In recent years, evidence suggests that land use practices drive emerging, re-emerging and further spreading of zoonotic diseases [1, 2, 3]. For instance, high percentages of forest edges nearby herbaceous cover increased exposure to tick vectors . Vittor et al. (2006) found that the conversion of rain forests to agriculture has led to a re-emergence and increased incidence of malaria. Cleared land creates areas where water can become stagnant, providing breeding sites for mosquito species capable of parasite transmission . A low proportion of riparian forest has been identified as an important predictor of whirling disease, due to increase stream sedimentation, thus, increasing habitat for oligochaetes . Increased forest fragmentation was also associated with an increased risk of Lyme disease by creating suitable habitats for hosts . Another study demonstrated that increased forest deforestation reduced the prevalence of Trypanosoma cruzi, etiologic agent of Chagas disease in humans, in wildlife by reducing the abundance of mammalian reservoir hosts . Therefore, understanding the underlying mechanisms how the risk of disease relates to the land use is important, both for predicting disease dynamics, and to provide valuable insights into successful control measures. Despite growing awareness that disease emergence may be related, at least in part, to land use , this study focuses on vector-borne diseases, while there is a notable scarcity of scientific knowledge to show whether, and how, ecological change plays this role on aerosol-borne animal diseases. Here, we examined the relation between the proportion of land use and bovine tuberculosis (bTB) prevalence in cattle, and explored the underlying mechanisms accounting for these observations.
Bovine tuberculosis (bTB), caused by Mycobacterium bovis, is a zoonotic disease of human, livestock and wildlife [10, 11], and cattle serve as the main host for the bacteria . The disease is an important case of concern with potential public health, conservation and economic importance, since it can affect international trade of animals and animal products . Inhalation of M. bovis is the principal route to bTB transmission and is facilitated by direct or indirect contact between infected and healthy animals. The direct or indirect interaction between hosts has several points at which alteration of the landscape could influence disease dynamics. Land use/cover could potentially influence bTB prevalence through direct effects on the host community composition, host densities and host contact networks. For instance, low proportion of forest, grassland, and other natural habitat due to anthropogenic influence has been associated with a number of ecological complications, resulting in reductions of biodiversity [14, 15]. Recent studies have shown that a reduction in biodiversity may increase the prevalence and transmission of bTB, as non-competent mammalian hosts at higher levels of biodiversity may dilute pathogen transmission, a mechanism termed the dilution effect [16, 17, 18]. Thus, it is important to distinguish between these processes, to understand how changes in land use indirectly affect pathogen transmission, and to be able to formulate control actions that minimize future disease outbreaks.
One of the other important pathways by which land use changes alter ecosystems is by the introduction of non-native species into novel landscapes [19, 20]. Incursion of non-native species is one of the most serious concern to natural ecosystems worldwide [21, 22, 23]. Recent evidence showed that land use changes due to invasion by invasive plant species affect the risk of exposure to diseases. For example, invasive plant species may increase the risk of infection to tick-borne diseases by increasing the density as well as the rate of encounter rate between ticks and their vertebrate hosts [24, 25]. Similarly, land use changes as a result of plant species invasions increase the risk of mosquito-borne diseases by providing favourable habitat for vector species , and providing a high-quality nutritional fruits and leaves for mosquito larvae . Prosopis juliflora (further referred to as Prosopis) is among the most extremely invasive plants species globally, infesting over four million hectares of lands in arid and semi-arid part of Africa [28, 29, 30]. The species is native to Mexico, South America and the Caribbean. It was introduced in Ethiopia in the 1970s [31, 32] as a control mechanism to combat desertification [33; 34]. Around Awash in the Afar Region, about 30,000 ha of grassland, rangelands, water points and open Acacia woodland were estimated to be occupied by Prosopis . Currently, the species is a most powerful invasive species, threatening biodiversity and mammalian host community structure . As biodiversity loss is a predictor for the increase in bTB outbreaks [16, 17, 18], we expected that invasion of Prosopis might increase the bTB prevalence through loss of non-competent or spillover mammalian hosts. The invasion by Prosopis reduces availability of palatable herbaceous species  which could increase the movement of cattle herds in search for pasture and increase the probability of contact with infected hosts. Moreover, areas that are currently invaded by Prosopis were important sources of forage for livestock in the dry season, leading to shrinkage of the range- and grasslands, and altering local host densities, and increasing contact among cattle herds. For instance, increased host movement have been repeatedly identified as one of the major risk factors for bTB infection in African cattle populations [35, 36, 37]. We, therefore, hypothesize that increasing Prosopis invasion can increase the risk of bTB infection by increasing contact among cattle herds. However, no study, to our knowledge, examined the effect of Prosopis invasion on the risk of disease transmission. Here, we explore the impacts of Prosopis invasion on bTB prevalence in cattle, and how these invasions may influence the key parameters of this host–pathogen interaction, leading to changes in bTB prevalence.