Equine influenza vaccination - a complex issue

The use and timing of vaccination in an equine influenza (EI) outbreak is a controversial issue. There are strong arguments both for and against, and decisions around vaccination need to be kept under constant review. Vaccination strategies and decision making where EI is endemic are significantly different to strategies applicable to the control of an outbreak. New Zealand's unique disease-free status makes the extrapolation of many of the strategies used elsewhere inappropriate. Vaccination alone is insufficient to control an outbreak. It needs to be combined with strict movement control, quarantine and biosecurity precautions.

Vaccine types

There are three types of equine influenza vaccine available:

Inactivated vaccines contain virus that is rendered incapable of replication (usually by chemicals). These vaccines frequently include a number of immunologically distinct strains of the EI virus. The main advantage of these vaccines is their safety. They are administered by injection and usually require two injections four-to-six weeks apart. Vaccinated animals usually only become fully immune 7–14 days after the second dose. For ongoing protection from disease, a booster vaccination is given at six months, and thereafter yearly or more frequently, depending on the likely exposure of the horse to EI.

Live modified vaccines contain live, attenuated cold-adapted EI virus that has been adapted to multiply at the lower temperatures of the sinus and nasal passages but not at normal body temperature. Because this is live virus that actually multiplies in its host, there is a risk of viral spread between horses as well as the possibility that the adapted virus could regain its ability to cause disease. This type of vaccine is usually administered into the nose of the animal. A single dose is followed by boosters at six-month intervals. There is evidence of early protection (as early as seven days) following vaccination, though full immunity is claimed to occur later than this. These vaccines cannot be used in pregnant mares.

Modified vaccines. These are another form of 'live' vaccine. They include genetically modified strains of virus and are treated as a separate type of vaccine. The virus, because it has been modified, is not able to spread from horse to horse. The vaccine is given by injection and two initial doses are recommended with boosters at a six-month interval. Immunity has been documented at
14 days after administration of the first dose. This is the type of vaccine used in the Australian outbreak.

Which vaccine to use?

Like the human influenza virus, the EI virus is constantly changing. This is why horses can catch influenza more than once, or become infected even if they are vaccinated.

It is important to use the correct vaccine for the type of influenza virus you want to protect against (i.e., that the vaccine contains the correct virus types). Vaccine manufacturers update their vaccines in response to newer subtypes. When there is a new outbreak it is important to know the virus type you are dealing with, to be able to select the most appropriate vaccine. Most influenza vaccines have at least two subtypes and generally include American and European strains. Generally, horses that have been vaccinated correctly shed less virus for shorter periods and show fewer or no detectable clinical signs than horses that have not been vaccinated.

Immunity is short lived and costly

The immunity to EI in horses following vaccination is relatively short lived and therefore revaccination is important. It is generally accepted that for vaccination to limit the spread of EI, it is necessary to have a strict vaccination programme that ensures that at least 70 percent of the horse population is effectively vaccinated.

New Zealand has a horse population of about 120,000, which means we would need to vaccinate 84,000 horses. At a cost of about $375 plus GST for an initial three-dose course of inactivated vaccine per horse, the first year cost would be about $30 million.

If a vaccination programme is not maintained, then the horse population will become increasingly susceptible to EI again. This is one of the reasons why, in countries where vaccination is practised, there are still outbreaks of the disease.

Vaccinated horses can spread infection

Vaccinated horses may still become infected, but will show few or no clinical signs because they have some protection. Horses that have had EI or that have been exposed to the virus (even if vaccinated) can carry and shed the virus for up to 14 days and infect other animals. Therefore, the infection can be spread as horses that are not obviously ill are moved to different locations. This is often how the infection enters a country – in horses with immunity that are carrying and shedding the virus. Vaccination also makes confirming the presence of EI more difficult and more expensive.

Why use vaccines in an outbreak?

If used correctly in an outbreak, vaccines can slow the spread of the infection and reduce or minimise the severity of the disease in horses. In Australia, vaccination was used effectively to create buffer zones around areas where disease occurred, to minimise the chance of infection spreading outside these areas. However, it is important that the use of vaccines is strictly controlled, so that it is known which horses have been vaccinated and when. This ensures that vaccinated horses can be prevented from spreading infection and also ensures effective implementation of other response activities.

Vaccination was only one of the tools used in the Australian response. Other measures such as a movement ban and strict biosecurity procedures (e.g., cleaning and disinfection) to prevent the spread from property to property played a significant role in ensuring that the outbreak was brought under control.

Vaccination in non-emergency situations (where the infection is not present)

Following Australia's decision to use the EI vaccine in response to the outbreak, the issue of whether New Zealand should or should not vaccinate has resulted in vigorous debate.

There are a number of reasons why the vaccine is strictly controlled, and why using it preventatively in the absence of disease needs to be very carefully considered.

Possible uses for the vaccine in the absence of EI in New Zealand could include:

  • targeted vaccination of high-risk populations within New Zealand (e.g., horses that travel frequently out of the country)
  • pre-emptive vaccination of the country's horse population to reduce the impact of disease and to reduce the likelihood of spread should it enter New Zealand. If pre-emptive vaccination is used in New Zealand, at least 70 percent of the horse population needs to be immune, so that the virus does not spread should it enter the country. If lower percentages of horses are vaccinated, then this may not prevent outbreaks. Even with high vaccination rates there is no guarantee of protection, because other strains may emerge that vaccinated horses are not immune to. This could still result in outbreaks.

Implications of using vaccination in non-emergency situations

Impact on EI-free status for international trade

One of MAF Biosecurity New Zealand's (MAFBNZ's) main concerns around large-scale vaccination in the absence of disease (i.e., non-emergency use) is the potential impact on our EI-free status.

If a significant number of horses were vaccinated, then MAFBNZ could no longer rely on detecting clinical signs in infected animals. It is this ability, facilitated by passive surveillance carried out by owners and veterinarians and supported by emergency reporting and response capability, which provides New Zealand with the evidence required to support claims of disease freedom. Loss of EI freedom status could affect exports. New Zealand would still claim freedom, but this would be less credible to our trading partners, who are under no obligation to accept our claims of EI freedom at face value.

The World Organisation for Animal Health (OIE) has specific guidelines for EI-free countries where vaccination is used. To comply with their requirements and ensure that our claims of EI freedom are accepted by other countries, New Zealand's equine industry would need to undertake extensive and expensive surveillance programmes. This also means that to retain EI-free status, New Zealand would need to identify horses that have been vaccinated. This is why all horses vaccinated in Australia have to be microchipped.

'Silent spread'

Vaccination may hide signs of infection in horses that are still capable of spreading the virus. This may result in the so-called silent spread of infection. This would allow the infection to spread much more rapidly before it could be detected. By the time signs of disease are detected and reported in a non-vaccinated horse, it could potentially be very widespread.

Need for repeated, regular vaccination

Horses need to be vaccinated at regular intervals to ensure that effective immunity is built up and maintained. If vaccination is allowed to lapse (a real risk where people don't see evidence of disease after a period and decide not to revaccinate) the horse population is again at risk.

Vaccination and movement control in the event of an outbreak

There is a misconception that vaccinated horses will be exempt from movement controls during an outbreak of EI and will be allowed to participate in activities such as races, shows and events. This is generally not the case. Vaccination alone is insufficient to control an outbreak and needs to be combined with strict movement control, quarantine and biosecurity precautions, as was the case in the Australian outbreak.

Summary

A decision to vaccinate, especially where the disease is not present, is not a simple matter. Many factors require consideration. Vaccination will affect different sectors of the horse industry in different ways, and it is important that all views from potentially affected parties are considered before any decisions are made.

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Page last updated: 30 April 2008