With every day that passes, we are learning more about the porcine circovirus type 2 (PCV2) infections still circulating the world. Further information has come this year from several sources, not least the annual meeting of the American Association of Swine Veterinarians. These updates have helped to confirm that differing pictures and patterns of the disease are being seen in different continents.
Why these differences? They depend primarily on how long the disease has been endemic in that country. But they relate also to a variety of other factors, such as the nature of any other diseases present on that particular unit (or in the country where it is located) and the management control factors introduced to combat PCV2.
The situation on many units in North America currently is one of an acute phase of high mortality rates. Circovirus diseases are a relatively recent phenomenon in North American herds. Their occurrence on European sites has been much longer by comparison, from the late 1990's. It helps to explain why Europe seems to typify much of the rest of the world by experiencing the chronic phase of disease, with much lower mortality.
This chronic phase happens where immunity already has had chance to build up, primarily in the breeding herd. The presence of immune sows means an end to most of reproductive disease problems that were seen initially in the infection cycle, such as embryo deaths and early piglet mortality. The disease does not erupt in chronically affected herds until maternal antibodies start to wane. Often this is in pigs from 10 weeks of age. With a chronic case there is still a viral challenge at some stage to the health of the pigs on the infected farm. Depending on the system and the severity of the challenge, however, the effect on mortality is much less, together with a lower incidence of underweight pigs or slowed growth and poorer feed conversion efficiency.
At the same time, the disease can actually be far worse if a number of other infections co-exist with the circovirus on the farm. Mycoplasmal pneumonia, PRRS, Aujeszky's disease/pseudorabies and especially Actinobacillus pleuropneumoniae are strong candidates to add to the severity. Under these circumstances the lower-grade PCV2 infection challenge becomes masked and often is considered effectively sub-clinical (see Figure 1). Frequently, though, the disease is still there, causing a 1-2% mortality and 1-2kg drop in average bodyweight.
The severity of the disease in a farm is not only the result of simply having the PCV2 infection. It also reflects the infectious or challenge load. Work published in 2004 by Olvera and colleagues (see Figure 2) showed that the presence of high levels of virus in the blood (viraemia) was frequently associated with a more severe form of the disease, although there was some overlap.
Lower levels of viraemia caused mild levels of disease and were often associated with the skin form known as porcine dermatitis and nephropathy syndrome or PDNS. As the viraemic load or challenge increased, the disease became more severe.
This explains why the disease pattern changes on a farm. At first, when there is no immunity, the infection can explode in the pig and cause high levels of challenge that lead to severe disease (including a high mortality). Later on, however, the disease situation improves because of the presence of antibodies from chronically infected or vaccinated sow herds, coupled with improved management and reduced exposure. The mortality rate subsides, as does the number of slow-growing underweight pigs. The farm has entered the more chronic or milder, sub-clinical phase.
New information connects to this by showing why the piglet vaccines now coming to market are proving to be so successful. As a presentation to the AASV meeting demonstrated (see Figure 3), these vaccines reduce both the number of viraemic pigs and the number that have a high virus load or challenge. Maternally derived antibodies did not appear to interfere significantly with the vaccine's protective effect, which lasted the whole life of the finishing pig.
As time progresses and the vaccines roll out around the world, it is expected that there will be a steady improvement in overall production from the protected herds. The picture will improve significantly, even on units that are affected sub-clinically by a PCV2 circovirus infection. PIGI