Abstract
Nairobi sheep disease (NSD) is a hemorrhagic vector-borne disease of small ruminants caused by the Nairobi sheep disease virus (NSDV), also known as Ganjam virus (GV). NSDV and GV refer to the same virus. The NSDV has been identified in East Africa, India, Sri Lanka, and China, and NSDV vector ticks can be carried by birds. There is few research on the mechanism of the global cycle and spillover of NSDV. Based on the prediction of the high probability distribution areas of NSD by the maximum entropy model (MaxEnt), the possible passive transport routes of NSDV vector ticks by migratory birds were simulated for further evaluation. The transmission probability of NSDV vector ticks by migrating birds was calculated using evaluations of the parasitism intensity of ticks on migratory birds at start points, the flying burden of parasitized birds, and the attachment coefficient of ticks on birds during migration. A total of 31 potential transport routes were predicted, which, through interaction with each other, constitute a spreading network for NSDV. Seven species of migratory birds were predicted as intra or interregional carriers. Our study first provides measurable support for estimating the possibility of passive migration of NSDV vector ticks by migratory birds that may be potential carriers of ticks and proposes a transmission mechanism between all known natural foci and potential natural foci. These findings highlight the necessity of cooperation in the management of the NSDV in all known and potential natural foci located in different countries, with the aim of blocking global circulation in a cost-effective way. Furthermore, these findings may also contribute to the prevention of other similar diseases.