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.