1 | INTRODUCTION
Parasitic ascariasis has long been a threat to the health of humans, livestock and wildlife worldwide (Hotez, Fenwick, Savioli, & Molyneux, 2009). With the expansion of towns, cities, and the wild land-urban interface, geographic isolation is no longer an effective barrier for transmission of helminth infections. As a result, the risks for transmission of diseases once isolated in wildlife have never been greater (K. Kazacos & W. M. Boyce, 1989). Due to its wide distribution and long incubation period, soil-transmitted helminth eggs are easily transmitted between wildlife and livestock, and even to humans through contaminated feces or soil. In-depth studies of helminths in wildlife can provide information of relevance for identifying and detecting pathogens and instigate appropriate actions to deal with possible risks with broad and far-reaching implications for wildlife and human health.
Baylisascaris schroederi , a parasitic nematode specific for the giant panda (Ailuropoda melanoleuca ), is a soil-transmitted nematode and can directly infect the giant panda without passing through an intermediate host (Bethony et al., 2006; De Silva et al., 2003). Baylisascaris species also cause infection as patent or latent larva migrans (LM) in a variety of mammals (K. Kazacos & W. M. Boyce, 1989), birds (Wolf, Lock, Carpenter, & Garner, 2007) and humans (Murray, 2002; Wise, Sorvillo, Shafir, Ash, & Berlin, 2005), and are therefore considered zoonotic parasites with potential public health and safety risks. Even though not all details of the life cycle of B. schroederi have been established, it is known that eggs secreted with feces are very resistant and can survive for extended periods in the soil. Fertile eggs can become infective under suitable temperature conditions (12℃~22℃) and after being ingested by a panda, the eggs hatch in the small intestine, the larvae migrate to several organs, and eventually returns to the intestine where they mature into adult reproductive worms (Wang & Tao, 2018). Migration of larvae to different organs may cause serious damage to the organs, with different roundworm species being associate with different syndromes including visceral larva migrans (VLM), ocular larva migrans (OLM), neural larva migrans (NLM) and even severe pneumonia and hepatitis (K. R. Kazacos & W. M. Boyce, 1989; Papini, Renzoni, Malloggi, & Casarosa, 1995; Wildt, Zhang, Zhang, Janssen, & Ellis, 2006; L. Zhang et al., 2011). Infection by Baylisascaris species can in addition cause severe baylisascariasis, intestinal blockage, and even fatal bowel rupture (Schaul, 2006; GY Yang, 1998). Compared with other roundworms, B. schroederi is smaller in size and is mainly found in the small intestine of giant pandas. Giant pandas have typical carnivorous intestinal characteristics (short and thick small intestines), but eat bamboo, a diet with low digestibility and absorption. This challenges the nutrient absorption ofB. schroederi for survival in the small intestine. Based on available epidemiological data of the giant panda, B. schroederiis the leading cause of death from primary and secondary infection in wild and captive populations (H. Hu et al., 2018; D. Li et al., 2014). Moreover, the problem of increased resistance to anthelmintics is likely to be seriously underestimated. Giant pandas in captivity are regularly dewormed (every 60 days). According to investigations, B. schroederi eggs can still be detected in the feces 10 to 15 days after treatment with anthelmintics (D. Li, He, & Deng, 2015), indicating that a development of drug-resistant subtypes had occurred in the B. schroederi population, and that B. schroederi variants with resistance to a variety of anthelmintics had survived. These variants may potentially become anthelmintic-resistant pathogens.
Although B. schroederi poses threats to both wild and captive giant pandas, current studies are limited to morphological and single or multiple gene analyses, preventing in-depth exploration of genetic mechanisms of adaptations and further prevention and control of infections (Xie et al.). Here, we report a chromosome-scale reference genome of B. schroederi , which is also the first chromosome-scale reference genome of ascaridoids. Based on the genome, we explored possible genetic mechanisms of the adaptation of B. schroederi to the intestinal environment, especially the specific diet of the giant panda, as well as the potential genetic basis of drug resistance. Finally, potential drug target proteins were identified, which provides new insights into the potential disease management ofBaylisascaris and related roundworms.