Quantitation of sheep-associated malignant catarrhal fever viral DNA by competitive polymerase chain reaction

Malignant catarrhal fever: An emerging yet neglected disease in captive sika deer (Cervus nippon) herds in China

Malignant catarrhal fever (MCF) is a fatal lymphoproliferative disease that represents a serious problem in the deer-rearing industry. To better understand an MCF-like disease that has emerged in northern China since 2015, we investigated ten cases by documenting clinical and epidemiological data and analysing causative agents and histopathological changes. In addition, a retrospective screen for Macavirus DNA and a questionnaire-based survey were conducted. Epizootic MCF in Chinese sika deer herds has emerged with a low morbidity of 3.8% (95% CI: 2.5%-5.1%) and a high mortality of 93.2% (95% CI: 86.6%-99.9%). The disease course varied from 3 to 12 days. Aetiologically, OvHV-2 was predominant in the MCFV, accounting for most MCF cases (21/23). In contrast, only two CpHV-2 isolates were phylogenetically closely related to CpHV-2. Diarrhoea and nasal discharges were the most frequent manifestations, although clinical signs varied in some cases. Pathologically typical lesions of haemorrhage, necrosis and lymphoid cell infiltration were readily observed in a variety of organs. Vasculitis caused by vascular and perivascular lymphoid cell infiltration was common. The retrospective survey suggested a low positive rate (3/275) of MCFV DNA in peripheral blood lymphocytes (PBL). The questionnaire-based survey suggested the disease was neglected by local veterinarians, who did not acknowledge the risk of co-rearing deer with reservoir species. Collectively, the emerging epizootic MCF in Chinese sika deer herds remains neglected, emphasizing the urgency of initiating full-field diagnoses and control strategies.

Malignant Catarrhal Fever in Brazilian cattle presenting with neurological syndrome

Malignant Catarrhal Fever (MCF) was investigated in the central nervous system of cattle with neurological syndrome. Two-hundred-ninety samples were analyzed by histology, and molecular methods to detect ovine herpesvirus type 2 (OvHV-2) were optimized and validated. The qualitative polymerase chain reaction (qualitative PCR) analytical sensitivity was 10¹ DNA copies/μL and found 4.8% (14/290) positive for OvHV-2. The quantitative polymerase chain reaction (qPCR) analytical sensitivity was 10⁰ DNA copy/μL and 5.9% (17/290) positivity, with 47.1% (8/17) of the positive samples presenting histological evidence of non-purulent meningo-encephalitis. The qualitative PCR products (422 bp of the ORF75 region) were sequenced and submitted to phylogenetic analysis. Identity matrices showed 100% similarity in OvHV-2 samples obtained in this study and those recovered from GenBank, corroborating other studies.

Sheep-associated malignant catarrhal fever virus: prospects for vaccine development

Sheep-associated malignant catarrhal fever is emerging as a significant problem for several ruminant species worldwide. The inability to propagate the causative agent, ovine herpesvirus 2, in vitro has seriously hindered research efforts in the development of effective programs for control of the disease in clinically susceptible hosts. Recent molecular technologic advances have provided powerful tools for investigating this difficult-to-study virus. Identification of the infectious virus source, establishment of experimental animal models and completion of sequencing the genome for ovine herpesvirus 2 have put us in a position to pursue the development of vaccines for control of the disease. In this review, the authors briefly describe the current understanding of ovine herpesvirus 2 and prospectively discuss vaccine development against the virus.

The evolutionary processes of canine coronaviruses

Since the first identification of the virus in 1971, the disease caused by canine coronavirus (CCoV) has not been adequately investigated, and the role that the virus plays in canine enteric illness has not been well established. Only after the emergence in 2002 of SARS in human has new attention been focused on coronaviruses. As a consequence of the relatively high mutation frequency of RNA-positive stranded viruses, CCoV has evolved and, with the biomolecular techniques developed over the last two decades, new virus strains, serotypes, and subtypes have been identified in infected dogs. Considering the widespread nature of CCoV infections among dog populations, several studies have been carried out, focusing upon the epidemiological relevance of these viruses and underlining the need for further investigation into the biology of CCoVs and into the pathogenetic role of the infections. This paper reports the evolutionary processes of CCoVs with a note onto recent diagnostic methods.

Validation of nonnested and real-time PCR for diagnosis of sheep-associated malignant catarrhal fever in clinical samples

Sheep-associated malignant catarrhal fever (SA-MCF), a frequently fatal disease primarily of certain ruminants, is caused by ovine herpesvirus 2 (OvHV-2). Molecular diagnosis of SA-MCF in affected animals has relied on detection of OvHV-2 DNA using a nested PCR, which has significant potential for amplicon contamination as a routine method in diagnostic laboratories. In this report, a nonnested and a previously developed real-time PCR were validated for detection of OvHV-2 DNA in samples from clinically affected animals. Three sets of blood or tissue samples were collected: 1) 97 samples from 97 naturally affected animals with evidence of clinical SA-MCF; 2) 200 samples from 8 animals with experimentally induced SA-MCF; and 3) 100 samples from 100 animals without any evidence of clinical SA-MCF. Among 97 positive samples defined by nested PCR from clinically affected animals, 95 (98%) were positive by nonnested PCR and 93 (96%) were positive by real-time PCR, respectively. One hundred percent of the samples from the animals with experimentally induced MCF were positive by real-time PCR, while 99% were positive by nonnested PCR. Neither nonnested PCR nor real-time PCR yielded a positive result on any of the 100 nested PCR-negative samples from animals without evidence of clinical MCF. The data confirmed that both nonnested and real-time PCR maintained high specificity and sensitivity for the detection of OvHV-2 DNA in clinical samples.

Shedding of ovine herpesvirus 2 in sheep nasal secretions: the predominant mode for transmission

Ovine herpesvirus 2 (OvHV-2), the major causative agent of malignant catarrhal fever in ruminant species worldwide, has never been propagated in vitro. Using real-time PCR, a striking, short-lived, peak of viral DNA, ranging from 10(5) to over 10(8) copies/2 microg of DNA, was detected in nasal secretions from over 60.7% of adolescent sheep (n = 56) at some point during the period from 6 to 9 months of age. In contrast, only about 18% of adult sheep (n = 33) experienced a shedding episode during the study period. The general pattern of the appearance of viral DNA in nasal secretions was a dramatic rise and subsequent fall within 24 to 36 h, implying a single cycle of viral replication. These episodes occurred sporadically and infrequently, but over the 3-month period most of the 56 lambs (33, or 60.7%) experienced at least one episode. No corresponding fluctuations in DNA levels were found in either peripheral blood leukocytes or plasma. In a DNase protection assay, complete, enveloped OvHV-2 virions were demonstrated in the nasal secretions of all sheep examined during the time when they were experiencing an intense shedding episode. OvHV-2 infectivity in nasal secretions was also demonstrated by aerosolization of the secretions into OvHV-2-negative sheep. The data herein show that nasal shedding is the major mode of OvHV-2 transmission among domestic sheep and that adolescents represent the highest risk group for transmission.

Experimental transmission of ovine herpesvirus-2 in sheep

Transmission of ovine herpesvirus-2 (OvHV-2) in sheep via natural contact and nasal secretions was examined. OvHV-2-free lambs were produced by separating newborn lambs from their mothers within 5 days of birth and raising them in an isolation facility. Transmission experiments via natural contact were conducted by keeping OvHV-2-free lambs with OvHV-2-infected sheep of different ages. Six of the infected ewes in this experiment were pregnant and gave birth during the experimental period. OvHV-2 was not transmitted from the adult sheep, though viral DNA was consistently detected in their peripheral blood leukocytes (PBL). On the other hand, OvHV-2 was transmitted from recently infected lambs to sheep at 10 or 12 weeks after the onset of contact. In addition, we attempted the experimental transmission of OvHV-2 via nasal secretions, by transferring nasal washings from infected sheep to the nostrils of uninfected sheep. Sheep receiving the nasal washings from infected adult sheep maintained their negative status for 15 months, whereas sheep receiving nasal washings from recently infected lambs acquired OvHV-2 by 8 months. The results of these experiments support that OvHV-2 is more easily transmitted to negative sheep by recently infected lambs than by adult sheep. Further, it is supposed that the nasal cavity is a portal for entry and shedding of infectious OvHV-2 in sheep.

Quantitation of canine coronavirus RNA in the faeces of dogs by TaqMan RT-PCR

A TaqMan^® fluorogenic reverse transcriptase-polymerase chain reaction (RT-PCR) assay was developed for the detection and quantitation of canine coronavirus (CCoV) RNA in the faeces of naturally or experimentally infected dogs. The CCoV fluorogenic RT-PCR assay, which targeted the ORF5 (M gene), was more sensitive than a conventional RT-PCR assay targeting the same gene, showing a detection limit of 10 copies of CCoV standard RNA, and was linear from 10 to 10⁸ copies, allowing quantitation of samples with a wide range of CCoV RNA loads. A total of 78 faecal samples of diarrhoeic dogs were tested simultaneously by conventional and fluorogenic RT-PCR: 29 were negative by both techniques, whereas 27 tested positive by conventional RT-PCR and 48 by the established CCoV fluorogenic assay. One sample, which was positive by conventional RT-PCR, gave no signal in the fluorogenic assay. In addition, by the fluorogenic assay CCoV shedding in the faecal samples of an experimentally infected dog was monitored for 28 days. The high sensitivity, simplicity and reproducibility of the CCoV fluorogenic RT-PCR assay, combined with its wide dynamic range and high throughput, make this method especially suitable for efficacy trials on CCoV vaccines.

Demonstration of sheep-associated malignant catarrhal fever virions in sheep nasal secretions

Ovine herpesvirus-2 (OvHV-2) is the causative agent for sheep-associated malignant catarrhal fever, which has never been propagated in vitro. Previous studies from this laboratory demonstrated significantly high levels of OvHV-2 DNA in sheep nasal secretions, suggesting a likely avenue of transmission. In the present study, real-time PCR was used to identify sheep experiencing an episode of intense OvHV-2 DNA shedding in their nasal secretions. A nuclease-resistance assay was used to examine the secretions for the presence of intact cell-free enveloped OvHV-2 virions. The results revealed that all nasal secretion samples from five selected individuals experiencing intensive shedding events contained cell-free OvHV-2 virions. Virions could not be identified in secretion samples from 11 OvHV-2 infected sheep that were not experiencing a shedding event. This is the first unequivocal demonstration of cell-free OvHV-2 virions. These results suggest that OvHV-2 lytic infection occurs in the epithelium of certain tissues in the upper respiratory tract of the natural host.

Mural folliculitis and alopecia caused by infection with goat-associated malignant catarrhal fever virus in two sika deer

Two sika deer from a zoo in Florida were examined because of chronic hair loss and skin lesions. No common causes of alopecia were identified in either deer. One deer was treated with prednisone, but the condition worsened when the dosage was decreased. Both deer were euthanatized after several months because of continued disease. The predominant histologic lesion in skin specimens was granulomatous mural folliculitis. Serologic testing and sequencing of fragments produced with a consensus polymerase chain reaction assay indicated that both deer were infected with caprine herpesvirus-2, a newly recognized member of the malignant catarrhal fever group of viruses. Disease in these deer was substantially different from that typically seen following infection with ovine herpesvirus-2, the sheep-associated malignant catarrhal fever virus. Findings in these deer establish the pathogenicity of caprine herpesvirus-2 in sika deer and illustrate the ability of this group of complex herpesviruses to cause a wide variety of clinical abnormalities in diverse species.

Effect of passive transfer of maternal immune components on infection with ovine herpesvirus 2 in lambs

OBJECTIVE

To define the role of passively tranferred immunity in protection against early infection with ovine herpesvirus 2 (OvHV-2) in lambs.

ANIMALS

15 adult sheep and 34 lambs.

PROCEDURES

2 groups of animals were used, including 15 lambs born to OvHV-2-free ewes and 19 lambs born to OvHV-2-positive ewes. After nursing colostrum, all lambs and their dams were introduced into a flock positive for OvHV-2. Blood was obtained from the lambs every 2 weeks and examined by PCR assay and competitive inhibition ELISA.

RESULTS

None of the animals had positive results by PCR analysis for samples obtained approximately 2 weeks after introduction into the flock. In the group of lambs from OvHV-2-infected ewes, 5 of 19 had positive results at 1 month of age and 17 of 19 by 5 months of age. In the group of offspring from OvHV-2-negative ewes, only 1 of 15 had positive results at 1 month of age, and the number reached 12 of 15 by 5 months of age. All lambs in both groups had positive results by 6 months. An active antibody response to the virus was detected in animals within 3 weeks after viral DNA became detectable in the blood.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis suggests that passively transferred immunity does not play an important role in the delay of infection with OvHV-2 in lambs. Age also does not seem to influence susceptibility. The rate of infection in young lambs may simply be a reflection of the intensity of viral exposure in their environment.

Levels of ovine herpesvirus 2 DNA in nasal secretions and blood of sheep: implications for transmission

A recently developed competitive PCR for ovine herpesvirus 2 (OvHV-2) was used to examine the levels of viral DNA in nasal secretions and peripheral blood leukocytes (PBL) of lambs and adult sheep. Viral DNA first appeared in the PBL of most lambs after about 3 months of age and the levels remained relatively constant thereafter. In most of the lambs (83%, n=12), viral DNA was undetectable by PCR in nasal secretions prior to 5 months of age. A dramatic rise of OvHV-2 DNA levels in the nasal secretions occurred starting at 5-6 months of age, which peaked at approximately 7 months. The highest level recorded in lamb nasal secretions was 7.5x10(8)copies/2microg DNA which were 75,000-100,000-fold higher than the levels in PBL of the same lambs. In adult sheep (n=10), the viral DNA levels in both PBL and nasal secretions were relatively stable over the 13-month period of the study, which included a lambing season. The data strongly suggest that neonatal lambs are not an important source for the transmission of OvHV-2 to clinically susceptible species, and that the nasal cavity is an important portal for shedding of infectious OvHV-2 in sheep. Furthermore, this study failed to identify a seasonal pattern in levels of viral DNA in nasal secretions or PBL of adult sheep that would provide a basis for the traditionally held belief that clinical cases of malignant catarrhal fever are significantly associated with lambing ewes.

Quantitative fluorogenic PCR assay for measuring ovine herpesvirus 2 replication in sheep

A fluorogenic PCR specific for ovine herpesvirus 2 (OvHV-2) DNA was developed and compared to a previously established conventional seminested PCR. Testing of a total of 152 blood samples from both positive and negative animals revealed that the results of both assays corresponded to each other in 100% of the cases. A second fluorogenic PCR for genomic sheep DNA was required to normalize the quantity of viral DNA in the sample. Separate standard curves had to be constructed for each PCR. The analytical sensitivity of the new PCRs ranged between at least 10 copies and sometimes even 1 copy of target DNA per reaction mixture. In dilution series of the target DNAs, linear decreases of the signals were observed over 7 orders of magnitude. Thus, it was possible to calculate the amounts of viral DNA in relation to the amounts of cellular DNA by normalizing the absolute quantity of OvHV-2 DNA with the amount of genomic sheep DNA. By this technique, it was possible for the first time to quantitatively characterize the course of OvHV-2 replication in naturally infected sheep.

Epizootic malignant catarrhal fever in three bison herds: differences from cattle and association with ovine herpesvirus-2

Three bison herds in Colorado experienced high mortality from malignant catarrhal fever (MCF). In comparison with cattle, the bison had a more rapidly progressive disease, fewer clinical signs, and milder inflammatory histologic lesions. There was consistent association with ovine herpesvirus-2 (OHV-2). Contact with sheep was not consistent. Of 17 animals in herd A, 15 died of acute MCF; 1 was slaughtered while healthy; and 1 developed clinical signs of MCF, was treated with corticosteroids and antibiotics, and died of fungal abomasitis and rhinitis after 5 months. In herds B and C, approximately 300 of 900 and 18 of 20 died of MCF following brief clinical disease. The nearest sheep were 1 mile away from herd A, but direct contact with sheep could be documented in herds B and C. Complete gross and histologic examinations were conducted on 34 animals, including all animals in herd A, and MCF was diagnosed in 31. In addition, field necropsies were performed on all dead animals in herd B and most in herd C and MCF was diagnosed on the basis of the gross lesions in most animals. Clinical signs of each animal in herd A were recorded. Illness was brief, usually 8-48 hours. Clinical signs were subtle; separation from the herd was often observed. In all 3 herds, hemorrhagic cystitis and multifocal ulceration of the alimentary tract were consistently found at necropsy. Mild lymphocytic vasculitis was present in multiple organs. Ovine herpesvirus-2 was found by polymerase chain reaction (PCR) in 71 of 105 formalin-fixed tissue specimens from 29 of 31 animals with MCF. In herd A, blood samples from 13 animals were collected at 5 time points and tested by PCR for the presence of OHV-2 viral sequences in peripheral blood leukocytes. Nine bison with a positive PCR test and 4 with negative results prior to clinical illness died of MCF.

Transmission of ovine herpesvirus 2 among adult sheep

Previous studies from this laboratory have defined the pattern of acquisition of ovine herpesvirus 2 (OHV-2) in lambs under natural flock conditions. This study examined the question of whether OHV-2 could be transmitted between adult sheep. Two potential routes of transmission were examined: (1) direct inoculation of either viable leukocytes or whole blood from OHV-2 positive sheep, and (2) horizontal transmission through natural contact with OHV-2 positive sheep. Two groups of OHV-2 negative adult sheep were inoculated with material from infected sheep, one with 5x10(8) viable peripheral blood leukocytes (PBL), and the other with 100 ml of whole peripheral blood. No PCR signals were detected in any of the three sheep inoculated with the PBL during the 20 weeks following inoculation. In the group of five sheep inoculated with whole blood, two became PCR-positive at 7 and 8 weeks post-inoculation, respectively, and the remaining three sheep maintained their negative status until termination of the experiment at 20 weeks post-inoculation. In two experiments conducted in different flocks, a total of 20 adult sheep were used to examine horizontal transmission by contact; all animals became PCR-positive within 12 months of mixing the uninfected and infected animals. The results of these experiments support two conclusions. First, the susceptibility to OHV-2 is not limited to young lambs; adult sheep remain fully susceptible. Second, the fact that whole blood, but not PBL, from infected sheep was able to transmit the infection to only two of five inoculated sheep suggests that the infection in peripheral blood cells may be largely non-productive.