Pros & Cons of EI Vaccination

25th September 2007

The Consultative Committee on Emergency Animal Diseases, a special body within the Australian Quarantine and Inspection Services (AQIS), says there are strong arguments for and against vaccination as part of Australia’s horse flu eradication program. A cost-benefit analysis is being developed.

Vaccination might assist in some circumstances but it does not offer a quick-fix national program.

As a precaution, contingency planning is being undertaken for possible use of vaccines in particular situations.

Horse flu is due to infection with the H3 or H7 influenza A viruses. The outbreak in Australia is caused by an H3 virus. Although all H3 viruses that infect horses are of the sub-type H3N8, they are not all the same.

They are subject to the same evolutionary process as all influenza A viruses: they are continually changing in a process called antigenic drift.

This is why animals can catch influenza more than once, even in consecutive years, and why vaccines (in humans) are changed annually according to predicted antigenic requirements.

Horse flu is endemic in nearly all countries with equine industries, and vaccination is used to control the disease in those countries. Australia (until recently) and New Zealand are the only countries with large equine populations free of the disease and therefore do not vaccinate.

Although vaccination can prevent the illness, the available equine influenza vaccines does not fully prevent infection or transmission of the virus.

However, vaccinated horses, in response to infection, shed less virus for shorter periods and show fewer or no clinical signs compared with fully susceptible horses.

Better protection will result when there is a close relationship between the field virus and the strain used in a vaccine.

Immunity following vaccination is much shorter than that resulting from natural infection. Sporadic outbreaks of illness still occur in countries using vaccination. This happens through inadequate vaccine coverage or when new strains emerge or are introduced.

Vaccination considerations

FOR a country like Australia, the decision to vaccinate or not is complex, and many issues need to be considered. Unlike endemically infected countries, horse flu has been introduced into Australia only recently and affects only part of the country and some sectors of the horse industry (mainly the recreational sector).

Vaccination will not produce protective immunity until about two weeks after the main course is completed. All available vaccine programs recommend a primary course of two vaccinations, generally four to six weeks apart. This means that immunity will not be optimal until about six weeks after vaccination.

Types of vaccine

For inactivated vaccines, a primary course consisting of two vaccinations four to six weeks apart is required before vaccine-based immunity is effective. Optimal immunity is not achieved until at least seven to 14 days after the second dose. For ongoing protection from illness, manufacturers recommend a booster vaccination six months after the primary course and then annually.

Live attenuated vaccines and recombinant vaccines, while still not providing complete protection, produce quicker and stronger immunity than inactivated vaccines. Live attenuated vaccines are not being considered for use in Australia because of the potential for introduction of other strains of horse flu virus.

The recombinant vaccine is under consideration here. It is the preferred choice as it has been shown that this vaccine can provide protection and reduce shedding of virus 14 days after a single dose. A second booster vaccination is required to ensure adequate duration of immunity. This vaccine was successful in the 2003 EI outbreak in South Africa.

Procedures

VACCINATORS must maintain stringent hygiene procedures to avoid spreading horse flu between properties. A cold chain would be required for the storage of vaccine. Vaccine use under emergency permits may require stringent administrative control of vaccine issue and use.

Broodmares can be vaccinated during pregnancy and lactation (in accordance with manufacturer’s recommendations), but vaccination in the month before foaling will not allow time for adequate production of antibody in colostrum, resulting in poor passive antibody protection in newborn foals.

In countries where horse flu is endemic, vaccination of broodmares is used to ensure maternal antibody to horse flu in colostrum, providing passive immunity for newborn foals.

There is no evidence that vaccination of horses already incubating influenza is harmful, but vaccination of clinically ill horses is not recommended.

Some manufacturers recommend against vaccinating young foals because in endemic countries the presence of colostral antibodies will interfere with development of immunity. This would not be a concern in the current Australian situation. Advice from equine experts at the Animal Health Trust in Britain is that vaccinating young foals in naive populations is likely to reduce clinical impact if these were to be exposed to infection.

Australian policy

AUSTRALIA’S response to a range of emergency animal diseases is contained in a series of technical and scientific response manuals called Ausvetplan. For horse flu, Ausvetplan states that vaccination will not be used if an outbreak is detected early and can be contained by movement controls. However, Ausvetplan recognises that vaccination may be appropriate where the illness is widespread when detected, numbers of horses are at immediate risk or initial control methods have failed and the illness has spread beyond the original restricted area and is likely to become endemic in the general equine population.

Ausvetplan identifies the following strategies for the use of vaccination in the face of an outbreak:

• Mass vaccination – this would involve widespread vaccination of horses to build up herd immunity.
• Ring vaccination – this is carried out locally in a ring around identified sources of infection to limit further spread by producing an immune buffer.
• Predictive vaccination – this targets enterprises and populations that could be expected to contribute most to future spatial transmission of infection.

Vaccination strategies

VACCINATION has not, on its own, eradicated horse flu anywhere in the world, and the use of stringent biosecurity measures and movement controls would still be required. Therefore vaccination must be conducted in conjunction with identification, record keeping, quarantine and movement control measures.

Vaccination may be used to protect animals in important sub-sectors or regions of the horse industry and/or to reduce the economic impact of the current approach on these sub-sectors or regions.

Vaccination may be used to reduce the impact of the illness on horses required to move for competition and other reasons, such as breeding.

Horses would be required to be vaccinated before movement, and because vaccinated horses can still become infected, stringent quarantine and movement controls would be necessary between infected and uninfected zones.