Since the late 1990’s vaccines – both live attenuated and inactivated – have been available. Whilst a useful contributor to control of disease, the nature of the virus is such that it is undergoing continual variation and in time this moves the field strain away from the original strain used in vaccines. Unfortunately, the veterinary licensing authorities in Europe do not permit regular alteration to licensed products (unlike that which occurs, for instance, in human influenza vaccines). This means that should vaccine need to be changed, it would have to undergo full licence scrutiny of safety, quality and efficacy. The costs involved preclude this happening on a regular basis.

PRRS therefore remains a major threat to pigs and anecdotal comments for NADIS reporting vets regularly highlight the role of the disease in reproductive failure (abortion, premature farrowing and poor quality of newborn pigs) and in Porcine Respiratory Disease Complex in growing pigs.

Within the population surveyed currently by NADIS, 56 per cent of all herds are reported to be infected with PRRS virus and of these, 40 per cent use vaccine in either breeding or feeding herds. The regional picture (graph 1) is interesting in that it highlights:

1. A high proportion of herds in the North East infected with virus but only 33 per cent vaccinating.
2. Much lower prevalence in the South West where only 24 per cent of herds are infected but all of them use vaccine.

Within breeding units the lowest percentage of herds affected falls within breeding only units – not unexpected, as herds without growing pigs are more likely to see the virus burn out. However, surprisingly, finisher herds show lower levels of infection than either nurseries or nursery finishers (graph 2).

Higher prevalence of PRRS infection is seen in indoor and slatted systems for sows with straw based systems and outdoor farms much lower (graphs 3 and 4). Certainly in the outdoor environment, there is less opportunity for virus recycling, especially if weaners are well separated from sows leading to sero-negative breeding populations.

The analysis of infections between continuous flow and batch systems suggests only a small difference (55 per cent versus 63 per cent of herds infected) but very surprisingly it is the batch system with a higher prevalence of infection. In the surveyed population batch production has not typically led to elimination of the virus (graph 5).

The reports of presence of PRRS virus within the herds are based on the clinician’s opinion. Clinical signs and laboratory tests form the basis of much of these assessments but the known presence of disease within the supply chain – whether that is replacement gilts and boars or incoming weaners – is an important element of health assessment (graph 6).

Further Reading

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Find out more information on PRRS by clicking here.