Sarcocystis arctosi sp. nov. (Apicomplexa, Sarcocystidae) from the brown bear (Ursus arctos), and its genetic similarity to schizonts of Sarcocystis canis-like parasite associated with fatal hepatitis in polar bears (Ursus maritimus)

Abstract

The tissues of herbivores are commonly infected with cysts of parasites belonging to the apicomplexan genus Sarcocystis, but such sarcocysts are rare in bears. Here, we describe a new species, Sarcocystis arctosi, based on the mature sarcocysts identified in two brown bears (Ursus arctos) from Alaska, USA. Microscopic sarcocysts (37-75 × 20-42 µm) had thin walls (<1 µm). The outer layer of the sarcocyst, the parasitophorous vacuolar membrane (pvm), was wavy in outline and had minute undulations that did not invaginate towards the sarcocyst interior; these undulations occurred at irregular intervals and measured up to 100 nm in length and up to 60 nm width. The ground substance layer beneath the pvm was smooth and lacked microtubules. Longitudinally cut bradyzoites measured 5.6-6.8 × 0.7-1.8 µm. A major portion of nuclear small subunit rDNA sequence obtained from these sarcocysts was similar to that previously obtained from the hepatic schizonts of a S. canis-like parasite from polar bears (Ursus maritimus).

Fatal Hepatic Sarcocystosis in a Captive Black Bear (Ursus Americanus) Associated with Sarcocystis Canis-Like Infection

“…with 100% homology to both S. canis and the newly described S. arctosi. 12 Sarcocysts were not observed in the present case, so no morphological comparison could be conducted to determine whether this organism might have characteristics of S. ursusi n. sp., as reported in black bears from Pennsylvania.…”

“…With the aid of molecular techniques, mature sarcocysts of S. arctosi in the musculature of 2 brown bears could not be distinguished from S. canis-like schizonts in a polar bear liver, prompting the authors to hypothesize that S. arctosi and S. canis could represent the sarcocyst and schizont stages, respectively, of the same etiological agent. 12 Moreover, there is no published genetic basis for differentiating S. canis from S. arctosi because the 18S rRNA sequences of these 2 proposed species are identical in the regions where both have been sequenced (994/994 base pair sequence identity). In the current report, molecular analysis suggests that the same S. canis-like agent is capable of causing fatal hepatitis in black bears and represents the first time this etiologic agent has been definitively identified in black bears by molecular techniques.…”

“…10 Mature sarcocysts have also been identified in 2 brown bears (Ursus arctos) from Alaska, and the name Sarcocystis arctosi was proposed. 12 Sporadic reports of fatal protozoal hepatitis associated with Sarcocystis canis-like infection have been reported in a variety of animals, including dogs, 1,2,9,13,24 a chinchilla, 21 marine mammals, 20,22,26 a horse, 7 a black bear, 27 and 2 polar bears (Ursus maritimus). 17 Reported herein is a case of fatal protozoal hepatitis in a captive mature black bear associated with a Sarcocystis spp.…”

“…display a narrow range of intermediate hosts. 12 It should be noted that currently no tests are available to unequivocally diagnose S. canis, 8 and past diagnoses have been based on organism morphology, location of schizonts within hepatocytes, and immunohistochemical staining that is positive for Sarcocystis spp. and negative for S. neurona, Neospora spp., and T. gondii.…”

“…11 Although the genus Sarcocystis includes more than 100 species, the life cycles of most have not been fully characterized. 11,12 Sarcocystis spp. employ a 2-host, preypredator life cycle in which asexual development occurs in an intermediate host (herbivore) and sexual reproduction occurs in the definitive host (carnivore/omnivore).…”