected full reports were screened independently by two authors based on the inclusion and exclusion criteria. Discrepancies were resolved by consensus.
Test for the difference of proportions was used to compare the positive pools between studies with vertical and horizontal transmission cycle. The values of the minimum infection rate (MIR) of each study were extracted manually and organized in an excel sheet. When the work did not include the MIR, it was calculated with the formula: the ratio between the number of positive pools and the total number of mosquitoes tested, multiplied by 1000.
Host frequencies identified in blood meals of Ae. albopictus were extracted from each work and organized in an excel sheet. Statistical analyses were performed using R statistical programming language version 4.0.2 and results were considered statistically significant when p≤ 0.05.

Chronological order of the first reports of Ae. albopictus in the Americas
The current distribution of Ae. albopictus encompasses 21 of 44 countries in the Americas, although the colonization pattern is different in each country (Table 1). Chile and Peru being the only continental countries where it has not been reported to date. Previously, Kramer and collaborators [3] conducted a global compendium of the distribution of Ae. albopictus and described its presence in 16 countries of the Americas. According to reports, the mosquito has presented an erratic distribution, but with great rapidity in its movement through America. The introduction of Ae. albopictus in America was divided into four periods. In the first period (1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990), the Asian mosquito was reported in three countries. The first report occurred in 1983 in the EE.UU., when a single adult of Ae. albopictus was captured in a cemetery in Memphis, Tennessee [44]. Three years later, in Brazil (1986), five male and six female mosquitoes with similar characteristics to the Asian mosquito were captured and their identity was confirmed as Ae. albopictus [45]. In Mexico, the Asian mosquito was reported for the first time in 1988, the larvae were collected in tires [46]. In the second period (1993 to 1998), the Asian mosquito was reported in six countries including the Dominican Republic, Cuba, Guatemala, Cayman Islands, Colombia, and Argentina [47][48][49][50][51][52]. Reiter [44] mentions that Ae. albopictus was reported in Bolivia and El Salvador, but there are no reports that confirm it. Their presence in these countries is not currently recognized. In the third period (2000 to 2010), the mosquito significantly expanded its distribution to ten countries including Bermuda, Canada, Trinidad and Tobago, Panama, Uruguay, Nicaragua, Costa Rica, Venezuela, Belize, and Haiti [53][54][55][56][57][58][59][60][61][62]. In the fourth period (2011-2021), the presence of the mosquito was only reported in Ecuador in 2017 and in Jamaica in 2018 [63,64]. It is well documented that the introduction of Ae. albopictus into America occurred through tires and bamboo stumps from Japan. It is also hypothesized that the massive distribution of the mosquito occurred through the export of used tires between countries in the Americas, Europe and Asia [1,[44][45][46]. Within countries, automobiles are believed to contribute to the distribution [65].  [44] 1986 Brazil Captured five males and six females [45] 1988 Mexico Larvae collected in tires [46] 1993 Dominican Republic Larvae collected in tires [48] 1995 Cuba Larvae collected [47] 1995 Guatemala Larvae collected in tires, glass bottles, and metal drums. [49] 1997 Cayman island Larvae collected [50] 1998 Colombia Captured adults [51] 1998 Argentina Larvae and pupae collected [52] 2000 Bermuda Island Larvae collected [53] 2001 Canada Two adults captured [55] 2002 Trinidad and Tobago Eggs collected with ovitrap [56] 2002 Panama Larvae collected [54] 2003 Uruguay Adults captured [57] 2003 Nicaragua Larvae collected [58] 2007 Costa Rica Larvae collected [59] 2009 Venezuela Larvae collected [60] 2009 Belize Adults captured [61] 2010 Haiti Larvae collected [62] 2017 Ecuador Captured five males and sixteen females [64] 2018 Jamaica Six females captured [63] 4. Blood feeding pattern of Ae. albopictus In total, there are 11 published papers on the blood feeding pattern of Ae. albopictus; nine of them were carried out in the EE.UU. and two in Brazil. The first four studies used the serological precipitin test and ELISAs to identify the identity of the vertebrate hosts. Seven publications used PCR to identify host DNA. In total, 1,925 individual mosquitos were tested. In 85.56% (1,647/1,925) of the mosquitos, the host was identified at the species level, which comprised 16 species of mammals and 5 species of birds (Table 2). Despite the ability of Ae. albopictus to feed on the blood of different vertebrate taxa, 98.70% (1900/1925) corresponded to mammals. The human (Homo sapiens), the domestic dog (Canis lupus), the brown rat (Rattus norvegicus) and the domestic cat (Felis silvestris) are the most frequent hosts in the publications and with more specimens analyzed.
The frequency of blood feeding of Ae. albopictus on a particular host determines the risk of pathogen transmission. According to studies published mainly in the EE.UU., the Asian mosquito has an anthropophilic tendency, although in the absence of humans it can feed on 15 other species of mammals and five species of birds. The method and the place of capture of Ae. albopictus was decisive to identify DNA of hosts in the blood meals of the mosquito. Most females of Ae. albopictus with human blood were captured with the human bait method and aspirated from mosquitoes inside and outside the houses [35,37,39,41]. The other works captured Ae. albopictus in the forest or habitats with abundant vegetation. For this reason, the number of wild species in the blood meals of the Asian mosquito was very diverse [14,34,36,38,40,42,43]. In the EE.UU., the feeding frequency of Ae. albopictus on birds and wild mammals partly explains the isolation of zoonotic arboviruses such as VEEE, Keystone

Natural infections of Ae. albopictus with arboviruses
In the Americas there are 24 published papers on the identification of arboviruses in wild populations of Ae. albopictus with the potential to infect humans and animals. Ten of the findings were made in Brazil, six in the USA, four in Mexico, three in Colombia and one in Costa Rica ( Table 3) Table 3).
Notably, 66.66% (16/24) of the publications reported the genome of the dengue virus in the Asian mosquito, although in only four studies the presence was confirmed by viral isolation. In decreasing order, the most frequent serotypes in the publications are DENV-2 (n = 8), DENV-3 (n = 5), DENV-1 (n = 4) and DENV-4 (n = 3). On the other hand, in six studies carried out in Brazil (n = 4) and Mexico (n = 2), the Zika virus was identified in Ae. albopictus.
Aedes albopictus has wide distribution in America, despite this only in five countries has been reported natural infection of Ae. albopictus with arboviruses of medical and veterinary importance. Currently, eight arboviruses have been isolated in the Asian mosquito (Table 3) [66][67][68].
In 2013, the chikungunya virus (CHIKV) emerged in the Americas and caused local outbreaks of chikungunya fever. To date, no natural infection with this virus has been reported in Ae. albopictus [69,70]. Aedes albopictus is an efficient vector of the epidemic mutant strain CHIKV_0621 of the East-Central-South African (ECSA) genotype [71], which, caused autochthonous cases of CHIKV in Indian Ocean [72]. Today, the circulation of the mutant strain in America is not reported.
Aedes albopictus is most abundant in the forest than Ae. aegypti and may be involved in virus transmission in rural areas or urban places with a lot of vegetation. Evidence suggests that the Asian mosquito is an extremely important vector because once established it can participate in the transmission of local arboviruses. In United States of America (USA), Ae. albopictus is a competent vector of endemic arboviruses such as VEEE, Keystone virus, La Crosse Virus, West Nile virus, and Cache Valley virus [10][11][12]14,15,27]. On the other hand, more than 70% of the publications of Ae. albopictus naturally infected with the dengue and Zika viruses come from Brazil, Mexico, Colombia, and Costa Rica, which are dengue endemic countries and between 2014 and 2018 there was active transmission of the Zika virus. Notably, in nine out of ten studies carried out in Brazil, dengue (all serotypes), Zika and yellow fever viruses were transmitted by transovarial route. Future studies should focus on finding out if there is an evolutionary relationship of arbovirus adaptation with vertical transmission of Ae. albopictus.

The minimum infection rate estimated in vertical and horizontal transmission of arboviruses
Twelve studies reported only vertical transmission and ten only horizontal transmission. Two studies carried out in Costa Rica, and Colombia reported both types of transmission. First findings of dengue virus in the Asian mosquito were through vertical transmission. In Brazil, in 1993, DENV-1 was isolated from two pools of mosquito larvae. Two years later during a dengue outbreak in Mexico, DENV-2 and DENV-3 were isolated from a pool of 10 males of Ae. albopictus. In Brazil in 1999, DENV-3 was again identified in three larval pools (Table 3).
In many geographic areas of the Americas, Ae. albopictus occupies the same ecological niches as Ae. aegypti. It is difficult to incriminate the tiger mosquito as the cause of autochthonous arbovirus outbreaks [13,26]. In horizontal transmission, Ae. aegypti is considered the main vector of dengue, Zika and chikungunya viruses in American countries [1,69,70]. In contrast, the evidence suggests that Ae. albopictus plays a secondary role in horizontal transmission of dengue and Zika viruses but is very efficient in transmitting them to their progeny [21]. Notably, 11 out of 14 publications refer to transovarial transmission of dengue virus. This has several aspects; the dengue virus can remain and persist silently during inter-epidemic periods [21,33]. The dispersal of eggs and larvae of Ae. albopictus infected with dengue and Zika viruses can cause the emergence and re-emergence of arboviruses and modify the local epidemiological pattern [23,24,[27][28][29][30]32]. Transovarial transmission ensures the presence of arboviruses in Ae. albopictus regardless of blood feeding on viremic hosts. The occurrence of male mosquitoes infected by transovarial transmission suggests equal probability of infection of the females of the same batch. Females of Ae. albopictus would not have to go through the extrinsic incubation period to transmit the virus to humans, which would enhance the dynamics of dengue transmission [25]. In addition, serotypes and genotypes not associated with autochthonous outbreaks have been detected during transovarial transmission. In Brazil, genotype III of DENV-3 was detected in larvae of Ae. albopictus collected in 1999 [28]. However, DENV-3 (genotype III) was first isolated as an autochthonous case from a 40-year-old woman residing in Sao Paulo, Brazil [73]. Which suggests that this serotype was present in Brazil one year before its detection. Similarly, DENV-3 was detected in males of Ae. albopictus in years when no human autochthonous cases of this serotype were recorded in São Paulo, Brazil [33].

Concluding Remarks and Future Prospects
Despite the importance of Ae. albopictus as a vector and reservoir of dengue virus, few studies have evaluated the vectorial capacity in the Americas. Studies should focus on gonotrophic cycle length, dispersion range, daily survival probability, parity index and the proportion of bites made by females on humans. Likewise, in Asian mosquito populations, the susceptibility status and genes associated with resistance to insecticides used by local health services should be monitored. Finally, Ae. albopictus is an invasive mosquito with wide phenotypic plasticity to adapt to broad and new areas, high vectorial competence to transmit several arboviruses mainly by transovarian transmission, it can participate in the endemic transmission, and serve as a bridge vector for emerging arboviruses between sylvan, rural, and urban areas. According with the data that of the MIR, which was 3 times greater in vertical (MIR=3.44) than horizontal transmission (MIR= 1.08), this means that Ae. albopictus could be maintaining a viral cycle vertically, and also could be useful as a sentinel species to monitor dengue virus in inter-epidemic periods. .
Author Contributions: CMBB wrote the first draft of the manuscript with considerable assistance from JGR, NCT and JCN. All authors contributed to manuscript revision. All authors have read and agreed to the published version of the manuscript.
Funding: This study was funded by the Universidad Internacional SEK, Quito, Ecuador. Grant DII-UISEK P011617_2.