Characterization of ovine herpesvirus 2-induced malignant catarrhal fever in rabbits

Malignant catarrhal fever (MCF) is a frequently fatal lymphoproliferative disease syndrome primarily of ruminant species, caused by gammaherpesviruses in the genus Macavirus. Ovine herpesvirus 2 (OvHV-2), carried by sheep, causes sheep-associated MCF worldwide, while Alcelaphine herpesvirus 1 (AlHV-1), carried by wildebeest, causes wildebeest-associated MCF, mainly in Africa. Diseases in rabbits can be induced by both viruses, which are clinically and pathologically similar; however, recent studies revealed different expression of viral genes associated with latency or lytic replication during clinical disease between the two viruses. In this study, we further characterized experimentally induced MCF in rabbits by nebulization with OvHV-2 from sheep nasal secretions to elucidate the course of viral replication, along with in vivo incorporation of 5-Bromo-2'-Deoxyuridine (BrdU), to evaluate lymphoproliferation. All six rabbits nebulized with OvHV-2 developed MCF between 24 and 29 days post infection. OvHV-2 DNA levels in peripheral blood leukocytes (PBL) remained undetectable during the incubation period and increased dramatically a few days before onset of clinical signs. During the clinical stage, we found that predominantly lytic gene expression was detected in PBL and tissues, and both T and B cells were proliferating. The data showed that the viral gene expression profile and lymphoproliferation in rabbits with OvHV-2 induced MCF were different from that in rabbits with AlHV-1 induced MCF, suggesting that OvHV-2 and AlHV-1 may play a different role in MCF pathogenesis.     

Immune complexes associated with infection of cattle by the herpesvirus of malignant catarrhal fever

Associated with the fatal lymphoid proliferation induced by the herpesvirus of African malignant catarrhal fever (MCFV) was the deposition of immune complexes comprising immunoglobulin, complement and conglutinin in the kidneys of cattle with terminal pyrexia due to infection with this virus. Such deposits were common in glomeruli and were also seen in renal arteries with and without lesions. Absence of lytic complement and a significant depletion of conglutinin in terminal sera confirmed the activation of complement. However, virus antigen was only rarely detected and virus-specific antibody could not be demonstrated in the deposits. These disparities and the importance of the observations are discussed.     

Antibodies in carrier wildebeest to the lymphoproliferative herpesvirus of malignant catarrhal fever

Six types of antibody to malignant catarrhal fever virus (MCFV) were measured in 132 sera collected from Wildebeest in Kenya Masailand. The titre of all types of antibody declined slowly with increasing age of the wildebeest. A significantly greater proportion of wildebeest calves had higher titres of antibodies to MCFV early antigens, IgM antibodies to MCFV late antigens and complement-fixing antibodies, than did older animals. One seronegative calf, reared in isolation without colostrum, became seropositive 4 1/2 weeks after birth but did not show any clinical signs indicative of MCFV infection. Similarities between MCFV infection of wildebeest calves and other inapparent infections with lymphoproliferative herpesviruses are discussed.     

Characteristics of the herpesvirus of malignant catarrhal fever isolated from captive wildebeest calves

A herpesvirus was isolated from buffy coat cells from a newborn wildebeest (Connochaetes gnou) and from tissues of a 12-day-old wildebeest during the 1982 calving season of a captive, inbred herd maintained in a zoologic collection. Both wildebeests were clinically healthy, and there was no herd record that malignant catarrhal fever (MCF) existed. Each viral isolate produced cytopathologic changes in bovine kidney cell cultures (intranuclear inclusions and massive syncytia). The viral-infected cell cultures contained antigens of MCF virus detected by immunofluorescence. The morphology of each viral isolate as determined by electron microscopy was that of a herpesvirus. Suspensions of 4 to 5 ml of disrupted cell culture material which contained virus from each wildebeest were inoculated (IV) into white-tailed deer (Odocoileus virginianus). Each deer became clinically ill within 28 days. Both deer had mucoid catarrh and a febrile response (40.5 to 41 C). Each also seroconverted to MCF virus. The histopathologic change in the tissues from the 2 inoculated deer was vasculitis. At 16 to 17 days after the deer were inoculated, a syncytial-forming virus was isolated from each deer from buffy coat cells fused with polyethylene glycol (1000) to bovine fetal kidney cells. The virus was identified as MCF virus by immunofluorescence and production of antibody to MCF virus. The presence of virus in the inbred wildebeest herd established this species as a reservoir or latent carrier of African MCF virus at the zoologic park.     

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.     

Malignant catarrhal fever in cattle experimentally inoculated with a herpesvirus isolated from a case of malignant catarrhal fever in Minnesota USA

A malignant catarrhal fever (MCF)-like syndrome was experimentally induced in three steers, which were under immunization trials with a herpesvirus previously isolated from a case of MCF in a cow in Minnesota USA. The clinical signs observed in the three steers, and the pathological and histological lesions observed in two of these steers which succumbed to the disease syndrome were indistinguishable from those described for MCF. Although seroconversion was readily demonstrated in the three animals, virus was not re-isolated from the blood leucocytes, secretions and tissues obtained from the two animals which succumbed to the syndrome during the course of the disease and after death. However, a herpesvirus which showed cell rounding cytopathic effects (cpe) in bovine thyroid cells (Bth), was re-isolated from the one steer which survived the disease.     

The malignant catarrhal fever complex

Malignant catarrhal fever (MCF) is defined as a clinicopathological syndrome caused by related herpesviruses and acquired from persistently infected wildebeest and sheep. There is convincing epidemiologic and virologic evidence that Alcelaphine herpesvirus 1 (AHV1) causes the wildebeest-derived disease (WD-MCF). Present knowledge suggests that a herpesvirus related to AHV1 may be associated with some cases of the non-wildebeest-associated disease (NWA-MCF). However, this virus possibly represents a passenger virus not related with the ultimate cause of the disease. Moreover, evidence for the role played by sheep as the reservoir for the agent of NWA-MCF is not convincing and awaits confirmation.     

Neutralising antibodies to wildebeest-derived malignant catarrhal fever virus in African wildlife

A total of 2,722 sera collected between 1963 and 1983, from 43 different species of wildlife in 11 African countries was examined for neutralising antibodies against the wildebeest-derived strain of malignant catarrhal fever (MCF) virus. Antibodies were demonstrated in 10 species of Bovidae which included eight species from the sub-family Hippotraginae and one species each from Bovinae and Antilopinae. Neutralising antibodies were also recorded in hippopotamus. It is suggested that the high prevalence of antibodies recorded in sera from waterbuck and reedbuck indicate infection with MCF. However, titres in other species may be due to antigenically related viruses.     

Shedding of malignant catarrhal fever virus by wildebeest calves

Malignant catarrhal fever virus (MCFV) was isolated from nasal and ocular secretions of wildebeest calves up to 3 months old. The virus was also isolated from explant cultures of cornea and nasal turbinates. It is suggested that MCFV replicates in the cornea and turbinates of young wildebeest calves less than 4 months old.MCFV was not isolated from secretions of calves older than 3 months, but virus neutralizing antibodies were found in their nasal secretions. The appearance of antibodies in the nasal secretions coincided with the cessation of virus shedding. The failure of calves over 3 months old to shed MCFV might explain the seasonal nature of bovine MCFV infection.     

The location of primary replication of the herpesvirus of bovine malignant catarrhal fever in rabbits

The herpesvirus of malignant catarrhal fever (MCFV) was isolated from the spleen of $\text{2}\text{3}$ rabbits 4 days after the intravenous inoculation of infectious lymph node suspension, while no virus could be isolated at 2 and 6 days post inoculation (p.i.). Indirect immunofluorescence identified the antigen-positive cells at 4 days p.i. as medium sized lymphocytes lying in the venous sinuses of the spleen, lower numbers of fluorescing cells being seen in the paracortical areas of lymph nodes and in the thymus. Scanty fluorescence, without isolation of virus, was evident in the spleen at 6 days p.i. but by day 8 p.i. virus could be isolated irregularly from spleen and lymph nodes and at 12 days p.i. was found in all lymphoid tissues in those rabbits with lymphadenitis and pyrexia. The primary replication in the spleen at 4 days is compared with other herpes induced lymphoproliferative disorders.     

Ocular lesions of bovine malignant catarrhal fever

The ocular lesions of bovine malignant catarrhal fever were characterized in 15 naturally occurring and eight experimentally induced cases of the disease. Consistent findings included: lymphocytic vasculitis of retinal, scleral, posterior ciliary, and uveal vessels; uveitis, especially involving ciliary processes, ciliary body, and iris; and keratitis with corneal edema, neovascularization, and epithelial and endothelial degeneration. Lymphocytic ciliary neuritis and optic meningitis were found less frequently. Ultrastructural examination of the ciliary body and iris from one experimental calf confirmed that most infiltrating mononuclear cells were lymphocytes. The uveitis, vasculitis, and keratitis of malignant catarrhal fever were probably immune-mediated.     

Antibodies to ovine herpesvirus 2 glycoproteins decrease virus infectivity and prevent malignant catarrhal fever in rabbits

Ovine herpesvirus-2 (OvHV-2) is the etiological agent of sheep-associated malignant catarrhal fever (SA-MCF), a fatal lymphoproliferative disease of many species in the order Artiodactyla. Development of a vaccine is critical to prevent mortality. Because OvHV-2 has not been cultured in vitro, SA-MCF research is hindered by the lack of in vitro tools to study viral constituents and specific host immune responses. As an alternative, in this study the neutralizing activity of antibodies against OvHV-2 glycoproteins gB and gH/gL was evaluated in vivo using rabbits. OvHV-2-specific antibodies were developed in rabbits by immunization using biolistic delivery of plasmids expressing the genes of interest. A lethal dose of OvHV-2 was incubated with the antisera and then nebulized into rabbits. Virus neutralization was assessed by measuring infection parameters associated with the virus infectious dose. Anti-gB or anti-gH/gL antibodies alone blocked infection in five out of six rabbits (83%), while a combination of anti-gB and anti-gH/gL antibodies protected all six rabbits (100%) from infection. These results indicate that antibodies to OvHV-2 gB and gH/gL are capable of neutralizing virions, and consequently, reduce virus infectivity and prevent SA-MCF in rabbits. Thus, OvHV-2 gB and gH/gL are suitable targets to be tested in a SA-MCF vaccine aimed at stimulating neutralizing antibody responses.