Herpesviruses are grouped as α-, β-, and γ-herpesviruses. Although several α- and γ- herpesviruses have been found in equids, there is currently no known member of the β-Herpesviridae family that affects horses.
γ-Herpesviruses share general characteristics with α-herpesviruses but are also distinctly different. One of the most important characteristics of all Herpesviridae is their ability to cause lifelong infection in a host. A second, generally accepted characteristic is their species specificity. A “lifelong” infection is typically divided into two periods of different replicative activity: (1) acute infection with high replicative activity followed by gradually decreasing activity over time, and (2) latency without replication. It is assumed that during the lifetime of an infected host, the virus spends most of the time in latency. Prominent clinical signs are usually evident during periods of high replicative activity. It is advantageous to the virus to produce large quantities of progeny that can be passed on to other members of the infected species. Spread is facilitated when viral replication occurs in an organ system that directly connects with the environment. The respiratory tract is a common site for herpesvirus replication, and nose-to-nose contact guarantees easy transmission of virus. Because of species specificity, it is important for the virus that clinical signs in the host are mild to moderate at best; an infection should not cause the host’s death, and virus transmission should be easy to allow spread within a population. The equine γ-herpesviruses (γ-EHVs) fit these requirements. It is assumed that a low-grade replicative phase is relatively brief with the α-herpesviruses, but given the ease of isolating γ-herpesviruses from peripheral blood mononuclear cells (PBMCs), it is possible that there is a prolonged stage of low-grade chronic persistent replication with these viruses and that it takes the γ-herpesviruses longer to settle into the immunologically silent phase of latency than it does their α counterparts.
To date and across all equine herpesviruses, it remains unclear what triggers recrudescence from latency. Stressful conditions of transportation and periods of strenuous exercise are frequently named but ill-defined triggers; however, other factors may exist. It appears that detection of γ-EHV, and in particular EHV-2, significantly increases after corticosteroid administration.
Most herpesviruses are known for benign coexistence with their hosts. Incidental cases, in which life-threatening or chronic-debilitating disease develops, are more likely the outcome of an immune-mediated phenomenon, rather than a solely virus-driven pathology.
Five different γ-herpesviruses have been identified in horses, ponies, and donkeys. EHV-2 and EHV-5 have predominantly been found in horses. Three members are primarily found in donkeys and are called asinine herpesviruses (AHV-2, -4, and -5). Throughout this chapter, the abbreviation γ-EHV will be used to refer generally to both equine and asinine γ-herpesviruses. However, for information pertaining specifically to the asinine γ-herpesviruses, the abbreviation γ-AHV will be used.
Because research efforts have traditionally focused on α-EHV-1, -3, and -4, less is known about γ-EHV, and even less information is available on γ-AHV. This relative scarcity of information on γ-EHV results from the general opinion that these viruses cause little pathology. For decades, disease was consistently associated with mild respiratory tract infection in young, adolescent horses. Furthermore, damage caused by γ-EHV replication has been assigned a “doorman function,” by enabling secondary pathogens to cause clinical disease. For example, it has been suggested that EHV-2 in foals facilitates infection with Rhodococcus equi. EHV-2 has been studied in greater detail in its debatable role in causing keratoconjunctivitis in horses. There is also thought (disputed at present) that EHV-2 and EHV-5 are associated with some exercise-intolerant horses by playing a role in chronic lower airway inflammation. The problem with these discussions lies in the ubiquitous presence of γ-EHV in the horse population, from a general lack of experimental infection data conducted with any of γ-herpesviruses in sufficiently large numbers of horses, and from the inability to quantify viral loads until recently. There is evidence of EHV-5 association with a usually fatal disease of horses known as equine multinodular pulmonary fibrosis (EMPF). Since 2007, EMPF has been described as an incidental disease in various countries in Europe and North America. Lung lesions in affected horses consistently harbor EHV-5, and less frequently, the concurrent presence of other γ-EHVs; however, little is known regarding whether these cases reflect causality—opportunistic infection, an innocent bystander reaction, or an exacerbated, immune-mediated phenomenon.
Recent Advances in Knowledge About Equine γ-Herpesviruses
The development of rapid, sensitive, and specific detection methods (e.g., polymerase chain reaction [PCR]) and the technology of rapid genome sequencing analysis has greatly advanced our knowledge of γ-EHV epidemiology and pathophysiology; however, some results of the intensified research into these viruses have added to the confusion. Unlike α-EHV, it appears that genome variation for EHV-2 or EHV-5 is rather common, leading to significant strain variation. Different strains have been identified within and between cohorts of infected foals, and this variation may be one factor that explains the great variability of clinical signs seen during respiratory disease outbreaks among foals. Serodiagnostic testing (serum antibody detection) is offered by diagnostic laboratories; however, because of the inability of many assays to distinguish between the γ-EHV members and a generally high seroprevalence in equine populations, they are not much used. With the availability of PCR assays, both EHV-2 and EHV-5 have been identified in equids on both hemispheres, and, interestingly, in the otherwise α-EHV–seronegative horse population in Iceland.
Equine γ-Herpesvirus and Disease
Among γ-EHVs, most of the research has focused on EHV-2, in part because of the ease with which the virus could be isolated in cell culture systems before the introduction of PCR. In contrast, EHV-5 is difficult to isolate in the laboratory. EHV-2 has been found in respiratory tract fluids (e.g., nasal secretions, tracheal fluid, and bronchoalveolar lavage fluid), in PBMCs, and in conjunctivae of horses. EHV-2 dwells (although not solely) in PBMCs during latency. EHV-5 is found less frequently, but is also detected, in respiratory tract fluids; it appears to be more consistently associated with EMPF. The location for EHV-5 latency is unclear to date and may involve multiple locations: PBMCs, pulmonary macrophages, dendritic cells, and various nervous system locations have been listed. The concurrent presence of EHV-2 and EHV-5 in the same animal has been described, and EHV-5 had been detected with γ-AHV in horses with EMPF.
To the author’s knowledge, only two experimental infection studies with EHV-2 have been conducted. Results of at least one study were hampered by preinfection detection of virus in most of the study horses. Prospective studies in foal cohorts followed from birth to weaning have been conducted. In these studies, EHV-2 was found in the majority of enrolled foals, in the respiratory fluids and in PBMCs, and these findings were in conjunction with moderate clinical disease, signs of which included fever, nasal discharge, pharyngeal follicular hyperplasia, and mandibular lymphadenopathy. When both EHV-2 and EHV-5 were found, disease tended to be more severe.
Cross-sectional studies have been performed to confirm either presence or absence of EHV-2 or EHV-5 in defined horse populations: in horses with acute respiratory disease and fever, and in respiratory tract fluid (tracheobronchial or bronchoalveolar lavage fluid) samples obtained from horses with cytologic evidence of inflammatory airway disease but no information regarding fever. A surveillance study in which multiplex PCR was used on nasal swabs from febrile horses with infectious upper respiratory tract disease revealed the copresence of EHV-2 and EHV-5 in about half of the samples analyzed that were positive for EHV-1, EHV-4, equine influenza virus, or Streptococcus equi subsp equi. About half of the samples that were negative for any of the listed infectious organisms contained EHV-2 or EHV-5. Conclusions were either that γ-EHV had causality or that other infectious organisms not yet identified or tested for were causing disease. In horses with inflammatory airway disease, tracheal or bronchoalveolar lavage fluids were more likely to harbor EHV-2 (detected by PCR) when the neutrophilic percentage was high. However, adding to the confusion is the fact that EHV-2 and EHV-5 have also been identified in respiratory tract fluids from apparently healthy adult horses, raising again the question whether EHV-2 and EHV-5 are a primary or secondary cause of disease, or whether their role is that of an innocent bystander.