Serological Evidence of Dengue and West Nile Virus Human Infection in Juarez City,Mexico

Abstract

Arboviruses are significant causes of human and animal diseases, globally. In the Rio Grande Valley of the United States–Mexico border region, endemic transmission of Dengue (DENV), Zika (ZIKV), and West Nile (WNV) viruses have been documented as a cause of human disease. Otherwise, very little is known about the distribution of arboviruses and their possible cause of human disease in other areas of the United States–Mexico border region. Therefore, a pilot serosurvey was conducted to determine if there was evidence of DENV and WNV infection among a human cohort in Anapra, Ciudad Juarez, Mexico. Baseline blood samples were obtained from 78 participants during May–June, 2015 and from 60 of the same participants again during November–December, 2015, and all the samples were tested for DENV and WNV indirect immunoglobulin G antibodies by an enzyme-linked immunosorbent assay and plaque reduction neutralization test (PRNT). The results showed that 14.1% (n = 11) of the 78 participants had neutralizing antibody to DENV and 5.13% (n = 4) had WNV-neutralizing antibody. Among 48 of 60 participants who were negative for DENV and WNV antibody during the baseline survey, 10.4% (n = 5) had acquired antibody to DENV (n = 4) and WNV (n = 1) by the second survey during November–December, 2015. These data support the local transmission of DENV and WNV in the Anapra, Ciudad Juarez community and therefore warrant further epidemiological studies to better understand the dynamics of transmission of these viruses in this United States–Mexico border city.

Introduction

Dengue fever is caused by the mosquito-borne dengue virus (DENV), serotypes 1, 2, 3, and 4 of the Flaviviridae family, genus Flavivirus (Westaway et al. 1985). Among the mosquito-borne viruses, DENV is the most significant cause of human disease (Bhatt et al. 2013, Murray et al. 2013). An estimated 390 million DENV infections occur annually, of which about 96 million are symptomatic infection, with approximately 20,000 deaths. Most DENV infections are asymptomatic; however, the spectrum of illness may range from nonsevere to severe dengue that can progress to fatal dengue hemorrhagic fever and/or dengue shock syndrome and can be caused by any of the four DENV serotypes. (Burke et al. 1988, Rodriguez-Figueroa et al. 1995, Endy et al. 2002, Gubler 2011, Guzman et al. 2013). The primary vector of DENV is Aedes aegypti, whereas Ae. albopictus is the most important secondary vector, with both species being common throughout the tropics and subtropics regions (Gubler 1998a, 1998b, Hotta 1998, Gratz 2004).

All four DENV serotypes cause an estimated 40 million infections annually throughout most of Latin America, with about half of these in Brazil and Mexico (MX) alone (Bhatt et al. 2013, Burke et al. 1988, Rodriguez-Figueroa et al. 1995, Gubler 1998a, 1998b, 2011, Hotta 1998, Endy et al. 2002, Gratz 2004, Guzman et al. 2013). Since the recognition of the four serotypes in MX in 1995, the annual incidence of nonsevere dengue in MX has increased from 1.72/100,000 in 2000 to 14.12/100,000 in 2011, with a marked increase in the incidence at locations close to the border with the United States (U.S.) (CDC 1987, Díaz et al. 2006, Brunkard et al. 2007, Ramos et al. 2008, Carrillo-Valenzo et al. 2010). The reason for the increase is not fully understood, but most likely reflects the circulation of multiple dengue serotypes, an increase in clinical recognition, and the establishment of the national dengue surveillance program (Dantés et al. 2014).

All four serotypes of DENV are endemic in urban communities of the Rio Grande Valley (RGV) of Texas that borders Mexico. One or more serotypes have circulated on the Texas side of the border periodically since 1980, but fewer cases of dengue have been reported on the U.S. side than on the MX side (Hafkin et al. 1982, Malison and Waterman 1983, CDC 2007). Most of the cases have been reported during sporadic outbreaks of dengue primarily in the urban communities of Matamoros, MX and in the Brownsville, Texas (Brunkard et al. 2007, CDC 1996, Ramos et al. 2008, Thomas et al. 2016). The outbreaks on the Texas side were associated with concurrent dengue epidemics in Tamaulipas, MX, and the presence of Ae. aegypti and Ae. albopictus (Champion and Vitek 2014, Vitek et al. 2014). Outbreaks of dengue were also reported from 2007 to 2014 along the United States–Mexico border in Sonora, MX, with 93 travel-documented cases reported during 2014 in Yuma, Arizona with all infections apparently acquired in MX (Jones et al. 2016). Although Ae. aegypti inhabits the entire border region, with a sporadic distribution pattern for Ae. albopictus, DENV has not been reported from other communities in the border region (Hahn et al. 2016).

West Nile virus (WNV) is a zoonotic mosquito-borne virus of the family Flaviviridae genus Flavivirus (Simmonds et al. 2017). The virus is transmitted primarily by Culex species mosquitoes to a variety of wild avian species that serve as the virus amplifying host and to man and equine as dead-end hosts. Since the 1990 s, outbreaks of West Nile (WN) fever and encephalitis have occurred globally, and the virus is now enzootic in Africa, Asia, Australia, the Middle East, Europe, United States, Canada, and Central and South America. WNV was first recognized as a cause of human disease during 2002 in Texas and has since been the cause of annual epidemics with a total of 5254 cases from 2002 to 2016 (ArboNET 2016, Texas Department of State Health Services 2016). The first human cases of WN were reported during 2003 in El Paso, Texas, where WNV has repeatedly been isolated from Cx. quinquefasciatus and Cx. tarsalis, the primary vectors of this virus in the El Paso community (Cardenas et al. 2011, Mann et al. 2013). Estimates of cases based on passive surveillance indicated that 271 cases of WN occurred in El Paso from 2003 through 2016, with cases occurring each year during late June to early November, and peaking in August (Cardenas et al. 2011, ArboNET 2016, Gonzales, F, 2017, unpublished data).

The El Paso/Ciudad Juarez region is considered one of the largest binational metropolitan areas in the United States–Mexico border, where humans (12,258,192 pedestrians and 19, 982, 407 personal vehicle passengers) cross the border, annually (annual border crossing in 2015; available from URL: www.bts.gov/content/border-crossingentry-data). Ae. aegypti and Cx. quinquefasciatus mosquitoes were first reported during 2005 to be abundant in Ciudad Juarez (de la Mora-Covarrubias et al. 2008, de la Mora-Covarrubias et al. 2010). WNV has been isolated from Cx. quinquefasciatus, but evidence of WNV human infection and/or disease has not been reported from Ciudad Juarez, but eight cases of human WN disease have been reported from other areas of MX (CDC 2001, Blitvich et al. 2003, Estrada-Franco et al. 2003, Fernández-Salas et al. 2003, Elizondo-Quiroga et al. 2005, Rios-Ibarra et al. 2010, Rodríguez et al. 2010). The absence of WN cases in Ciudad Juarez and the low number of cases in other areas of MX may reflect underreporting because of limited surveillance and resources for testing human samples.

Anapra is one of the most impoverished communities in Ciudad Juarez with insufficient water supply, sanitation, and waste collection (Ruiz-Hernandez 2015). Therefore, water is stored in artificial containers that provide suitable mosquito breeding habitat close to human dwellings that pose a risk for DENV infection as reported previously in other United States–Mexico border communities in Brownsville and Matamoros (Brunkard et al. 2007). Previous studies in Anapra reported that a high population density of Cx. quinquefasciatus was correlated with the low quality of housing, income, and population density (de la Mora-Covarrubias and Granados 2007) and that dengue viral ribonucleic acid (RNA) was detected in Ae. aegypti mosquitoes in this community (de la Mora-Covarrubias et al. 2010). However, since there has not been any reported association of WNV and DENV with humans, this study was conducted to determine if DENV and WNV were causing human infection and/or disease in Anapra, Ciudad Juarez, a United States–Mexico border community.

Materials and Methods

Study site

Ciudad Juarez has an area of 188 km² and is located in the state of Chihuahua, MX. The estimated population size is 1.32 million inhabitants (INEGI 2014). The climate is arid with annual mean temperature of 18.3°C reaching extreme temperatures (41°C) during the summer season. The rainy season extends from July to September, with an annual precipitation of 264.5 mm (INEGI 2015). Anapra is a neighborhood in Ciudad Juarez with an estimated population of 16,990 inhabitants, most of whom lack sanitary services (water and sewer). Also, most residents work in manufacturing plants, the main economic activity of the community.

A serosurvey was performed for DENV and WNV antibodies among a convenience subsample of humans in Anapra between May and June 2015 and between November and December 2015, or during the peak activity of Ae. aegypti and Culex mosquitoes. One member of each family among 87 households was selected to participate in the survey. The first family was chosen from the geographic center (centroid) of the Anapra neighborhood. Subsequent households of separate families located between 100 and 200 meter radius were selected according to accessibility, security, and presence of inhabitants (de la Mora-Covarrubias and Corral-Díaz 2011). The survey was explained to all the household members and one member per house was invited to participate in the study. The inclusion criteria included household participants >18 years of age. One member of each of the 87 houses agreed to participate in the study. When possible, two blood samples were collected from each of the participants. The first blood sample was used as a baseline to determine the DENV and WNV antibody prevalence rate and the second sample was used to measure the incidence of seroconversion to these viruses as evidence of infection. All the participants signed a consent form approved for this survey by the Ethics Committee at the Institute of Biomedical Sciences of the Autonomous University of Ciudad Juarez, and all except nine participants completed a questionnaire to provide demographic information and any history of travel outside of Ciudad Juarez. These nine participants were excluded, and therefore a total of 78 participants were included in the study.

Blood samples were obtained by venipuncture using standard aseptic techniques with a vacutainer collection tube containing an anticoagulant. All samples were collected from each of the participants in their homes, and then placed in an Igloo container on ice packs and transported to the laboratory. Samples were centrifuged at 1200 Gs at 4°C for 10 min. The plasma samples obtained were transferred to sterile vials and stored at −20°C until tested for immunoglobulin G (IgG) and neutralizing antibodies.

Indirect IgG enzyme-linked immunosorbent assay

Plasma samples were tested by an indirect enzyme-linked immunosorbent assay (ELISA) for IgG antibodies to DENV and WNV using lysate of infected Vero cells as an antigen coated to the bottom of the wells of 96 microtiter plates as previously described (Ansari et al. 1993). Each plasma sample was diluted 1:100 in blocking buffer (5% skim milk, 1%Tween, in phosphate-buffered saline [PBS] 1X pH 7.4) and tested in duplicate against a pool of DENV antigens (DENV1–DENV4) or WNV antigen and uninfected lysate cells were used as control antigen. Then, 96-well microplates coated with the cell lysates (100 μL) were incubated overnight at 4°C. The next day, microplates were washed with PBS 1× Tween 0.1% and the diluted plasma samples (100 μL/well) were added. Then, 100 μL of a secondary antibody (Horseradish peroxidase [HRP]-conjugated mouse anti-Human IgG) was added to each well of the 96-well microplates, followed by the addition of a colorimetric substrate ABTS (2,2'-Azinobis [3-ethylbenzothiazoline-6-sulfonic acid]-diammonium salt). After incubation for 30 min, the optical density (OD) values at 410 nm were recorded. The cutoff OD value was calculated as the mean of six antibody-negative controls plus three times the standard deviation of the negative plasma samples (Ansari et al. 1993). Plasma samples diluted at 1:100 with an OD higher than the cutoff value were considered antibody positive and then were retested at dilutions ranging from 1:100 to 1:6400 to determine the IgG antibody titers. Negative and positive control antibodies had a titer lesser than 1:100 and higher than 1:6400, respectively.

IgM-capture ELISA to DENV

Dengue-specific IgM antibody was determined by an IgM-capture ELISA, as previously described (Innis et al. 1989), for samples that were positive for DENV IgG antibodies. Briefly, 96-well microplates were coated with anti-human IgM antibody and incubated at 4°C overnight. Plasma test samples were diluted 1:100 in blocking buffer as described above for testing for IgG antibody. The next day, microplates were washed, and the plasma samples were added into the microplate. Attempts to detect IgM antibody was performed by the addition of dengue viral antigen, followed by virus-specific hyperimmune ascitic fluid and HRP-conjugated rabbit anti-mouse IgG. After adding the colorimetric substrate, OD values were read at 410 nm for each sample. The procedures for the calculation of the OD cutoff value and antibody-positive samples and titers were performed as described above for WNV and DENV IgG antibody.

Plaque reduction neutralization test

Plasma samples that were reactive in the indirect IgG ELISA to DENV or/and WNV were tested by plaque reduction neutralization test (PRNT) to each of the DENV serotypes: DENV 1 (16007), DENV 2 (16681), DENV 3 (H87), DENV 4 (1036); and WNV (NY-99) as previously described (Morens et al. 1985). Briefly, four-fold dilutions of heat-inactivated plasma samples were incubated at 4°C overnight with 30–60 plaque-forming units (PFU) of either DENV or WNV suspensions. The next day, mixtures of plasma/virus were inoculated on baby hamster kidney cells, clone 15, or Vero cells for DENV and WNV neutralization assays, respectively. After 3–7 days of incubation, cells were fixed and stained with Naphthol Blue–Black solution. Virus dose was determined as the mean number of PFU recorded on 12-well cells infected with 30–60 PFU based on testing of an equal volume of a dilution of the virus stock and antibody-negative control human plasma. Plaques were counted, and the dilution of plasma that reduced 70% of the virus dose was considered as the antibody titer. Samples with titers 1:40 or higher were considered antibody positive.

Seroconversion was defined as the first plasma sample being antibody negative for either DENV or WNV and the second plasma sample from the same individual having a four-fold or higher rise in the antibody titer and confirmed by PRNT to DENV or WNV.

Results

A total of 78 study participants provided a blood sample during the May and June 2015 survey period and 60 of the 78 participants provided samples during the November and December 2015 survey. Eighteen participants (23.1%) were not available to participate in the study after the first survey period. Most of the participants were females (66.7%), with primary education (79.5%) and 48.7% were working as housekeepers (Table 1). The results of interviewing participants during the baseline survey revealed that 30.8% (n = 24) had a history of travel outside Ciudad Juarez. All of the plasma samples positive for DENV and/or WNV IgG antibodies during the first (n = 14) and second survey (n = 17) were confirmed by PRNT. DENV IgM antibody with titers of 1:400 was detected in 2 of the 17 IgG antibody-positive samples during the second survey.

Table 1.

Description of the 78 Participants Enrolled in the 1st Survey Period, (May–June 2015) in Ciudad Juarez, Mexico

Sociodemographic characteristics	N (%)
Sex
Male	26 (33.3%)
Female	52 (66.7%)
Occupation
Housekeeper	38 (48.7%)
Manufacturing worker	16 (20.5%)
Merchant	12 (15.4%)
Unemployed	2 (2.6%)
Retired	2 (2.6%)
Professional	1 (1.3%)
Others	7 (9.0%)
Education
None	4 (5.1%)
Elementary school	32 (41.0%)
Middle School	30 (38.5%)
High school	10 (12.8%)
Professional/Technician	2 (2.6%)
House construction
Wood	9 (11.5%)
Concrete	11 (14.1%)
Mixture	58 (74.4%)
Years living in house
0–5	22 (28.2%)
6–10	15 (19.2%)
11–15	11 (14.10%)
16–20	15 (19.2%)
>21	15 (19.2%)
Sewage
Sewer	61 (78.2%)
Septic tank	17 (21.8%)
Travel history outside Ciudad Juarez
Yes	24 (30.8%)
No	54 (69.2%)

In the first survey period, the prevalence of DENV- and WNV-neutralizing antibody was 14.1% (11/78) and 5.13% (4/78), respectively; with one of those being positive for both DENV and WNV (Table 2). Four of the 11 DENV antibody-positive samples had neutralizing antibodies to DENV 1, 2, and 3. Only four participants had monotypic antibodies (one for DENV 2 and three for WNV). The remaining participants (n = 6) had polytypic responses to DENV and/or WNV.

Table 2.

Neutralizing Antibody to Dengue and West Nile Virus Infection Among a Human Cohort in Ciudad Juarez, Mexico, 2015

Serotype	1st period (n = 78)	2nd period (n = 60)	Seroconversion ^a (PRNT₇₀ titer) ^b
DENV 1	0	1 (1.66%)	^c1 (DENV 1, 1:320)
DENV 2	1 (1.28%)	2 (3.33%)	1 (DENV 2, 1:1280)
DENV 1, DENV 2	2 (2.57%)	4 (6.66%)	^c1 (DENV 1, 1:640; DENV 2, 1:80)
DENV 1, DENV 2	2 (2.57%)	4 (6.66%)	1 (DENV 1, 1:1920; DENV 2, 1:320)
DENV 1, DENV 3	2 (2.56%)	2 (3.33%)	0
DENV 1, DENV 2, DENV 3	4 (5.13%)	2 (3.33%)	0
DENV 1, DENV 2, DENV 3, DENV 4	1 (1.28%)	1 (1.66%)	0
DENV 1, DENV2, DENV 3, WNV	1 (1.28%)	1 (1.66%)	0
WNV	3 (3.84%)	4 (6.66%)	1 (WNV, 1:1280)
No antibody	64 (82.05%)	43 (71.66%)	—

Seroconversion, four-fold or greater rise in antibody titer in paired sera.

PRNT ₇₀ titer, 70% plaque reduction neutralization test titer.

Number of individuals DENV IgM positive.

A total of 60 participants of the 78 included in the first survey period were enrolled in the second survey period (Table 2). Among these 60 participants, 17 had neutralizing antibodies; 12 participants had the same distribution of DENV and/or WNV antibodies as during the baseline survey. Of those 12 participants, 8 had neutralizing antibody to DENV only. Among 48 participants who were negative for antibody during the first survey, 10.4% (n = 5) acquired antibody to WNV or DENV. Of these five participants, four seroconverted to DENV (one individual seroconverted to DENV 1 (PRNT₇₀ titer: 1:320), one seroconverted to DENV 2 (PRNT₇₀ titer: 1:1280) and the other two seroconverted to DENV 1/DENV 2, with DENV 1 neutralizing antibody titers four-fold higher than the DENV 2 antibody titers), and 1 individual seroconverted to WNV (PRNT₇₀ titer: 1:1280) (Table 2). Also, as stated in the comparison of DENV and WNV antibody prevalence and incidence among the human cohort in Ciudad Juarez (Table 3), the five participants who seroconverted did not have any travel history outside Ciudad Juarez during the study. Finally, two of the four individuals who seroconverted to DENV 1 and/or DENV 2 were positive for DENV IgM antibody (Table 2).

Table 3.

Comparison of Dengue and West Nile Virus Antibody Prevalence and Incidence of Seroconversion Among Human Cohort During May to December, 2015 in Ciudad Juarez, Mexico

Variable	% of Prevalence ^a (No. positive ^b /No. tested ^c )		% of Incidence ^d (No. positive ^b /No. tested ^c )
Sex	DENV	WNV	DENV	WNV
Male	15.38 (4/26)	3.85 (1/26)	11.11(2/18)	5.55 (1/18)
Female	11.54 (6/52)	5.76 (3/52)	6.66 (2/30)	0/30
Occupation
Housekeeper	10.53 (4/38)	7.89 (3/38)	9.52 (2/21)	0
Manufacturing worker	12.5 (2/16)	0/16	8.33 (1/12)	8.33 (1/12)
Merchant	25 (3/12)	0/12	0/6	0/6
Unemployed	50 (1/2)	50 (1/2)	100 (1/1)	0
Professional	100 (1/1)	0/1
Retired	0/2	0/2	0/1	0/1
Others	0/7	0/7	0/7	0/7
Education
None	25 (1/4)	25 (1/4)	0/1	0/1
Elementary school	15.62 (5/32)	3.13 (1/32)	5.56 (1/18)	0/18
Middle school	6.66 (2/30)	6.66 (2/30)	9.09 (2/22)	4.55 (1/22)
High school	20 (2/10)	0/10	16.6 (1/6)	0/6
Professional/Technician	50 (1/2)	0/2	0/1	0/1
House construction
Wood	22.22 (2/9)	22.22 (2/9)	20 (1/5)	0/5
Concrete	18.18 (2/11)	0/11	0/6	0/6
Mixture	12.08 (7/58)	3.45 (2/58)	8.11 (3/37)	2.7 (1/37)
Years living in house
0–5	18.18 (4/22)	9.09 (2/22)	16.67 (2/12)	0
6–10	6.67 (1/15)	0/15	8.33 (1/12)	8.33 (1/12)
11–15	18.18 (2/11)	0/11	0	0
16–20	20 (3/15)	0/15	12.5 (1/8)	0/8
>21	7.14 (1/14)	14.29(2/14)	0/10	0/10
Sewage
Sewer	8.2 (5/61)	4.92 (3/61)	5.13 (2/39)	2.56 (1/39)
Septic tank	35.29 (6/17)	5.88 (1/17)	22.22 (2/9)	0/6
Travel history^e
Yes	20.83 (5/24)	0	0/16	0/16
No	11.11 (6/54)	7.4 (4/54)	12.5 (4/32)	3.13 (1/32)

Prevalence, individuals with evidence of past WNV or DENV infection.

No. Positive, number of seroconversion to DENV or WNV.

No. Tested, the total number of samples available.

Incidence, individuals who contract WNV or DENV infection during May–December, 2015.

Travel history outside Ciudad Juarez.

Discussion and Conclusions

Our findings are the first evidence of DENV infections in humans in Ciudad Juarez, MX, where dengue RNA was detected during 2005 in field-collected Ae. aegypti mosquitoes (de la Mora-Covarrubias et al. 2010). In addition, even though the sample size of this study was small (n = 78), the serological data supported the local circulation of DENV in Ciudad Juarez because of the detection of neutralizing antibodies to DENV 1 and DENV 2 serotypes and DENV IgM antibody in two of four study participants who seroconverted to DENV infection. Also, the individuals with DENV seroconversion did not report any travel history outside Ciudad Juarez, and three of the four individuals stated that most of their time was spent at home during the survey, suggesting that DENV infection occurred at home (Table 3). However, further studies are needed to confirm this possibility. An understanding of our observations for two of the participants, who were negative during the first survey for DENV antibody, but were positive for DENV IgG antibody and negative for DENV IgM antibody during the second survey, is unknown. However, it is possible that the IgM antibody waned to undetectable levels for these two individuals during the 5–6-month interval between the initial and follow-up survey period.

Our results represent the first evidence of endemic DENV transmission along the United States–Mexico border west of the DENV endemic areas in Brownsville, Texas and Matamoros, MX and surrounding urban communities in the RGV. Seroconversions in our study as evidence of a recent DENV infection was detected in 8.3% (4/48), and DENV IgG antibody as evidence of past infections was detected in 14.1% of the study participants. Although the sample size was much lower in our study, the rate of recent infections was higher than the 2% and 7.3% rates reported during 2004 for residents of Brownsville and Matamoros, respectively (Brunkard et al. 2007). However, the 14.1% DENV seroprevalence rate for past infection in Ciudad Juarez during this study was lower than the 40% rates reported during 2004 in the residents of Brownsville and 78% rate reported in Matamoros (Brunkard et al. 2007). In Nuevo Laredo, MX, the rate of recent DENV infection during an outbreak, in 1999 was 16%, but in Laredo, Texas, the rate of 1.3% was substantially lower than our finding of 8.3% rate in Ciudad Juarez (Reiter et al. 2003). Also, the seroprevalence rate for past infection in the Ciudad Juarez community (14.1%) was lower than the 23% rate reported for Laredo, Texas, and 48% in Nuevo Laredo (Reiter et al. 2003). Overall, the seroprevalence rates as evidence of DENV infections have been reported to vary in the border communities. Hotez (2008) estimated that more than 100,000 dengue infections occur annually among the entire border population of 10 million people (Pew Hispanic Center 2015), based on the observation that 2% of humans in Brownsville had a recent DENV infection (Brunkard et al. 2007).

As an example of the possible underestimation of dengue cases, the RGV is the better-known region of endemic DENV transmission on both sides of the border. Still, the understanding of the ecology and epidemiology is insufficient because of the lack of any systematic and poorly designed studies that have failed to provide an accurate estimate of the incidence and the public health impact of the DENV infections. As an example, during 1980–1999, there were 65,514 cases of dengue reported from the MX side of the border in the RGV as compared with only 64 cases on the U.S. side of the border (Gubler 2001, Reiter 2001, Brunkard et al. 2007). One study suggested that this disparity could be attributed to the human behavior and the air conditioning system on the U.S. side of the border (Reiter et al. 2003). However, a subsequent study revealed that dengue cases were being underreported in the United States near the Mexican border (Brunkard et al. 2007). Efforts to understand possible reasons for the underestimation of cases indicated that this could be attributed to passive surveillance (Hafkin et al. 1982). For example, during the 1980 outbreak in Texas, passive surveillance failed to detect any dengue cases, but 63 cases were detected by active surveillance at outpatient clinics, including 52 (83%) in counties adjacent to the Texas–Mexico border. Another example related to misdiagnosis of cases was during a dengue outbreak in 2013 in Brownsville when less than half of the cases were reported to the Texas Department of State Health Services because commercial laboratory diagnostic testing results were false negative for about one in five of the cases (Thomas et al. 2016). Other possible reasons were that dengue cases might go unrecognized because DENV strains caused mostly silent or subclinical infections and mild diseases. Also, 59% of Brownsville residents cross the border into MX for medical care, which is likely to limit disease reporting on the U.S. side of the border (CDC 2001, Brunkard et al. 2007)

In this study, serological evidence of past (n = 4) and recent (n = 1 seroconversion) WNV infections were detected in humans in Ciudad Juarez, where WNV-positive Cx. quinquefasciatus mosquitoes were reported in a previous study (Mann et al. 2013). These results support endemic WNV transmission in Ciudad Juarez similar to observations in the neighboring El Paso border community (Cardenas et al. 2011) and northern MX (Nuevo Leon) (Rodríguez et al. 2010).

Since our testing was for only performed DENV and WNV antibodies, we cannot exclude the possibility that other Flaviviruses were circulating in Ciudad Juarez, such as St Louis encephalitis virus or ZIKV because of the possibility of cross-reactivity that has been reported among Flaviviruses, especially among humans who had secondary infections (Lanciotti et al. 2008). However, ZIKV has not been reported from the Ciudad Juarez community but has been documented in other areas, especially in Chiapas, MX (Guerbois et al. 2016). In addition, the distribution of neutralizing antibodies to DENV and WNV in the 12 individuals with paired samples in this survey were similar for the baseline (May–June 2015) and the second survey (November–December 2015), suggesting that ZIKV and/or other Flavivirus infection were not involved in the neutralizing antibody response in those 12 individuals.

Our results suggested that further studies are warranted to determine the incidence and clinical outcome of DENV infections in the Ciudad Juarez community. This observation is further supported by a recent report of 1 and 9 dengue cases in this community during 2016 and 2017, respectively. (DGE 2016, DGE 2017), and therefore, is likely to reflect an increasing trend in dengue cases. In case of WNV, the lack of WN cases reported in Ciudad Juarez and other Mexican areas could be related to the limited resources for testing human samples, or else the endemic level of WNV is low. However, further studies are needed to determine the clinical outcome of WN in the Ciudad Juarez community. Also, longitudinal cohort studies and mosquito surveillance could lead to a better understanding of the transmission dynamics and public health importance of DENV and WNV in this border community and provide critically needed data to develop more effective mosquito control programs.

Footnotes

Acknowledgments

This work was funded by the Office of Research and Sponsored Project at the University of Texas at El Paso. The study protocol was approved by the University Autonomous of Ciudad Juarez Institutional Review Board in compliance with all applicable Federal regulations governing the protection of human subjects.

Author Disclosure Statement

No competing financial interests exist.

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