Human brucellosis is a serious public health problem in the global livestock husbandry areas, and the seasonal distribution of the disease is strongly associated with climate variability [2,7]. The critical findings of our study were that temperature, sunshine duration, and evaporation were key determinants of disease seasonality in Yulin, and similar associations could easily be expanded over wider regions in northern China. Meanwhile, the lagged effects of climatic factors could provide sufficient time to develop targeted interventions for decreasing the disease burden in humans and livestock.
Seasonal climatic conditions may affect multiple aspects of climate-sensitive infectious diseases [15]. Consequently, climatic factors can be used to forecast the occurrence of disease outbreaks within periods of weeks to months [16]. Public health professionals use mathematical models to investigate the complex relationships between disease epidemics and climate data [17]. The DLNM was conducted in the study to examine the quantitative relationships and exposure-lag-response effects between climate variability and human brucellosis. This model has been effectively adopted to assess the non-linear relationships between meteorological factors and infectious diseases such as dengue, malaria, hand-foot-mouth disease, tuberculosis, and mumps [18-23].
The results of the present study suggest that changes in temperature may have a greater impact on the epidemics of human brucellosis than other climatic factors. The increasing temperature increased Brucella spp. development and replication in the host and increased the frequency of exposure of susceptible animals and humans [7]. Therefore, the transmission and persistence of Brucella spp. may be enhanced in warmer conditions, as demonstrated in our study, in which the maximum cumulative effect was highest at 15.2 °C in 4 months. In this respect, higher temperatures in late spring and early summer increase husbandry activities for sheep and goats, including shearing, breeding, processing of meat products, and commercialization of sheep products, consequently increasing the exposure of susceptible animals to contaminated animal products [11,24]. In contrast, the RR of brucellosis transmission decreases during winter because colder temperatures may limit the development of infective organisms [25]. In addition, Yulin is a region known for cashmere production and sheep farming. Meanwhile, the quarantine measures in sheep farms, adequate management of animal abortion products, and human protective measures are all deficient, causing more people to be exposed to the contaminated animal products [26-27].
The variability in monthly cumulative evaporation and sunshine strongly affects the transmission of Brucella spp., which develops intracellularly in the host and outside the host in dust, soil, and water [11,24,25]. The high incidence of brucellosis may be partly due to sheep breeding and abortion in subsequent lambing [7,21]. Considering that the gestation period in sheep is 6 months, our results suggest that the increased sunshine duration in early winter could promote estrus in sheep, resulting in the birth of animals more susceptible to infections with the adoption of high-risk breeding activities [12]. In addition, higher levels of evaporation and sunshine may favor drought, limiting plant germination. Meanwhile, dry environments may cause human skin dryness and fissures, increasing the exposure risk. As for China, the national policy of converting farmland into forests has limited field grazing, and breeding methods for sheep and goats have been adapted to raising livestock in pens [26]. However, the levels of minerals in stored grasses cultivated are unbalanced, making sheep and goats immunologically compromised and more susceptible to diseases.
This study aimed to investigate the effects of climatic factors on the incidence of human brucellosis, and several limitations should be acknowledged. First, the data were obtained from a passive surveillance system, in which some cases of human brucellosis might not have been reported because of milder clinical symptoms or delayed reporting in rural areas [28]. Second, the temporal dynamics of brucellosis are affected by non-climatic factors as well, including the number of affected sheep, immunity of the local population, human activities and movements, commercialization of sheep products, and eating habits. The analytical capability of model may be limited because of omitted covariates or changes in environmental and human factors. These limitations should be further investigated.