Disease occurrence in small mammals from Northern Borneo: Histopathology and selected pathogens

Small mammal species have been the study object for various scientific disciplines because their adaptation skills into different habitats, the basic position they occupy in the trophic pyramid and the role of multiple of these species as host and reservoir for parasites and zoonotic pathogens. From a veterinary point of view, there is a lack of information on the diseases of wild small mammal species and the pathogens they might carry, especially in degraded and anthropogenically modified environments, which could have a negative effect on the animal's health. For this reason, this project investigated 15 different small mammal species and a total number of 346 individuals captured between 2012 and 2013 along a landuse gradient in Northern Borneo from the tropical rainforest to the urban area of Kota Kinabalu. The animals belonged to the families Muridae, Sciuridae, Soricidae and Tupaiidae and were classified according to their habitat preference into forest species, i.e. forest rodents and treeshrews, the generalist Sundamys muelleri found along all habitat gradient and urban species as Rattus spp. and the shrew Suncus murinus. After capture animals were immediately euthanized, dissected and tissue samples were collected. Subsequently all animals were examined for subclinical histopathological changes followed by a scoring of their lesions to estimate the individuals' health status. Additionally, molecular analyses and transmission electron microscopy were conducted to identify selected pathogens.
The histopathological assessment showed that endoparasitic lesions were the most prevailing finding. Comparing the occurrence of pathological changes of the different organs the lung was predominantly affected with 89% (277/310), follow by lesions in liver (60% (192/321)), gastrointestinal tract (38% (102/266)) and striated muscle (sarcosporidiasis) (35% (108/309)). In all animals, the organs affected to a lesser degree were kidney and heart in 28% (89/314) and 19% (60/324), respectively. One the more notable findings were dense bacterial layers on the mucosal surface of the stomachs in the forest rodents Maxomys spp. and Niviventer sp., which so far was only described in laboratory rodents. Further, the first time a Klossiella-like infection in the kidneys of two Sundascirurus lowii squirrels was described. As the majority of pathological changes were observed in individuals from all species, the exposure to pathogens seems to occur regardless their chosen habitat. However, although lesions with parasitic origin were detected in forest species as well as in the generalist Sundamys muelleri, they were more predominant in urban individuals from the Rattus genus. Possible reasons could be the high density and the colony social structure, which favour parasite occurrence and transmission.
The evaluation and scoring of the health status, based on the histopathological findings showed that the majority of the individuals were estimated to be moderately affected 66% (219/331) by their lesions, while 22% (74/331) had no or only mild lesions and were classified as clinically healthy. In contrast to this, only 12% (38/331) of all individuals were assumed to have a severely affected health status. Comparing the ecological groups, the health status was similar, however, there were distinct differences of the health status between certain species living in the same habitat. Among forest species, treeshrews were healthier than forest rodents, which health status was similar to the generalist species Sundamys muelleri. Amongst urban species, the shrew Suncus murinus presented a better health status than individuals from the Rattus genus. These findings suggest that in general the habitat has not a significant influence of the health status. But intraspecific characteristics based on the behaviour, predation pressure and/or social structure may have an impact in the health of the species.
Species living in urban areas, mainly Rattus species, as well as the generalist Sundamys muelleri, were predominantly infected with Leptospira interrogans. While in contrast to this, infections of forest species showed a higher diversity of the detected Leptospira spp. DNA of the blood parasite Trypanosoma spp. was found in approximately 9% (24/277) of all investigated individuals. Amongst them, the potentially zoonotic Trypanosoma lewisi was predominantly found in Sundamys muelleri and urban Rattus species, meanwhile forest individuals were infected with either novel or unclassified Trypanosoma species. Histopathological investigations revealed that 30% (108/309) of all animals were infected by Sarcocystis. Further examination of the ultrastructural features of the Sarcocystis walls revealed the presence of three different Sarcocystis species (S. singaporensis, S. zuoi and S. villivillosi) in the forest species Sundascuirus lowii and Maxomys whiteheadi, and the generalist Sundamys muelleri. Additionally, a novel species, Sarcocystis scandentiborneensis sp. nov., was detected in treeshrews, corroborated by unique ultrastructural wall characteristics and phylogenetic evaluations. The morphological evaluation of Sarcocystis spp. showed a higher diversity among forest species and Sundamys muelleri, supporting that a broad range of small mammal species are intermediate hosts for the Sarcocystis spp. The occurrence of herpesvirus DNA was similar to Sarocystis spp. (29% (46/156) of all individuals), and although due to rather short fragments of herpesvirus DNA strain identification was not possible, sequence results indicated a wide diversity due to marked genetic heterogeneity of herpesviruses among animals from all habitats.
For the first time this study investigated multiple small mammal species trapped along a landuse gradient not only by molecular techniques, but by histopathology. This allowed to not only detect the presence of a given pathogen, but to get an estimate of the individuals' health status. Moreover, it lead to the identification of a novel parasite species (S. scandentiborneensis sp. nov.). Such investigations are only possible with an interdisciplinary approach, but to be able to better understand the interconnectivity of multiple animal species and pathogens in an ecosystem such approaches are indispensable.