Study design flowchart.

IntroductionFamilial clustering of HTLV-1 and related diseases has been reported in Brazil. However, intrafamilial transmission of HTLV-1 based on molecular analysis has been studied only in a few communities of Japanese immigrants and African-Brazilians.ObjectiveTo investigate the familial clustering of HTLV-1 infection and to determine the likely routes of transmission through epidemiological and genetic analyzes.MethodsMedical records of 1,759 HTLV-1+ patients from de the Center for HTLV in Salvador, Brazil, were evaluated to identify first-degree relatives previously tested for HTLV-1. Familial clustering was assumed if more than one member of the same family was HTLV-1+. LTR regions of HTLV-1 sequences were analyzed for the presence of intrafamilial polymorphisms. Family pedigrees were constructed and analyzed to infer the likely transmission routes of HTLV-1.ResultsIn 154 patients at least one other family member had tested positive for HTLV-1 (a total of 182 first-degree relatives). Of the 91 couples (182 individuals), 51.6% were breastfed, and 67.4% reported never using a condom. Of the 42 mother-child pairs, 23.8% had a child aged 13 years or younger; all mothers reported breastfeeding their babies. Pedigrees of families with 4 or more members suggests that vertical transmission is a likely mode of transmission in three families. Three families may have had both vertical and sexual transmission routes for HTLV-1. The genetic signatures of the LTR region of 8 families revealed 3 families with evidence of vertical transmission, another 3 families (spouses) with sexual transmission, and one family with both transmission routes. HTLV-1 sequences belonged to Cosmopolitan subtype HTLV-1a Transcontinental subgroup A.ConclusionSexual and vertical transmission routes contribute to the intrafamilial spread of HTLV-1 in the state of Bahia.</div

Sociodemographic and clinical profile of index cases and family members with HTLV-1.

Sociodemographic and clinical profile of index cases and family members with HTLV-1.</p

Composition of families of HTLV-1-infected patients (index cases).

Composition of families of HTLV-1-infected patients (index cases).</p

Pedigrees of 8 families with likely routes of HTLV-1 transmission based on epidemiological data, sexual behavior and genetic sequencing of the HTLV-1 LTR region.

Family members highlighted in red represent, partners, children, or grandchildren of the index case who provided secondary information about sexual behavior and other risk factors.</p

Pedigrees of 20 families with likely routes of HTLV-1 transmission based on epidemiological data and sexual behavior.

Pedigrees of 20 families with likely routes of HTLV-1 transmission based on epidemiological data and sexual behavior.</p

Global incidence in Brazil per 100,000 inhabitants of CS (A) and SiP (B) according to SINAN.

Joinpoint regression analysis using APC (Annual Percentage Change) calculations identified three distinct periods for each form of syphilis.</p

Spatiotemporal distribution of SiP cases (2001 to 2017) in Brazil, based on cases by microregions.

Digital maps in the public domain (publicly available) were obtained from IBGE cartographic database in shapefile format (.shp), which was subsequently reformatted and analyzed using QGIS version 3.10. Authors specify that this figure is licensed under CC BY 4.0.</p

Spatial distribution of CS and SiP (2001 to 2017) in microregions.

The top 15 states or microregions with the highest cumulative rates per 100,00 live births for CS and 100,000 inhabitants SiP. Digital maps in the public domain (publicly available) were obtained from IBGE cartographic database in shapefile format (.shp) and reformatted and analyzed using QGIS version 3.10. Authors specify that this figure is licensed under CC BY 4.0.</p

Spatiotemporal distribution of reported numbers of CS cases.

(XLSX)</p

Analysis of changes in sociodemographic variables by region, stratified according to two periods of CS notifications.

Digital maps in the public domain (publicly available) were obtained from IBGE cartographic database in shapefile format (.shp), which was subsequently reformatted and analyzed using QGIS version 3.10. Authors specify that this figure is licensed under CC BY 4.0.</p