What is the aim of a vaccination programme?
As we saw in previous units, although PRRS and PMWS are both caused by viruses, there are however, differences between them (refer to unit 2 and 3 if you think you want to refresh your memory). These differences will have an effect when developing vaccines and in the application of vaccination programmes.
Porcine reproductive and respiratory syndrome (PRRS): Although the first vaccine was released in 1994, the high mutation rate of the virus and the lack of complete understanding of the disease, have made the development of a reliable vaccine difficult. There are two main types of PRRS vaccine in the market: modified live vaccine (which is applied to piglets from 2 weeks of age) and killed-virus vaccines. As might be expected, both have advantages and disadvantages:
Modified live vaccines (MLV):
- Offer protection against infection with the same strain. Sometimes they can offer protection against different strains, but protective immunity may not be complete against all strains
- It takes time to develop immunity (around 4 weeks). If vaccinated pigs get challenged by PRRS virus before these 4 weeks, there will be no antibodies and it would be almost as if the pig has not been vaccinated.
- MLVs are not completely stable and there is a risk of recombination of the farm virus with the vaccine virus (See 'what is virus recombination?' in unit 2 if you want to refresh your memory). This was experienced in the last outbreak in China.
- After vaccination an infective virus can be shed by individual animals, so it is important to vaccinate all pigs at the same time in the pen.
- There is no risk of recombination between the vaccine virus and the one circulating in the farm. Therefore killed-virus vaccines are considered safer.
- They have limited efficacy against different strains and sometimes even against the same strain.
Post-weaning multi-systemic wasting syndrome (PCV-2):Vaccines have been recently developed and started to be used in early 2008. There are vaccines available for sows and for piglets. All currently available PCV2 vaccines are killed-virus vaccines, and they are produced by various manufacturers. The vaccines have proved to reduce clinical signs and reduce mortality. An increase in average daily weight of 34 g has been reported from nursery to finishing. Mortality reduction seems to vary between countries and according to PMWS severity before vaccination, but a mean absolute mortality reduction of 5% has been estimated in vaccinated herds. However the vaccine against PCV-2 does not give sterilising immunity.
What is sterilising immunity?
Sterilising immunity is when the pathogen is completely eliminated from the infected pigs. In other words the 'army' which is the immune system fights the pathogen and totally eliminates it.
In the case of PMWS (and many other diseases) the vaccine does not give a sterile immunity, which means that the pathogen is not totally eliminated from the infected pigs, but nevertheless the vaccine will help to fight the pathogen to reduce disease burden.
So, is vaccination the solution for PMWS?