EHV-1 – How Best to Utilise Vaccinations

Basics first

Vaccination is the mainstay prophylactic measure against many viral pathogens. EHV-1 is a respiratory tract virus that initially affects and replicates in the respiratory tract. The respiratory tract is an ‘open system’ that allows airflow into the lungs, while at the same time providing a mechanical barrier and having an ingenious immune system with innate and specific immune functions. However, the respiratory tract is not impenetrable, and its defence mechanisms can be overwhelmed specifically by an infectious dose of a pathogen. Any specific immunity is the result of previous encounter with an antigen, whether by vaccination or natural infection. While vaccine-induced immunity is never ‘as good’ as infection-induced immunity, both inducers will mount different degrees of humoral and cellular immunity based on the antigen presentation as well as memory in the individual animal, which may differ also based on age, health/nutrition status and other factors. Any quality and quantity of specific immunity will decrease over time, while memory will fade, which will mean that during pathogen encounter it will take longer to come up with immunological defence mechanisms [1,2].

EHV-1 infection – primary respiratory disease and viraemia-associated complications
EHV-1 infection in the adult horse starts with benign clinical signs of respiratory disease. Primary infection only may be followed by viraemia, a dissemination of virus in mononuclear cells via the bloodstream, and simultaneously accompanied by a high fever. Current belief is that some horses develop a specific immunological response profile as early as with respiratory tract infection, and only in those with viraemia will EHV-1 abortion, ‘neurological herpes’ or EHM (EHV-1 associated myeloencephalopathy) occur [3]. As we see different percentages of EHM in viraemic horses, risk factors for EHM are likely to be involved. Risk factors are infectious dose and dose over time, immunity, age, sex and breed [4,5]. EHV-1 herd immunity of >85% is likely necessary to decrease the amount of circulating virus during an outbreak. However, we are aware of individual ‘super spreaders’ during outbreaks that dramatically increase the infection pressure on neighbouring horses.

The crystal ball

EHV-1 is a dsDNA virus, and it is not prone to mutations in its genome. On the other hand, it is quite species-specific, and a strategy of coevolution with immune evasion has evolved. There are different products available of modified live inactivated vaccines, and multivalent vaccines (especially in the USA). Currently, there are no RNA-based vaccines available. Licensing requires EHV-1 specific parameters following an experimental infection (a vaccination challenge) like antibody titres, improved clinical outcome as a result of decreased nasal replication and shedding, or absence of EHV-1 viraemia. Recommendations of basic immunisation and booster shot intervals are often based on licensure study outcome or are extrapolated from ‘other’ respiratory tract viruses.

How to best utilise EHV-1 vaccination

Vaccines licensed as an aid to the prevention of EHV-1 related abortions recommend three vaccinations at intervals of 2 months during pregnancy, starting at the beginning of the fifth month. The rationale behind this is that most EHV-1 abortions occur in the second half of pregnancy, and 8-week booster vaccination may provide active immunity, albeit over a short period of time only [6]. This has led to a dramatic decrease of abortions on farms with vaccine coverage that approaches 100% of animals. However, it must be emphasised that good biosecurity protocols need to be in place to prevent spread of virus from an aborted fetus.

Vaccination intervals for EHV-1 vaccinations for the show horse and recreational horse population have been once or twice per year, possibly out of convenience, as 6- or 12-month intervals have been adopted from influenza vaccination protocols. Based on epidemiological findings that most EHM outbreaks are during winter and spring months, protocols may need adjustments; however, scientific evidence must be provided first prior to any vaccination schedule adjustments. Some regions prefer more frequent vaccinations under the assumption that ‘more is better’; however, there is little science behind that. This is a concerning practice, as some studies suggest a unique (and one-time only) decrease in antibody response after the second booster when an EHV-1 vaccine was applied every 8 weeks [7].

Especially inactivated vaccines have shown to decrease numbers of viraemic animals in vaccine challenge experiments [8,9]. Absence of viraemia means absence of secondary complications, abortions and EHM; however, most of these studies have been done in naïve yearling horses or ponies, and immediately/2 weeks after the second or third immunisation.

Take home message

• We need more studies that target the population that needs to be protected from EHV-1 (primary) infection and complications.

• Novel RNA-based and subunit vaccine technology should be explored in the future.

• Good veterinary practice and biosecurity measures (testing strategies and removal of a shedding horse from the herd) will still be necessary to prevent or mitigate an EHV-1 outbreak.

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