An 11-yr-old Peron's tree frog (Litoria peronii) was presented with neurologic signs progressing from head bobbing to circling and incoordination. Postmortem and histologic evaluations demonstrated the presence of a cerebral xanthoma, a large cholesterol granuloma expanding the leptomeninges. The nodule in the cerebral cortex consisted of arrays of cholesterol clefts surrounded by foamy vacuolated macrophages. The factors leading to focal cerebral xanthomatosis are unclear, and the relationship between trauma, dietary lipid composition, hypercholesterolemia, and xanthomas in amphibians requires further investigation.
Peron's tree frogs (Litoria peronii) are small, locally abundant inhabitants of forest, grassland, and open areas in eastern Australia with an estimated adult population of over 50,000. They are often referred to as the maniacal cackle frog because of their characteristic high-pitched vocalizations. Despite their local abundance in the wild and presence in captive collections, little information has been documented on their health and susceptibility to disease.
Xanthomas (cholesterol granulomas) are focal or multifocal, nodular lesions in human and veterinary cases that consist of accumulations of cholesterol and other lipids in tissues, surrounded by granulomatous inflammation (Ginn et al., 2007; Hargis and Ginn, 2007; Kumar et al., 2010). Cholesterol deposition in tissue incites granuloma formation, characterized by sheets of vacuolated macrophages and numerous multinucleated giant cells. Disruption of surrounding tissues occurs because of compression by the impinging mass as it enlarges, or by inclusion of the affected tissue within the granuloma.
Whereas cutaneous manifestations predominate in humans, cats, and birds (Schmidt et al., 2003; Ginn et al., 2007; Kumar et al., 2010), cerebral xanthomas (also referred to as cholesteatomas or cholesterol granulomas) have been reported in horses, meerkats, reptiles, and amphibians (Jackson et al., 1994; Allan et al., 2006; Maxie and Youssef, 2007). Occurrences of cerebral xanthomas in herpetofauna have been reported in Cuban tree frogs (Osteopilus septentrionalis), geckos, and green water dragons (Physignathus cocincinus) (Carpenter et al., 1986; Russell et al., 1990; Garner et al., 1999; Kummrow et al., 2010), although most taxa are likely susceptible.
An 11-yr-old Peron's tree frog (Litoria peronii) presented to the Melbourne Zoo veterinary hospital for a left-sided head tilt of 1 wk duration. The frog demonstrated intermittent head bobbing behavior, which progressed to incoordination, circling to the left, and severe head tilt. Radiographs were unremarkable and the frog was euthanized for necropsy. Postmortem examination found the frog was in moderate body condition, had bilateral opacities in the lenses, and a poorly aerated right lung containing a pale yellow nodule 3 mm in diameter. A range of tissues was placed in buffered 10% formalin for histopathology.
After fixation of the brain, a single yellow nodule (3 mm diameter) was noted on the dorsal surface of the cerebral cortex. Histologically, this nodule corresponded to a large cholesterol granuloma expanding the leptomeninges (Fig. 1). The nodule consisted of arrays of cholesterol clefts surrounded by foamy vacuolated macrophages (Fig. 2). The diagnosis was cerebral xanthoma.
The liver showed diffuse hepatocellular vacuolation consistent with fatty change, and an increased accumulation of melanomacrophages. The lung contained a small focal bronchoalveolar proliferation suggestive of a bronchoalveolar neoplasm, which in view of the absence of respiratory signs was considered incidental. All remaining tissues (including eye) appeared normal, with no evidence of cholesterol or lipid deposition.
This report of central vestibular disease associated with a single cerebral meningeal xanthoma extends the findings of Carpenter et al. (1986), who reported a large xanthoma arising from the region of the third ventricle in a Cuban tree frog that displayed decreased mobility. The histologic features in the present case are essentially the same as those described for xanthomas in humans, other mammals, birds, amphibians, and reptiles (Carpenter et al., 1986; Schmidt et al., 2003; Allan et al., 2006; Ginn et al., 2007; Kumar et al., 2010; Kummrow et al., 2010). Neurological signs reported in reptiles with cerebral xanthomas include opisthotonous, horizontal head bobbing, and incoordination in green water dragons (Kummrow et al., 2010); torticollis and opisthotonous in a Northern gecko (Naultinus grayi); and seizures in a leaf-tailed gecko (Uroplatus henkeli) (Garner et al., 1999).
The Peron's tree frog in this report was housed in an exhibit with its female conspecific and two pink-tongued lizards (Tiliqua gerrardii), and had been observed for 5 months before clinical signs were noted. In horses, choroid plexus xanthomas are often subclinical (presenting as incidental postmortem findings), or affected animals may present with subtle progressive neurological deficits (Jackson et al., 1994). In amphibians, subtle changes in muscle tone, proprioception, or mental acuity may be easily overlooked, thus limiting the likelihood for an early diagnosis.
In this case report, the xanthoma was found over the cerebral cortex. Although the majority of intracranial xanthomas reported in amphibians and reptiles are in the ventricles, significant neurologic deficits may arise from either location (Carpenter et al., 1986; Kummrow et al., 2010; Russell et al., 1990). Few reports of asymptomatic intracranial xanthomas in amphibians are published; however, subclinical cases are likely to be underreported because of a lack of diagnostics or failure to examine brain tissue postmortem (Carpenter et al., 1986). Further investigation is required to determine the relative contribution of xanthoma location, growth rate, size, and small brain size to the occurrence of clinical signs in amphibians compared to mammals.
Although many xanthomas are idiopathic, particularly if solitary, it has been suggested that the development of xanthomas may be triggered by aberrant lipid metabolism or excessive dietary lipid intake (Gross et al., 2005; Ginn et al., 2007; Hargis and Ginn, 2007; Kumar et al., 2010). It has also been suggested that trauma may predispose an animal to the formation of cutaneous xanthomas, though specific triggers leading to focal cerebral xanthomatosis are unclear. In horses, the xanthomas are most often found in the choroid plexus, and are interpreted to be an age-related degenerative change, possibly related to chronic intermittent congestion, edema, and hemorrhage (Jackson et al., 1994; Maxie and Youssef, 2007). In the present case, serum cholesterol and other lipids were not measured in view of the poor prognosis and rapid neurologic deterioration of the animal, and there was no gross or histologic evidence of extracranial cholesterol deposition. The fatty change seen in the liver may simply reflect catabolism of body fat stores secondary to anorexia, or it may indicate disordered lipid metabolism.
A previous report in Cuban tree frogs also described abundant fat in hepatocytes in addition to disseminated xanthomatosis, atherosclerosis, and corneal lipidosis with keratitis, with the corneal lesions being among the early clinical signs (Carpenter et al., 1986). In the present case, the Peron's tree frog had bilateral opacities in the lenses, but there was no histologic evidence of corneal lipidosis or keratitis, despite the well-developed nature of the cerebral lesion. Clinical evidence of corneal lipidosis or keratitis has not been noted in Peron's tree frogs in this collection, though long-lived individuals are susceptible to age-related degenerative changes of the lenses.
The potential contribution of dietary lipid has been discussed in previous reports of xanthomas in frogs, and the severity of xanthoma lesions is often directly related to the degree and duration of hypercholesterolemia (Carpenter et al., 1986; Russell et al., 1990). Peron's tree frogs are generalized insectivores, and in this collection are fed predominantly crickets. The lipid content of adult crickets, based on ether extract analysis, is approximately 13.8% on a dry matter basis (DM), compared to 18.4 and 32.8% DM in mealworm beetles and larvae, respectively (Bernard and Allen, 1997). Variation due to developmental stage of the insects may lead to excess consumption of dietary lipid even on well-planned diets if immature forms are fed out with adults. The relationship between dietary lipid composition and hypercholesterolemia in amphibians requires further investigation.
The occurrence of cerebral xanthomatosis may be underreported where brain is not routinely examined at necropsy or by histopathology.