Baylisascaris spp.

Sapp, Sarah G. H., Gupta, Pooja, Martin, Melissa K., Murray, Maureen H., Niedringhaus, Kevin D., Pfaff, Madeleine A. & Yabsley, Michael J., 2017, Beyond the raccoon roundworm: The natural history of non-raccoon Baylisascaris species in the New World, International Journal for Parasitology: Parasites and Wildlife 6 (2), pp. 85-99 : 90-93

2.2. Baylisascaris spp. of bears

2.2.1. Baylisascaris transfuga

Baylisascaris transfuga was originally described by Rudolphi (1819) as Ascaris transfuga and re-described as a Toxascaris species in 1922 ( Baylis and Daubney, 1922). Ultimately Sprent (1968) formally described the Baylisascaris genus and designated Baylisascaris transfuga as the type species for this genus. Morphological characteristics and/or molecular techniques can be used to distinguish this species from other Baylisascaris spp. ( Table 3). Baylisascaris transfuga adults can be morphologically distinguished from other species by their spicule length (estimated between 0.80 and 0.92 mm), having between 46 and 70 precloacal papillae, rounded posterior margin of the pericloacal area, salient alae, denticles in equilateral triangles, and a saddle shape of the median lobe of the lip ( Testini et al., 2011; Sprent, 1968; Baylis and Daubney, 1922). Eggs of B. transfuga are morphologically similar to other Baylisascaris spp. eggs and are largely considered indistinguishable ( Sprent, 1968; Kazacos and Turek, 1983; Testini et al., 2011) ( Table 1).

Of interest is that morphometrics of parasites reported as B. transfuga from numerous hosts across several continents show variation ( Table 3). Measurements have been made on specimens from black bears ( Ursus americanus ) in Canada, multiple subspecies of brown bear ( Ursus arctos ) from across sites Eurasian and North American sites, multiple captive polar bears ( Ursus maritimus ) in various sites, sloth bears in India ( Melursus ursinus ), and a captive sun bear ( Helarctos malayanus ) ( Canavan, 1929; Baylis and Daubney, 1922; Sprent, 1968; Testini et al., 2011; Moudgil et al., 2014). Sprent (1968) reported a maximal length of 120 mm for males and 240 mm for females; other studies have found smaller size ranges for both males and females ( Table 3). Egg dimensions are also variable and have been reported as low as ~57 M m wide up to ~94 M m long, although fertilization or embyronation status may influence this morphology ( Table 3).

These morphologic differences in a limited number of parasites examined across a wide geographic and host range suggest that these “ B. transfuga ” may represent several distinct species. Currently there are two distinct Baylisascaris species apart from B. transfuga reported within Ursidae , B. schroederi of giant pandas ( Ailuropoda melanoleuca ), and the recently-described B. venezuelensis from spectacled bears ( Tremarctos ornatus ). Further support is provided by preliminary molecular data on B. transfuga samples from Alberta, Canada and West Virginia that suggest these parasites are genetically distinct across locations (L. Camp, pers. comm.). Careful morphologic analysis combined with molecular characterization of B. transfuga samples from a diverse geographic and host range is needed to address parasite diversity in the Ursidae .

2.2.1.1. Epidemiology. Worldwide, B. transfuga has been reported in all extant species of bear in the family Ursidae excluding the spectacled bear although unidentified ascarid eggs have been detected in spectacled bear feces ( Figueroa, 2015). Baylisascaris transfuga infections have been reported from American black bears ( Ursus americanus ), sloth bears, polar bears, brown bears ( Ursus arctos ), Malayan sun bears ( Helarctos malayanus ), Asiatic black bears ( Ursus thibetanus ), and giant pandas ( Ailuropoda melanoleuca ) although these reports are most likely B. schroederi ( Sprent, 1968; Rudolphi, 1819; Canavan, 1929; Baylis and Daubney, 1922). No non-bear definitive hosts are known.

The prevalence of B. transfuga in bears varies widely among studies ( Table 4). In North America, the majority of studies have been conducted on black bears but natural infections have been reported in brown bears and in captive polar bears. Prevalence of B. transfuga in black bears appears to be highest in Alberta, Canada and the Great lakes regions of the USA. In brown bears, prevalence of infection with B. transfuga was higher in the Wyoming and Montana, USA compared to Alaska and Canada.

A few studies have attempted to investigate seasonal trends in prevalence with some conflicting results. One, based on fecal floatation, detected a higher prevalence in spring compared to the fall in black bears in Quebec, Canada ( Frechette and Rau, 1978). Another, based on collection of nematodes at necropsy in black bears and grizzly bears in western Canada observed the opposite seasonal trend with peaks in the fall but this study was ( Frechette and Rau, 1978; Catalano et al., 2015). However, the seasonal association with prevalence in black bears was weakly significant (p = 0.04) and sample sizes were relatively small (n = 40); sample size for grizzly bears was too small for statistical analysis. Finally, Rausch (1954) and Rogers (1975) found that bears were shedding eggs in the spring soon after torpor and found evidence of egg shedding just prior to denning, so it remains unclear whether or not infections are cleared during winter torpor. Additional studies are needed with greater sample sizes and age class representation to accurately assess the seasonal ecology of B. transfuga in bears.

In the bear host, B. transfuga infections do not typically cause clinical disease, but heavy infections have been reported causing clinical disease or death. Reports of disease in the natural bear host include peritonitis in a brown bear in Europe and suggestions of enteric impactions ( Mozgovoi, 1953; Szczepaniak et al., 2012). Subclinical effects including reduced host condition have also been reported in infected bears ( Fu et al., 2011).

2.2.1.2. Non-de fi nitive hosts. The risk of B. transfuga to cause clinical disease as a result of larva migrans in aberrant hosts is considered low compared to other Baylisascaris species. This is partly due to their slower growth rate and smaller size as well as the decreased penetration of the intestinal wall by B. transfuga larvae resulting in fewer larvae being detected in visceral organs compared to other species ( Sprent, 1952b, 1953a; Sato et al., 2004; Schaul, 2006).

Despite a reduced migratory capacity relative to other Baylisascaris spp. , experimental infections in laboratory mice, Mongolian jirds ( Meriones unguiculatus ), guinea pigs, rabbits, and chickens indicate that B. transfuga can occasionally cause VLM, OLM, and/or NLM; however, in most hosts, clinical disease was mild or not apparent ( Sprent, 1952b, 1955; Papini et al., 1993; Papini et al., 1994, 1996a; Papini and Casarosa, 1994; Sato et al., 2004; Matoff and Komandarev, 1965). There is substantial variation among hosts in disease severity.Laboratory mice developed only mild clinical disease with granulomas in the brain. Mongolian jirds developed severe clinical signs with malacia and lack of host immune reaction ( Sato et al., 2004). Rabbits displayed a loss of appetite, dyspnea, and depression but no neurological signs ( Papini et al., 1996a). No clinical signs were observed in experimentally-inoculated chickens ( Papini et al., 1993). In guinea pigs, Matoff and Komandarev (1965) showed that B. transfuga larvae migrate into the intestinal wall and either encyst in the wall, penetrate the intestinal wall and enter the abdominal cavity, or travel to the lungs, heart, and/or skeletal muscles through the lymphatics and systemic circulation. However, larvae were not noted in brains. Sprent (1953a) reported that larvae were still alive in experimentally-infected mice one year after infection.

Despite experimental studies suggesting a wide range of hosts may develop larva migrans, only one presumed report of natural infection of a paratenic host has been reported. Japanese macaques ( Macaca fuscata ) housed near American black bears at a zoo in Japan developed fatal neurological disease; however, identification of the parasite was not definitive in this case and identification of the parasite was based only on histology ( Sato et al., 2005). While there are no confirmed reports of larva migrans in humans following B. transfuga infection, experimental evidence with other species shows that given a sufficiently high infection, larva migrans in people may be possible.

2.2.1.3. Treatment and control. Baylisascaris eggs present in the environment or in captive animal facilities are difficult to eliminate or kill. Similar to other Baylisascaris species, eggs of B. transfuga become infective after ~2 weeks and can remain infective for at least 15 months under artificial conditions ( Papini and Casarosa, 1994). Eggs have reported to persist in the environment for up to five years, and infected bears can pass between 100 and 19,800 eggs per gram of feces so environments can become contaminated with large number of eggs quickly ( Abdel-Rasoul and Fowler, 1979; Vercruysse et al., 1976). In captivity, the prevalence of B. transfuga is higher in certain species ( U. maritimus , Melursus ursinus ), although this could be sampling bias, the substrate (e.g. sand or soil) used in enclosures, or the housing of bears in groups ( Schaul, 2006). Strict, routine quarantine and treatment of bears in captive settings can be an effective way to reduce shedding and prevent subsequent infections.

Treatment of bears infected with B. transfuga has only been done in captive situations. Numerous anthelmintics have been used to manage B. transfuga infections in captive bears; however, efficacy is variable and dose-dependent ( Clark et al., 1969; Moudgil et al., 2014). Dichlorvos (19 mg /lb) rapidly (1‾2 days post treatment) reduced fecal egg counts (FEC) to zero in many bear species; however, these animals became reinfected within months after treatment, emphasizing the need to clean the environment ( Clark et al., 1969). Orally-administered fenbendazole (10 mg /kg) on three consecutive days was unable to reduce fecal egg counts to zero in a sloth bear, however, this infection was cleared with 15 mg / kg for three days followed by the original treatment ( Moudgil et al., 2014). Mebendazole was used successfully to treat five polar bears infected with B. transfuga ( Vercruysse et al., 1976) . Macrolides, benzimidazoles, and tetrahydropyrimidines have all been used in North American zoos to treat bears but efficacy data were not provided (Schaul, 2009).

Due to concerns about larva migrans, it is important to determine if larvae could be killed prior to entering the CNS. Laboratory mice given one dose of 2 mg /kg ivermectin had fewer lesions and resulted in fewer B. transfuga larvae recovered in visceral organs compared to those without treatment ( Papini et al., 1996a,b,c). In another study, single doses of levamisole or ivermectin were administered subcutaneously and intramuscularly, respectively, to groups of inoculated laboratory mice 3 DPI or 14 DPI ( Fu et al., 2011). Upon necropsy, reduced larval burdens were observed in treatment groups compared to controls. Levamisole resulted in an 81% decrease in “migrating” larvae (at 3 DPI) but only a 49% overall decrease in “encapsulated” larvae (at 14 DPI).Although ivermectin had similar activity against larvae at 3 DPI (88% reduction), it had greater activity against larvae at 14 DPI (75% reduction). Levamisole-treated mice at 14 DPI had fewer larvae within the brain (43% versus 24% in ivermectin group) and appeared to ameliorate the severity of neurologic signs in two of five mice displaying clinical disease after 17 DPI ( Fu et al., 2011). These data suggest that larvicidal activity will vary depending on the age of infection, resulting in changes in susceptibility to the drugs during migration, and in encapsulation status. Pharmacokinetics of anthelmintics also influence treatment efficacy; for example, ivermectin does not cross the blood-brain barrier whereas levamisole appears to do so ( Fox, 2006; Lin and Tsai, 2006). Given this differential susceptibility and drug efficacy, it seems the best option for treating B. transfuga larva migrans in paratenic hosts is to use multiple drug classes in order to maximize larvicidal activity in brain, viscera, and skeletal muscle.

2.2.2. Baylisascaris venezuelensis

A new species, Baylisascaris venezuelensis , originating from a South American spectacled (Andean) bear ( Tremarctos ortnatus ) in western Venezuela was recently described (Ṕerez Mata et al., 2016). A female spectacled bear in poor body condition was found dead and at necropsy, a large number of Baylisascaris were present in the gastrointestinal tract. Other gross pathologic findings included congestion and hemorrhagic foci in the lungs. The authors suggest that the high worm burden was the cause of mortality.

An adult male and female nematode were examined morphologically. Fewer post-cloacal papillae (n = 44) were present compared to Nearctic and Palearctic B. transfuga worms, which have an average of 66 ( Sprent, 1968). Other morphologic features were similar to B. transfuga , including overall length, a stout appearance, salient cervical alae, similar length spicules, and a rounded posterior margin of the pre-cloacal area (with B. venezuelensis having a “little process” on this margin) (Ṕerez Mata et al., 2016). Molecular analysis supported the separation of B. venezuelensis as a separate species. Combined ITS-1 and ITS-2 sequences were only 91.8% and 90.6% similar to B. transfuga and B. schroederi , respectively (Perez ́Mata et al., 2016). There were also three nucleotide differences in the highly conserved region of 5.8S rDNA that differentiated B. venezuelensis from the two other ursid-associated species and other Baylisascaris spp. Phylogenetic analysis of both ITS regions included B. venezuelensis in a clade containing the other two ursid-associated species along with B. ailuri from red panda (Ṕerez Mata et al., 2016).

A few instances of previously detected ascarid eggs in fecal examinations of captive and free-ranging spectacled bears may represent B. venezuelensis infections ( Schaul, 2006; Figueroa, 2015). Eggs designated as “roundworm” eggs were present in 9/25 (36%) of spectacled bear fecal samples from zoos across the United States, although it is impossible to determine if this is B. venezuelensis or native B. transfuga acquired from other bears in the captive environment ( Schaul, 2006). Ascarid eggs resembling those of Baylisascaris or Toxocara were reported in 6/28 (21%) of T. ornatus scats from northern Peru, but measurements of the eggs were not provided ( Figueroa, 2015). It is currently unknown if B. venezuelensis is usually pathogenic for spectacled bears. The type host is believed to have died from the nematode infection, but most previously reported positive spectacled bears were presumably asymptomatic. Although most Baylisascaris spp. , including B. transfuga , rarely cause mortality in their definitive hosts, B. schroederi from pandas is a major cause of morbidity and mortality so additional research on the potential risk of B. venezuelensis to spectacled bears is needed ( Zhang et al., 2008).

The finding of a new, seemingly valid Baylisascaris species in a relatively isolated population of ursids further supports the idea that “ B. transfuga ” represents an assemblage of species globally, and highlights the need for further molecular and morphologic work to characterize possibly cryptic species. Field surveys are also necessary to determine the prevalence as well as definitive and paratenic host range of this new tropical species.