In comparison to other captive murids, the captive Australian native tree-rats and stick-nest rats presented differences in their leukocyte morphology, haematology and serum biochemistry. The haematology and serum biochemistry values were relatively consistent between individuals, despite the use of different analysis equipment and regardless of some differences in collection methods between individuals. WBC counts were higher in females in both species. Both species also had high N:L ratios (tree-rat ratios were almost even). HCT was higher in male stick-nest rats than females. Differential leukocyte counts and leukocyte morphology was consistent with previous descriptions in other murids and between individuals. Blood biochemistry values were unremarkable except for the high level of globulin in stick-nest rats when compared to previous murid research (Bradley et al. 1988; Kemper et al. 1987; Monamy 1995; Old et al. 2005, 2007; Thrall et al. 2012).
Healthy specimens of both species had elevated total WBC counts in comparison to the other murids (Bradley et al. 1988; Kemper et al. 1987; Monamy 1995; Old et al. 2005, 2007; Thrall et al. 2012). Tree-rats had a mean WBC count that was almost double that reported previously for murids, while the stick-nest rats were within the expected range for murids, but at the higher end. Stick-nest rats had a higher WBC count when compared to other murids (Bradley et al. 1988; Kemper et al. 1987; Monamy 1995; Old et al. 2005, 2007; Thrall et al. 2012) and had a small standard deviation, suggesting the values are likely to be a true indication of ‘healthy’ stick-nest rat WBC counts. The differences in tree-rat mean WBC counts were different between the two sexes, females having higher counts. A larger sample size is needed to accurately determine species reference values (Table 2).
Both species had neutrophilia, as animals were classified as ‘healthy’ and did not show signs of inflammation, the cause of the condition can be assumed to be physiologic as a result of epinephrine or from stress (Harvey 2012). Neutrophils, usually make up 20–30 % (Provencher Bolliger and Everds 2012) of leukocytes. In tree-rats (44 %) and stick-nest rats (64 %) numbers of neutrophils were much higher than anticipated. Lymphocytes are usually the predominant leukocyte and can be as high as 70–80 % of the differential WBC count in the laboratory mouse (Provencher Bolliger and Everds 2012), however in the black-footed tree-rat lymphocytes were just below 50 % and made up 32 % of all WBCs in the greater stick-nest rats.
High neutrophil to lymphocyte ratios are useful indicators of poor health or stress (Old et al. 2005). On average both species had high N:L ratios, possibly a result of neutrophilia. A fifth of the stick-nest rats were skewed (6.6–11.5), while all other ratios were <4.0, which may account for the high mean ratio. Compared to other captive murids (Bradley et al. 1988; Kemper et al. 1987; Monamy 1995; Old et al. 2005, 2007; Thrall et al. 2012), both species had very high N:L ratios, with tree-rats three times and stick-nest rats six times larger than previously reported murid N:L values.
Both species were rarely handled or removed from their enclosure for any medical procedure. The stress of being handled prior to anaesthesia may explain the irregularities in the values as it can increase the number of neutrophils (Hedrich 2012). Anaesthesia, specifically isoflurane, can have an effect on the percentage of neutrophils found in C3H mice, with 30 min exposures leading to a 15.4 % reduction in the number of circulating WBCs, and specifically a 26.9 % reduction in neutrophils up to 48 h after exposure (Colucci et al. 2013; Jacobsen et al. 2004). Exposure to 4 % isoflurane, if administrated for a duration longer than 5 min may also have had an effect on erythrocytes parameters (Nahas and Provost 2002). The length of time the murids in this study were under anaesthesia is unknown.
The morphological appearance of leukocytes in the two species was similar to that described previously for other murids including the brown rat (Rattus norvegicus) (Thrall et al. 2012), plains rat (Pseudomys australis), spinifex hopping-mice (Notomys alexis) (Old et al. 2005) and the central rock-rat (Zyzomys pedunculatus) (Old et al. 2005). Neutrophils of both species in this study were larger in diameter when compared to the house mouse (Mus musculus) and brown rat (Thrall et al. 2012). Lymphocyte size greatly fluctuates from the size of erythrocytes to neutrophils (Thrall et al. 2012). Both species’ lymphocytes did not exceed the size of neutrophils. Monocyte size and morphology were similar to that previously described for other murid species (Bradley et al. 1988; Kemper et al. 1987; Monamy 1995; Old et al. 2005, 2007).
The low numbers of eosinophils and basophils was not unexpected. Eosinophil numbers are normally only elevated under certain conditions such as eosinophilia during an allergic response or in individuals with parasites (Harvey 2012). As the two species in this study were both from captive populations it is unlikely they would have had high parasite loads (due to regular treatment), and if allergic reactions were evident, would likely have been recorded in the clinical notes. In mammals, basophils are generally never found in high numbers and in some species can be absent (Latimer 2011).
Globulin values include levels of enzymes, antibodies, and fibrous and contractile proteins. The stick-nest rat had a mean of 30.2 g/L globulin, 8.9–17.8 g/L above the current reported murid range (Bradley et al. 1988; Kemper et al. 1987; Monamy 1995; Old et al. 2005, 2007; Thrall et al. 2012). The cause or effect of high globulin in rodents has not been investigated in detail. However in humans, high globulin can indicate chronic inflammation, an infectious disease, leukaemia, diseases of the liver or kidneys, or an autoimmune disease (Willard and Tvendten 2012). Stick-nest rats over the age of 4 years did display a higher globulin level than their younger counterparts, presumably as they had been exposed to more pathogens than the younger animals. As the expected longevity of free-ranging stick-nest rats is 4 years (Jackson 2007), advanced age (or the wide range of ages of murids in this study) is a reasonable explanation for these high values.
ALP is associated with measurements of skeletal growth and can be used as an indicator of age, with levels decreasing as the animal reaches adulthood (Calabuig et al. 2010). Tree-rat ALP was higher in older animals than younger animals and was not consistent with previous murid values (Bradley et al. 1988; Kemper et al. 1987; Monamy 1995; Old et al. 2005, 2007; Thrall et al. 2012). Stick-nest rat ALP values were low in young individuals, peaked around 2.5 years, and dropped again when animals reached 4 years. In quolls (Stannard et al. 2013) and other murid species (Old et al. 2005) ALP levels varied greatly between individuals. High ALP has been seen as an effect of captivity in the black vulture (Aegypius monachus), as well as poor health (Villegas et al. 2002). Whilst higher ALP values have also been reported in healthy captive southern hairy-nosed wombats (Lasiorhinus latifrons) compared to wild wombats (Gaughwin and Judson 1980). A larger number of samples with a wider range of ages are needed to determine the reasoning behind the variability in the results and whether captive management is affecting the ALP values of these species.