Bullet points on the FMD vaccine.
Compiled By Dr Ruth Watkins (clinical virologist) on 19th June 2001 from information gleaned from Prof Fred Brown FRS, Drs Paul Sutmoller, Simon Barteling and Paul Kitching (world experts on FMD).
Foot and mouth disease virus.
•The virus is an RNA virus (the viral nucleic acid is RNA).
•The FMD virus, an aphthovirus, is in the Picornavirus family.
•Viruses in this family are not enveloped but consist of nucleic acid surrounded by a ‘protein coat’, called the capsid.
•These viruses are hardy in the environment though the aphthovirus genus is sensitive to acid PH (the particle ‘explodes’ when exposed to PH <5).
•The 7 serotypes each require by definition type-specific neutralising (protective) antibody to be rendered non-infectious.
•Neutralising antibody reacts with the external protein coat, the capsid. Even one molecule complexed with the virus locks the nucleic acid within the capsid so that a neutralised virus particle cannot replicate even should it enter a cell.
•Each serotype is stable so that the vaccines prepared for each have remained effective (the exception is type C for which the vaccine was reformulated).
•The virus causing this outbreak is serotype O, Pan Asia subtype- the serotype O vaccine (prepared from the Manisa subtype) is effective in protecting against this epidemic strain.
The FMD Vaccine.
•It is a killed (inactivated) vaccine.
•It induces immunity but does not infect the vaccinated animal.
•This type of FMD vaccine has been used for 40 years and it is well tested in the field.
•The modern vaccine is safer than the old vaccines because it is rendered non-infectious by a single step inactivation with BEI (rather than formaldehyde).
•Modern quality control in vaccine production also ensures a safe product (one that does not contain live or infectious virus).
•Virtually 100% of animals respond to the oil-based vaccines, given IM, by making protective antibody that is specific to the serotype of FMD virus in the vaccine. It is prepared from virus particles harvested from tissue culture.
•The vaccine is licensed for use in the UK.
•Protection can be conferred after one dose of high potency vaccine or after two doses 3 to 4 weeks apart of commercial vaccines.
Vaccine protects against disease and halts spread of infection.
•The response to vaccination protects against disease.
•Response to vaccine does not protect completely against infection, like all killed vaccines in humans and animals. They are nevertheless successful in controlling and eradicating infections.
•A vaccinated animal can only be infected with a very large dose of virus e.g. from a non-vaccinated pig that is acutely infected.
•A vaccinee has a modified infection because of pre-existing immunity at the time of exposure.
•A vaccinee sheds lower titres of virus than a non-vaccinated infected animal.
•A vaccinee sheds virus complexed with neutralising (protective) antibody.
•A vaccinated animal is protected against infection from another acutely infected vaccinee.
•A vaccinee has not been shown to infect another animal whether vaccinated or not.
Vaccinated carriers do not pose a problem.
•Persistent infections arise in cattle, sheep and goats in a proportion of infected animals- called carriers.
•Though these carriers cannot be proved to spread infection in laboratory conditions they have been held responsible for outbreaks in a few historical instances on circumstantial evidence.
•Vaccinated carriers have been shown to arise in cattle. Sheep have not been studied.
•The period of carriage is shorter and the number of carriers less in vaccinated animals than unvaccinated animals.
•Vaccinated carriers have never been shown to spread infection to another animal. It remains a hypothetical possibility only.
Control and elimination of virus by vaccination.
•Vaccination of the herd or flock (at least 80%) will prevent further transmission of infection (whilst vaccine of high potency stimulates the protective immune response there may be one more cycle of infection in an already infected flock).
•Transmission of virus is halted by vaccination.
•There were no further outbreaks 5 days after vaccination around the outbreaks in Holland.
•Vaccinated animals will prevent recrudescence of outbreaks from sources such as material contaminated with infectious virus, acutely infected wildlife and the rare unvaccinated infected carrier that ‘reactivates’, the 1 in a million chance.
•All carriers in domestic animals eventually stop carrying virus- sheep and goats by about 9 months and cattle and Water Buffalo by 3 years. Pigs are not thought to become carriers.
•Thus vaccination leads to elimination of virus from the population or country.
There is no veterinary reason to slaughter vaccinated animals.
•Vaccinated animals are not infected by vaccination, nor are they infectious to other animals if infected after vaccination.
•Exposure of unvaccinated new stock to contaminated material or infected wildlife will result in another outbreak.
•Vaccinated animals prevent further acute infection after restocking.
•Vaccinated animals are safe to eat or for the consumption of products.
•We have consumed hundreds of millions of vaccinated animals in Europe and milk from them and continue to do so now in other parts of the world
•Once all acute infections is halted, and 3 weeks have elapsed in those animals that have been acutely infected to allow clearance of the acute infection (whether or not the virus persists as a carrier), we could open markets and allow animal movements within the UK (though we may not trade as FMD-free for a period of time).