Rosa M. Sanchez-Casas

Global genetic diversity of Aedes aegypti

Mosquitoes, especially Aedes aegypti, are becoming important models for studying invasion biology. We characterized genetic variation at 12 microsatellite loci in 79 populations of Ae. aegypti from 30 countries in six continents, and used them to infer historical and modern patterns of invasion. Our results support the two subspecies Ae. aegypti formosus and Ae. aegypti aegypti as genetically distinct units. Ae. aegypti aegypti populations outside Africa are derived from ancestral African populations and are monophyletic. The two subspecies co‐occur in both East Africa (Kenya) and West Africa (Senegal). In rural/forest settings (Rabai District of Kenya), the two subspecies remain genetically distinct, whereas in urban settings, they introgress freely. Populations outside Africa are highly genetically structured likely due to a combination of recent founder effects, discrete discontinuous habitats and low migration rates. Ancestral populations in sub‐Saharan Africa are less genetically structured, as are the populations in Asia. Introduction of Ae. aegypti to the New World coinciding with trans‐Atlantic shipping in the 16th to 18th centuries was followed by its introduction to Asia in the late 19th century from the New World or from now extinct populations in the Mediterranean Basin. Aedes mascarensis is a genetically distinct sister species to Ae. aegypti s.l. This study provides a reference database of genetic diversity that can be used to determine the likely origin of new introductions that occur regularly for this invasive species. The genetic uniqueness of many populations and regions has important implications for attempts to control Ae. aegypti, especially for the methods using genetic modification of populations.

Chikungunya Virus as Cause of Febrile Illness Outbreak, Chiapas, Mexico, 2014.

Since chikungunya virus (CHIKV) was introduced into the Americas in 2013, its geographic distribution has rapidly expanded. Of 119 serum samples collected in 2014 from febrile patients in southern Mexico, 79% were positive for CHIKV or IgM against CHIKV. Sequencing results confirmed CHIKV strains closely related to Caribbean isolates.

First Report of Aedes aegypti Transmission of Chikungunya Virus in the Americas

During a chikungunya fever outbreak in late 2014 in Chiapas, Mexico, entomovirological surveillance was performed to incriminate the vector(s). In neighborhoods, 75 households with suspected cases were sampled for mosquitoes, of which 80% (60) harbored Aedes aegypti and 2.7% (2) Aedes albopictus. A total of 1,170 Ae. aegypti and three Ae. albopictus was collected and 81 pools were generated. Although none of the Ae. albopictus pools were chikungunya virus (CHIKV)-positive, 18 Ae. aegypti pools (22.8%) contained CHIKV, yielding an infection rate of 32.3/1,000 mosquitoes. A lack of herd immunity in conjunction with high mosquito populations, poor vector control services in this region, and targeted collections in locations of human cases may explain the high infection rate in this vector. Consistent with predictions from experimental studies, Ae. aegypti appears to be the principal vector of CHIKV in southern Mexico, while the role of Ae. albopictus remains unknown.

Detection of West Nile virus-specific antibodies and nucleic acid in horses and mosquitoes, respectively, in Nuevo Leon State, northern Mexico, 2006–2007

In the last 5 years, there has been only one reported human case of West Nile virus (WNV) disease in northern Mexico. To determine if the virus was still circulating in this region, equine and entomological surveillance for WNV was conducted in the state of Nuevo Leon in northern Mexico in 2006 and 2007. A total of 203 horses were serologically assayed for antibodies to WNV using an epitope‐blocking enzyme‐linked immunosorbent assay (bELISA). Seroprevalences for WNV in horses sampled in 2006 and 2007 were 26% and 45%, respectively. Mosquito collections in 2007 produced 7365 specimens representing 15 species. Culex mosquitoes were screened for WNV RNA and other genera (Mansonia, Anopheles, Aedes, Psorophora and Uranotaenia) were screened for flaviviruses using reverse‐transcription (RT)‐PCR. Two pools consisting of Culex spp. mosquitoes contained WNV RNA. Molecular species identification revealed that neither pool included Culex quinquefasciatus (Say) (Diptera:Culicidae) complex mosquitoes. No evidence of flaviviruses was found in the other mosquito genera examined. These data provide evidence that WNV is currently circulating in northern Mexico and that non‐Cx. quinquefasciatus spp. mosquitoes may be participating in the WNV transmission cycle in this region.

Natural transmission of dengue virus by aedes albopictus at Monterrey, Northeastern Mexico

Abstract. Dengue cases occur frequently at Nuevo Leon, Mexico, where Aedes aegypti (L.) and Ae. albopictus (Skuse) are present. Ae. albopictus is considered the second vector of dengue. Because it bites humans outdoors during the day, the mosquito plays an important role in transmission of dengue virus (DENV). However, no previous studies at Nuevo Leon indicated the role of the mosquito outdoors. To assess Ae. albopictus for dengue virus, mosquitoes were collected from April to October 2010 at five localities at Guadalupe and Santiago, Nuevo Leon, (Northeast) Mexico, by using two methods: engine backpack aspirator and ovitraps. In total, 1,836 Ae. albopictus and 833 Ae. aegypti mosquitoes were collected by ovitrap and engine backpack aspirator methods. Groups of mosquitoes were processed by RT-PCR. Examination for DENV infection of mosquitoes showed one positive group of four female Ae. albopictus from an ovitrap. This research provided information that showed transovarial transmission of dengue virus in Ae. albopictus occurred naturally, maintaining endemic levels of disease at a study site.

Mammalophilic feeding behaviour of Culex quinquefasciatus mosquitoes collected in the cities of Chetumal and Cancun, Yucatán Peninsula, Mexico

The studie describes the blood‐feeding behaviour of mosquitoes in Mexico, to understand host–vector relationships and dynamics of disease transmission.

Detection of Dengue Virus Serotype 2 in Aedes aegypti in Quintana Roo, Mexico, 2011

Abstract. In October 2011, the State Health Department announced that several laboratory-confirmed cases of dengue had occurred among residents in two neighborhoods of Benito Juarez, Quintana Roo State, Mexico. To identify the dengue virus serotype(s) temporally and spatially associated with the cases, entomologic-based virus surveillance was initiated in October 2011 in both neighborhoods. Adult mosquitoes were collected from 88 houses by CDC-backpack aspirator, and all female Aedes aegypti L. (n = 419) were individually homogenized and assayed in pools of as many as 10 by reverse transcriptionpolymerase chain reaction (RT-PCR) using dengue virus-specific primers. Five (12%) of 41 pools were positive for dengue virus RNA. The individual mosquitoes that comprised the pools were analyzed separately by RT-PCR using dengue virus serotype-specific primers. Six mosquitoes were positive for dengue virus serotype-2 (DENV-2) RNA, three of which were collected in the same house. The mean number of female Ae. aegypti collected in each house was 4.76 ± 6.19. The overall dengue virus-infection rate in female Ae. aegypti was 1.4%. Interestingly, most (60%) of mosquito females were collected only from 15 (17%) houses. In summary, we provide evidence of recent DENV-2 transmission in Quintana Roo State.

Aedes aegypti Mosquitoes at Nonresidential Sites Might be Related to Transmission of Dengue Virus in Monterrey, Northeastern Mexico

Abstract Traditionally the major risk environment for transmission of dengue virus has been assumed to be households. In Mexico, dengue outbreaks continue year after year despite intense control efforts. Nonresidential sites (public and private spaces) infested with Aedes aegypti (L.) were evaluated. In total, 141 nonresidential sites were sampled for the presence of potential and active oviposition sites and adult mosquitoes. Eighty percent of the sites were oviposition sites; Ae. aegypti adults were recovered at 94.7% of nonresidential sites. Most female Ae. aegypti, 21.6 and 10.4, were at schools and recreational sites, respectively. Chi-squared indicated no significant differences in the dengue vector to categories of sample sites (X2 = 17.76, df = 9, P = 0.38). Indoor-use patterns of adult mosquitoes indicated bathrooms and classrooms were preferred resting sites. Reverse transcription polymerase chain reaction (RT-PCR) assay did not identify dengue virus nucleic acids from a group of 221 pools containing 1,521 female Ae. aegypti. Daytime human activities; e.g., school and work, synchronize with the bimodal biting pattern of Ae. aegypti, increasing the chance of transferring dengue virus.

Detection of Aedes aegypti Mosquitoes Infected with Dengue Virus as a Complementary Method for Increasing the Sensitivity of Surveillance: Identification of Serotypes 1, 2, and 4 by RT-PCR in Quintana Roo, Mexico

Abstract. Sensitivity of monitoring Aedes aegypti (L.) populations was determined to identify the distribution of dengue virus (DENV) during epidemics in Quintana Roo. From September to November 2012, we used a motorized aspirator to collect 2,144 female Ae. aegypti from 569 homes. These were grouped into 220 to use semi-nested RT-PCR for DENV, and positive groups were analyzed individually. Five groups (2.27%) were positive for DENV. Individual analysis yielded eight groups that tested positive, six with DENV-2, one DENV-1, and one DENV-4. The latter was not reported by the surveillance system that year. The mean number of female mosquitoes per household was 3.77 ± 5.71, and the rate of viral infection of Ae. aegypti was 0.4%. Most infected mosquitoes (49%) were concentrated in 10% of the houses. Monitoring Ae. aegypti infected with DENV has the potential to complement the current system of clinical and entomological surveillance.

West Nile Virus Survey of Birds, Horses, and Mosquitoes of the Pacific Coast, Southern Mexico

Abstract. Serology of West Nile virus vectors and non-human reservoirs was surveyed at Acapulco, Jose Azueta, and Ometepec, three Pacific Coast localities of Guerrero State, Mexico. The objectives of this study were to use enzyme-linked immnosorbent assay (ELISA) to assess West Nile virus antibodies of bird and equine serum samples and use reverse transcription of polymerase chain reaction (RT-PCR) to detect the virus in field-collected resting mosquitoes. Forty birds trapped using mist nets yielded 10% seroprevalence. Similarly, 18.6% of 102 equine blood samples had West Nile virus. In addition, 4,854 mosquitoes were caught using motorized backpack aspirators and grouped into 116 pools. Of the 16 species and seven genera, no mosquito was positive for West Nile virus. Our study demonstrated West Nile virus seroprevalence on resident birds and equines in Guerrero State, Mexico.