New Test: Serpentovirus (reptile nidovirus) RT-PCR

Heindrich Snyman, Patricia Bell-Rogers, Hugh Cai

Animal Health Laboratory, University of Guelph, Guelph, ON (Snyman, Bell-Rogers, Cai)

AHL Newsletter, 2021;25(4):20. 

Respiratory disease in captive snakes is an all too common and significant concern for pet owners and the exotic animal veterinarians.  It is a complex and multifactorial problem resulting from complex interactions between environmental husbandry factors (e.g. temperature, humidity), host anatomical factors (elongate trachea and poorly developed mucociliary escalator apparatus, large sac-like lung, lack of diaphragm), and infection with a variety of different primary and opportunistic pathogens.  Reptiles are also cryptic creatures, and disease is often unapparent to the owner until the problem is quite advanced and open-mouth breathing is observed.  This can make treatment particularly frustrating, especially when multiple coinfections are present.  A variety of bacterial (often opportunistic gram-negative upper respiratory tract commensals), parasitic (lung mites, lung worm), fungal, and primary viral (e.g. ophidian paramyxovirus, inclusion body disease, nidoviruses) pathogens are recognized.  In recent years, reptile nidoviruses (also known as serpentoviruses) have emerged as particularly significant respiratory pathogens in reptiles.

With this in mind, the Animal Health Laboratory has developed a real-time PCR assay for the detection of nidoviruses with the goal of helping veterinarians to better differentiate nidovirus as a possible cause for respiratory disease, and to assist them in their management of reptile collections.  The RT-qPCR assay is based on published methods (3,6).  Upon request, PCR sequencing can be performed for further identification.  Lung and distal trachea collected at the time of postmortem is the preferred sample for testing deceased snakes, while deep tracheal swabs or trans-tracheal lung lavage samples are preferred for testing clinical cases.

The majority of nidovirus infections to date have been documented in snakes within the Pythonidae (pythons) family, but infections have also been documented in Boidae (boas), Colubridae (the largest family of snakes), and Homalopsidae (water/mud snakes), as well as in a few individual lizard species (e.g. Australian shingleback lizards and veiled chameleons) and some testudines (e.g. Bellinger and Murray River turtles).  Nidovirus infection typically results in a prominent proliferative interstitial pneumonia, and is now commonly referred to as nidovirus-associated proliferative disease (NPD).  Although the lung is the primary site of infection and lesions, the virus also has tissue tropism for the oral cavity and upper alimentary tract, and this can be helpful for differentiation from other viral infections (Fig. 1).  Therefore, submission of either the whole head or the excised epiglottis, surrounding oral tissue and upper trachea and esophagus is essential when submitting tissues for histological confirmation of NPD.   AHL

Figure 1. Lung and oropharynx in an adult Burmese python (Python bivittatus) with nidovirus-associated proliferative disease (NPD). A. Lung: Faveolar and sub-faveolar septal walls are thickened with increased amounts in intervening smooth muscle and fibrous connective tissue that markedly narrow and constrict the air spaces.  Hyperplastic pseudo-follicular lymphoid aggregates are scattered throughout the lung. B. & C. Lung: The lining respiratory epithelium consists of tall columnar ciliated epithelium arranged in a variably thickened pseudostratified to stratified layers with scattered areas of epithelial swelling and degeneration.  Constricted air spaces occasionally contain entrapped homogenous eosinophilic proteinaceous fluid and there is widespread loss of apical cilia. D. Oropharynx: The oropharyngeal mucosa is similarly thickened and hyperplastic with large submucosal lymphocyte aggregates.  H&E stain.

References

1. Comolli JR, Divers SJ. Respiratory diseases of snakes. Vet Clin North Am Exot Anim Pract 2021;24(2):321-340.

2. Dervas E, et al. Serpentoviruses: More than respiratory pathogens. J Virol 2020; 31(94):18.

3. Dervas E, et al. Nidovirus-associated proliferative pneumonia in the Green Tree Python (Morelia viridis). J Virol 2017;13(91):21.

4. Parrish K, et al. Nidoviruses in reptiles: A review. Front Vet Sci 2021;21(8):733404.

5. Hoon-Hanks LL, et al. Respiratory disease in ball pythons (Python regius) experimentally infected with ball python nidovirus. Virology. 2018; 517:77-87.

6. Hoon-Hanks LL, et al. Longitudinal and cross-sectional sampling of serpentovirus (nidovirus) infection in captive snakes reveals high prevalence, persistent infection, and increased mortality in pythons and divergent serpentovirus infection in boas and colubrids. Front Vet Sci 2019;3(6):338.