FINAL DIAGNOSIS

Sheep-associated Malignant Catarrhal Fever
(by histopathology and PCR)

Signalment:   9-month-old Holstein  heifer

Clinical history:

  Short history of pyrexia, lethargy and nasal discharge.  History also indicated this animal was in close association with sheep on the farm.

Gross necropsy findings:

 Coronary bands of both front limbs were hyperemic and mildly swollen.  There were multiple mucosal erosions/ulcerations of the oral cavity, esophagus, gastrointestinal tract.  Nasal turbinates were hyperemic and covered with fibrinonecrotic material.  Mesenteric lymph nodes were enlarged and, on cross section, were dark red with areas of necrosis.  The meninges were congested.  The urinary bladder was hyperemic.  The mucosa was covered in some areas by fibrinonecrotic material.

Histopathology findings:

 Histopathology revealed lymphocytic to necrotizing/ulcerative lesions associated with vasculitis in oropharyngeal, esophageal and abomasal sections.  Histopathology of the cerebrum and cerebellum revealed a moderate lymphocytic meningitis.  The meninges were infiltrated by large lymphoblastic-type lymphocytes, and fewer macrophages that extended along Virchow-Robins space and concentrated around cerebral vessels.  In some areas, the infiltrates extended into the adjacent neuropil.  The urinary bladder was denuded of surface epithelium and overlain by a layer of fibrinohemorrhagic material.  The propria submucosa was infiltrated by moderate numbers of lymphocytes, fewer plasma cells and macrophages.  The tunica media and adventitia of several small to medium caliber arteries were infiltrated by large lymphoblastic-type lymphocytes.  In some vessels, the tunica media was segmentally necrotic and expanded by fibrin deposits.  Endothelial cells were swollen.

Morphologic diagnosis:

Multiple tissues; Multifocal ulcerations, erosions, marked, with lymphocytic to necrotizing vasculitis Brain; Lymphocytic meningitis and  Vasculitis

Etiology:

Sheep-associated Malignant Catarrhal Fever virus

Discussion:

 Malignant catarrhal fever (MCF) is the clinical manifestation of infection of certain ruminant species with one of a group of pathogenic gammaherpes viruses known as MCF viruses.  The disease is sporadic to epidemic and is distributed worldwide.  Most domestic cattle and numerous exotic species, such as banteng and gaur are susceptible to clinical disease.  Bison, moose and some species of deer are highly susceptible.

 The disease syndromes associated with these viruses range from acute, severe inflammatory disease with a short clinical course to a more chronic syndrome.  The acute disease is characterized by high fever, lymph node swelling, and widespread inflammation of mucosal surfaces.  Lymphoproliferation and vasculitis are the main histologic lesions.

 There are two known pathogenic viruses that are etiologically associated with MCF-1)  the sheep-associated and 2) the wildebeest- associated.  These two viruses are closely related antigenically and genetically.  The virus that is endemic in and well-adapted to wildebeest alcelaphine herpesvirus 1 (AHV-1), can be readily isolated.  Wildebeest are the principal reservoir hosts.  The virus that is endemic in sheep, though never isolated, has been designated ovine herpesvirus 2( OHV-2).

 The gross lesions of MCF are similar to those associated with bovine viral diarrhea, infectious bovine rhinotracheitis and bluetongue.  In the case described here, indirect immunofluorescence assays did not detect antiviral antibodies or antigens in lung, spleen and kidney tissue samples for these viruses.  Sheep-associated malignant catarrhal fever was highly suspected based on gross and histologic lesions of lymphocytic to fibrinonecrotizing vasculitis in various tissues, and was confirmed by OHV-2 specific PCR.  Lymphocytic to necrotizing vasculitis in the brain of cattle, together with fibrinonecrotizing vasculitis in several tissues are characteristic of MCF.  The recent development of molecular diagnostic assays has provided effective diagnostic tools for MCF viruses.  A serological assay, a competitive-inhibition enzyme-linked immunosorbent assay (CI-ELISA) and PCR for the OHV-2 and AHV-1 strains of MCF viruses have dramatically improved the accuracy of diagnosis of MCF in clinically infected animals.

 The pathogenesis of MCF is not well understood, but the early involvement of cytotoxic lymphocytes has been proposed.  Sheep are latent carriers of OHV-2 and close association with cattle must be avoided to avoid transmission.

-by Victoria Owiredu-Laast, DVM,

 ADDL Graduate student