By Dr. Roman KREJCI, Swine Corporate Technical Manage, CEVA Animal Health, Libourne, France
Porcine pleuropneumonia induced by Actinobacillus pleuropneumonia is one of the most important respiratory diseases of pigs in both acute and chronic forms. The infection is highly prevalent and difficult to handle. A good knowledge of the pathogenesis can help to diagnose the intensity of the disease according to the frequency of the typical lesions and also to develop a good strategy for the prevention.
PREVALENCE AND ECONOMIC IMPORTANCE
A. pleuropneumonia (A.p.) is widely spread particularly in Europe, Latin America and South-East Asia. A.p. was diagnosed in all European countries with different prevalence. The recent epidemiological studies indicate a very high rate of exposure reaching up to 100% of seropositivity of investigated farms. Of those: 90% in Northwest Germany (Ganter et al 1998), 96% in Belgium, 89% in Spain, 100% in Italy (Maes 2010) and 100% in Belarus (Ceva internal data). In Asia, 18 out of 22 tested farms in Thailand and 18 out of 69 farms in Philippines were positive (Torres et al 2006, Assavacheep et al, 2003). Those data suggest that the real prevalence is often underestimated and appropriate screening diagnostic methods are necessary.
Dramatically increased mortality due to haemorrhagic necrotic pneumonia, as high as 10-20% during outbreaks, is a threat to the economics of the farm. However, enormous losses due to lower growth performance are also recorded in chronically infected herds that often have subclinical course of infection. Pleuropneumonia can cause a 34% decrease in daily gain and a 26% decrease in feed efficiency (Sorensen et al 2006).
PATHOGENESIS AND PLEURISY SCORING
The lesions in the respiratory tract induced by A.p. are mostly associated with the direct effect of RTX toxins named Apx I, Apx II and Apx III. These Apx toxins exert cytotoxic effect on various cell types including endothelial cells and macrophages. Apx together with lipopolysaccharides (LPS) of A.p. induce the local over-production of proinflamatory cytokines, complement and toxic oxygen metabolites. Damage of endothelial cells by Apx and the action of above mentioned factors lead to constriction of pulmonary airways, vasodilatation, activation of coagulation pathway, localized ischaemia and subsequent necrotic pneumonia and pleuritis characteristic for porcine pleuropneumonia (Bossé et al 2002). Those lesions are localized predominantly in the dorsal part of caudal lung lobes.
This typical lesion localisation is used as a useful indicator for the differential diagnosis. Recently the new SPES method for screening the pleurisy at the slaughterhouse was described by Dottori et al (2007). It is used for semiquantitative evaluation of lesions suggestive for previous chronic infection by A.p. SPES together with the serological examination and clinic-pathological history of the farm, completes the picture of incidence and severity pleuropneumonia in the herd.
Using SPES, Meyns et al (2010) evaluated various risk factors associated with the prevalence of pneumonia and pleurisy on 60 farms in Belgium, where 20.8% of examined lungs exhibited pleurisy suggestive for A.p. infection. Percentage of A. pleuropneumonia seropositivity and also number of pigs/pen during the nursery period were positively correlated with the pleurisy score, while age at weaning was negatively correlated.
The fact that convalescent pigs are almost completely immune to homologous challenge indicated the possibility of active immunisation against A.p. For the effective protection against various serotypes, the immunity against all major factors of virulence is necessary.
If none of the A.p. serotypes produce all of the Apx toxins at the same time, various serotypes may be present in a farm, a region, a country. For this reason, most of the factors of virulence including the three Apx I, II and III must be present in an effective vaccine. In numerous experimental and field trials with the toxoid based A. pleuropneumonia vaccine Coglapix® (Ceva SA) containing all App I, II, III and a controlled amount of LPS, an effective protection was accomplished in vaccinated pigs indeed. Tenk et al (2011) described a significant reduction of lung lesions in pig vaccinated with Coglapix® in an experimental high dose challenge study. The efficacy of the vaccine Coglapix® was calculated to be of 76.7%, while the competitor’s one was close to 0%. This confirmed previous results (Tenk 2010) when Coglapix® conferred high level of protection against the experimental challenge (Tab 1.)
Tab 1 Average clinical and pathological scores of vaccinated and non vaccinated pigs
In farm conditions pigs vaccinated with Coglapix® performed significantly better than non vaccinated control animals in terms of survivability and growth as seen in Tab 2. (Krejci. R., et al 2011).
Tab 2 Losses and performance in fattening pigs.
A. pleuropneumonia is a typical primary pathogen. The immunosuppressive factors such as PCV2 infection can obviously negatively impact the health status of the batch, co-infected by bacteria. Even in PCV2 stabilized herds, however, A.p. infection has to be well controlled with the appropriate attention. Krejci et al (2011) described that pigs vaccinated against PCV2 were still susceptible to A.p. infection and performed worse in the fattening period, compared to pigs vaccinated against A. pleuropneumonia by Coglapix® - Tab 3.
Tab 3 Performance in fattening pigs vaccinated either agianst PCV2 of with Coglapix® against A. pleuropneumonia
A. pleuropneumonia is widely spread in most pig herds. SPES scoring method is a highly indicative tool for evaluating chronic lesions induced by A.p. Successful prevention of the losses due to mortality and cuilling as well as descreased growth performance can be achieved by vaccination using the effective vaccine, Coglapix®. Coglapix® contains major virulence factors-derived immunogenic components, including all Apx toxoids and a well controlled level of LPS. This vaccine confers a high degree of protection against both experimental and natural infections of A.p.
(Source: "Axis - Ceva Swine Tour 2011 Special Edition" - July 2011)