Signalment:  

1-year-4-month-old male rock hyrax (Procavia capensis).This rock hyrax was found dead without premonitory signs. There were no other deaths in the group in the preceding four months. A second, four month old, rock hyrax was euthanized due to similar lesions 10 days after this individual died.

Gross Description:  

There are multifocal to coalescing, thick, tenacious, white to pale yellow plaques adhered to the mucosal surfaces of the aryepiglottic folds, the vocal folds, the laryngeal pouches and the nasopharynx and extending into tonsillar crypts. The aryepiglottic folds are markedly thickened, resulting in severe reduction in the diameter of the glottis (pinpoint). There is multifocal red discoloration of the mucosa in areas not covered by plaques. There were no other significant findings.

Histopathologic Description:

Larynx/pharynx: There is multifocal and extensive ulceration of the laryngeal mucosa, a thick overlying pseudomembrane and dense inflammatory infiltrates in the submucosa. Intact epithelium adjacent to areas of ulceration is infiltrated by variable numbers of neutrophils, is mildly hyperplastic and expanded by edema. These areas often contain large syncytial cells which, in addition to individual epithelial cells, have large amphophilic intranuclear inclusion bodies that either fill the entire nucleus leaving a thin band of peripheralized chromatin, or are surrounded by a clear halo encircled by peripheralized chromatin. The overlying pseudomembrane is composed of fibrin, wispy basophilic material (mucin), viable and intact neutrophils, cellular debris, sloughed epithelial cells (occasionally with intranuclear inclusion bodies) and large, dense colonies of mixed bacteria which are most prevalent on the surface. Inflammation expanding the submucosa directly subtending ulcers is composed of predominantly neutrophils with fewer lymphocytes and plasma cells; the latter predominate in the deeper submucosa and more peripherally. There is multifocal marked submucosal edema and submucosal capillaries are markedly congested.

Morphologic Diagnosis:  

Larynx/Pharynx: Laryngitis/pharyngitis, fibrinosuppurative, ulcerative, subacute, multifocal to coalescing, extensive, marked, with intralesional intranuclear viral inclusion bodies, intralesional viral syncytia and superficial mixed bacteria.

Lab Results:  

Aerobic culture, larynx: moderate coagulase negative Staphylococcus spp., moderate Pasteurella multocida

Anaerobic culture, larynx: Few Prevotella spp.

Condition:  

Hyrax herpesvirus-1

Contributor Comment:  

The histological findings in this case, specifically mucosal ulceration with intralesional intranuclear inclusion bodies and syncytia formation, were consistent with infection by the previously described hyrax herpesvirus.(1) The severity of lesions resulted in near-obstruction of the larynx and presumably contributed to the death of this hyrax.

Investigations into this virus performed by Galeota et al.(1), including biological behavior and molecular characteristics, supported the inclusion of hyrax herpesvirus in the Alphaherpesvirinae subfamily and Simplexvirus genus. At our institutions we have seen oral ulcers on numerous occasions in rock hyraxes (often as incidental lesions), and previous molecular investigations from individuals in this colony have shown the offending organism to have 100% shared identity with published hyrax herpesvirus DNA polymerase gene sequences. 

As with multiple other members of the Alphaherpesvirinae, lesions associated with the hyrax herpesvirus are the result of epithelial necrosis and ulceration with secondary inflammation and, in some cases, bacterial infection. Systemic lesions associated with this virus have not been confirmed, but one published case was described as having nonsuppurative meningoencephalitis1 and we have seen neuronal necrosis in one of our cases. As with other herpes viruses, it is suspected that the hyrax herpesvirus causes long-term infection with occasional episodes of recrudescence. As no new animals were recently introduced into this group prior to the death of these two animals, recrudescence was suspected in these cases. The colony had recently been transferred to their indoor winter housing which likely introduced stressors and may have induced shedding of the virus in previously infected animals and overt lesions in these younger individuals.

JPC Diagnosis:  

Larynx/Pharynx: Laryngitis/pharyngitis, ulcerative and fibrinosuppurative, acute, multifocal, marked, with eosinophilic intranuclear inclusion bodies, viral syncytial cells, submucosal edema and colonies of bacteria.

Conference Comment:  

Herpesviruses belong to the order Herpesvirales, which contains three families: Herpesviridae (herpesviruses of birds, mammals and reptiles), Malacoherpesviridae (oysters) and Alloherpesviridae (fish and frogs).(2) Herpesviruses are double-stranded, enveloped DNA viruses with worldwide distribution. Replication occurs within the nucleus, resulting in intranuclear inclusion bodies; the viral envelope is acquired via budding through the nuclear membrane. These viruses usually have a narrowly restricted host range and are known for the ability to establish latent infections. The family Herpesviridae is divided into three broad subfamilies: alpha, beta and gammaherpesvirinae.(2) Betaherpesviruses, also known as cytomegaloviruses, replicate slowly, have a highly restricted host range and often produce greatly enlarged cells. When latent, they are sequestered in secretory cells, lymphoreticular organs, and the kidney, however they are more associated with continuous viral shedding than periodic reactivation (as opposed to alphaherpesviruses).(2) Betaherpesvirus is normally found in many species, however it usually only causes disease in immunosuppressed individuals, such as SIV infected monkeys. Betaherpesviruses also cause inclusion body rhinitis in swine (suid herpesvirus 2) and salivary gland inclusions and cytomegaly in guinea pigs (caviid herpesvirus 2).(2) Gammaherpesviruses, such as ovine herpesvirus-2 and alcephaline herpesvirus-1 (causative agents of malignant catarrhal fever) or saimiriine herpesvirus 2 (Herpesvirus saimiri) replicate in lymphoblastic cells and induce lymphoproliferative response.(2)

Alphaherpesviruses tend to lyse host cells and typically result in widespread or localized necrosis, as in this case. They grow rapidly, producing Cowdry type-A intranuclear inclusion bodies (and viral syncytial cells) and establishing lifelong latent infections in both the lymphoreticular system and the trigeminal ganglion.(4) Gallid herpesvirus 2 (Mareks disease) is a rare example of an alphaherpesvirus which acts more like a gammaherpesvirus in that it induces a lymphoproliferative response.(2) Table 1 summarizes other alphaherpesviruses of veterinary importance.(2,4)

Directional spread of alphaherpesviruses within the nervous system and the establishment of latency is a critical component of the viral lifecycle. The virus initially replicates peripherally in the skin or mucus membranes, where the innate immune response provides the first line of defense. Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns (PAMPs) expressed by the virus. Activated TLRs result in the production of cytokines, which recruit macrophages and/or induce proteins that degrade mRNA and inhibit translation. The adaptive immune response also works to prevent viral spread via viral specific CD8+ cytotoxic T-cells.(3) Although the initial infection is typically cleared within a few weeks, the virus spreads along axons to the sensory nerve ganglion, where the viral genome is maintained indefinitely. During latency, normally productive viral genes are quiescent and unproductive and LATs (latency associated RNA transcripts) accumulate in neuronal nuclei. LATs are Bcl-2 analogs (Bcl-2 is an anti-apoptotic regulator protein) that confers resistance to apoptosis, allowing viral persistence in sensory neurons.(5) Thus, in times of stress or immune suppression, the virus can reactivate and cause clinical disease.

Table 1: Alphaherpesviruses of veterinary importance.^2,4

Bovine herpesvirus 1	Infectious bovine rhinotracheitis, infectious pustular vulvovaginitis
Bovine herpesvirus 2	Bovine mammillitis/pseudo-lumpy skin disease
Bovine herpesvirus 5	Bovine herpes meningoencephalitis
Porcine herpesvirus 1	Pseudorabies/Aujeszkys Disease
Equine herpesvirus 1	Equine abortion
Equine herpesvirus 3	Equine coital exanthema
Equine herpesvirus 4	Equine rhinopneumonitis
Equine herpesvirus 5	Multinodular pulmonary fibrosis
Gallid herpesvirus 1	Avian infectious laryngotracheitis
Gallid herpesvirus 2	Mareks disease
Psittacid herpesvirus	Pachecos disease
Anatid herpesvirus 1	Duck Plague
Feline herpesvirus 1	Feline viral rhinotracheitis
Canine herpesvirus 1	Canine herpesviral disease
Macacine herpesvirus 1	B-virus of macaques
Saimiriine herpesvirus 1	Herpes tamarinus

References:

1. Galeota J, Napier J, Armstrong D, Riethoven J, Rogers D. Herpesvirus infections in rock hyraxes (Procavia capensis). J Vet Diagn Invest. 2009;21:531-535.

2. MacLachlan NJ, Dubovi EJ eds. Fenners Veterinary Virology. 4th ed. London, UK; 2011:179-201.

3. Kramer T, Enquist LW. Directional spread of alphherpesvirus in the nervous system. Viruses. 2013;5:678-707.

4. Zachary JF. Mechanisms of microbial infections. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO: Mosby; 2012:212-238.

5. Zerboni L, Che X, Reichelt M, Qiao Y, Gu H, Arvin A. Herpes simplex virus 1 tropism for human sensory ganglion neurons in the severe combined immunodeficiency mouse model neuropathogenesis. J Virol. 2013;87(5):2791-2802.

Click the slide to view.

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3-1. Larynx

3-2. Larynx

3-3. Larynx

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