Feline Panleukopenia in Free Ranging Bobcats (Lynx rufus) From 3 States

Joseph K. Gaydos and John R. Fischer
Southeastern Cooperative Wildlife Disease Study; College of Veterinary Medicine, The University of Georgia, Athens, Georgia 30602, USA

Abstract: Feline panleukopenia is caused by a parvovirus and is known to infect many members of the family Felidae. Very little is known about the effects of this disease on free ranging populations of bobcats (Lynx rufus). Here we review three cases of feline panleukopenia among wild bobcats submitted to the Southeastern Cooperative Wildlife Disease Study over the last 17 years. We also review two of five cases of feline panleukopenia diagnosed during a bobcat study conducted by the California Department of Fish and Game between 1976 and 1984. These case reports demonstrating feline panleukopenia as a mortality factor in sub-adult and adult animals suggests that feline panleukopenia has the potential to significantly effect free ranging bobcat populations.

Key words: Bobcat, Lynx rufus, Feline Panleukopenia, Parvovirus

Feline panleukopenia is an acute viral disease caused by a parvovirus and is known to infect members of the family Felidae and some animals in closely related families.¹ Many reports of feline panleukopenia in non-domestic felids have been from captive animals with very few reports coming from free ranging cats.² Although it is generally believed that bobcats (Lynx rufus) are susceptible to infection with feline panleukopenia, few reports exist and little is known about the effects of this disease on populations of free-ranging bobcats. Fowler³ reports confirming this disease in bobcats by histopathologic testing, but case reports are not given and it is not known if these were captive or free ranging bobcats. Panleukopenia in free-ranging bobcats was reported in Highlands County, Florida (USA) in 1979,⁴ where eight of 17 bobcats fitted with radiotransmitter collars died. Four of eight bobcats that died during the study were examined and found to have gross and microscopic lesions consistent with feline panleukopenia. Serologic testing and viral isolation was not attempted to confirm the presence of the virus. ⁴

The lack of information of the effects of feline panleukopenia on populations of free ranging bobcats prompted a review for diagnosis of feline panleukopenia among wild bobcats submitted to the Southeastern Cooperative Wildlife Disease Study (SCWDS, College of Veterinary Medicine, The University of Georgia, Athens, Georgia, USA) between 1 January 1981 and 31 December 1998. Whole animals usually refrigerated but often frozen, were available for examination in most instances, however, in some cases only formalin-fixed tissues were submitted. Personnel of state and federal wildlife agencies submitted all cases. Because necropsies were oriented toward determination of the cause of morbidity or mortality, diagnostic procedures varied among cases. Over the 17-year period, 14 bobcats from four states were submitted. Most of the bobcats were from Georgia (n=5) with fewer from West Virginia (n=4), Arkansas (n=3) and South Carolina (n=2). Naturally acquired feline panleukopenia was diagnosed in three (21%) bobcats and was second only to trauma (36%) as the most frequent diagnosis.

Case reports of the three free-ranging bobcats diagnosed with feline panleukopenia at SCWDS are presented (cases 1-3). Additionally a case report (case 4) is given for two of five free ranging bobcats diagnosed with feline panleukopenia during a bobcat study conducted by the California Department of Fish and Game between 1976 and 1984.

CASE 1:

On July 28, 1998, a male bobcat, determined to be a young adult (13-24 months) based on skull measurements,⁵ and weighing 6.5 kg, was found dead under a bush in a private citizen’s yard on Kiawah Island, South Carolina. The animal was frozen by personnel from the South Carolina Department of Natural Resources and submitted to the SCWDS for necropsy on August 24, 1998. The bobcat was in good physical condition on gross examination. Scant watery feces were present throughout the intestine and two superficial erosions were present on the right margin of the tongue. Sections of major organs were fixed in neutral buffered 10% formalin, embedded in paraffin, sectioned at 5 µm, and stained with hematoxylin and eosin (H&E) for histologic examination. Aseptically collected sections of intestine were submitted to the Athens Diagnostic Laboratory (Athens, Georgia, USA) for aerobic bacterial culture and fluorescent antibody (FA) testing for feline panleukopenia virus. Sections of the cerebrum including hippocampus were submitted to the same laboratory for FA testing for rabies virus. Bone marrow was also submitted for FA testing for feline leukemia virus and serum was submitted for enzyme-linked immunosorbent assay (ELISA) for feline immunodeficiency virus. Gastric contents were submitted to the University of Pennsylvania, New Bolton Center Laboratory of Toxicology (Kennett Square, Pennsylvania, USA) for gas chromatography – mass spectroscopy organic chemical screen.

Histologic examination of small intestine revealed moderate depletion of crypt epithelium and occasional crypt abscesses. Moderate multifocal colonies of bacteria of mixed morphology also were present. In stomach sections, mild multifocal hemorrhages were present in the submucosa and muscularis. A single small focus of chronic mucosal inflammation was also present. Moderate lymphoid depletion and moderate hemosiderin accumulations in macrophages characterized the spleen.

Fluorescent antibody testing using canine parvovirus FA conjugate^a was positive. Subsequent repeated attempts to isolate a parvovirus using feline kidney cells^b were unsuccessful. Likewise, repeated attempts to demonstrate parvoviral particles in intestinal contents by transmission electron microscopy were unsuccessful. Frozen sections of intestine were submitted to the James A. Baker Institute for Animal Health (Ithaca, New York, USA) where attempts to detect feline parvovirus and canine parvovirus were unsuccessful using both hemagglutination and polymerase chain reaction (PCR) testing.

Bacterial culture yielded Salmonella enterica, serovar alexander using the Spicer- Edwards test. The brain sections were negative for rabies, the bone marrow was negative for feline leukemia virus and the serology was negative for feline immunodeficiency virus. Toxic compounds were not detected in stomach contents.

Based on histopathology and positive FA testing, this bobcat was diagnosed with feline panleukopenia virus. Because Salmonella enterica was not cultured from an extra-intestinal source, it is not known if this bobcat was a non-clinical carrier of the bacterium or if infection actually played a role in the gastrointestinal disease of this animal. As a group, parvoviruses are highly immunosuppressive⁶ and it is impossible to say definitively, but panleukopenia viral infection may have predisposed this bobcat to a Salmonella infection or caused activation of a latent Salmonella infection.

CASE 2:

In 1989, a young male bobcat weighing 5.3 kg and estimated to be 1 year old was found dead in Randolph County, West Virginia. The animal was frozen and later necropsied by personnel from the West Virginia Department of Natural Resources. The most striking gross necropsy finding was hemorrhagic enteritis. Routine tissues were aseptically collected for bacterial and viral isolation. Replicate samples were placed in neutral buffered 10% formalin. All samples were submitted to the SCWDS for further diagnostic work. Formalin fixed samples were prepared for histologic exam and aseptically collected sections of small intestine and spleen were submitted to the Athens Diagnostic Laboratory for transmission electron microscopy and virus isolation.

Histologic examination revealed extensive mucosal necrosis throughout the intestinal tract. Severe villous atrophy and nests of sloughed epithelial cells in crypts characterized sections of small intestine. Severe necrosis was present in gastrointestinal associated lymphoid tissue. Medial hypertrophy of arteries and hyperplasia of bronchiolar epithelium characterized lung. Mucus was present in many airways and alveolar spaces were often edematous. Large numbers of marginating inflammatory cells were present in pulmonary vessels The mature cell population appeared to be absent in the bone marrow and granulopoietic components were sparse. Relative numbers of megakaryocytes appeared increased.

Transmission electron microscopy examination of intestinal contents revealed parvoviral particles. Frozen sections of spleen were submitted to the James A. Baker Institute for Animal Health where attempts to isolate parvoviruses were unsuccessful. Based on histopathology and demonstration of parvoviral particles on transmission electron microscopy, this bobcat was diagnosed with feline panleukopenia virus.

CASE 3:

A radio-collared 2-3 year old female bobcat weighing 4.9 kg was found dead in Barnwell County, South Carolina on February 24, 1981. Radio-telemetry patterns revealed consecutive identical locations for the three preceding days. Prior to this, the pattern was considered normal. The animal was submitted to the SCWDS for necropsy. On gross necropsy, the animal was in fair physical and postmortem condition. The perineal hair was matted with wet feces and the eyes were sunken. The stomach and small intestine were partially filled with green-yellow liquid. The intestinal wall was edematous with yellow mucous present. A number of tapeworms and nematodes were present in the stomach and intestines. Nematodes were also present in the lungs. Mild serous atrophy of cardiac fat was present and the liver was congested. Routine tissues were aseptically collected for bacterial and viral isolation. Replicate samples were formalin fixed and processes for histologic examination. Aseptically collected sections of small intestine and spleen were submitted to the Athens Diagnostic Laboratory for bacterial culture and FA testing for feline panleukopenia respectively. Brain was submitted to the Department of Health and Human Service, Center for Disease Control (Lawrenceville, Georgia, USA) for rabies testing.

Histologic examination was hindered by postmortem autolysis. A decreased crypt to villus ratio was present in the duodenum with dilatation of crypts and acini present in small intestine. Congestion and interstitial pneumonia were present in lungs. Aleurostrongylus abstrusus nematodes were noted in lung bronchioles, alveoli and deep in the tracheal mucosa. Germinal centers in the spleen were comprised of plasma cells.

Sections of brain were negative for rabies by FA test. Bacterial culture of intestine failed to yield significant growth. Sections of spleen were negative for feline panleukopenia virus by FA test. Attempts to isolate feline panleukopenia virus from intestinal content were unsuccessful. Based on histologic findings this bobcat was diagnosed with suspected feline panleukopenia.

CASE 4:

From 1976 to 1984, researchers from the California Department of Fish and Game studied two populations of bobcats in San Diego County. Sixty-six bobcats were trapped, collared and re-captured from a low harvest-pressure population (El Capitan Study Area) and 46 from a heavily trapped population (Laguna Mountains Study Area). Of 33 documented mortalities in the low harvest-pressure population, five (15%) were attributed to feline panleukopenia. This was the third highest cause of mortality with predation (36%) and unknown causes (18%) ranking higher. Of 17 recorded deaths in the high harvest-pressure population, none were attributed to feline panleukopenia. The low harvest-pressure bobcat population cycled more strongly than the high harvest-pressure population and panleukopenia was only diagnosed during times of high bobcat population densities. The low harvest-pressure site also contained some semi-rural housing and free ranging dogs and cats where as the high harvest-pressure population did not.

Bobcat SD54 was a male and weighed 5.3 kg when it was first trapped on December 29, 1981. It was found dead on January 15, 1982 near a road. Gross lesions consistent with vehicular trauma were present in addition to gross and microscopic changes consistent with feline panleukopenia.

Bobcat SD55 weighed 5.3 kg when first trapped on January 16, 1982. On January 28, 1982 it was found dead and weighed only 3.2 kg. It was extremely emaciated and dehydrated. The stomach was empty and the small intestines were inflamed and hemorrhagic. The colon was filled with black-tarry stool. Moderate numbers of roundworms and tapeworms were present in the small intestine. The blood was thin. Histology revealed microscopic lesions consistent with feline panleukopenia. A patchy purulent alveolar pneumonia was also seen. Rabies examination was negative.

These four independent cases were not obtained in a manner that would allow an accurate assessment of the relative importance of natural mortality factors in bobcats. The submission of cases was influenced by many unknown factors, and the data are undoubtedly biased. Despite this bias, four cases reports demonstrating feline panleukopenia as a mortality factor in sub-adult and adult animals suggests this disease has the potential to significantly effect free ranging bobcat populations. Despite these four case reports where feline panleukopenia caused mortality in wild bobcats, a 41% prevalence of feline panleukopenia-reactive antibodies in wild bobcats from seven Florida counties,⁴ suggests that not all bobcats infected with the virus develop fatal infections.

The epidemiology of feline panleukopenia in free ranging bobcats has not been investigated. Anecdotal evidence from case 4 implies that population density and exposure to domestic dogs and cats may play a role in transmission, but this is not known. Feline panleukopenia can be transmitted by direct contact, but the virus can also survive for several years outside the host, ⁶ permitting fomite infection as well. Further investigation is necessary to better understand the role of this disease in free-ranging populations of bobcats.

a. American BioResearch, Sevierville, TN (USA).

b. CRFK cells, ATCC CCL-94, ATTC, Rockville, MD (USA).

The authors would acknowledge the assistance of Drs. Jim Crum, C. Edward Couvillion (deceased), and Gail Mahnke for their work on several of these cases. Dr. Wayne Roberts and the staff at the Athens Diagnostic Laboratory were also extremely helpful with diagnostics and their time and effort is much appreciated. We appreciate Dr. Colin Parrish of the James A. Baker Institute for Animal Health, Ithaca, New York doing the hemagglutination and PCR assays for feline panleukopenia virus. We would also like to thank Mr. Gordon I. Gould, Jr. and Mr. Michael Lembeck of the California Department of Fish and Game for their contribution of case 4 to this paper. Dr. W. R. Davidson and Dr. Jane Huffman were also very helpful in reviewing this paper. This work was supported through the sponsorship by fish and wildlife agencies of Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Puerto Rico, South Carolina, Tennessee, Virginia, and West Virginia. Funds were provided by the Federal Aid to Wildlife Restoration Act (50-Stat. 917) and through Grant Agreement 14-45-0009-94-906, National Biological Service, U.S. Department of the Interior.

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