With porcine circovirus type 2 (PCV2) infections still circulating the world, we are seeing different pictures and patterns of the disease in different continents. This difference depends primarily on how long the disease has been endemic in that country and also on a variety of other factors, such as what other diseases are present in the farm and country and the management control factors introduced to reduce the disease.
In North America, they are going through the acute, high mortality phase of the disease, whereas in the rest of the world, such as Europe, we are in the chronic phase of the disease with much lower mortality, as we have had the disease since the late 1990s. This chronic phase, where immunity has primarily built up in the breeding herd means that early disease problems such as embryo mortality and early piglet mortality are largely over and the disease occurs when maternal antibodies start to wane, often in the finishing pigs from 10 weeks of age and above. There is still a challenge going on in the infected farm at some stage but depending on the system and the severity of the challenge, the effect on mortality and the incidence of under-weight pigs or slowed growth and poorer feed conversion efficiency, is much lower. If there are a number of other infections on the farm, such as enzootic pneumonia, PRRSV, Aujeszky's disease and especially Actinobacillus pleuropneumoniae, the disease can be much worse and the lower grade PCV2 infection challenge becomes masked and often considered effectively sub-clinical (see Graph 1). Frequently though, the disease is still there causing a 1-2% mortality and 1-2kg drop in average bodyweight.
Graph 1. Acute, chronic and sub-clinical phases and patterns of PMWS/PCVAD in finishers with mixed infections
The severity of the disease in a farm is not only the result of simply having the PCV2 infection but also the infectious or challenge load. Work by Olvera and others (2004) 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.
Graph 2. PCV2 viraemic load (log scale) in relationship to severity of disease
(after Olvera and others, 2004)
Lower levels of viraemia caused mild levels of disease and were often associated with the skin form, porcine dermatitis and nephropathy syndrome (PDNS - see Photo 1). As the viraemic load or challenge increased, the severity of the disease increased. This explains why the disease pattern changes on a farm. When there is initially no immunity the infection can explode in the pig causing high levels of challenge and severe disease, including a high mortality. However, with the presence of antibodies from chronically infected or vaccinated sow herds, coupled with improved management and exposure reduction, the disease ameliorates and the mortality and the number of slow growing under-weight pigs subsides and the farm enters the more chronic or milder, sub-clinical phase.
Photo 1. Porcine Dermatitis and Nephropathy Syndrome (PDNS)
The reason that piglet vaccines are proving to be so successful is that they reduce both the number of pigs having a viraemia (Opriessnig and Halbur, 2007) and also the number of pigs with a high virus load or challenge. Maternally derived antibodies did not appear to significantly interfere with the vaccine's protective effect, which lasts the whole life of the finishing pig.
Graph 3. Effect of piglet vaccination on reducing virus infection in pigs
(from Opriessnig and Halbur, 2007)
As time progresses and the vaccines roll out around the world, it is expected that there will be a steady improvement in overall pig production and, even in the sub-clinically affected herds, the picture will significantly improve.
Olvera, A., Sibila, M., Calsamiglia, M., Segales, J. and Domingo, M. (2004) Comparison of porcine circovirus type 2 load in serum quantified by a real time PCR in postweaning multisystemic wasting síndrome and porcine dermatitis and nephropathy syndrome naturally affected pigs. Journal of Virological Methods 117, 75-80
Opriessnig, T. and Halbur, P.G. (2007) Influence of anti-PCV2 passively-acquired antibodies on efficacy of Suvaxyn® PCV2 vaccination in pigs experimentally-infected with PCV2. Proceedings of the American Association of Swine Veterinarians Meeting Orlando, Florida, USA, pp 299-300
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