June 2014
Case A
History
Tissues from a 16-year-old male cat in good health with a subcutaneous mass on its forehead.
Example Histopathological Description
Skin (pinna): Extending from near the dermoepidermal junction, through the dermis, subcutis and skeletal muscle and almost to the underlying cartilage of the ear, is a focally-extensive, moderately circumscribed, unencapsulated inflammatory mass. It comprises an infiltrate of predominantly pleomorphic epithelioid macrophages, large numbers of multinucleated giant cells, and small numbers of lymphocytes and plasma cells separating remnants of dermal collagen and adnexa. Within the cytoplasm of many macrophages and multinucleated giant cells are multiple, round to collapsed, organisms, 10-20 μm in diameter, with lightly-stained outlines and faintly basophilic or clear centres. Some of these bodies are divided into wedge-shaped segments (endospores), consistent with algae (Prototheca/Chlorella). The area of granulomatous inflammation extends almost to one lateral surgical margin. The overlying epidermis is intact and mildly acanthotic and contains a small focus of serocellular crusting.
Skin (face): There are similar changes in this skin section to those in the pinna (above). The epidermis is intact and increased from 2 to 5 cells deep over an irregularly-circumscribed nodule of inflammation extending from the dermoepidermal junction through the dermis into the subcutis. The mass comprises a similar population of macrophages and multinucleated giant cells containing large numbers of intracytoplasmic organisms consistent with algae.
Lymph node: The extracapsular stroma and subcapsular, trabecular and medullary sinuses are expanded by epithelioid macrophages and some multinucleated giant cells with grey granular cytoplasm containing faintly detectable algal organisms. The extracapsular stroma also contains scattered lymphoplasmacytoid foci. The lymph node cortex contains numerous follicles with prominent germinal centres and the paracortex and medullary cords are expanded by plasma cells.
Morphological Diagnosis
Skin (face and pinna): Dermatitis; granulomatous, chronic, focally extensive, severe, with intralesional algal organisms.
Lymph node: Lymphadenitis; granulomatous, chronic, regionally diffuse, severe, with intralesional algal organisms.
Aetiological Diagnosis
Protothecal dermatitis and lymphadenitis.
Comments
Prototheca spp are colourless (achlorophyllic), non-photosynthetic variants of the unicellular green alga Chlorella. Protothecal lesions are characterised by diffuse infiltrates of predominantly epithelioid macrophages and large numbers of organisms. They can be identified in histological sections by their refractile cell wall (which is argyrophilic and PAS positive) and distinctive ‘criss-cross’ internal septation resulting from the formation of wedge-shaped endospores (their ‘cut-pie’ appearance is diagnostic).
Prototheca spp and Chlorella spp can be differentiated by special stains. Chlorella, whose natural green pigmentation is removed during fixation and normal tissue processing, contains large starch granules (Prototheca does not), which are argyrophylic and PAS positive (diastase negative).
Some features useful for the histological differentiation of some fungal infections from algal infections are as follows.
Cryptococcus and Blastomyces exhibit budding. Prototheca and Chlorella algae do not.
Cryptococcus is round to oval (3.5-20 μm diameter), usually with a wide, clear capsule (which may increase the organism’s overall diameter to 40 μm). In cats, large numbers of small, poorly-capsulated Cryptococcus organisms may be grouped in clear spaces, and possibly resemble Sporothix.
Histoplasma and Sporothrix are similar in size to each other and small (2-5 μm). Sporothrix (slightly larger than Histoplasma) is typically cigar-shaped and can exhibit budding.
Dermatophytic pseudomycetomas caused by Microsporum canis in cats have a distinctive histopathological appearance, with multiple aggregates of tangled, delicate hyphae and bulbous thick-walled dilatations resembling spores; these aggregates are imbedded in amorphous eosinophilic material (Splendore-Hoeppli reaction).
June 2014
Case B
History
Adult female pig died after a two day history of pyrexia. It was in a small pig herd co-housed with sheep and chickens.
Example Histopathological Description
Brain (probably brain stem): This is a fragmented section of brain (without meningeal or ventricular edges) comprising intermixed grey and white matter with several aggregations of intact large neurons. Irregularly scattered throughout all areas, are many small to medium-sized blood vessels with transmural and perivascular (within Virchow Robin spaces) infiltration by large numbers of lymphocytes, with some macrophages and small numbers of granulocytes, including some eosinophils. Perivascular cuffs are 2-15 cells thick and in some cases the inflammatory cells are loosely associated and separated by eosinophilic, oedematous stroma and cellular debris. There is minimal extension of inflammation into the surrounding neuropil. Neuronal degeneration is not detected.
Morphological Diagnosis
Encephalitis; non-suppurative, subacute, multifocal, moderate, vasocentric, with vasculitis and perivascular cuffing.
Aetiological Diagnosis
Viral encephalitis
Comments
The pig was housed in Fleming County, Kentucky USA. It was treated with Banamine and Predef on Dec 30 and 31, 2006 while its temperature was 106 F. It had been treated in September for pneumonia and died January 1, 2007. The animal was co-housed in a barn with sheep and other pigs. Other pigs in the group had similar signs but did not die. Illness was not observed in the co-habited sheep. FA tests for pseudorabies virus, porcine parvovirus, PRRS virus, porcine circovirus and BVD virus were negative. Molecular biological tests produced a 100% match with ovine herpesvirus 2.
The non-suppurative encephalitis in this pig is suggestive of a viral infection. Vasocentric inflammation (with vasculitis) is a feature of Malignant Catarrhal Fever/Ovine Herpes Virus 2 (MCF/OHV-2) infection and Classical Swine Fever (CSF). This case did not have the neuronal degeneration or viral inclusions that can be a feature of some viral encephalitides (e.g. lyssaviral infection, pseudorabies). However, virological and molecular testing is necessary to definitively diagnose a viral cause.
Potential viral diagnoses in pigs include:
Endemic to Australia:
- Malignant catarrhal fever (MCF) – ovine herpesvirus 2 (OHV-2): for histological support of his diagnosis, other organs should be examined for evidence of vasocentric, non-suppurative inflammation. Infection source would be in-contact sheep.
- Porcine teschovirus (PTV) encephalomyelitis (Porcine teschoviral/enteroviral encephalomyelitis, formerly Talfan disease, porcine enterovirus encephalomyelopathy). Overseas, porcine teschovirus serotypes (PTV-1, -2, -3, -4, -5, -6, -9, -10) have caused Talfan disease (milder form disease). In Australia, serotypes PTV-1, -2, -5, -8 have been isolated, and at least PTV-1 strains have caused disease.
- Porcine haemagglutinating encephalomyelitis virus (HEV) encephalitis – coronavirus.
- Rhabdoviral encephalitis – Australian bat lyssavirus (ABL).
- Encephalomyocarditis virus (EMC) encephalitis – unlikely cause of such a florid vasocentric encephalitis in a pig.
Exotic to Australia:
- Porcine pestiviral encephalitis – Classical swine fever (CSF) – a non-suppurative encephalitis resulting from vasculitis, with endothelial swelling, eccentric vessel cuffs of lymphocytes and macrophages often undergoing necrosis.
- Porcine paramyxoviral encephalitis: Nipah virus and Blue-eye paramyxovirus.
- Porcine teschovirus (PTV) encephalomyelitis (formerly Teschen disease) caused by virulent strain of porcine teschovirus serotype PTV-1.
- Flaviviral encephalitis: Japanese encephalitis –usually no clinical signs of encephalitis in pigs.
- West Nile Virus (Kunjin virus) encephalitis – swine less susceptible than horses.
- Porcine encephalitis associated with PRRSV infection – neurovirulent especially in young pigs and can cause encephalitis under natural conditions.
- Porcine alphaherpesviral encephalitis – Aujeszky’s disease/pseudorabies (suid herpesvirus 1). Causes panencephalitis with focal gliosis and neuronal degeneration; intranuclear inclusions should be sought in neurons and astroglia.
- Swine vesicular disease (SVD) encephalitis.
- Swine influenza encephalitis.
- African swine fever (ASF)