Malignant catarrhal fever: a review

Malignant catarrhal fever (MCF) is a fatal lymphoproliferative disease of cattle and other ungulates caused by the ruminant γ-herpesviruses alcelaphine herpesvirus 1 (AlHV-1) and ovine herpesvirus 2 (OvHV-2). These viruses cause inapparent infection in their reservoir hosts (wildebeest for AlHV-1 and sheep for OvHV-2), but fatal lymphoproliferative disease when they infect MCF-susceptible hosts, including cattle, deer, bison, water buffalo and pigs. MCF is an important disease wherever reservoir and MCF-susceptible species mix and currently is a particular problem in Bali cattle in Indonesia, bison in the USA and in pastoralist cattle herds in Eastern and Southern Africa.MCF is characterised by the accumulation of lymphocytes (predominantly CD8⁺ T lymphocytes) in a variety of organs, often associated with tissue necrosis. Only a small proportion of these lymphocytes appear to contain virus, although recent results with virus gene-specific probes indicate that more infected cells may be present than previously thought. The tissue damage in MCF is hypothesised to be caused by the indiscriminate activity of MHC-unrestricted cytotoxic T/natural killer cells. The pathogenesis of MCF and the virus life cycle are poorly understood and, currently, there is no effective disease control.Recent sequencing of the OvHV-2 genome and construction of an AlHV-1 bacterial artificial chromosome (BAC) are facilitating studies to understand the pathogenesis of this extraordinary disease. Furthermore, new and improved methods of disease diagnosis have been developed and promising vaccine strategies are being tested. The next few years are likely to be exciting and productive for MCF research.     

Bilateral perirenal hemorrhage in two Stone's sheep (Ovis dalli stonei): a possible manifestation of malignant catarrhal fever

Malignant catarrhal fever (MCF)-like disease was diagnosed at postmortem in 2 Stone's sheep (Ovis dalli stonei). On gross examination, the predominant abnormality in both sheep was severe perirenal hemorrhage and multiple renal infarcts. Microscopically, there was severe, multisystemic lymphocytic arteritis. Both sheep were positive for Ovine herpesvirus 2 (OvHV-2) on polymerase chain reaction, and partial sequencing of the viral DNA polymerase genes from the 2 sheep revealed >99% homology, with 96% similarity to the reference GenBank OvHV-2 viral sequence. Based on the histological lesions, polymerase chain reaction results, and viral DNA polymerase gene sequencing, a diagnosis of OvHV-2-mediated MCF was made. Massive perirenal hemorrhage has not been described previously as a manifestation of MCF.     

Experimental induction of malignant catarrhal fever in pigs with ovine herpesvirus 2 by intranasal nebulization

Malignant catarrhal fever (MCF), a frequently fatal herpesviral disease primarily of ruminant species, has been sporadically reported in pigs. All cases of naturally occurring porcine MCF reported to date have been linked to ovine herpesvirus 2 (OvHV-2), a gammaherpesvirus in the genus Macavirus carried by sheep. Experimental induction of MCF by aerosolization of the virus in nasal secretions collected from infected sheep has been successful in bison, cattle and rabbits. The goals of this study were to determine the susceptibility of pigs to MCF following experimental intranasal inoculation of OvHV-2, and to characterize the disease. Twelve pigs in four groups were nebulized with 10(5), 10(6), 10(7), or 10(8) DNA copies of OvHV-2 from sheep nasal secretions. Three control pigs were nebulized with nasal secretions from uninfected sheep. Three additional pigs were inoculated intravenously with 10(7) DNA copies of OvHV-2 to evaluate this route of infection with cell-free virus. Seven of twelve intranasally challenged pigs became infected with OvHV-2. Five of these seven, all in higher dose groups, developed MCF. Lesions resembled those reported in natural cases of porcine MCF. The most striking and consistent histological lesions were in trachea, lung, kidney and brain. These comprised mucopurulent tracheitis, interstitial pneumonia, necrotizing arteritis-periarteritis, and nonpurulent meningoencephalitis. No infection was established in the intravenously challenged or control groups. The study showed that MCF can be experimentally induced in pigs by aerosol challenge using sheep nasal secretions containing OvHV-2. Domestic pigs are a natural clinically susceptible host for sheep-associated MCF. They represent a useful, cost-effective model for MCF research.     

Intra-nasal inoculation of American bison (Bison bison) with ovine herpesvirus-2 (OvHV-2) reliably reproduces malignant catarrhal fever

Sheep-associated malignant catarrhal fever (MCF) due to infection with ovine herpesvirus 2 (OvHV-2) is common in commercial herds of American bison ( Bison bison). Inability to propagate OvHV-2 in vitro has been a constraint on experimental studies of the disease. We sought to establish whether nasal secretions from sheep that shed OvHV-2 might induce the disease in bison and to define a minimum challenge dose. Fourteen bison were nebulized with sheep nasal sections containing 10(3)-10(7) OvHV-2 deoxyribonucleic acid (DNA) copies. Most challenged bison (11/14, 78.6%) developed clinical signs at 29-52 days postnebulization (DPN). The mean incubation time was 42.18 (+/-7.33 SD) DPN. Using real-time polymerase chain reaction, we detected OvHV-2 DNA in peripheral blood leukocytes at 21-31 DPN. All bison that developed MCF had antibodies against the MCF group viruses. Gross and histologic lesions were typical of the acute disease. There was no morphologic evidence of a dose-related difference in the severity or distribution of lesions. This is the first successful reproduction of MCF in bison using a nasal route of exposure. Experimentally challenged bison are more susceptible to MCF, compared with experimentally challenged domestic cattle in a previous experiment. Bison are a pertinent ruminant species in which the pathogenesis of the disease can be investigated.     

Oral immunisation against classical swine fever (CSF): onset and duration of immunity

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.     

Evaluation of ovine herpesvirus type 2 infections, as detected by competitive inhibition ELISA and polymerase chain reaction assay, in dairy cattle without clinical signs of malignant catarrhal fever

OBJECTIVE: To monitor ovine herpesvirus type 2 (OvHV-2) infection status and the association between OvHV-2 infection and development of clinical signs of malignant catarrhal fever (MCF) in cattle. DESIGN: Longitudinal study. ANIMALS: 30 mature adult cows and 18 cattle submitted for necropsy. PROCEDURE: Blood and milk samples were collected at monthly intervals from 30 adult cows for 20 consecutive months. Nasal and ocular swab specimens were also collected during months 9 through 20. Polymerase chain reaction (PCR) assay for detection of OvHV-2 was performed on blood, milk, nasal swab, and ocular swab specimens. Competitive inhibition ELISA (CI-ELISA) for detection of antibodies against MCF viruses was performed on serum samples obtained prior to study initiation and monthly during the last 12 months. Tissues obtained from herdmates without clinical signs of MCF that were submitted for necropsy were analyzed for OvHV-2 DNA via PCR assay for possible sites of latency. RESULTS: Initially, 8 of 30 cows had positive CI-ELISA results. Seroconversion was detected in 4 cows. Ovine herpesvirus type 2 DNA was intermittently detected in blood, milk, nasal secretions, or ocular secretions from 17 of 30 cows. Twenty-one cows had positive CI-ELISA or PCR assay results. No cattle in the study developed clinical signs of MCF. Results of PCR assays performed on tissue samples from 2 of 18 animals submitted for necropsy were positive for OvHV-2. CONCLUSIONS AND CLINICAL RELEVANCE; OvHV-2 infection can occur in cattle without concurrent development of clinical MCF. Ovine herpesvirus type 2 DNA was detected intermittently, suggesting fluctuating viral DNA loads or reinfection in subclinical cattle. A definitive site of latency was not identified from tissues obtained during necropsy.     

Gamma herpesvirus carrier status of captive artiodactyls

Between 1998 and 2000, 103 individuals of 19 species of the order Artiodactyla at Whipsnade Wild Animal Park were tested for evidence of infection with gamma herpesviruses in order to distinguish between species which are susceptible to malignant catarrhal fever (MCF), caused by alcelaphine herpesvirus-1 (AlHV-1) of wildebeest (Connochaetes sp.) or ovine herpesvirus-2 (OvHV-2) of domestic sheep, and species which carry related viruses sub-clinically. Gamma herpesvirus DNA was detected in the known, or suspected, carrier species: roan antelope (Hippotragus equinus), scimitar-horned oryx (Oryx dammah), gemsbok (Oryx gazella), musk ox (Ovibos muschatus) and mouflon (Ovis musimon). In six other species: lowland anoa (Bubalus depressicornis) yak (Bos grunniens), sitatunga (Tragelaphus spekei), greater kudu (Tragelaphus strepsiceros), waterbuck (Kobus ellipsiprymnus) and Nile lechwe (Kobus megaceros), DNA was present in some newborn calves and over 30% of adults, strongly suggesting a carrier state. In contrast five Père David's deer (Elaphurus davidianus) and two swamp deer (Cervus duvauceli) died of MCF during the study. A virus isolated from scimitar-horned oryx calves produced cytopathic effects in scimitar-horned oryx kidney cell-culture and caused MCF in a rabbit.     

Virus in sheep's skin

In a double sense, the ovine gamma herpesvirus type 2 (OvHV-2) is a virus in sheep's skin. Not only is it present world wide in all sheep breeds but also it causes malignant catarrhal fever (MCF) in cattle pigs, elk, and bison. OvHV-2 cannot be propagated in cell culture. Therefore, new results from OvHV-2 research are based on molecular techniques and may be summarized as follows. OvHV-2 is transmitted by respiratory routes as well as by sexual intercourse. Lambs are infected within the first few months of life. Leucocytes, primarily latently infected lymphocytes, are responsible for disseminating the virus over the entire organism. On rare occasions, virus particles could be visualized by electron microscopy in explanted lymphocyte cultures. Structural antigens were detected by immunohistology in M-cells of diseased rabbits. Immunologically and cell biologically active genes have been detected on the viral genome.The products arising from those are thought to fine balance, the number of latently infected cells in sheep and to keep them alive without causing harm. Thus, it seems that this balance has been found through co-evolution, favoring both virus and natural host. In contrast, other host species that were exclude from the process of co-evolution, are bound to fall from MCF.     

Experimental aerosol infection of cattle (Bos taurus) with ovine herpesvirus 2 using nasal secretions from infected sheep

Infection of clinically susceptible ruminants, including domesticated cattle and American bison, with ovine herpesvirus 2 (OvHV-2) can result in the fatal lymphoproliferative and vasculitis syndrome known as malignant catarrhal fever (MCF). A reliable experimental infection model is needed to study the pathogenesis of MCF and to develop effective vaccination strategies to control the disease. An experimental aerosol infection model using sheep, the natural carriers of OvHV-2, has been developed (Taus et al., 2005). Using the protocol and OvHV-2 inoculum established in the previous study, eight calves were nebulized with four different doses of OvHV-2 in nasal secretions from infected sheep. Two control calves were nebulized with nasal secretions from uninfected sheep. Infection status of all calves was monitored using competitive inhibition ELISA, PCR and clinical parameters. Six of eight nebulized calves became infected with OvHV-2. One calf receiving the highest dose of virus developed typical clinical, gross and histological changes of MCF. This study showed that nasal secretions collected from sheep experiencing OvHV-2 shedding episodes were infectious for cattle and capable of inducing MCF. The data also indicate that cattle are relatively resistant to disease following infection. The use of more susceptible species as experimental animal models, such as bison and selected cervid species should be examined.     

The prevalence of ovine herpesvirus-2 in 4 sheep breeds from different regions in South Africa

About 90% of bovine malignant catarrhal fever (BMCF) PCR-positive cases in South Africa are caused by alcelaphine herpesvirus-1 (AlHV-1) and the other 10% by ovine herpesvirus-2 (OvHV-2). The prevalence of OvHV-2 in different sheep breeds in South Africa was determined in order to investigate whether the lower incidence of BMCF caused by OvHV-2 in comparison with AlHV-1 can be ascribed to a low incidence of the virus in sheep. A single-tube hemi-nested PCR was developed, evaluated and applied to detect OvHV-2 DNA. The prevalence of the virus in 4 sheep breeds from various regions in South Africa was shown to be 77%. No statistically significant difference was found amongst the sheep breeds tested.     

Development of a management program for a mixed species wildlife park following an occurrence of malignant catarrhal fever

During late 2001 and early 2002, a mixed species wildlife park in North Carolina experienced an acute outbreak of morbidity and mortality in Pere David's deer (Elaphurus davidianus), axis deer (Axis axis), blackbuck antelope (Antelope cervicapra), white-tailed deer (Odocoileus virginianus), and Rocky Mountain elk (Cervus elaphus). Clinical signs varied from fulminant disease, progressing from depression to bloody scours to death in fewer than 4 days in Pere David's deer, to a more protracted form of disease, ranging from 2 wk to 3 mo, in axis deer. In moribund axis deer, high levels of anti-malignant catarrhal fever (MCF) virus antibody by competitive-inhibition enzyme-linked immunosorbent assay were detected. Ovine herpesvirus 2 (OvHV-2) DNA was detected in peripheral blood leukocytes of the affected axis deer. No other MCF viruses were detected. Retrospective examination of frozen tissue samples from the affected Pere David's deer and blackbuck antelope also confirmed the presence of OvHV-2 DNA. Initial control efforts were directed at preventing further deaths of clinically susceptible animals by removing MCF virus reservoir species, particularly ovine species. The most prevalent ovine species in the wildlife park was mouflon sheep (Ovis musimon). All sheep were removed from the park by June 2002, and the last MCF death occurred in October 2002. Since mouflon sheep had been a prominent attraction in the wildlife park, the owner wanted a means to reintroduce this species to the park. Derivation of OvHV-2-uninfected mouflon lambs was undertaken using the previously described program for production of OvHV-2-free sheep (Ovis ovis). The rederived MCF virus-negative mouflon sheep were introduced into the park in approximately January 2004. As of December 2007, no further cases of MCF have occurred since the removal of OvHV-2-positive mouflon sheep and reintroduction of the virus-free lambs. This paper describes the successful management and control of MCF in a densely populated mixed species animal park.     

Experimental transmission of sheep-associated malignant catarrhal fever from sheep to Japanese deer (Cervus nippon) and cattle

The assumption that sheep carry ovine herpesvirus-2 (OvHV-2), the causative agent of sheep-associated malignant catarrhal fever (SA-MCF), is widely accepted, albeit OvHV-2 has not been isolated. We attempted experimental contact transmission of MCF from Japanese sheep persistently infected with OvHV-2 to Japanese deer (Cervus nippon) and cattle. In Experiment 1, a deer was kept in close quarters with an infected ewe. In Experiment 2, a second deer was kept with the same ewe. In Experiment 3, two cows were each kept with two infected wethers. In Experiment 1, the deer developed clinical signs at 138 days after first contact and then died. OvHV-2 genes by polymerase chain reaction (PCR) and fluorescent antibodies to Alcelaphine herpesvirus-1 were detected in the affected deer. Moreover, sequences of PCR products (423bp), obtained by amplification of materials from the sheep and from the affected deer, coincided. These results clearly confirmed that the sheep was a carrier of OvHV-2, and that this virus had induced SA-MCF in a deer. In other experiments, no OvHV-2 infection occurred in deer and cattle during the 6-18 months periods of contact, though viral genes were detected in the nasal swabs and white blood cells of the sheep. To our knowledge, this is the first report on successful experimental transmission of MCF from OvHV-2-infected sheep to deer.     

Transmission of caprine herpesvirus 2 in domestic goats

Caprine herpesvirus 2 (CpHV-2) is a recently recognized gammaherpesvirus that is endemic in domestic goats and has been observed to cause clinical malignant catarrhal fever (MCF) in certain species of deer. In this study, transmission of CpHV-2 in goats was examined. A total of 30 kids born to a CpHV-2 positive goat herd were selected and divided into two groups: group 1 (n=16) remained in the positive herd; group 2 (n=14) was separated from the herd at 1 week of age after obtaining colostrum. Peripheral blood samples from each kid were examined regularly by competitive ELISA for MCF viral antibody and by PCR for CpHV-2 DNA. Fifteen out of 16 goats (94%) that remained with the positive herd seroconverted and became PCR-positive for CpHV-2 by 10 months of age. In contrast, all kids (100%) that were separated from the positive herd at 1 week of age remained negative until termination of the experiment at 1 year of age. Additional transmission experiments revealed that all CpHV-2-free adult goats were susceptible to CpHV-2 or ovine herpesvirus 2 (OvHV-2) infection. The data indicate that the transmission pattern of CpHV-2 in goats is similar to the pattern of OvHV-2 in sheep and that CpHV-2-free goats can be established by early separation of kids from positive herds, which has significant implications for MCF control programs.     

Occurrence of ovine herpesvirus type-2 infection in sheep and cattle in Samsun Province, Turkey

July 2004, a cow with clinical signs of ovine herpesvirus type-2 infection which is known as sheep associated malignant catarrhal fever (SA-MCF) was reported in Samsun Province in Turkey. Blood samples were collected from the suspected cow, 10 sheep housed with it, and from 150 healthy sheep and 29 healthy cattle randomly selected from different places in Samsun Province. Nested polymerase chain reaction (n-PCR) was used to detect ovine herpesvirus type-2 (OvHV-2) DNA in the suspected cow and competitive- ELISA (c-ELISA) kits were used to detect antibodies against OvHV-2. The suspected cow was found to be n-PCR positive and c-ELISA negative. The serological results were as follows: All 10 (100%) of sheep housed with the suspected cow and 18 of 29 (62%) of the randomly selected cattle were found seropositive. All 150 randomly selected healthy sheep were seronegative. The overall percentage of seropositivity was 14.7% (28/190). OvHV-2 DNA was detected in the peripheral blood leucocyte (PBL) samples of the cow and of the 10 sheep housed with the suspected cow.     

A simpler, more sensitive competitive inhibition enzyme-linked immunosorbent assay for detection of antibody to malignant catarrhal fever viruses.

An earlier competitive inhibition enzyme-linked immunosorbent assay (CI-ELISA) was developed for detection of specific antibody against malignant catarrhal fever (MCF) viruses (MCFV) in ruminants. In this study, the indirect CI-ELISA was improved by conjugating the monoclonal antibody 15-A directly with horseradish peroxidase and by developing a method of producing precoated, dried antigen plates. This new test is referred to as a direct CI-ELISA. The reformatted test yielded a significantly improved sensitivity, and the time required was reduced to about one-sixth of the previous time. Of 37 MCF cases in cattle that were confirmed by histopathology and polymerase chain reaction (PCR) assay, 37 (100%) were positive by the new test, whereas the indirect CI-ELISA detected only 23 (62%). The direct CI-ELISA detected antibody to MCFV in 100% of 48 sheep that had been defined as infected with ovine herpesvirus 2 (OvHV-2) by PCR, whereas the indirect CI-ELISA detected only 41 (85%). Comparison of antibody titers measured by the 2 assays for sera collected from OvHV-2-infected sheep and from cattle, bison, and deer with clinical sheep-associated MCF revealed that the direct CI-ELISA offered a 4-fold increase in analytical sensitivity over the indirect format. The number of seropositive animals detected by the direct CI-ELISA among apparently normal cattle and bison was 2-3 times greater than the number detected by the indirect CI-ELISA, indicating that a significant percentage of normal cattle and bison are subclincally infected with MCFV.     

Malignant catarrhal fever-like disease in sheep after intranasal inoculation with ovine herpesvirus-2.

A malignant catarrhal fever (MCF)-like disease was induced experimentally in 3 sheep after aerosol inoculation with ovine herpesvirus-2 (OvHV-2). Each of 3 OvHV-2-negative sheep was nebulized with 2 ml of nasal secretions containing approximately 3.07 X 10(9) OvHV-2 DNA copies from a sheep experiencing an intensive viral-shedding episode. Ovine herpesvirus-2 DNA became detectable by polymerase chain reaction in the peripheral blood leukocytes of all 3 sheep within 3 days, and all 3 seroconverted between 6 and 8 days postinfection (PI). The sheep developed clinical signs, with copious mucopurulent nasal discharge and fever around 14 days PI. One of the 3 clinically affected sheep was euthanized at 18 days PI. Major lesions at necropsy were multifocal linear erosions and ulcers in mucosa of the cheeks, tongue, pharynx, and proximal esophagus and mild disseminated pneumonia. Microscopically, there was extensive moderate superficial histiocytic-lymphocytic rhinitis with epithelial dissociation and degeneration. Moderate multifocal histiocytic bronchointerstitial pneumonia was associated with loss of terminal bronchiolar epithelium. Lymphocytic vasculitis was present only in the lung. The remaining 2 sheep recovered clinically, approximately 25 days PI. The study revealed that clinical signs and lesions resembling MCF can develop when uninfected sheep are exposed to a high dose of aerosolized OvHV-2.