Baylisascaris schroederi

Li, Junqiang, Karim, Md Robiul, Li, Jun, Zhang, Liping & Zhang, Longxian, 2020, Review on parasites of wild and captive giant pandas (Ailuropoda melanoleuca): Diversity, disease and conservation impact, International Journal for Parasitology: Parasites and Wildlife 13, pp. 38-45 : 39-41

3.1. Baylisascaris schroederi and baylisascariasis

The first documented roundworm in giant pandas, initially described as Ascaris schroederi , was discovered in 1939 (McIntosh, 1939). Ascaris schroederi was renamed as Baylisascaris schroederi in 1968 (Yang, 1998; Li et al., 2013). The morphology of B. schroederi has been described by many researchers. The adult B. schroederi is a thick nematode with white or grayish brown color. The egg of B. schroederi is characteristic yellow to brown, sub globular (67.5–83.7 μm × 54.0–70.7 μm), and symmetrical (Kong and Yin, 1958; Zhang et al., 2010; Hu et al., 2018).

Baylisascaris schroederi is a soil-transmitted parasite that mainly infects through the fecal-oral route. Baylisascaris schroederi eggs are excreted in the stool with strong survival ability in the environment (Li et al., 2013). The egg/larvae develops most suitably at 22–28 ◦ C; and the development stops when the temperature is below 4 ◦ C (Li et al., 2013), however maintains infection activity for a long time. Baylisascaris schroederi developmental stages in vitro have been well described (Wu et al., 1985a, 1985b). The visceral larval migrans stage of B. schroederi has been observed in mice infection models (Li, 1990).

Baylisascaris schroederi is a parasite specific to the giant panda, causing baylisascariasis (Zhang et al., 2008). The parasite is found mainly in the small intestine, and has also been found in the pancreatic and bile ducts connected to the intestinal tract (Ye, 1989). The clinical presentation of baylisascariasis comprises some unspecific symptoms, such as weight loss, pale mucous membranes, indigestion, diarrhea or constipation, poor activity, abdominal pain, and disheveled fur (Yang, 1998; Li et al., 2013). Baylisascaris schroederi larval migration causes mechanical injury, which results in gastroenteritis, cholangitis, pancreatitis, gastrointestinal obstruction, and even secondary infections that may lead to death (Li et al., 2013). In wild and captive giant pandas, the most common and harmful larval migration is VLM, which is responsible for more than half of the deaths reported in China during 2001–2005 (Zhang et al., 2008).

Currently, B. schroederi detection is mainly based on the morphology of eggs and/or adult worms either at necropsy or in feces or vomit, and some limited molecular tools ( Table 2). In case of microscopic examination of B. schroederi eggs, the undigested bamboo fibers in giant panda’ s feces may challenge the detection, sometimes contribute repeated ‘negative’ fecal test results. Hence, test sensitivity appears to be relatively low, in spite of the high reproductive index of B. schroederi

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(Wang et al., 2018). PCR-based molecular techniques can overcome this issue. With the research works regarding the molecular detection of B. schroederi in giant pandas, the complete mitochondrial genomes (Xie et al., 2011), microRNA sequences (Zhao et al., 2013) and some other genes came out. Subsequently, several sensitive and suitable molecular detection methods have been developed based on specific genes, such as the internal transcribed spacer-1 (ITS-1) (Lin et al., 2012), internal transcribed spacer-2 (ITS-2) (Zhao et al., 2012), ATPase subunit 6 (atp6), mitochondrial 12S rRNA (Zhou et al., 2013b), mitochondrial COII (Zhang et al., 2012), mitochondrial cytochrome c oxidase subunit I (Xie et al., 2014), and mitochondrial cytochrome c oxidase subunit II (Wang et al., 2013). The molecular studies reported that the B. schroederi isolates in giant pandas exhibit low genetic structure and a rapid evolutionary rate, indicating that there is no geographical separation among the populations (Zhou et al., 2013a; Xie et al., 2014). Other than the microscopic and molecular assays, some progress has been made on developing serological detection methods. For instance, an antibody detection enzyme-linked immunosorbent assay (ELISA) employing a B. schroederi glutathione S-transferase antigen was established for the detection of anti- B. schroederi serum antibody (IgG) in experimentally infected mice (Xie et al., 2015).

Baylisascaris schroederi is the most prevalent parasite in giant pandas, and the infection rate in both wild and captive animals ranges from 7.1 (1/14) to 100% (33/33) ( Table 3). The high parasite burdens were widely observed (Ye, 1989; Yang, 1998; Zhang et al., 2010), with the highest documented number of B. schroederi up to 3204 in a single giant panda (Zhang et al., 2010) . Higher prevalence of the parasite was reported in the wild and/or dead giant pandas (Kong and Yin, 1958; Feng and Zhang, 1991; Yu et al., 1998), while lower prevalence was reported in captive giant pandas in zoos (Wang et al., 2001; He et al., 2012).

In terms of the infection rate and infection intensity of parasitic diseases including baylisascariasis, captive giant pandas and wild giant pandas are quite different. In captivity, giant pandas receive good veterinary care, resulting in minimal rates of infection, and intensity of parasitic diseases, however these rates are substantially higher amongst wild giant pandas. In captive giant panda populations, the transmission of B. schroederi depends on various factors, such as housing system, hygiene, management practices and anthelmintic treatment. However, the current short-term control strategies of this parasitic infection are mainly based on monthly coprological examination of the parasitic eggs and a mass anthelmintic treatment. A number of anthelmintics have practically been used, such as pyrantel pamoate, albendazole, fenbendazole, mebendazole; ivermectin, milbemycin oxime, doramectin and selamectin (Wang et al., 2018). Usually, multiple (2–4 times) treatments are given until an individual panda ceases to expel worms and/or eggs in the feces. However, the possibility or likelihood that drug resistance in Baylisascaris could emerge as a problem has stimulated the search for alternative methods of prevention and control. One possibility could be to develop a vaccine against baylisascariasis (Wang et al., 2008; Xie et al., 2013). Apart from work directed towards a vaccine against B. schroederi , efforts have also been made to understand aspects of the molecular biology and genetics of this parasite.

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General protocols of microscopic diagnosis: The feces, vomit, intestinal contents (for necropsy), blood or tissue samples, or surface skin samples of giant pandas were obtained, and then subjected to the direct, smear, or stain observation under the microscopy. The parasites were preliminarily identifed based on the morphology, size, coloration, refraction of the eggs/oocysts/cysts/larva, or adult of the parasites, as well as the biological characteristics of the parasitic host.

General protocols of PCR diagnosis: The total genomic DNAs of the suspected samples were extracted, and then amplifed in vitro (PCR instrument was usually used) based on the specifc gene sequences (such as, SSU rRNA, ITS). The amplicons were identifed by the electrophoresis, and sequencing if necessary.