While there is no doubt about the participation of Zika virus in microcephaly, its epidemiology is not entirely clear and doubts remain about the intervention of other factors. In studies on the epidemiology of dengue, the infestation by Aedes aegypti peridomiciliary and the population density are the main determinants for viral spread. However, in Rio Grande do Norte state (RN), the counties that have confirmed cases of microcephaly overlapped the river basins regions surrounded by agriculture and livestock. In addition, the prevalence of microcephaly at the end of the first year of the epidemic was higher in small towns than in larger ones, elements that seem to contradict what is known about the epidemic by other arboviruses. Methods: 234 cases of microcephaly were analyzed from three states and 144 counties. Results: An exponential trend of higher prevalence of microcephaly in the smaller cities (r2=0,7121) was found.The correlation coefficients (R) between the Prevalence of microcephaly and the variables that measured the density of animals in the territory ranged from moderate to strong. Discussion: Concerning microcephaly, studies in progress point to the possibility of association between the Zika Virus and the BVDV, a virus known to produce birth defects in farm animals but perceived as innocuous in humans. Conclusions: The overlap of cases of microcephaly in river basins, their higher prevalence in smaller cities, the strength of the correlation coefficient, render necessary new etiological and pathophysiological studies.