Abstract
Within the European Endangered Species Program (EEP) cheetah (Acinonyx jubatus) population, numerous cases of hind limb paresis have occurred in cheetah cubs over the past years. This paper describes a nerval disease of cheetah cubs in three different clusters during the period from 1995–1997. This disease is clinically characterized by an acute onset of hind limb ataxia-paresis. On histopathology, mild to severe demyelination, predominantly of the dorsal spinocerebellar tracts and the sulcomarginal tracts, was found throughout all levels of the spinal cord. The onset of ataxia was regularly in chronologic relation to sneezing, ocular and nasal discharge. Serum samples, conjunctival and nasal swabs confirmed the presence of feline herpes virus 1 (FHV1). The authors suggest a possible viral etiology. Ataxia progression was inhibited by the application of the anti-herpes drug acyclovir and prednisolone. A primary-latent oro-nasal FHV-1 infection may, under stressful conditions, trigger an immunologic process leading to ataxia with myelin destruction.
Introduction
Within the European Endangered Species Program (EEP) cheetah (Acinonyx jubatus) population, numerous cases of hind limb paresis have occurred in cheetah cubs over the past years. In reports made to the International Cheetah Studbook, various facilities acknowledge hind limb paresis as a cause of death in related juvenile animals.6 These cases have occurred in certain focal clusters. Taking the data obtained by an EEP questionnaire amongst the cheetah holders, eight locations in Europe, one in Namibia, and one in Dubai were identified (Walzer 1997, unpublished data). Hind limb paresis in cubs is at the present one of the most important limiting factors in the growth of the EEP cheetah population and the major veterinary concern of the cheetah EEP and European Feline Taxon Advisory Group (TAG).
Material and Methods
This paper describes spontaneous ataxia in four litters in three separate clusters (Nüremberg, Germany; Salzburg, Austria; Fota, Ireland) within the Cheetah EEP population. A very detailed description of the clinical and serologic findings has been published previously.10
In Nüremberg and in both litters in Salzburg it was noted that the ataxia occurs spontaneously (within minutes to hours). In two litters (Nüremberg and Salzburg #1) ataxia occurred following a stressful experience for the individual or the litter. In all three clusters ataxia was preceded by ocular and/or nasal discharge, as is commonly seen in clinical FHV-1 infection.
From the two litters in Salzburg serologic evidence of an FHV-1 infection was determined. FHV-1 was isolated in cell culture from nasal and ocular swabs. PCR methods were established using primers selected on the basis of FHV-1 sequences in the GenBank database. The amplified products of the PCRs were sequenced and compared with the FHV-1 sequences in the GenBank showing a 99% identity compared to the published sequences. A cross-neutralization test using FHV-1 isolates from a domestic cat and a cheetah, as well as the corresponding antisera, yielded no significant titre differences. These results conclusively demonstrate that the isolates from the cheetah and the domestic cat are very closely related.
Complete histopathology has to date been carried out on six ataxic cubs in the three clusters. Though the degree of the lesions varies with the time lapse between ataxia onset and histopathology it is by and large identical in all cubs. Neuropathology of brain, spinal cord, sciatic nerves and brachial plexus revealed only lesions of the spinal cord. A severe demyelination, predominantly of the dorsal spinocerebellar (ascending) tracts and the sulcomarginal (descending tracts of the extrapyramidal system) tracts, was found throughout all levels of the spinal cord. The demyelination was associated with severe astrogliosis, some degree of axonal swelling and focal small non-suppurative inflammatory infiltrates in the white matter and the leptomeninges. Intriguingly, the ascending tracts of the dorsal funiculus (gracile and cuneate fascicle) were consistently unchanged. Brain and peripheral nerves showed no pathologic changes.
CNS tissue (cerebrum, cerebellum, hippocampus and brain stem) from cub FD90 were examined with the thymidine-kinase-PCR. All the tissue samples proved positive. Negative controls were all negative. However, when inoculating these same organ samples on CRFK cell cultures, no CPE was observed.
Ataxia in litter 2 in Salzburg appears to have resolved upon therapy consisting of acyclovir (Zovirax®, Glaxo-Wellcome GmbH Vienna, Austria) and prednisolone (Solu-Dacortin®, E. Merck, Darmstadt, Germany).
Of the five animals in the Nüremberg litter, one cub was hand raised and therefore had no contact to the dam and other littermates, this animal never developed any clinical ataxia and no CNS lesions could be identified at necropsy.
Discussion
This report describes a nerval disease of cheetah cubs which is characterized by acute onset hind limb paresis and ataxia. Literature reports on this subject are very scarce. Ataxia in cheetah cubs is mentioned only by Brand1, Zwart12 and Hafner2. These papers discuss ataxia as a result of a copper deficiency and one paper compared it to enzootic ataxia of goats.12 The demyelinating lesions described in this paper are confined to the spinal cord, which is in contrast to lesions in enzootic ataxia in sheep and goats, where neurons in the brainstem and spinal cord are also affected.8 Furthermore on neurologic examination the sheep and goats affected with enzootic ataxia demonstrate flaccid paresis, hypotonia and hyporeflexia (lower motor neuron disease) which is in clear contrast to the considered upper motor neuron disease established in cubs in litter #1 Salzburg. Considering the non-responsiveness of the ataxia to aggressive copper therapy as carried out by Gaukler in the Nüremberg Zoo (A. Gaukler, personal communication), and similar experiences encountered in Fota (L. Guerin, personal communication), we believe that copper deficiency can be discounted as a primary etiology in this nosological entity.
In adult cheetahs there are reports on animals with a spinal demyelinating disease located between T6 and L3. A hereditary myelopathy with a late onset was suggested as a possible cause.10 The main difference between this description in adult cheetahs and the ataxia in the cubs is in the distribution of the lesions in the spinal cord and the progressive onset in the adult animals. Furthermore, as the cubs in the three clusters were born of different parents, a hereditary degenerative disease is unlikely.
The pattern of the neuropathologic lesions and the absence of distinct inflammatory changes seem to exclude a viral etiology. On the other hand, various facts collected in the three clusters suggest an infectious agent as the cause of the ataxia. In Nüremberg, four parent-raised cubs developed ataxia, whereas, the hand raised littermate did not. The onset of ataxia was regularly in chronologic relation to sneezing, ocular and nasal discharge. The inhibition of ataxia in litter 2 in Salzburg, with the application of the anti-herpes drug—acyclovir—suggests a viral agent as etiology.
Some interesting parallels seem to exist between cheetah cub ataxia and multiple sclerosis (MS), the most common human demyelinating disease. Although the diseases have significant morphologic differences and cannot be compared per se, the symptoms of MS result from recurrent inflammatory attacks on the CNS.7 Litter 2 in Salzburg experienced two separate onsets of ataxia. Similar to the spontaneous clinical onset in the cheetah, over 50% of the MS patients report an abrupt onset of the neurologic symptoms; this although, the destruction of the myelin is a complex process that most likely takes several days to complete.7 As in the cheetah ataxia, MS occurs in clusters. One assumes that an environmental factor is responsible for the geographic link.5 MS may begin when the immune system is activated by some environmental event, a virus or other infection.4 The difficulties encountered in trying to reconcile the myelin destruction observed in MS with primary involvement of CNS antigens has led to a theory of possible cross reacting infectious antigens.7 Recently it has been demonstrated that T-cells cloned from MS patients recognize not only the CNS antigen myelin basic protein (MBP) but also numerous antigens from various infectious agents such as herpes simplex, influenza and Epstein-Barr virus.11
It can be speculated that a primary-latent oro-nasal FHV-1 infection may trigger an immunologic process similar to that in MS, leading to ataxia with myelin destruction.
At the present, in situ hybridization work and immunohistochemistry is ongoing to attempt to explain the immuno-pathologic process involved in cheetah cub ataxia.
Acknowledgments
The authors wish to thank the following colleagues: Dr. S. Kölbl for serologic examinations, Dr. K. Melzer for CDV diagnostics, Dr. T. Hänichen and PD Dr. P. Schmidt for providing path samples from the Nüremberg animals, Dr. A. Gaukler for clinical information on the Nüremberg litter, Dr. L. Guerrin and Dr. N. Stronach for the information and discussions on the Fota cubs, Dr. Sean Callanan for performing the necropsy on the Fota cub, Prof. Dr. P. Zwart for providing histo slides, Dr. S. Kneissl for radiologic evaluation, the keepers Joe Angermann and Baskhim Kameri for their support in the extensive sampling procedures.
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