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EQUINE FLU VACCINATION-THE ONGOING DEBATE

The use and timing of vaccination in an Equine Influenza (EI) outbreak is always going to be a contentious issue.

There are strong arguments both for and against vaccination and our decisions around vaccine use need to be constantly under review. With the news that Australia is to import vaccine for use in "buffer zones" around EI hotspots we can expect another round of discussion and opinion on the merits of vaccination.

Vaccines are developed to trigger immunity against a specific infectious organism ie a virus or bacteria that causes disease. A vaccine aims to mimic the disease without producing any clinical signs.

There are three types of Equine Influenza vaccines available. These are:

Killed or inactivated vaccines - the virus is killed usually by chemicals and then mixed with a liquid carrier (adjuvant). The vaccine may use the whole virus, various virus proteins or viral DNA or RNA. These vaccines frequently include multiple strains of influenza virus A2 representing the major circulating strains. The main advantage of these vaccines is the absence of pathogenicity, virus replication and subsequent spread between hosts. They are administered by intramuscular injection and a course of two injections four to six weeks apart is recommended. Optimal immunity is not present until at least 7-14 days after the second dose. For ongoing protection from disease, manufacturers recommend a booster vaccination at 6 months. Annual or more frequent boosters, depending on the likely exposure of the horse to circulating virus, are then required.

Live modified vaccines - these vaccines have been made safe through a process called attenuation (decreasing the virulence of the virus for the horse). Horses that have been infected by EI virus have been shown to be protected from reinfection for about 12 months so it would be expected that a live virus vaccine would give superior protection to the inactivated vaccines. However the vaccine did not provide this level of protection and while they did provide quicker and stronger immunity than inactivated vaccines anecdotal evidence suggests they have not had widespread uptake. They do present the risk of viral spread following their use. An intranasal cold adapted modified live equine influenza virus vaccine based on a Kentucky 1991 A/Equi 2 virus is available in North America. It is licensed for vaccination of non-pregnant animals over 11 months of age using a single dose of vaccine followed by boosters at 6 month intervals. There is evidence of early onset of protection (as early as 7 days) following the use of this vaccine.

Recombinant vaccines - these vaccines are another form of "live vaccine" but because they require different technology and approvals process it is convenient to treat them as a separate type. In these vaccines selected genes from the equine 2 influenza virus are inserted into another "safe" non disease causing virus. A recombinant canary pox vector based equine influenza vaccine is available in Europe and the USA. The vaccine is given by intramuscular injection and a two dose priming regime is recommended with boosters at a six month interval. The onset of immunity has been documented at 14 days after administration of the first dose and this is probably one of the reasons that Australia is planning on importing to import this type of EI vaccine for use in creating their "buffer zones".

Inactivated vaccines take too long to generate immunity in the horse and the real risk that vaccinated horses can still contract the disease, albeit with fewer or no symptoms, and excrete virus thus potentially adding to the spread of the disease is of real concern.

Most of the vaccines we encounter as humans are very effective and impart long standing immunity against the disease they are designed to protect against. When we look at the available vaccines against equine influenza and also human influenza this is not the case.

To understand why, we have to look both at the virus and the vaccine. The Equine Influenza virus is constantly changing how it is seen by horse's immune system. These subtle changes are called antigenic drift (the virus presents a different face to the horse's immune system).

Every so often there is significant change in the virus and a new subtype is identified. New subtypes are named after the place where they were first identified so we see names like Prague, Suffolk, Ohio, Miami, Kentucky, Wisconsin etc. attached to the influenza virus. These constant subtle changes in the virus are the reason why horses can catch influenza more than once, even in consecutive years.

Although vaccination can prevent disease, the available EI vaccines neither fully prevent infection nor transmission of the virus. However, vaccinated horses, in response to EI infection, shed less virus for shorter periods and show fewer or no detectable clinical signs than fully susceptible horses.

The immunity generated in horses after vaccination is relatively short lived so unless a horse is constantly challenged its immunity wanes and it can be reinfected quite quickly. There is no carrier state ( ie the virus does not remain in the normal recovered horse as is the case with Equine Herpes Virus) in horses that have been infected with Equine Influenza.

In countries where the virus is endemic it is because the infection keeps circulating through the horse population. For vaccination to have much impact on the rate at which EI circulates and reinfects horses, in a country in which the disease is endemic, it is necessary to have a strict vaccination program involving about 70% of the horse population. If we were to apply this to New Zealand with about 120,000 horses, we would need to vaccinate 84000 and at a cost of approximately $375 +GST for an initial three dose course of killed vaccine per horse the first year cost would be about $30 million.

With these subtle changes to the virus, vaccine manufacturers are always chasing a moving target. They are constantly upgrading their vaccines with the latest subtypes. This process takes a couple of years at least. So when there is a new outbreak it is important to firstly type the virus and then select the most appropriate vaccine.

Most influenza vaccines have at least two subtypes and generally include American and European strains. We have no word yet from Australia as to the subtype they have but if it is the Wisconsin 2003 subtype as widely suggested, vaccines containing Kentucky 1997 subtype would be as close as we could get and we understand it would be relatively effective.

This combination of poor immunity, antigenic drift and the use of out of date or wrong subtype vaccines gives rise to a real problem we have already alluded to when confronted with this disease. Vaccinated horses will be reinfected and as they have some protection they will show few if any symptoms (ie be subclinical) but still shed virus and be a source of new infections. This makes detection more difficult and more expensive as we have to rely on a wider range of laboratory tests.

In the face of an outbreak there is the confusion created with positive blood results as to whether they are due to vaccine or natural infection. In those countries where vaccination is practised there are regular episodes of disease and every five or so years there is a major outbreak.

It must be pointed out that vaccination on its own has not ever resulted in EI eradication. It may have a place alongside stringent biosecurity measures and movement controls.

Vaccination may be used to protect animals in certain sub sectors /regions of the horse industry and /or to reduce the economic impact of this disease.

Advantages and disadvantages of vaccination for EI

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BUFFER ZONES
Containing horse flu is similar to fighting a bushfire; buffer zones, like firebreaks, work best where there are natural barriers and a low level of fuel. Susceptible horses are the fuel for the Equine Influenza fire. Buffer zones are being established in NSW in areas of low density of horses and the susceptibility of horses in the buffer zones will be dampened down by vaccination.
Buffer zones are likely to be an important component in NSW DPI’s plan to contain Equine Influenza by stopping the property-to-property spread. Owners of horses within the buffer zones will have two alternatives:

Voluntary movement of their horses out of the buffer zone or
compulsory vaccination and an indefinite standstill.
It is important that horses vaccinated in the buffer zone stay where they are. Vaccinated horses can still get infected and spread the virus.

It will be increasingly vital for horse owners to comply with movement restrictions and to practise stringent biosecurity. Recent EI outbreaks at Warwick Farm, Brisbane or Scenic Hills Riding Ranch are almost certainly due to spread of infection by humans. This disease is highly infectious. The safest policy is not to allow anyone who has contact with other horses onto your property.

Although the total number of infected properties is growing steadily, these are within our expectations. Apart from the new infected properties found around Armidale, the infected properties are still within the containment lines and buffer zones.

SITUATION UPDATE
There are currently 2242 Infected Properties, 340 Dangerous Contact Properties and 312 Suspect Properties. Now that zoning has been implemented, we will no longer report on Restricted Areas, unless the current number changes. Some adjustments at the interface of the amber and red zones are now taking place.

As part of the NSW DPI’s buffer zones strategy, development of vaccination handling orders and protocols is under way for use of vaccine in the two major buffer zones north of Newcastle and south of Sydney. Vaccination is planned to commence on the weekend following the delivery of vaccine to Sydney on Thursday. Vaccination will only be permitted under the approval of NSW DPI’s Chief Veterinary Officer.

An intensive survey to confirm the number of horses and their locations in the southern buffer has been completed. The process for the northern buffer commenced today.

As reported yesterday we are now starting to see properties that suffered the first wave of infection coming out the other side, with projected outbreak scenarios predicting a continued high incidence of Infected Properties for the next month or so followed by a variable rate of decline over succeeding months.

The Community Education branch is currently establishing extension teams, with the help of Forward Command Posts and industry representatives, to arrange community consultative committee meetings. These meetings will be held in strategic locations throughout NSW.

The distribution of community information packages and posters continues, with all NSW local councils and NSW universities receiving these. Over 50 organisations are now distributing these packages to their members and to the public.>

Situation
1. To date, almost all spread after the 25th August has been at the local level with the main risk factor for spread being high density of horse population.
2. The disease containment situation in NSW is very encouraging and there is justification at this stage to be optimistic about the prospects for eradication.
3. Almost all spread to regions occurred inadvertently before the standstill movement restrictions of the 25th August were in place.
4. As warmer, sunnier and windier conditions develop as the state moves from winter to summer, conditions for virus maintenance and transmission are anticipated to become considerably reduced and containment at the local level will be considerably enhanced.

Comments
1. The 1986 South African outbreak is similar in many respects to the outbreak Australia is currently experiencing. The South African outbreak was severe and extensive, yet was eradicated in circumstances much more difficult than Australia is facing.
2. There is no evidence of widely dispersed continuing infection.
3. Newly infected properties are largely confined to areas of existing infection where density of horse populations is high (Figures 1, 2 and 3).
4. State-wide and regional epidemic curves are showing promising declines in numbers of new infected properties (Figures 4 and 5). We are confident that the curves show effective disease containment rather than underreporting, based on feedback from industry liaison officers. Targeted surveillance may become necessary if community enthusiasm for reporting wanes.
5. Rapid escalations in the number of infected properties recorded in the past few weeks were from local spread between horse properties in densely horse populated areas, particularly in the special restricted areas where horse movements are permitted.
6. There is mounting evidence that spray-drift-like airborne transmission over mainly short distances, but up to 3 km (as opposed to longer distance wind transmission) is contributing to local spread in some areas. Small groups of infected coughing horses at the top of valleys, produce clouds of aerosolised virus that drift down valleys aided by atmospheric conditions of cold, still night air and temperature inversions.
7. The important factors that appear to have driven recent spread are (i) density of horse populations combined with (ii) local airborne spread and (iii) nose to nose over fence contact in these areas. There are, however, numerous instances where spread by people and fomites has occurred.
8. Disease containment is likely to be further aided as the onset of warmer weather and longer daylight act to reduce virus survival and airborne transmission.
9. Insect activity may increase as summer approaches but is highly unlikely to contribute to virus spread under hotter, sunnier conditions. Spread by insects has not been a feature of overseas outbreaks.
10. Disease spread has halted at natural barriers such as forests and ranges that tend to delineate high density horse populations – these barriers are being used to advantage to strategically place vaccination zones (bands).
11. The bands serve to protect moderate to higher density horse populations by covering escape routes from restricted and special restricted areas.

Recommendations
1. Continuing effort must be made to encourage reporting of suspect cases, particularly in control areas.
2. Continuing effort must be made to encourage high standards of personal biosecurity and observance of quarantine and movement restrictions by horse owners and service providers. Both are vitally important so as not to undo the considerable gains made in containing the disease to date.