FHV-1 existing in snow leopards has been found by next-generation sequencing using serum and rectal swab samples 5; however, there is no clinical case report about snow leopards. Herein, three snow leopards infected with FHV-1 presented different lesions and clinical passages. FHV-1 often causes feline viral rhinotracheitis, ocular disease, ulcerative dermatitis, and pneumonia3; 7; 15. The pathogenesis of FHV-1 is based on two different mechanisms16. The first mechanism is that FHV-1 as a cytolytic virus can damage the epithelial cells of mucosae and cornea leading to ulceration. The second mechanism is immune-mediated reaction driven by antigenic stimulation16. In this study, all of the snow leopards with obvious sialorrhea and sneezing symptoms have been first confirmed to be infected with FHV-1 using real-time qPCR.
In the case of No. 1, foam cells and hemosiderosis showed that cerebral infarction was old17; 18. The meningoencephalitis was in the acute stage and an obvious demyelination reaction was visible in the white matter according to the clinical neural symptom of No. 1. It has been reported that one white-handed gibbon died of cerebral infarction and myocardial fibrosis with herpes simplex Ⅰ and Epstein-Barr virus has been reported; however, the viral infection was not considered the main cause of death19. In cats and dogs, non-suppurative meningoencephalitis is frequently found and these pathogens include porcine herpesvirus 1, parvovirus, feline infectious peritonitis virus, feline leukemia virus, West Nile virus, and encephalomyocarditis virus are found in the central nervous system of dogs and cats with non-suppurative meningoencephalitis by immunohistochemistry. Therefore, the primary or virus-triggered secondary immune-mediated mechanisms cannot be ignored 20. FHV-1 has also been reported as a causative agent of severe nonsuppurative meningoencephalitis in domestic cats 21. In this study, though FHV-1 was not detected in organs other than the lung of No. 1 using PCR, the vascular cuff reactions and demyelinating lesions were generally suggestive of a viral etiology in the brain22. Additionally, herpesvirus is a common causative agent in humans and animals23–25. Thus, the FHV-1 infection may be relative to the non-suppurative meningoencephalitis of No. 1.
In the case of No. 2, lung, kidney, and urinary bladder presented the obvious pathological change. Histopathology examination showed renal failure owing to uroschesis and interstitial pneumonia. Similarly, only the lung was positive for FHV-1 using PCR and immunohistochemistry. Owing to the cataract of the right leg, the activity of No. 2 might be limited which would induce neurothlipsis of the urinary bladder 26. This may lead to acute postrenal renal failure, according to the symptoms of anuria. To date, urinary system diseases with FHV-have not been reported. Using a pathological examination, immunohistochemistry, and PCR, pneumonia with FHV-1 infection became clear. Firstly, the lung was characterized by interstitial pneumonia, which was primarily caused by a viral infection in morphology, and immunohistochemistry demonstrated the antigen of FHV-1 located in the epithelial cells of a bronchiole22. Secondly, detection of the gD gene was also positive. According to the report, FHV-1 targets both respiratory epithelial cells and pneumocytes and enters the lung. Moreover, the FHV-1 can make infectious cells dead via apoptosis or inducing neutrophil infiltration 3. In this study, FHV-1 was mainly located in the epithelial cells of the bronchiole with little neutrophil infiltration. However, massive necrotic cast-off cells are visible in the bronchiole. That also proves that the pathway of FHV-1 shedding is primarily the respiratory tract. Therefore, it is considered that No. 2 died of the combined effect of renal failure and FHV-1 pneumonia.
However, No. 3 recovered after 2 weeks of treatments and then died 5 months later. Since it was neither autopsied nor stored in any samples, the relationship between its death and FHV-1 infection was not clear. However, No. 3 was infected with FHV-1 and presented obvious clinical symptoms. Different from No. 1 and No. 2, No. 3 had no background disease clinically. No. 3 recovered gradually from infection underlying treatment before death. This may be relative to the reactivation of latency of FHV-1 that is a hallmark of alphaherpesvirus biology7. When animals suffer stress, viral reactivation easily occurs spontaneously 27. In this case, FHV-1 poses a greater threat to sick snow leopards, similar to that FHV-1 primarily affects kittens and juvenile cats and persists lifelong of the host1. Therefore, the animal recovered from FHV-1 infection should be fed separately for further observation.
Furthermore, a nasal swab of No. 2 was used for the isolate of FHV-1 in F81 cells, and the obvious CPE was visible. Immunohistochemistry showed that FHV-1 isolate replicated in cells. The nucleic acid of FHV-1 was also detected in infected cells. The results further confirmed that snow leopards were infected with FHV-1.
Research shows that FHV-1 has only one serotype and is relatively homogenous genetically 1. To explore the infectious origin, the sequence of gD and gE genes was analyzed and phylogenetic trees were constructed. The gD protein probably had host-selective and stimulated the host to produce high cellular immunity and anti-gD antibody28. While the gE protein is mainly related to the virulence of FHV-1 29. The results showed SL/QH/2019 was highly homologous with the mainly epidemic isolates of cats in China. Thus, the origin of FHV-1 infecting snow leopards was probably from feral cats in the zoo. Additionally, the identity between these isolates and tiger isolate is lower compared to cat isolates. The genomic variation is not necessary for cross-species transmission of FHV-1 and FHV-1 from a domestic cat that can directly infect other wild felids.
The severity and symptoms of FHV-1 infection are not relative to the viral genome variants, and it is most likely due to other factors such as host response 30. Therefore, the different pathological changes of No. 1 and No. 2 snow leopards may be the result of different background diseases.