Circo PCV2 receives a Greek update
350 experts from over 20 countries met to discuss how to avoid the virus's damage.
Athens in Greece was the setting chosen by Merial for the company's first separate forum on the porcine circovirus PCV2. With 350 swine-specialist veterinarians from over 20 countries attending, talk flowed around issues concerning how to avoid the damage that the virus does to the pig.
But the backdrop to all discussions about practical control measures was the recognition of the value of vaccinating against the circovirus. In fact, one presenter suggested that the ratio of return on investment from vaccine use may be as high as 7:1.
This was the case when one of Poland's largest producers decided to try PCV2 vaccination for his farrow-finish enterprise with over 2500 sows, explained Professor Zygmunt Pejsak of Poland's National Institute of Veterinary Research. The end of 2007 had brought a severe outbreak of wasting syndrome PMWS with big death losses in nursery and grower pigs. Each of the vaccine protocols tried against the problem was helpful but the greatest return on investment was secured from vaccinating the sows.
Three protocols were tested in this comparison. The first to be attempted meant administering the vaccine only to the sows, the second involved vaccinating only their piglets and the third was a combination of sow and piglet injections. While the best results overall came from the sow + pig approach, Pejsak commented, this may have been due to successive rounds of vaccination having lowered the viral load on the farm. Also, each series had been accompanied by improvements in management and environment. On balance, it seemed likely that every euro spent on vaccinating both sows and piglets had yielded around 3 euros in return, compared with a 4:1 ratio from applying the vaccine only to piglets.
Practitioner presentations from three countries of western Europe added to evidence of the benefits available from vaccinating. From the UK, veterinarian Jake Waddilove gave the example of late-stage PCV2 problems affecting the finishing herd at around 60-80kg liveweight, resulting mainly in pneumonia and the sudden deaths of good pigs although some others did show the classical signs of wasting or weight loss.
Where this occurred at a unit of 4,000 pig places receiving pigs from a 1,300-sow outdoor herd at 45kg to take to 95-98kg, he reported, vaccinating sows and gilts was followed by a halving of the mortality rate in finishers and an extra 4.5kg of weight for pigs of the same age at slaughter.
The Netherlands experience described by Dutch veterinarian Cees Veldman centred on a unit of 740 sows and 7,600 finishers beset by drops in growth during the second half of the finishing period, with 3.5% mortality. At the end of 2008, on an experimental basis, its advisers introduced a trial of vaccinating piglets at eight weeks old. Death losses have since dropped by half and the unit has also cut its use of antibiotics by 35%.
Those calculations of the financial benefit from vaccine usage did not take into account any possible uplift in breeding herd performance but the Merial forum heard confirmation that such improvements have been observed in herds that now vaccinate their sows. From France, Dr Julian Avon spoke of a northern French herd that had beaten high losses and improved growth rates in nursery pigs around 7 weeks old by vaccinating piglets on weaning day. Afterwards, it added sow vaccination.
Retrospective analysis showed that this application of the vaccine to sows ended a previous decline in liveborn litter size. So the clinical signs of PCV2 had occurred after weaning, Dr Avon remarked, but the vaccination had brought to light that there was also an unsuspected effect on reproduction.
In light of the increasing international availability of proven vaccines against the circovirus, at what point do health problems observed in the herd suggest PCV vaccination as a possible remedy?
The difficulty with answering this question is that circo diseases can have many faces. The original focus on PMWS has broadened to a wider span of signs, even while the health professionals admit they still cannot fully explain the physiological mechanisms for the wasting or weight loss behind the wasting-syndrome reference in the PMWS name.
Debate similarly continues over whether the skin condition PDNS can be counted as part of the circo story, according to remarks in Athens. They underlined, too, that although ear necrosis is a frequent observation among infected pigs, using a PCV vaccine does not always stop it.
It certainly seems more justified than ever to speak of PCVD, signifying a range of diseases in which the virus is believed to play a role. However, several Athens delegates noted the drawback attached to assuming a wide-ranging approach, which is that PCV2 will become blamed for every health problem encountered on the pig unit.
Take-home messages from the meeting started with the ability of the circovirus to upset the pig's natural immune systems. Two different outcomes are possible from infection with the same virus, delegates heard. Although exposure can activate an immunity, the field virus also is able to inhibit immune functions so no protective antibodies are produced.
PCV2 is highly unusual in producing large amounts of a double-stranded replicative form of DNA, leading to problems of immune defence development. The virus replication cycle takes an unusually long time of 36 hours to complete. Although the circovirus binds quickly to receptors, it then needs to enter the cell and degrade in order to liberate its DNA so that new viral material can be formed in the nucleus of the cell. Moreover, membranes around the nucleus and cell need to be breeched before the new virus is released. There are occasions when the virus simply stays within the cell.
By inhabiting cells associated with recognising an invader and initiating an immune defence, it can interfere with the recognition of danger signals and moderate immunity. This makes them become more tolerant of other infections.
The interference is from the viral DNA that fills these cells, however, so inhibition depends on the presence of a threshold minimum concentration of the replicative type of DNA.
Merial team members underlined that the virus infects piglets very early in their life. There is then a confrontation with protective maternal antibodies. These may not be at high levels in the herd, or possibly are not transferred fully in colostrum at suckling time.
In any event, their concentration declines naturally. At some point the balance is upset so there is insufficient protection passed from the sow to shield the piglets.
High viral load
The young pig may be faced with a high viral load in its environment even while its immunity system is compromised by the virus. But when sows are vaccinated, the team continued, this has the effect of reducing their shedding of virus so the challenge to the piglet is reduced. Vaccinated sows have been demonstrated to have functional cells for specific immunity that are transferred effectively to piglets in the colostrum. The specific cells are detectable in the litter from soon after birth.
Vaccination, therefore, not just for maternal antibodies, delegates heard. It is also for the transfer of cells priming the immune system. Sow vaccination, therefore, should be seen as establishing long-term protection in the piglets.
Of course, some PCV vaccines are marketed for administering to piglets rather than sows. The advice at the Merial meeting was to consider piglet vaccination as an option if late-stage finishing disease occurred that traced back to an inadequate vaccination process in the sow herd. It would also be practicable for units sourcing their pigs from various herds and unable to influence the immunising of sows.