Zoonotic potential of pseudorabies virus series: An Emerging Threat – Part 1

Posted on June 8, 2023 by

Introduction 

Pseudorabies virus (PRV) is a double stranded α-herpes virus that is widely distributed throughout the world. While pseudorabies, or ‘mad itch’, was originally described in cattle in 1813, the etiologic agent was identified as a virus by Aladar Aujesky in 1902.1  Dr. Aujesky was a Hungarian physician and veterinary scientist whose lab was actually dedicated to investigating rabies. However, because PRV caused a clinical presentation that was similar to rabies, he often saw cases of pseudorabies as part of his research. He concluded that the cases of pseudorabies he saw were not rabies because they had characteristics that were ‘strictly different from rabies’, and the virus he isolated from these cases was ‘not identical with rabies virus’.1 Because of his early work on PRV, pseudorabies is often also known as ‘Aujesky’s Disease’.

Much of the literature in the 1800s focused on the disease process in cattle, however the primary reservoir for PRV is swine, and the disease is widely distributed throughout the world. Other species affected by PRV include sheep, goats, dogs, cats, mink, and fox.2 Horses and reptiles seem to be resistant to the virus.3 Higher primates, including humans, are also thought to be resistant to PRV. However, prior to 2017, there had only been sporadic cases of possible zoonotic transmission of PRV from animals to people.3,4 Most of these cases produced only mild disease, or were in people with compromised immune systems, and many considered the zoonotic potential of PRV to be minimal to non-existant in immunocompetent humans.3,5

However, starting in 2018, a series of case reports and case series emerged in China detailing human cases of encephalitis and endophthalmitis. Virtually all of the affected individuals worked in the pork industry. Many of these individuals had a history and/or physical evidence of work-related skin injuries at the time of diagnosis. In these reports, PRV was detected in samples of cerebrospinal fluid (CSF), vitreous humor, and blood via metagenomic next-generation sequencing (mNGS).6–9 This diagnostic tool is being used increasingly to identify potential causes of infectious disease, especially neurologic infections.10 The process of mNGS works by amplifying any nucleic acid strands present in parallel, producing multiple copies of complementary DNA (the process can also be modified to sequence RNA), then sequencing the amplified strains using rapid bioinformatics; the entire process usually takes 24-48 hours.11 This relatively new diagnostic method has several advantages to previous molecular diagnostics, most significantly that it detects nucleic acid in an unbiased, agnostic approach versus PCR or Sanger Sequencing which rely on specific nucleotide primers. It also has a relatively quick turnaround time, and since it doesn’t require specific primers, can be used to identify a broad range of viral, bacterial, protozoal, and multi-cellular pathogens.

In most diagnostic samples that could be analyzed using mNGS, the amount of host nucleic is usually several orders of magnitude greater than the amount of any foreign nucleic acid present. This amounts to the equivalent of trying to find a needle in a haystack. The process compensates for this by greatly amplifying the amount of nucleic acid present (essentially producing many more needles). However, this can result in very small amounts of nucleic acids, that were present as a contaminant, creating a spurious result. Notwithstanding, when the process is able to detect foreign sequences in the sample, it can be a powerful way to identify pathogens that are either difficult to detect, are not typically considered in the pathologic process being studied, or would otherwise require long turnaround times to diagnose.11 Furthermore, in cases of encephalitis or meningitis, CSF is an ideal sample for mNGS as it is supposedly sterile and would have a lower amount of host nucleic acid creating background signals (less straw to sift through).11

The timing of these case reports is intriguing, as they occurred soon after China experienced a large outbreak of pseudorabies in its pork industry.12,13 This outbreak, which occurred between 2012 and 2017, was significant both in its magnitude, and due to the fact that it occurred in farms that had been previously vaccinated with the Bartha-K61 PRV vaccine. This modified-live vaccine had been used successfully to control outbreaks of porcine PRV in China between 1990 and 2011.13 Genetic analysis of viral samples from the swine outbreak revealed the emergence of novel strains with characteristics that appeared to enable them to evade vaccine derived immunity.12,13

While the detection of PRV in the CSF of patients with encephalitis is suggestive of possible emergence of a new strain of PRV with greater zoonotic potential; the case reports in and of themselves do not provide sufficient evidence of a causal relationship. However, in 2021 researchers were able to isolate and culture a viable strain of PRV from the CSF of a human patient with encephalitis. Subsequent laboratory testing on this strain supported the hypothesis that it was closely related to local swine variants and had the potential to be pathogenic in people, however further research is necessary to confirm the existence of a new zoonotic threat from PRV.

Continue Reading Part 2 of the Zoonotic Potential of Pseudorabies Virus Series: An Emerging Threat

Zoonotic potential of pseudorabies virus series: An Emerging Threat – Part 2

References:

  1. Mettenleiter TC. Aujeszky’s Disease and the Development of the Marker/DIVA Vaccination Concept. Pathogens. 2020;9(7):563. doi:10.3390/pathogens9070563

  2. Tan L, Yao J, Yang Y, et al. Current Status and Challenge of Pseudorabies Virus Infection in China. Virol Sin. 2021;36(4):588-607. doi:10.1007/s12250-020-00340-0

  3. Sawitzky D. Transmission, species specificity, and pathogenicity of Aujeszky’s disease virus. In: Kaaden OR, Czerny CP, Eichhorn W, eds. Viral Zoonoses and Food of Animal Origin. Springer Vienna; 1997:201-206. doi:10.1007/978-3-7091-6534-8_19

  4. Mravak S, Bienzle U, Feldmeier H, Hampl H, Habermehl KO. Pseudorabies in man. The Lancet. 1987;329(8531):501-502. doi:10.1016/S0140-6736(87)92104-0

  5. Jack SW, Box PO, Godwin KC, Services UAW, Fw POD. Serologic evidence of Brucella and pseudorabies in Mississippi feral swine.

  6. Ai JW, Weng SS, Cheng Q, et al. Human Endophthalmitis Caused By Pseudorabies Virus Infection, China, 2017. Emerg Infect Dis. 2018;24(6):1087-1090. doi:10.3201/eid2406.171612

  7. Yang X, Guan H, Li C, et al. Characteristics of human encephalitis caused by pseudorabies virus: A case series study. Int J Infect Dis. 2019;87:92-99. doi:10.1016/j.ijid.2019.08.007

  8. Wang D, Tao X, Fei M, et al. Human encephalitis caused by pseudorabies virus infection: a case report. J Neurovirol. 2020;26(3):442-448. doi:10.1007/s13365-019-00822-2

  9. Yan W, Hu Z, Zhang Y, Wu X, Zhang H. Case Report: Metagenomic Next-Generation Sequencing for Diagnosis of Human Encephalitis and Endophthalmitis Caused by Pseudorabies Virus. Front Med. 2022;8:753988. doi:10.3389/fmed.2021.753988

  10. Wilson MR, Sample HA, Zorn KC, et al. Clinical Metagenomic Sequencing for Diagnosis of Meningitis and Encephalitis. N Engl J Med. 2019;380(24):2327-2340. doi:10.1056/NEJMoa1803396

  11. Simner PJ, Miller S, Carroll KC. Understanding the Promises and Hurdles of Metagenomic Next-Generation Sequencing as a Diagnostic Tool for Infectious Diseases. Clin Infect Dis. 2018;66(5):778-788. doi:10.1093/cid/cix881

  12. Gu J, Hu D, Peng T, et al. Epidemiological investigation of pseudorabies in Shandong Province from 2013 to 2016. Transbound Emerg Dis. 2018;65(3):890-898. doi:10.1111/tbed.12827

  13. Sun Y, Liang W, Liu Q, et al. Epidemiological and genetic characteristics of swine pseudorabies virus in mainland China between 2012 and 2017. PeerJ. 2018;6:e5785. doi:10.7717/peerj.5785

  14. Liu Q, Wang X, Xie C, et al. A Novel Human Acute Encephalitis Caused by Pseudorabies Virus Variant Strain. Clin Infect Dis. 2021;73(11):e3690-e3700. doi:10.1093/cid/ciaa987