Recent dengue virus infection: epidemiological survey on risk factors associated with infection in a medium-sized city in Mato Grosso

ABSTRACT BACKGROUND: Dengue is considered to be the most important arbovirus worldwide, with important complications that increase its lethality. In Brazil, an endemic country, the disease reaches significant incidence levels, with occurrences of serious cases and high costs of hospitalizations for its treatment. OBJECTIVE: To analyze risk factors among individuals with recent histories of dengue infection in a medium-sized city in Mato Grosso. DESIGN AND SETTING: Descriptive cross-sectional study, of epidemiological-survey type, conducted among the urban population of a city located in mid-northern Mato Grosso. METHODS: A seroepidemiological survey using questionnaires and collection of biological material was conducted among 596 adults aged ≥ 18 years who had been selected through a cluster sampling process. Positive dengue cases were those with positive results from anti-dengue immunoassays (ELISA). Statistical analyses with descriptive and inferential techniques were used, with 95% confidence intervals and a 5% significance level. RESULTS: The seroepidemiological profile of the study participants was predominantly female, with ages between 18 and 39 years, self-declared non-white race/color, not more than eight years of education and not living with a companion. Among the sanitary factors analyzed, the following were risk factors for dengue virus infection: no running water at home; no water supply from the public piped network; no waste from drains or toilets sent to the sewage network; endemic disease combat agents visiting the home; and presence of mosquito breeding sites at home. CONCLUSION: Low schooling levels and previous dengue virus infection were associated with current dengue virus infection.

Brazil is ranked second highest for dengue cases in this country, and the average incidence rate in the state of Mato Grosso is 30%. [19][20][21] Mato Grosso has high temperatures and low humidity. It has a diversity of environments, comprising areas of the Cerrado, Pantanal and Amazon biomes, which are undergoing constant environmental transformation. These factors may favor occurrences of human infections caused by numerous pathogens. 21,22

OBJECTIVE
In the light of this scenario, the aim of the present study was, through an epidemiological survey, to analyze risk factors among individuals with recent histories of dengue infection in a medium-sized city in Mato Grosso.

METHODS
This was a descriptive and cross-sectional study, of epidemiological-survey type, conducted among the urban population of a city located in mid-northern Mato Grosso. Data collection was performed between January and March 2018, by a previously trained team, and a pilot study was first conducted in a census tract that did not form part of the final sampling.
A cluster sampling process was carried out in two stages (census tracts and households). Initially, these tracts (delimited by the Brazilian Institute for Geography and Statistics) were assessed to identify inhabited homes (households), i.e. to determine whether these tracts contained households, shops, wasteland or other types of land use, as shown in Figure 1. From this, an updated listing of census tracts was obtained. Proportional numbers of households were then systematically intercalated for each census tract sampled.
To estimate the serological prevalence of dengue in the population, with a margin of error of five percentage points, a sample of 660 individuals was required. Thus, a standard error of 0.025 was adopted for a sample of 400 individuals, and 10% loss was added to this number, plus inflation of 1.5, to reach this total of 660 individuals.
The study included individuals aged 18 years or older who were already living in the urban area of this city before April 1, 2016, and who remained as residents until the time of data collection in 2018. Women of childbearing age and pregnant women were also included. Institutionalized individuals were excluded from the study. Among the 660 individuals drawn, 64 left the study due to refusal or absence. Thus, the final sample for the study comprised 596 individuals. Selection of these individuals was carried out by means of simple random sampling among the residents of the selected households. Data collection only began after the potential participants had signed an informed consent statement.
Data collection was performed in the households, and took approximately 40 minutes per household. The interview consisted of application of a structured questionnaire, composed of questions that addressed sociodemographic, health and sanitary factors.
The dependent variable of this study was recent seropositivity for dengue (immunoglobulin M, IgM). The independent variables analyzed were sex, age group, schooling, race/color, marital status, type of household, running water, main form of water supply, bathroom drain, garbage destination, use of mosquito nets, visits by endemic disease combat agents, presence of mosquito breeding sites and hospitalization due to dengue.
For serological analysis, peripheral venipuncture was performed in an antecubital region, to collect 6 ml of blood in a vacuum collection tube containing separator gel. These samples were then sent to the laboratory of the city's Epidemiological Surveillance Department. There, they were centrifuged and cryopreserved in order to transport them to the Virology Laboratory of the Medical School of the Federal University of Mato Grosso in Cuiabá, Mato Grosso, to be kept in an ultra-freezer (-80 °C).
Serological analyses were performed at the Central Public Health Laboratory of Mato Grosso, in Cuiabá. These were done in accordance with biosafety standards, through an immunoenzymatic    collection for the present survey. 28 In this context of cross-reactivity, there is protein homology between DENV and ZIKV, which are both flaviviruses and share all the essential structural characteristics, such as capsids, envelopes, membrane protein precursors and quaternary structures. This consequently enables considerable immunological cross-reactivity. [29][30][31][32] Simultaneous detection of antibodies to DENV and CHIKV is also observed, and these can respond either through cross-reactivity or through coinfection. 33 Although the municipality studied here and Odisha, in India, had similar prevalences, which were both higher among women, the prevalences according to age groups were different: in Tangará i.e. significant at the 5% level) ( Table 3).

The association between low schooling level and occurrences of
DENV infection that was observed in the city of the present study has also been one of the sociodemographic factors correlated with positive DENV cases elsewhere. However, this was not as an isolated factor, but as part of a larger set of sociodemographic, economic and poor health factors that influence occurrences of dengue cases. Schooling has been pointed out as an important aspect of overall awareness levels regarding dengue. This awareness includes knowing about the vectors that transmit the disease, identifying and implementing disease control and prevention methods within the population and recognizing the signs and symptoms of the disease process. [34][35][36][37] Individuals with low levels of education were also among those with lower levels of knowledge about the disease. Their situation was the inverse of people who had attended college/university and who, therefore, formed part of the group with the highest percentage of seronegativity for DENV. 38 A study conducted in Sri Lanka  Table 3. Variables in the final model and prevalence ratios adjusted using robust Poisson regression (rPR) that presented associations with positive and negative dengue cases (immunoglobulin M, IgM), with their respective 95% confidence intervals (CI) and P-values, in a city in Mato Grosso, Brazil, 2018 rPR = prevalence ratio adjusted using robust Poisson regression model with variable selection by means of backward method. CI: confidence interval; *Significant at the level of 5%. Note: The garbage destination variable remained in the model as the adjustment variable, although the P-value was greater than 0.05.
on the level of awareness of dengue among schoolchildren (13 to  44,45 Traditionalism in the control model is characterized by lack of innovation in the strategies to cope with endemic disease, combined with little improvement in infrastructure and little investment in new means of empowering healthcare professionals or embracing the community. 3 It is of paramount importance to know the factors that are associated with DENV infection, given its already-known association with social vulnerability, and also its impact on society as a whole.
The present study is important insofar as it provides knowledge of yet another dengue distribution profile, in a medium-sized city in a state covering an immense area.
The following limitations of the present study need to be taken into consideration. Children (under the age of 18 years) were not included in this survey, given that children, along with the elderly, make up the risk group for dengue cases. The serotypes circulating among the participants were not identified, although this is necessary in order to ascertain how the disease is circulating in the community. No other analyses such as the reverse transcriptase polymerase chain reaction (RT-PCR) were performed to confirm recent cases of dengue, although that could also have helped in identifying cross-reactivity between DENV and the Zika and chikungunya arboviruses that circulate simultaneously with dengue in this city.

CONCLUSIONS
In this cross-sectional study conducted in a city located in midnorthern Mato Grosso, the findings showed that there were no significant associations between DENV infection and the sani-