Epidemiology, Disease and Control of Infections in Ruminants by Herpesviruses – an Overview

INTRODUCTION This paper gives an overview of the biology of infections by currently recog-nised ruminant herpesviruses. There are 128 ruminant species in the family Bovidae. The family Herpesviridae contains 3 subfamilies, the Alpha-, Beta-and Gamma-herpesvirinae based on their biological characteristics. Alphaherpesviruses are characterised by rapid, lytic growth cycles; they primarily establish latency in neurons of sensory ganglia and have a variable but broad host range. Beta-herpesviruses are slow-growing, cause enlargement of infected cells (cytomegaly), have a restricted host range and establish latency in lymphoid cells. Gammaherpes-viruses may be oncogenic, are slow-growing and have specificity for either T or B lymphocytes and establish latency in lymphoid cells 74. There are at least


INTRODUCTION
This paper gives an overview of the biology of infections by currently recognised ruminant herpesviruses.There are 128 ruminant species in the family Bovidae.The family Herpesviridae contains 3 subfamilies, the Alpha-, Beta-and Gamma-herpesvirinae based on their biological characteristics.Alphaherpesviruses are characterised by rapid, lytic growth cycles; they primarily establish latency in neurons of sensory ganglia and have a variable but broad host range.Betaherpesviruses are slow-growing, cause enlargement of infected cells (cytomegaly), have a restricted host range and establish latency in lymphoid cells.Gammaherpesviruses may be oncogenic, are slow-growing and have specificity for either T or B lymphocytes and establish latency in lymphoid cells 74 .There are at least 16 recognised herpesviruses affecting ruminants.Of the 16 recognised viruses, 6 belong to the subfamily Alphaherpesvirinae and 8 to the subfamily Gammaherpesvirinae while buffalo herpesvirus (BflHV) and ovine herpesvirus type 1 (OvHV-1) remain officially unclassified 18 .These unclassified viruses do not appear to be important pathogens and will not be considered further as little published information is available for them.It should be emphasised that BflHV is antigenically related to bovine herpesvirus-1 (BHV-1) but the viruses have distinct restriction endonuclease DNA fingerprints 5 .The recognised ruminant alphaherpesviruses are BHV-1, BHV-2, BHV-5, caprine herpesvirus-1 (CapHV-1), cervine herpesvirus-1 (CerHV-1), and CerHV-2.
The currently known gammaherpesviruses comprise 8 malignant catarrhal fever (MCF)-related viruses and BHV-4.The MCF-related virus group comprises 4 viruses known as alcelaphine herpesvirus-1 (AlHV-1), -2 (AlHV-2), hippotragine herpesvirus-1 (HipHV-1), ovine herpesvirus-2 (OvHV-2) and 4 recently identified viruses.These newly discovered lymphotropic viruses have been provisionally named caprine herpesvirus-2 (CapHV-2), cervine herpesvirus-3 (CerHV-3), bovine lymphotropic herpesvirus (BLHV) and caprine lymphotropic herpesvirus (CapLHV) 34 .These viruses cause either no illness, mild to moderate or fatal disease in spite of sharing some common pathogenic mechanisms (see below).Accordingly, the economic impact of infection in the respective natural hosts by these viruses also varies.This paper will concentrate on diseases, epidemiology, and latency of these viruses and available immunoprophylaxis in natural hosts.There are more than likely herpesviruses, particularly in wild ruminants, that remain undiscovered since systematic surveys and monitoring of wild ruminants is costly and difficult to conduct.

Alphaherpesviruses
Infection by alphaherpesviruses mostly occurs through the respiratory tract via virus aerosol 18 , while infection of genital tract surfaces usually occurs after viraemia resulting from the respiratory tract infection.At both these sites virus replicates in epithelial cells.The released progeny virus infects fresh epithelial cells.This results in viraemia.The process continues for several days until immune intervention.Infected cells are usually destroyed.This damages the mucosae and results in inflammation.
Bovine herpesvirus-1 (BHV-1), commonly known as infectious bovine rhinotracheitis virus (IBR), is arguably the most important respiratory tract disease of cattle globally.IBR due to BHV-1 infection is an important cause of economic losses to the cattle industry worldwide 21 .BHV-1 causes IBR, infectious pustular vulvovaginitis (IPV) and infectious balanoposthitis (IPB) and infrequently other diseases in cattle 21 .The progeny virus shed by epithelial cells can also invade local neurons whose endings are embedded in the mucosae.Brain invasion is not a common consequence of BHV-1 infection 42 .There are conflicting reports as to whether the respiratory and the genital isolates of BHV-1 can be distinguished by restriction endonuclease DNA fingerprints 5 .These authors did not find obvious differences between the respiratory and the genital BHV-1 isolates.Besides cattle, BHV-1 also naturally infects sheep, goats 44 and pigs and a number of wild ruminants 21,48 .There is scanty published data on the prevalence, incidence and economic impact of BHV-1 infection of these other species.There is evidence of antibody to BHV-1 in sera from 20 species of free-living African ruminants, Australian water buffaloes (Bubalus bubalis) and American prong horn antelope (Antillocapra americana) but the antibody could be due to related ruminant alphaherpesviruses 48 .In cattle the main route of transmission is by virus aerosol 21,18 .This route is also likely to be the main mode of virus transmission in deer 18 .Experimentally, BHV-1 is transmissible to goats by intranasal inoculation and although infected goats shed virus in nasal mucus for 3-10 days they remained clinically normal 17 .BHV-1 is also transmissible to mule deer 10 .
Bovine herpesvirus-2 (BHV-2) mammillitis is not a common disease in temperate countries but the incidence is high in cattle in the tropics 39 .BHV-2 in the tropics or warmer countries is associated with a generalised skin disease known as pseudo-lumpy skin disease.Furthermore antibody to BHV-2 has been detected in 17 species of African wild ruminants 48 .BHV-2 causes mammillitis in heifers and young cows 39 .BHV-2 infection produces widespread erosive skin lesions on teats and udders.Erosion and ulceration leads to the formation of scabbed pustules.Similar lesions in the buccal mucosa may occur in suckling calves.Unlike other alphaherpesviruses, BHV-2 transmission occurs mechanically by milking machines and dairy personnel 39 .
Geographically, fatal encephalitis in calves due to bovine herpesvirus-5 (BHV-5) is restricted to Australia 20 , Argentina and United States of America (USA) and sporadically in other countries 43 .BHV-5 encephalitis is a sporadic and uncommon disease of cattle 5,20 .Encephalitis usually occurs without other clinical signs 5,43 .Similar to the closely related BHV-1 the main mode of BHV-5 transmission is via the aerosol route.The incidence of BHV-5 induced disease and the associated economic loss in countries where the virus is most prevalent is poorly documented.However, unlike BHV-1, upper respiratory tract infection by the closely related BHV-5 can lead to brain invasion via the trigeminal pathway even though BHV-1 and BHV-5 have similar growth profiles (extent and duration) in the nasopharynx of calves 43 .The inflammatory responses in the upper respiratory tract due to BHV-1 and BHV-5 are also different in infected calves.Thus, in BHV-5 infection of calves, unlike that of BHV-1 infection, pyrexia and nasal discharge are not commonly observed 43 .
Natural caprine herpesvirus-1 (CapHV-1) infections have only been recognised in goats 3,4,97 .CapHV-1 was isolated from goat kids (Capra hircus) 79 and the virus is distributed worldwide 45 .Cattle and sheep are susceptible to experimental infection by CapHV-1 3,4,18,53 .In adult goats most CapHV-1 infections are subclinical.Abortions due to CapHV-1 have been recorded after experimental intranasal or intravenous infection of pregnant does 4,79 .Natural or experimental infection of neonatal kids with CapHV-1 is often fatal 4,95 .Ten-to-16-day-old Boer goat kids died showing depression, abdominal pain, anorexia, nasal and ocular discharge, ulceration of the nasal septum, erosions in the nasal cavity and diarrhoea 95 .In natural fatal cases in kids ulcerative and necrotic lesions were found in the ileum, colon and caecum while experimentally these lesions were seen in the caecum and colon 4,95 .CapHV-1 is nonpathogenic for bovine cows 4 and nonpathogenic 4 or mildly pathogenic 53 for lambs.However, information on the prevalence and epidemiology of CapHV-1 is limited 91,92 .Since goats can be infected by both the intranasal and the intravaginal routes and virus from the respiratory infection can spread to the genital site, the mode of CapHV-1 transmission is likely to be via the oronasal route 88,92 .Experimentally, CapHV-1 causes an IBR-like disease in goats with shorter (1-6 days) periods of viral shedding compared to the 1-12 days of shedding in nasal mucus of calves infected by BHV-1 or BHV-5 17,43 .Importantly, the economic impact of CapHV-1 infection in goat rearing areas remains unreported but there could be considerable loss of goat kids due to CapHV-1 infection 97 .
Cervine herpesvirus-1 (CerHV-1) is widespread in free-living and farmed red deer (Cervus elaphus) 47 .It was isolated from an outbreak of ocular disease primarily involving copious mucopurulent discharge and supraorbital oedema 48 .CerHV-1 can also cause respiratory disease, notably ulcerative erosions of the nares and muzzle, fever, mucopurulent nasal discharge and conjunctivitis.Infected deer shed virus in eye and nasal secretions 26,47 .Experimentally, CerHV-1 was not transmissible to bovine calves by means of intranasal inoculation 49,69 .
The prevalence of rangeferine herpesvirus-1 (CerHV-2) is poorly documented and the virus is not associated with an overt clinical disease in reindeer, the natural host 19 .However, experimentally CerHV-2 does cause mild rhinitis in bovine calves accompanied by viral shedding for 6-9 days 49 .
A key mechanism in the disease process caused by some alphaherpesviruses in ruminants is progeny virus viraemia initiated from the site of primary mucosal replication (usually upper respiratory tract mucosa) to secondary sites within the body of the infected host.In general the systemic spread is by means of lymphocytes 18 .It should, however, be pointed out that the level of viraemia is low since experimentally it could barely be detected even in cases of BHV-1 and BHV-5 infections of calves where there is significant (8-9 log10 median tissue culture infective dose (TCID50) infectivity per mlll nasal swab) virus shedding by almost all infected calves.Systemic disease has been recorded in the case of BHV-1, BHV-2, BHV-5, CapHV-1, and CerHV-1 albeit with varying incidence rates and severity.

Gammaherpesviruses
Gammaherpesviruses have a tropism for either T or B lymphocytes 46 .A feature of gammaherpesvirus infection is the induction of lympoproliferation which may manifest itself as infectious mononucleosis, splenomegaly, or malignant catarrhal fever (MCF).In free-ranging animals, MCF is a fatal disease of cattle, deer, bison (Bison bison) and moose (Alces alces).Many other ruminant species become susceptible when in captivity.The presence of antibodies that react with AlHV-1 has consistently been detected in species belonging to 4 subfamilies of the family Bovidae although there is no evidence that any of them naturally spread to susceptible species and cause MCF.In natural hosts, MCF viruses are endogenous without causing disease but often cause disease in other secondary ruminant hosts 65,72,73 .The characteristic pathology in MCF is a widespread lymphoid cell proliferation and cell necrosis in which the pathognomonic lesions are widespread arteritis characterised by segmental fibrinoid necrosis 63,64,65 .MCF is caused by several different gammaherpesviruses, namely AlHV-1, OvHV-2, CaHV-2 and MCF virus of white tailed deer.
In Africa, MCF is mainly caused by the wildebeest-associated AlHV-1 while outside Africa ovine herpesvirus-2 (OvHV-2) is the main cause of sheep-associated 0038-2809 Jl S.Afr.vet.Ass.( 2008) 79(1): 8-14 MCF 65,73 .OvHV-2 is also present in Africa.AlHV-1 grows in tissue culture 62 thus allowing virus isolation and virus antibody tests required for virus epidemiology and pathogenetic studies.By contrast, OvHV-2 is at present not cultivable in tissue culture and hence studies of OvHV-2 have relied on tests using virus specific DNA probes 7,1,25 and recently a new monoclonal antibody based ELISA test 32,35 .
In Africa, AlHV-1 is carried by blue and black wildebeest (Connochaetes taurinus and Connochaetes gnou) in which it is innocuous and is shed in nasal mucus 65 .In cattle, sporadic but fatal cases occur at the time of wildebeest calving, the route of virus transmission being virus containing aerosol from shedding wildebeest.Wildebeest have been the source of AlHV-1 transmission to deer and many other antelope species in zoos 48,73 .AlHV-1 induced fatal MCF was reported in sika deer (Cervus nippon) 80 , barasingha deer (Cervus duvaucell) 22 , Père David's deer (Elaphurus davidianus) 97 , reindeer 48 , and white-tailed deer (Odocoileus virginianus) 101,103 .
Sheep older than 6 months are the healthy carriers of OvHV-2 25 , and are the source of OvHV-2 transmission to deadend, secondary ruminant hosts worldwide wherever sheep are in contact with them 65,71,73,25 .Goats are also a source of transmission of OvHV-2 24,102 .Fatal cases of sheep-associated OvHV-2 MCF have been recorded in cattle and deer species (see below).Recent data 51 show that OvHV-2 is not invariably fatal in cattle.OvHV-2 is reported to cause disease in pigs 36 .The mode of OvHV-2 transmission between sheep is via nasal secretions and the highest amounts of OvHV-2 DNA were detected in nasal secretions of sheep 25 , which could also be indicative of oronasal transmission to cattle, deer and other susceptible species.This has been directly demonstrated by intranasal inoculation of cattle (Bos taurus) and American bison (Bison bison) with sheep nasal secretions containing OvHV-2 DNA.Inoculated cattle and bison developed clinical MCF 52,90 .Experimental intranasal OvHV-2 transmission to sheep was also demonstrated using nasal secretions from infected sheep 89 .Losses in cattle are sporadic but those in deer are extensive.Unlike the situation in cattle, transmission from farmed red deer to other deer and rabbits has proved relatively easy 68 .The deer species susceptible to OvHV-2 are: farmed red deer 76,67,40 , rusa deer (Cervus timorensis) 14 , axis deer (Axis axis) 12 , mule deer 60 and Père David's deer 70 .In cattle, OvHV-2-induced MCF occurs mainly over several weeks following lambing 25 .The latter was explained on the basis of an increased reactivation of latent OvHV-2 during this period of increased social interaction among sheep 25 .
HipHV-1 was isolated from culture of explanted submandibular lymph node (LN) from a roan antelope (Hippotragus equinus).The explanted LN culture grew as a monolayer with cytopathic effects (CPE) 72 .The LN culture grown HipHV-1 induced clinical disease and pathological lesions characteristic of MCF in New Zealand white rabbits after intravenous inoculation but was not tested in cattle 72 .The natural host of AlHV-2 is the red hartebeest (Alcelaphus buselaphus) 66 .Experimentally AlHV-2 is transmissible to cattle and other ruminant species but produces a mild form of MCF 18 .
In addition to the known MCF-causing vir uses namely AlHV-1, OvHV-2, CapHV-2, and MCF virus of white tailed deer 33 described above, 4 new novel gammaherpesviruses related to AlHV-1 and OvHV-2 were discovered recently.The new viruses are BLHV 78 , CerHV-3 33 , CapHV-3 34 and CapLHV 34 .Discovery of novel MCF viruses has depended on targeting viral DNA polymerase gene with specific sequences for the PCR-based detection of new herpesviruses 34 .CerHV-3 causes classical MCF in white-tailed deer (Odocoileus virginianus) 33 while CapHV-2 which is carried by goats, causes chronic dermatitis and weight loss in sika deer (Cervus nippon) 34 , but how common these diseases are is unknown.For BLHV and CapLHV we are unable to find published literature for disease incidence and prevalence and also if other ruminant species are affected or not.This related group of gammaherpesviruses also includes HipHV-1 72 .
The pathogenesis of fatal MCF due to AlHV-1 and OvHV-2 in secondary ruminant hosts is suggested to arise from infection of a specific subset of lymphocytes which undergoes uncontrolled interleukin driven hyperplasia, mainly of CD8 + cells 81 and consequent widespread autoimmune destruction of tissues.It was suggested that AlHV-l, OvHV-2, HipHv-1 interfere with the immune system in a subtly different way 81 .In AlHV-1-induced MCF, there is deposition of immune complexes in lesions 55,81 .Consistent with the T-lymphocyte hyperplasia hypothesis is the fact that very little virus antigen expression is detected in lesions of AlHV-1-induced MCF.Thus, approximately 1 cell in 100 000 was positive for AlHV-1 antigen 54 or DNA 8 .
Bovine herpesvirus-4 (BHV-4) is a ubiquitous virus of cattle.Based on restriction enzyme cleavage pattern analysis of virus genomic DNA, BHV-4 isolates have been classified as American and European strains.However, BHV-4 isolates do not always comply with this geographical classification 9 .The mode of virus transmission in cattle and possibly other hosts such as American bison (Bison bison), water buffaloes (Bubalus bubalis) and African buffaloes (Syncerus caffer) is likely to be via the respiratory route since the virus is shed in nasal and ocular secretions 61 .BHV-4 infection in cattle is not associated with a significant disease and its economic impact is therefore not clear.Epidemiologically, the significance of the virus either alone or in association with other cattle viruses is difficult to assess despite considerable knowledge of BHV-4 molecular virology 93,94 .

Alphaherpesviruses
Some glycoproteins of BHV-1, BHV-5, CapHV-1, CerHV-1 and CerHV-2 share common epitopes and these viruses cross-react in several serological tests including also the most virus specific test namely the virus neutralisation (VN) test 50,37,29,100 .Limited data also confirm the serological findings of cross relationships at the level of genomic relatedness to a varying degree 16,82 .There is indeed shared homology of the genomic sequences and structure among these viruses but the data are limited to allow conclusions about their evolution and genetic changes in adaptation to their respective natural hosts 48,18,82 .Although most of the ruminant alphaherpesviruses share antigenic and genetic components there are differences between them.The genomic DNA of CerHV-1 and CerHV-2 are distinct from each other and also from the BHV-1 genome 82 .It also applies to CapHV-1 and BHV-1 16,82 .However, the closest antigenic and genetic similarity is between BHV-5 and BHV-1 and yet they vary markedly in the diseases they cause.The former can cause a fatal brain infection whist the latter is an acute febrile respiratory disease 20,18 .

Gammaherpesviruses
Recent studies suggested that ruminant MCF viruses should be defined by the presence of the 15-A antigen epitope and base sequence similarity of conserved regions of viral DNA polymerase gene 33,34 .The 15-A antigen epitope is present on all known MCF viruses 34,33 .An ELISA kit for serological detection of MCF using a monoclonal antibody to the 15-A MCF epitope is available*.Prior to these recent tests and probes, relatedness of MCF viruses was established by serological tests using polyvalent antisera from hyperimmunised and/or infected animals. 23,64,65,72,73,77.
We should, however, point out that not all of these viruses have been shown to induce MCF.
The ubiquitous BHV-4, despite considerable investigation of its genome and the protein makeup 93,94 remains the least understood with respect to its relationship with other ruminant gammaherpesviruses.BHV-4 has a genomic structure and arrangement closely resembling herpesvirus saimiri and the prototype virus of the subfamily Gammaherpesvirinae namely Epstein Barr virus 93,94 .Epidemiologically the significance of the virus either alone or in association with other cattle viruses is not clear despite much work.

Latency
All herpesviruses are able to establish latent infections.A requirement for an effective latent infection is that the cell harboring the virus is long-lived and is not destroyed either through apoptosis or immune elimination.Long living, non-dividing, terminally differentiated cells such as neurons and lymphocytes fulfill this requirement.Whilst the mechanisms central to the establishment and maintenance of a state of latency remain yet to be fully defined, it is clear that latently infected cells are not eliminated by the host's immune responses.Mechanisms triggering reactivation have also not been identified 18 .However, dysfunction of the cellular immune system and/or immunosuppression appears to be involved in this process 18 .The site of latency by most analysed alphaherpesviruses is primarily in neurons of the sensory ganglia close to the primary site of virus infection and replication.For some gammaherpesviruses notably AlHV-1 and OvHV-2, lymphocytes are probably the site of latent infection in carrier hosts.Virus reactivated from such cells after productive virus replication, usually in the mucosa of the respiratory tract, is considered the source for transmission to other susceptible hosts both natural and unnatural.Reactivation of HipHV-1 in explanted diced fragments of submandibular lymph node from an African roan antelope (Hippotragus equinus) was suggested as the mechanism of productive virus growth and CPE in these cultures which eventually produced a cell monolayer 72 .However, the molecular events leading to reactivation and those of transport of reactivated infectious progeny virus back to the portal of entry at the primary infection site from where virus is shed and transmitted to susceptible hosts are undefined.Thus, latency represents a long-term reservoir in an otherwise immune host.In the case of BHV-1 reactivation, infectious virus is transported intra-axonally back to the original portal of virus entry where virus replicates and is shed and may thus be transmitted 18 .

Virulence modulating genes
For alphaherpesviruses, mutations of some non-essential and essential genes were found to modulate virulence of the virus for the natural host.Deletion of several non-essential genes has led to decreased virulence of the mutated virus for the natural host.Notable examples are genes encoding glycoproteins (g) C, E, I, and M 11,31,74,98 , and virus-encoded enzymes such as thymidine kinase 30 .The virulence-associated genes of gammaherpesviruses currently remain uninvestigated.

IMMUNOPROPHYLAXIS Alphaherpesviruses
With respect to control of disease and economic loss due to infection by ruminant alphaherpesviruses, the major effort has been against BHV-1.The first live, attenuated BHV-1 vaccine was developed soon after the first report of BHV-1 isolation 38 .Scores of BHV-1 vaccines have been developed commercially and new products are still being offered to the market place 85,86 .Single and/or combined BHV-1 vaccines are available as both live and killed formulations.Subunit gD or gC and marker gE deletion (gE -) mutant virus vaccines have also been produced 85,86,99 .The reader should be made aware that none of the currently available BHV-1 vaccines completely prevents viral shedding and respiratory disease due to BHV-1 challenge 56,57,87,99 .Normally, natural BHV-1 outbreaks of combined respiratory and genital diseases are rare 11,22 .In spite of the fact that the respiratory BHV-1 isolates can produce genital disease and conversely, the genital isolates typically produce respiratory disease 41,84 , the efficacy of BHV-1 vaccines has largely been assessed against the respiratory disease 56,57,99 .With respect to vaccine efficacy, live BHV-1 vaccines are superior under some circumstances 99 .This is with respect to quicker onset of immunity and therefore live vaccines are more suitable for use in the face of an outbreak compared to killed BHV-1 vaccines 56,57,99 .Secondly, live BHV-1 vaccines afford better protection to unweaned passively immune calves 56,58 .
Interestingly, a BHV-1 vaccine crossprotected bovine calves against BHV-5 meningio-encephalitis 6,100 .This was, however, not the case in a recent controlled efficacy study with an experimental, live gE -BHV-1 vaccine 83 .Despite these contradictory findings, BHV-1 vaccines are in use in Latin America to immunise cattle against BHV-5 infection.Notwithstanding antigenic cross-reactivity between BHV-1 and CerHV-1 the prospect of protecting farmed red deer against CerHV-1 ocular disease 69 with BHV-1 vaccines needs to be experimentally demonstrated.
There are few examples of vaccines for other ruminant alphaherpesviruses.A live non-attenuated tissue culture grown BHV-2 vaccine has been used on farm premises where BHV-2 mammillitis is a persistent problem 39 .An experimental killed CapHV-1 vaccine, containing Montanide ISA 740 adjuvant, after 2 subcutaneous injections was found to protect goats against vaginal lesions and vaginal virus shedding after intravaginal challenge 92 .It was, however, not determined if the vaccine protected goat kids from fatal disease.

Gammaherpesviruses
To our knowledge, there is no immunoprophylaxis currently offered against ruminant gammaherpesviruses.A vaccine to prevent fatal MCF in cattle and deer due to AlHV-1, OvHV-2 and some of the new MCF-causing viruses would be a major step forward.

Alphaherpesviruses
BHV-1 infection in cattle is the most studied in this respect.Aspects of the immune response investigated comprise: (i) those following primary infection, (ii) re-infection (secondary infection), (iii) latent virus reactivation and after vaccinations with different (live, killed, subunit and DNA plasmids of virus glycoproteins and respective genes) BHV-1 vaccines 99 .BHV-1 infection normally induces a balanced Th2 (antibody) and Th1 (cellular) immunity.The latter is the key response in recovery from primary infection, while the former and particularly VN antibody has a role in limiting virus spread and generalisation within the infected host during secondary infection.The role of the many BHV-1 proteins in conferring immunity and aiding in recovery from an ongoing infection remains to be elucidated.It is, however, known that the major BHV-1 glycoproteins (gB, gC and 0038-2809 Jl S.Afr.vet.Ass.( 2008) 79(1): 8-14 gD) are important in stimulating protective antibody and cellular antiviral activities 2 .BHV-1 gC and to a lesser degree gB and gD are involved in virus attachment to cell receptor(s) and also have VN antibody epitopes.Neutralised virus is prevented from attaching to and hence gaining entry into the cell.Therefore virus replication is blocked.BHV-1 VN antibody is significantly effective in reducing virus replication in vivo.This was experimentally demonstrated in BHV-1 naive bovine calves that were inoculated intramuscularly with BHV-1 VN antibody globulin fractionated from serum pooled from 3 hyperimmunised calves 58 .This protective effect was also seen in calves with maternally derived antibody (MDA) 56 .In secondary infection (re-infection or latent virus reactivation) and in revaccinated cattle BHV-1 antibody neutralises extracellular virus and also virus inside damaged cells and thus restricts virus spread.BHV-1 antibody also has a pivotal role in binding to virus glycoproteins on surfaces of infected cells which are lysed by polymorphonuclear neutrophils (PMNs), macrophages and NK cells via antibody-dependent cell cytotoxicity (ADCC).Both interferon and complement also enhance the ability of PMNs to lyse virus infected cells by ADCC 2 .In addition to antibody, cellular immunity also plays an important role in recovery from secondary BHV-1 infection.
There is limited or no published data on protective immune responses by other ruminant herpesviruses in their respective natural hosts.Notwithstanding this limitation, it is reasonable to conclude that some of the above described mechanisms for BHV-1 are likely to be involved in controlling infections by related viruses such as BHV-5, CerHV-1, CapHV-1 and CerHV-2.

Gammaherpesviruses
The immunology of the T-lymphotropic gammaherpesviruses AlHV-1 and OvHV-2 is of particular interest since they are asymptomatic in their natural carrier host while causing a fatal disease in susceptible hosts 71,65 .AlHV-1 in wildebeest, the natural reservoir host, induces VN antibody 64 but AlHV-1 infected cattle produce poor or no VN antibody.The relevance of these observations 77 is not clear.There is also an antibody response against HipHV-1 in its natural host, the roan antelope 72 .It should be stated that AlHV-1 infected cattle do produce other AlHV-1 specific antibodies 77 .In fatally affected hosts, AlHV1 and OvHV-2 interfere with the homeostasis of cell-mediated immunity (CMI).More specifically, a hypothesis suggests that the hyperplasia in affected animals is mainly due to CD8+ lymphocytes in response to excessive IL-2 production resulting from deregulation of natural killer (NK) cells and the IL-2 activity.The NK cell dysfunction progressively increases as the infection generalises.The end result of the CMI system dysfunction is a widespread destruction of normal tissues 71,65 .That antibody is also involved in this destructive process is indicated by the formation of immune complexes in the terminal disease in cattle 55 .
Cattle infected with BHV-4 produce circulating antibodies but they are poorly neutralising 15 .No information is available on CMI induced by BHV-4.

CONCLUSION
In conclusion, much effort has been devoted to the study of BHV-1 biology at all levels and our knowledge of the virus is fast catching up with our understanding of herpes simplex virus, the archetype for the Alphaherpesvirinae.There are 128 ruminant species in the family Bovidae and therefore it is likely that some herpesviruses, particularly those in wild ruminants, remain undiscovered.Currently 6 known alphaherpesviruses occur only in their natural hosts and do not cross stably into other ruminant species.By contrast, 9 ruminant gammaherpesviruses appear to have a much broader host range which is probably, along with their biology, not fully unraveled at present.The MCFcausing viruses can cause disease under natural conditions in cattle, pigs, domesticated buffaloes, free-ranging deer and moose.In captivity, many different antelope species, African buffaloes and bison can develop MCF.
Gammaherpesviruses are innocuous in their natural hosts but have the potential to cause serious diseases in secondary ruminant hosts.
However, a disproportionately smaller effort has been made to understanding immune-pathological mechanisms causing MCF in cattle and deer following infection by AlHV-1 and OvHV-2.This is surprising since a well characterised animal model namely the rabbit exists for MCF caused by both AlHV-1 and OvHV-2 81 .An interesting question is whether the MCFcausing viruses are evolving, which is very likely.Such findings may also aid our understanding of human lymphoproliferative disorders due to gammaherpesviruses.