Equine ’flu and whole cell vaccine technology

01 October 2008, at 12:00am

PATRICK TRAILL reviews the types of vaccine available and the need for robust control strategies

THE consequences of the exposure of a naive population of horses to equine influenza are significant. While mortality in itself is not a major issue, the associated losses to the equestrian industry caused by morbidity, movement restrictions and event cancellation can be catastrophic, making equine ’flu one of the most important diseases from the point of view of the global horse economy.

To give this some context, during the 2003 South African outbreak, 49 race fixtures were cancelled in the month that followed. More recently the cost to the equestrian industry in the five months following the outbreak in Australia in 2007 was estimated at $A522 million (£240 million).

Naturally, those countries which adhere to a structured immunisation policy which attempts to maximise uptake of vaccine within the entire population will be best positioned in the face of an outbreak. The UK, unfortunately, is not yet at this stage.

With a horse population of over 1.35 million and an uptake of ’flu vaccination still below the optimal level at 35-40%, further outbreaks of ’flu remain a real and continuous threat. It is suggested that containing outbreaks of ’flu would rely on 70% per cent of the horse population being vaccinated.

When it comes to the equine vaccines themselves, they can be broadly divided into two main categories: those consisting of “whole cell technology” and those that fall into the category of “sub unit technology”.

Whole cell technology

The major difference between whole cell and sub unit technology is that while all vaccines present the antigenic surface determinants, namely haemagglutinin and neuramidase, some sub unit vaccines present only the heamagglutinin component. Only vaccines produced using whole cell technology include the nucleoprotein within their mix.

The benefit of the addition of the nucleoprotein in the vaccine is two-fold. Firstly, it aids in the stimulation of the humoral response, along with the antigenic surface proteins, and secondly it plays a role in its effect on the cell mediated arm of the immune chain. The nucleoprotein is important in stimulating cytotoxic T-lymphocyte activity. It is also important in its ability to up-regulate the entire immune response.

The induction of a cellular immune response to a conserved protein, such as the nucleoprotein, may potentially provide protection when the viral strains incorporated in the vaccine do not match circulating strains. This is an important consideration in the face of continuous strain changes, as is the case with equine ’flu.

Sub unit vaccines

Sub unit vaccines can be further divided into those that present both major antigenic surface proteins, namely haemagglutinin and neuramidase markers, and those using technology that presents the haemagglutinin alone. It has been suggested that haemagglutinin alone means a vaccine is more exposed to any future antigenic drift that is likely to occur.

At present, vaccines on the market that utilise genetic modification through the incorporation of ’flu genetics into a canary pox vector, only present haemagglutinin.

This is particularly important when considering influenza viruses as they are prone to mutate and evolve over time.

It is important to remember that all equine ’flu vaccines will also make use of a chemical adjuvant as part of their final presentation to the horse. Within the UK, ’flu-only vaccines will have one of two adjuvants, carbomer or QuilA.

The importance of having these within the vaccine make-up is for their ability to aid in stimulation of the overall immune response and effectively prime the horse’s immune system against the vaccinal challenge.

Both these chemicals are well proven and have been in use in human and animal medicine for over 30 years. ISCOM technology which uses QuilA as its basis is a technology that was established in the 1980s. Carbomer is found in both whole cell and sub unit vaccines.

Carbomer adjuvant potentiates production of a higher initial antibody response and slows down their natural decline, where high levels of antibody are required for protection against strains circulating in the field.

Although a lot of the discussion surrounding equine ’flu vaccinology revolves around strains within the vaccines and their associated technologies, it is important to remember that adhering to the basic principles of vaccine storage and administration holds equal worth.

If vaccines are not stored according to the manufacturer’s recommendations, 2-8 deg. C, these biological products can alter as proteins and nucleic acid denature, which will potentially reduce the efficacy of the immunising dose.

With the shuttling and movement of horses around the globe being common practice in today’s equine world, it is easy to see how there is increased risk of exposure to different and evolving ’flu strains, to such an extent that in 2005 the FEI (Federation Equestre International) changed its ruling from an annual to a six-monthly vaccination schedule.

This ruling applies independent of the type of vaccine technology being used, which certainly helps highlight the perceived threat ’flu holds for the global horse population.

It is also important to remember that one of the most fundamental aspects to disease control from a herd perspective is the ability to reduce viral shed. Due to the highly infectious nature of equine flu and associated morbidity, better uptake of all vaccines remains the cornerstone upon which any robust control strategy relies.