This paper investigated the relationship between meteorological factors and brucellosis in Jilin Province from 2005–2019 using a DLNM. The model has been effectively used to assess the relationship between meteorological factors and climate-sensitive infectious diseases, including influenza, hand, foot and mouth disease, dengue fever, chickenpox, etc (Wang et al. 2022; Hao et al. 2020; Xiang et al. 2017; Zhang et al. 2023). Using the same model, two previous studies examined the relationship between climatic factors and the risk of brucellosis. One of the studies examined the relationship between climate and brucellosis in northwest China (Yulin city, Shaanxi province) and concluded that temperature, sunshine duration, and evaporation contributed significantly to seasonal fluctuations in brucellosis within 6 months (Liu et al. 2020). Another study examined the relationship between brucellosis transmission and climate in northwest China (Yongchang city, Gansu province). It concluded that temperature, sunshine hours, and air pressure increase the risk of brucellosis at different lag times (Zheng et al. 2023). Our study examined the northeastern region of China, represented by Jilin province. The results showed that the risk of brucellosis had a non-linear relationship with meteorological factors, including temperature, sunshine duration, and wind speed, and there was also a certain lag effect.
Temperature affects the seasonality and intensity of infectious disease transmission (Harvell et al. 2002). The explanatory power of temperature for the spatially uneven distribution of brucellosis diminishes over time (Xu et al. 2022). Our study found that both low and high temperatures had a more significant effect on the spread of brucellosis in a shorter lag time, and the effect diminished as the lag time increased. For example, extreme cold temperatures (-17.2°C) reduced the risk of brucellosis transmission at a lag of 0 month (RR = 0.38, 95% CI = 0.19–0.76), while the effect was not statistically significant after a lag of 2.5 months. Brucella is a human-animal bacterium closely associated with animal husbandry. It reproduces faster and exhibits stronger biological activity during warm seasons compared to cold seasons (Chen et al. 2023). A Russian study found that increased temperature improves the chances of host survival and increases the risk of zoonotic infections (Revich et al. 2012). However, regression analysis by Iranian scholars showed a negative significant correlation between ambient temperature and the risk of brucellosis (Dadar et al. 2020). And a spatio-temporal study in Inner Mongolia, China, showed that regions with low temperature were more likely to be high-prevalence areas of human brucellosis (Liang et al. 2021). Our findings showed that low temperatures reduced the risk of brucellosis transmission, and high temperatures were a risk factor for brucellosis infection. Temperature can affect the survival and transmission of infectious pathogens in the environment, as well as influence human behavior and activities, thereby, to some extent affecting the development of infectious disease transmission (Lin et al. 2013; Imai et al. 2015). We believe that low temperature limit husbandry activities, including shearing, breeding, and free trade of livestock products such as dairy and meat, thereby reducing the risk of exposure to susceptible individuals. Most animals infected with brucellosis show no obvious signs and infect susceptible people mainly through abortion and secretions (Yang et al. 2020). Suitable high temperature environment is conducive to the propagation and growth of brucellosis (Li et al. 2013), and the warm season is conducive to the estrus of sheep and goats, which not only increases the density of sheep but also facilitates the spread of brucellosis among animals. It also increased livestock production and increased contact between humans and infected animals.
Our study found that extreme low wind velocity (1.65 m/s) at a lag of 0-2.5 months increased the risk of brucellosis transmission. A study investigated the effect of meteorological factors on brucellosis by developing a Markov switching model (MSM) and found that low wind speed (1.89m/s) at a lag of 1 month predicted the highest incidence of brucellosis (Mohammadian-Khoshnoud et al. 2021). Both had similar findings. High wind velocity can reduce disease because bacteria have a shorter life span in the air (Tapak et al. 2018). The higher the wind velocity, the higher the evaporation, which means that wind velocity can also indirectly affect the spread of brucellosis by affecting evaporation (Cao et al. 2020). However, high wind velocity can drive brucella hidden in animals and feces to the eyes and respiratory tracts of susceptible people. It can cause skin cracking, increasing the chances of pathogen invasion (Yuan et al. 2020). Our results showed that high wind velocity was not statistically significant in increasing the risk of brucellosis transmission.
Regarding the impact of sunshine hours on the spread of brucellosis, some studies have found that extreme weather with high sunshine hours reduces the transmission of brucellosis (Li et al. 2013; Chen et al. 2023). However, our results showed that high sunshine hours increased the risk of brucellosis and had a significant lag effect. The maximum relative risk was 1.85 (1.23–2.80) when the sunshine hours was 332.6h with two months lag. Our view can be supported by the fact that extreme weather with high sunshine often occurs in spring and summer, which are the most common periods of brucellosis. In addition, high levels of sunlight and evaporation can cause drought and limit plant germination (Liu et al. 2020). As a result, cattle and sheep have reduced feed intake and weight gain in spring and summer. They are more susceptible to gastrointestinal parasites due to high temperature and humidity conditions, leading to reduced immunity and increased risk of brucellosis infection (Poli et al. 2020).
Our study contributes to understanding the impact of meteorological factors on the risk of brucellosis, especially in northeastern China. For example, brucellosis activity will intensify in the next three months when the environment is hot or has low wind velocity or high sunshine hours. This may inform local health departments in developing effective prevention strategies. However, our study has certain limitations. Although monitoring data can better reflect the nonlinear relationship between meteorological factors and infectious diseases (Brownstein et al. 2010; Li et al. 2022), improving the accuracy of time series models requires research based on large amounts of data (Shumway et al. 2017). Although our study involved 15 years of data, the total data is still small because brucellosis is a monthly reported category B infectious disease in China. In addition, extreme weather conditions such as haze and dust storms have a great impact on the spread of infectious diseases, and it may be possible to add meteorological parameters related to air pollution to the model to more comprehensively and accurately investigate the impact of meteorological factors on the spread of brucellosis.