Detection of EHV-1 neuropathogenic strains using real-time PCR in the neural tissue of horses with myeloencephalopathy

Severe Neurologic Disease in a Horse Caused by Tick-Borne Encephalitis Virus, Austria, 2021

As evidenced by sero-epidemiological studies, infections of horses with the tick-borne encephalitis virus (TBEV) occur frequently in TBEV-endemic areas. However, there are only very few reports of clinical cases. A possible underreporting may be due to a variety of diagnostic challenges. In this study, ELISA and neutralization tests were applied to serum samples. Brain tissue samples were investigated for the presence of nucleic acids of TBEV, Equid alphaherpesvirus 1, Borna disease virus 1, West Nile and Usutu viruses, rustrela virus, as well as Eastern, Western, and Venezuelan equine encephalitis viruses with RT-qPCR, RT-PCR, and qPCR, respectively. TBEV-specific amplification products were subjected to Sanger sequencing. In addition, a direct fluorescent antibody test for rabies was performed. Clinical and patho-histological findings are reported. Using specific RT-qPCR and RT-PCR assays, TBEV nucleic acids were demonstrated in brain tissue samples. Sequencing revealed the Western (formerly Central) European subtype of TBEV as the etiological agent. A high titer of TBEV-specific neutralizing antibodies was found in the serum. RNAscope in situ hybridization revealed TBEV RNA confined to neuronal cell bodies and processes. No other pathogens or nucleic acids thereof could be detected. Diagnostic procedures need to be carried out early after the onset of neurological signs to allow for a final etiological diagnosis of acute TBEV infections in horses.

Investigation of an EHV-1 Outbreak in the United States Caused by a New H752 Genotype

Here we report on an EHV-1 outbreak investigation caused by a novel genotype H752 (histidine in amino acid position 752 of the ORF 30 gene). The outbreak involved 31 performance horses. Horses were monitored over a period of 35 days for clinical signs, therapeutic outcome and qPCR results of EHV-1 in blood and nasal secretions. The morbidity of the EHV-1 outbreak was 84% with 26 clinically infected horses displaying fever and less frequently anorexia and distal limb edema. Four horses showed mild transient neurological deficits. Clinically diseased horses experienced high viral load of EHV-1 in blood and/or nasal secretions via qPCR, while subclinically infected horses had detectable EHV-1 mainly in nasal secretions. The majority of infected horses showed a rise in antibody titers to EHV-1 during the outbreak. All 31 horses were treated with valacyclovir, while clinically infected horses further received flunixin meglumine and sodium heparin. This investigation highlights various relevant aspects of an EHV-1 outbreak caused by a new H752 genotype: (i) importance of early detection of EHV-1 infection; (ii) diagnostic challenge to assess H752 genotype; (iii) apparent benefit of valacyclovir use in the early stage of the outbreak; and (iv) weekly testing of blood and nasal secretions by qPCR in order to monitor individual infection status and lift quarantine.

Equine herpesvirus-1 genotype did not significantly affect clinical signs and disease outcome in 65 horses diagnosed with equine herpesvirus-1 myeloencephalopathy

The objective of this study was to determine if the genotype of equine herpesvirus-1 (EHV-1) impacted clinical disease and outcome of horses with laboratory confirmed equine herpesvirus myeloencephalopathy (EHM). Medical records from 65 horses diagnosed with EHM from 2011 to 2019 were reviewed for signalment, presence and severity of clinical signs (lethargy, fever, ataxia, urinary incontinence) and outcome. Horses were further grouped based on the EHV-1 genotype into neuropathic (D₇₅₂) or non-neuropathic (N₇₅₂) EHV-1 infection. Between the two EHV-1 genotype groups, age and sex distributions were similar, although breed distribution was different (Quarter Horses and Saddlebreds were overrepresented and Warmbloods were underrepresented in the EHV-1 D₇₅₂ group compared to the EHV-1 N₇₅₂ group; P = 0.009). Lethargy, fever, ataxia and outcome were not significantly different between the two EHV-1 genotype groups (P > 0.05). However, urinary incontinence was significantly more frequently reported in horses infected with the D₇₅₂ genotype of EHV-1 (P=0.04). Contrary to previous studies, the present study showed no difference in frequency of genotype (D₇₅₂ or N₇₅₂) among 65 horses with EHM and, with the exception of urinary incontinence, no difference in clinical disease or outcome related to the EHV-1 genotype.

Translation of a laboratory-validated equine herpesvirus-1 specific real-time PCR assay into an insulated isothermal polymerase chain reaction (iiPCR) assay for point-of-need diagnosis using POCKIT™ nucleic acid analyzer

Equine herpesvirus myeloencephalopathy (EHM), a major problem for the equine industry in the United States, is caused by equine herpesvirus-1 (EHV-1). In addition, EHV-1 is associated with upper respiratory disease, abortion, and chorioretinal lesions in horses. Here we describe the development and evaluation of an inexpensive, user-friendly insulated isothermal PCR (iiPCR) method targeting open reading 30 (ORF30) to detect both neuropathogenic and non-neuropathogenic strains on the field-deployable POCKIT™ device for point-of-need detection of EHV-1. The analytical sensitivity of the EHV-1 iiPCR assay was 13 genome equivalents per reaction. The assay did not cross react with ten non-target equine viral pathogens. Performance of the EHV-1 iiPCR assay was compared to two previously described real-time PCR (qPCR) assays in two laboratories by using 104 archived clinical samples. All 53 qPCR-positive and 46 of the 51 qPCR-negative samples tested positive and negative, respectively, by the iiPCR. The agreement between the two assays was 95.19% (confidence interval 90.48-99.90%) with a kappa value of 0.90. In conclusion, the newly developed EHV-1 iiPCR assay is robust to provide specificity and sensitivity comparable to qPCR assays for the detection of EHV-1 nucleic acid in clinical specimens.

New real-time PCR assay using allelic discrimination for detection and differentiation of equine herpesvirus-1 strains with A2254 and G2254 polymorphisms

A single-nucleotide polymorphism (A(2254) or G(2254)) in open reading frame 30 (ORF30) has been linked to the neuropathogenic phenotype of equine herpesvirus-1 (EHV-1). Identification of this polymorphism led to the development of a real-time PCR (rPCR) assay using allelic discrimination (E(2)) to distinguish between potentially neuropathogenic and nonneuropathogenic EHV-1 strains (G. P. Allen, J. Vet. Diagn. Invest. 19:69-72, 2007). Although this rPCR assay can detect and genotype EHV-1 strains, subsequent studies demonstrated that it lacks the sensitivity for the routine detection of viral nucleic acid in clinical specimens. Therefore, a new allelic discrimination EHV-1 rPCR assay (E(1)) was developed by redesigning primers and probes specific to ORF30. The E(1) and E(2) rPCR assays were evaluated using 76 archived EHV isolates and 433 clinical specimens from cases of suspected EHV-1 infection. Nucleotide sequence analysis of ORF30 was used to confirm the presence of EHV-1 and characterize the genotype (A(2254) or G(2254)) in all archived isolates plus 168 of the clinical samples. The E(1) assay was 10 times more sensitive than E(2), with a lower detection limit of 10 infectious virus particles. Furthermore, all A(2254) and G(2254) genotypes along with samples from three cases of dual infection (A(2254)+G(2254)) were correctly identified by E(1), whereas E(2) produced 20 false dual positive results with only one actual mixed A(2254)+G(2254) genotype confirmed. Based on these findings, E(1) offers greater sensitivity and accuracy for the detection and A/G(2254) genotyping of EHV-1, making this improved rPCR assay a valuable diagnostic tool for investigating outbreaks of EHV-1 infection.

Molecular and pathological investigations of EHV-1 and EHV-4 infections in horses in Turkey

The aim of the present study was to investigate abortion storms that occurred in the Marmara region of Turkey in 2008-2009 using a real-time PCR. Two aborted foetuses were necropsied and histo-pathological findings reported herein. Ten lungs, 3 brains and one nasal swab from 10 aborted foetuses, 6 nasal swabs and 3 vaginal swabs from aborting mares were included in this study. EHV-1 was isolated from the lung, liver and brain of 1 aborted foetus. EHV-1 DNA was detected in the lungs, livers and spleens of 2 necropsied foetuses and in 3 lungs from 10 foetuses submitted for diagnosis. A brain from one of the aborted foetuses was also positive for EHV-1 DNA. EHV-4 DNA was detected only in a nasal swab of one of the tested foetuses. Neither EHV-1 nor EHV-4 DNA was detected in the swabs of aborting mares. Sequence analysis of the glycoprotein B of the strains was performed and a phylogenetic tree was generated. The results indicated that 4 of the 5 Turkish EHV-1 strains (TR02, TR03, TR04 and TR05) clustered together; the fifth strain (TR01) was slightly removed from the group and clustered with other EHV-1 from various origins. Single nucleotide polyporphism (SNP in ORF30) associated with neuropathogenesis was not detected in any of the strains. At necropsy, sub-milier focal necrosis in the liver and spleen was observed. Microscopically, focal coagulation necrosis and marked eosinophilic intranuclear and intracytoplasmic inclusion bodies in the hepatocytes localised around the necrotic areas in the liver. Severe coagulation necrosis in white pulp of the spleen was also observed.

Evaluation of the usefulness of a PCR assay performed at a clinical laboratory for the diagnosis of respiratory disease induced by equine herpesvirus type 1 in the field

A PCR assay for the diagnosis of respiratory disease induced by equine herpesvirus type 1 (EHV-1) was performed at the clinical laboratory in the Racehorse Clinic of the Ritto Training Center of the Japan Racing Association from December 2007 to March 2008. The assay was performed without the trouble of contamination throughout the study and its turnaround time was approximately 6 hr. The PCR detection rates of EHV-1 among seroconverted horses were 22.2% for nasal swabs and 33.3% for blood samples. However, EHV-1 DNA was also detected in horses without seroconversion at a low rate. These results indicated that the PCR assay should be used as an adjunct method, but would help to make an early diagnosis of EHV-1 infection.

Detection of neuropathogenic strains of Equid Herpesvirus 1 (EHV-1) associated with abortions in Germany

A single nucleotide polymorphism within EHV-1 gene ORF 30, which encodes for the viral DNA polymerase, allows the differentiation of the neuropathogenic (G2254) from non-neuropathogenic genotype (A2254). The aim of our study was to investigate the distribution of the neuropathogenic and non-neuropathogenic genotype of EHV-1 isolates associated with abortions in Germany. To determine the nucleotide sequence at the polymorphic site the amplification product of ORF 30 gene specific nested PCR was digested with restriction enzyme SalI and sequenced. Thirty-two EHV-1 isolates from six abortion outbreaks and 34 archived isolates from individual cases were obtained between 1987 and 2009 from stud farms in different regions of Germany. 89.4% (59/66) of the EHV-1 abortion isolates was of non-neuropathogenic genotype (N752/A2254) and 10.6% (7/66) contained the neuropathogenic marker (D752/G2254). Two out of seven EHV-1 abortion isolates with the mutation (G2254) came from the same outbreak and were derived from mares with neurological signs. Interestingly, the semen of a stallion from the same stud was tested positive for the neuropathogenic genotype (G2254) too. The other five EHV-1 strains came from individual abortion cases with no neurological signs. In addition to the A2254 to G2254 substitution, two EHV-1 reference strains (Ab4 and RacH) and one field isolate from an individual abortion case showed an exchange of adenine to cytosine at position 2258. In sum, we confirmed coherence between the occurrence of abortions and the non-neuropathogenic EHV-1 (A2254), but 10.6% of the abortion strains carried the mutation (G2254). The relevance of this finding as well as the role of the additional mutation and of stallions as carriers should be further investigated.

Cervid herpesvirus 2 infection in reindeer: a review

Herpesviruses of the genus Varicellovirus are known to infect and cause disease in a variety of ruminant species, but the impact of cervid herpesvirus 2 (CvHV2) in reindeer (Rangifer tarandus) is mostly unknown. Reindeer is a circum-polar species with a total estimated number of more than 5 million animals. Mortality may reach high values, as in northern Norway, especially in calves (37%; 2005-2006), and disease can potentially account for some of this mortality. CvHV2 has been isolated during a natural outbreak of infectious keratoconjunctivitis, indicating an etiologal link. Serological screening has shown that CvHV2 infection is prevalent in Northern Norway and experimental infection studies have demonstrated that viremia, latency and vertical transmission occur for CvHV2. The present review aims at summarizing current knowledge on the epidemiology, pathogenesis and molecular virology of CvHV2.

Equine herpesvirus-1 myeloencephalopathy: a review of recent developments

Equine herpes myeloencephalopathy (EHM), although a relatively uncommon manifestation of equine herpesvirus-1 (EHV-1) infection, can cause devastating losses on individual farms or boarding stables. Although outbreaks of EHM have been recognized for centuries in domestic horse populations, many aspects of this disease remained poorly characterized. In recent years, an improved understanding of EHM has emerged from experimental studies and from data collected during field outbreaks at riding schools, racetracks and veterinary hospitals throughout North America and Europe. These outbreaks have highlighted the contagious nature of EHV-1 and have prompted a re-evaluation of diagnostic procedures, treatment modalities, preventative measures and biosecurity protocols for the disease. This review concentrates on these and other selected, clinically relevant aspects of EHM.