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Copyright, S. Karger AG, Basel

Acute Encephalopathy Induced in Cats
with a Stealth Virus Isolated from a
Patient with Chronic Fatigue Syndrome

W. John Martin*, R. Tom Glass~

*Department of Pathology, University of Southern California School of Medicine, Los Angeles, Calif., and ~Department of Oral Pathology and Pathology, University of Oklahoma, Colleges of Dentistry and Medicine, Oklahoma City, Okla., USA

Abstract

A simian cytomegalovirus-related stealth virus, isolated from a patient with the chronic fatigue syndrome, induced an acute neurological illness when inoculated into cats. Histological examination of brain tissue showed foci of cells with cytoplasmic vacuolization and an absence of any inflammatory reaction. Electron microscopy confirmed the presence of herpes-like viral particles and viral-like products in the brain of an inoculated animal. These findings support the role of stealth viruses in the pathogenesis of human neurological diseases and provide an animal model to evaluate potential antiviral therapy.

Molecular probe- and viral culture-based assays on patients diagnosed as having the chronic fatigue syndrome (CFS) led to the identification of a novel class of cytopathic viruses provisionally termed stealth viruses [1,2]. These viruses appear to have been mainly derived from human and animal herpesviruses and to lack antigenic components normally responsible for evoking an effective cellular inflammatory antiviral response [2,3]. Using appropriate culture conditions, these viruses produce a foamy cell cytopathic effect in fibroblasts derived form multiple species, including feline cells [2].

Stealth viruses have been isolated from patients with various neurological and neuropsychiatric diseases [2,4, unpubl. observations]. This has led to the suggestion that CFS is part of a spectrum of dysfunctional brain syndromes attributed to a stealth virus encephalopathy [5]. Clarification of this issue could be provided by a suitable animal model. Unpublished surveys have indicated a higher than expected frequency of unexplained neurological diseases in domestic cats belonging to CFS patients. This finding, together with the observation that feline fibroblasts were permissive for the stealth virus originally isolated from a CFS patient, suggested that cats might provide a suitable animal model for the study of stealth viral encephalopathy. This paper presents the initial clinical and histological observations using this model.

Methods

The isolation and characterization of the stealth virus from the CFS patient D.W. is described in detail elsewhere [2]. This virus shows a high degree of DNA homology with the Colburn strain of simian cytomegalovirus and is considered to be of African green monkey origin [3]. A second stealth virus isolate was obtained from a patient (initials T.M.) who had been diagnosed as having systemic lupus erythematosus with extensive cognitive impairment. The viruses were grown in culture tubes on human MRC-5 fibroblasts containing 2 ml medium 199 plus 7% fetal calf serum. The cells from cultures showing a moderate (2/4) cytopathic effect were suspended in the medium and shipped frozen to the University of Oklahoma. After thawing, the suspension was drawn through a needle into a syringe and 0.5 ml inoculated intravenously into each of 5 cats. Cells from an uninfected culture were similarly processed as a control inoculum into a single cat. Daily clinical evaluations of the cats were made by one of us (RTG), an animal technician and a licensed veterinarian. All of the animal studies were performed under an approved protocol at the University of Oklahoma.

Results

All 5 cats inoculated with infected cultures remained asymptomatic for 48 h. Thereafter, an acute illness developed in each of the animals. The early manifestations developing within the 1st week included gingivitis, bloody ocular and nasal discharge, lymphadenopathy, pupil dilation with photophobia (squinting in response to light) and nuchal hair loss from rubbing against the cage. There was a reduction in body temperature beginning in the 2nd week which averaged 0.6 F at 2 weeks and 0.8 F at weeks 3 and 4. Most striking were the marked behavioral changes in all of the virus-inoculated cats. They lost the playfulness that was present prior to injection and became reclusive and irritable. They resisted being handled and the animal caretakers resorted to wearing heavy leather gloves. On direct palpation, the enlarged lymph nodes and various muscle groups were identified as being painful for the animals.

The cats were euthanized at 1, 2, 4, 6 and 15 weeks after virus inoculation. Although only a few animals ere observed, the severity of the illness peaked at around 4 weeks with definite improvement noted in the cat surviving to 6 weeks. The cat that lived to 15 weeks had resumed normal activities by week 10 and appeared to be symptom-free.

The control cat that received material from an uninfected culture developed diarrhea with mild abdominal distention. Coccidia were detected in his stools which were eliminated with medication. Neurological signs were absent. This cat was necropsied 6 weeks after inoculation. The control cat that received the heat-inactivated virus-infected cells showed very minimal evidence of gingivitis with no behavioral changes. This animal received two subsequent inoculations of noninactivated virus at 3-week intervals without the development of any acute signs or symptoms, apart from a transient drop in temperature. This cat subsequently became acutely sick with neurological changes when inoculated with a stealth virus isolated from the patient T.M..

Histological examination of the virus-inoculated cats revealed widespread, although subtle, cellular changes throughout many tissues including the brain. Comparable changes were not apparent in the tissues from the control cat, or from animals used in nonrelated studies. Tissues from the virus-inoculated animals contained scattered foci of cells showing cytoplasmic and nuclear vacuolization with varying degrees of cellular disruption and degeneration. Strikingly, none of the tissues showed an inflammatory infiltrate, although occasional eosinophils were seen within the lymph nodes. The histological changes were apparent in all of the virus-inoculated cats, including the animal euthanized at 15 weeks. A section of brain tissue and of a lymph node illustrate the vacuolated nuclear and cytoplasmic changes seen in the tissues of the virus-inoculated animals. Electron microscopy of brain tissue from the 2-week-inoculated animal confirmed the presence of herpesvirus-like particles. Structures suggestive of incomplete viral capsids can also be seen in this section.

Discussion

These initial results are presented because they help establish the neuropathogenicity of the prototype stealth virus cultured from a CFS patient. They also confirm the essential lack of an overt inflammatory reaction accompanying infection with this virus [2]. Interestingly, the major clinical manifestation was altered behavior exemplified by irritability and reclusiveness. The dilated pupils and avoidance of light also pointed to a neurological illness. Prolonged rubbing against the cage leading to areas of complete nuchal hair loss was possibly an attempt to alleviate headaches. Similarly, the bloody ocular and nasal discharges were most likely due to repeated scratching in response to pain and irritation. These findings clearly establish that the simian cytomegalovirus-related stealth virus repeatedly isolated from a CFS patient [2,3] is pathogenic in cats.

The cat was chosen as a possible model for CFS and for other stealth virus-associated human illnesses. Certainly, the symptoms and signs observed in these animals are consistent with a viral encephalopathy, as has been postulated to be the underlying cause of CFS [5]. The vacuolated cellular changes seen histologically, and the paucity of any inflammatory reaction, are similar to the changes seen on brain biopsies of stealth virus-infected humans [unpubl. observation]. Although the animals' symptoms were mainly attributed to an encephalopathy, the histological changes indicated that the infection was widespread throughout the animal. It is likely, therefore, that neural cells are not intrinsically more susceptible than other cell types to infection, but rather that brain function is simply more sensitive to low-level viral-induced damage than most other organs.

While serving as a potential model system for stealth viral infections, the cat should also be considered as possibly involved in human to animal disease transmission. Stealth viruses have occasionally been isolated from both cats and dogs belonging to CFS patients [unpubl. observations], and a formal epidemiological study to address the issue is clearly warranted.

While based only on a single animal, it does appear possible to preimmunize against active infection by simply using a heated sample of viral infected cells. The immunized animal was not protected against another stealth viral isolate, but this was not unexpected since stealth viruses are known to be molecularly heterogeneous [2,3].

References

1. Martin WJ: Viral infection in CFS patients; in Hyde B (ed): The Clinical and Scientific Basis of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Ottawa, Nightingale Res Found, 1992, pp 325-327.

2. Martin WJ, Zeng LC, Ahmed K, Roy M: Cytomegalovirus-related sequences in an atypical cytopathic virus repeatedly isolated from a patient with the chronic fatigue syndrome. Am J Pathol 1994;145:440-451.

3. Martin WJ, Ahmed KN, Zeng LC, Olsen J-C, Seward JG, Seehrai JS: African green monkey origin of the atypical cytopathic stealth virus isolated from a patient with chronic fatigue syndrome. Clin Diagn Virol 1995;4:93-103.

4. Martin WJ: Stealth virus isolated from an autistic child. J Autism Dev Disord 1995;25:223-224.