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.
B. 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 larval migrants (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. Its eggs can develop directly into infective larvae
under appropriate conditions. Larvae can migrate to multiple organs of
the host and cause visceral larval 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). A large number of roundworm infections 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, 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 bamboo 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.