Expert opinion

4 commentslast update:May 14, 2009

A viral update – lots going on in the swine world

There is a lot going on in the swine world regarding viral infections over recent days and weeks – flu, PRRS and circovirus, so I thought it would be useful to give an overview for readers.

One 'flu' over the cuckoo's nest:
It amazes me that a human flu virus (H1N1), albeit one which has co-mingled with porcine and avian genetics has emerged as 'swine flu' rather than 'Mexican' or 'North American' (OIE) flu and is causing disruptions in pig trade and concern about pork consumption around the world. The media cuckoos have a lot to answer for all this hype. I must admit the early stories were of concern, with a high reported mortality and I expressed my concern at a Rotary meeting that this was potentially more serious than avian flu, as it had already been transmitted from human to human unlike the avian influenza strain (H5N1).

Fortunately, although the virus is highly contagious, it is not highly pathogenic (disease causing) at this stage and has probably been going on for some time in Mexico, as it appears well disseminated there. To reassort, to produce this new strain, a human and porcine flu virus have to come in contact in a cell, whether in humans and in pigs. In this case probably man. The N and H subtypes found in man and pigs are usually similar (see Table 1 – bold Xs) and are often very different from many of the avian subtypes. Therefore it is man to pig contact, which is important and the chances may actually be higher in backyard scenarios – as in Asia and Asian flu epidemics – rather than intensive husbandry situations where there is high pig but low man contact.

Table 1. Type A influenza surface subtype antigens (from Defra, 2005).


Haemagglutinin(H)  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
Human X X X                        


Avian X X X X X X X X X X X X X X X
Neuramidase (N)                               
Human X X                          
Swine X X                          
Avian X X X X X X X X X            

It is interesting that the UK flu experts forecasted that the next flu epidemic would likely come from H1-3 and N1-2 virus reassortments rather than the avian subtypes – how right they were.

Highly pathogenic PRRS – an Asian problem?
The spread of the flu virus around the world by passengers on planes highlights the easy movement of infections. The movement of breeding stock and semen is the nearest we get to intercontinental transmission and we are fortunate that the highly pathogenic strains of PRRS, which have caused 'high fever disease', are confined to China and parts of the Asian region rather than spreading to Europe and N. America. We are lucky that the pig flow is to Asia rather than the other way, but it also highlights the importance of surveillance and vigilance and import controls.

Circovirus – vaccine confusion:
We have just had reported major meetings in Athens, Lisbon and Singapore on the sow vaccine (Merial – Circovac®) and the introduction in Europe of a new piglet vaccine (Porcilis® PCV – Intervet/SP) and the continued roll-out in Asia of Boehringer Ingelheim's – Ingelvac CircoFlex®, respectively. The vaccines have been an undoubted success, with reports of 90% of the US market using vaccines in piglets, which resulted in shortages in other parts of the world. This situation is steadily improving now that new production is coming on stream.

There is still a dichotomy of opinion on the best use of vaccines either in sows or piglets but both seem to have benefits. Sow vaccines obviously protect the breeding stock and their subsequent litters from the virus. The maternally derived antibodies (MDAs) also protect piglets in the nursery stage for a period of time, which has not quite been determined. Their presence appears to neutralise a viraemic attack and will permit immunity to develop without disease, if the viraemia takes place at an early stage.

In systems where the viraemia is taking place later, especially in the finishing barns, the protection appears to be no longer sufficient. Piglet vaccines appear to give good protection both during the nursery and finisher stages (from two weeks after vaccination). These observations were confirmed in a recent survey of UK vets that the sow and piglet vaccines gave similar protection in the nursery (weaner/grower stages) but the sow showed advantages in the number of pigs born alive and the piglet vaccines gave markedly superior protection in the finishing stages (see Table 2).

Table 2. Survey of UK vets and the comparative improvement in results from sow and piglet vaccination.


Vaccine  Pre-weaning (pigs/litter improvement) Nursery (weaner/grower)mortality reduction (%) Finisher mortality reduction(%)
Sow 0.1 1.7 0.2
Piglet 0.0 2.0 3.6

From the American Association of Swine Veterinarians recent meeting there were many programmes of use described, such as using sow vaccines in piglets and piglet vaccines in sows. Vaccinating piglets at processing (below one week of age), one-shot or two-shot programmes and the effects of MDAs on vaccines were also reported and by the end there appeared to be almost a confusion of information.

I think the basic rules apply, if you use vaccines in sows to boost immunity don't be surprised if MDAs interfere with early piglet vaccination, therefore delay it for as long as possible, to at least weaning. There is a definite split between one-shot or two-shot use between the US and Europe. This may be a reflection of the size of farms, as in the US, compliance is a reported issue. In the EU we tend to have smaller farms and cost of vaccine and administration pushes us more towards the one-shot approach, if we can.

Viral infections do play an important role in pig production, and can have a devastating effect. We have wrestled with PCV2 over the last decade and are almost getting on top of it now but we are still struggling with PRRS virus. We have enough of our own problems, so it seems even more unfair that the media appear to be blaming us over 'Mexican flu'.


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    John Gadd

    Useful review, David, thanks.
    Yes, PRRS across the world seems as bad as ever. I get moans about it - past or present - on most farms I visit. But one little glimmer of light could be there and be down to management? I've been suggesting the idea of not pushing these genetically-precocious gilts we are lucky to have available these days into service too fast, so as to allow more time for her immune system to develop. I feel it is worth breeders being a little more patient and investing some money into this area.
    Revisiting farms one to two years later where they did put this into practice, it does seem that PRRS has been less of a problem to them - but these opinions from only a dozen big units is a small sample and, sure - might be down to coincidence, but I'm beginning to suspect that there might be something in it.
    I have a feeling too that 'parity segregation', where gilts and their progeny are kept well away from the rest of the unit in their own sections until after their second parity, is having a dampening effect on virus infection on those farms which have courageously spent the money on doing this. These young breeding animals, with their lower acquired immune pattern, especially if rushed into service to impatiently, could be shedding viruses to the main herd.
    Just two gut-feelings from a lay observer who is fortunate to be asked to visit a lot of big farms each year in many countries.
    What do you think ?

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    Uffe Nielsen

    Thank you, David, for a very good and precise comment.
    I have discovered Porcine Cyto Megalo Virus as a course of classical PCMV symptoms in Denmark in three herds and I think that it is the reason for problems in four more herds, that I know of.
    We do not know how to handle PCMV in Denmark.
    Can you, David, give some advice how to manage this disease.

    If I may comment - I think that the two MLV vaccines against PRRS work very well in Denmark. They are safe and easy and cheap to use.

    Greetings from Denmark

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    David Burch

    Dear John,
    Sorry for the delay in responding. Whether it is an effect on the immune system or just serving precocious gilts early is the problem one cannot be sure. Recent work going on in Holland suggests that gilts should not be served before 150kg so that they are better to withstand the effects of first and especially second litter problems, which might be interpreted as an effect of PRRS. Difficult one to split. Gilt segregation is an interesting concept. I have not personally experienced this. Certainly to stabilise immunity in a herd prior to eradication one closes the herd to gilt introduction to allow virus to die out. The persistent reintroduction of potentially virus excreting animals could be considered to have a destabilising effect and perpetuation of infection.
    There is also the risk of the introduction of new virus strains if the gilts are brought in rather than home reared.
    I hope this is of help
    Best regards

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    David Burch

    Dear Uffe,
    Again, I apologise for the delay in replying. I am glad to hear that the 2 MLV PRRSV vaccines are working well in Denmark. We do not have the US strains of PRRSV and have only the milder European strains, hence my concerns of the highly pathogenic form strains in Asia. Apparently, they are vaccine responsive but there was some confusion over the original diagnosis and differentiation from swine fever and circovirus infections.
    You raise an interesting question about porcine cytomegalovirus in Denmark. In the UK it was commonly referred to as pig inclusion body rhinitis virus and was to be differentiated from the classic progressive atrophic rhinitis, which has largely disappeared. Reports suggest that over 90% of herds in the UK are infected with PCMV and therefore we associate it with the more chronic form affecting young weaners and causing respiratory signs like sneezing etc. In naive herds, however it can be quite severe causing mortality in young piglets. Control is difficult as there are no vaccines and probably depend on immunity building up in the sow herd to protect the suckling piglets, leading to the relatively mild chronic form. Antibiotics to control the secondary bacterial infections, probably due to Pasteurella multocida, are considered the main supportive therapy.
    I hope this is of help.
    Best regards

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