Actinobacillus pleuropneumoniae, the aetiological agent of swine pleuropneumonia, is responsible for substantial morbidity and mortality causing substantial economic losses in the global pork industry. Pleuropneumonia in its peracute and acute forms is mainly characterised by severely affected well-being and increased mortality. Pigs that survive infection, including after antimicrobial treatment, are likely to develop chronic disease characterised by reduced activity and appetite. Acute-peracute clinical signs are generally clear and distinct plus chronic pleurisy is easily diagnosed via slaughterhouse investigations 1, 2, 3, 4. Pigs diagnosed with subacute pleuropneumonia show milder, less distinct clinical signs and lower fatality. While subclinical pleuropneumonia may involve pathological pneumonic lesions, no clinical signs may be apparent 3, 5. How accurate the distinction between the subclinical and subacute forms are in the individual case will rely highly on the frequency and quality of on-farm health monitoring. Some subclinical cases may very well be missed, for example, in farms where animals are co-infected with Mycoplasma hyopneumoniae. The subclinical form of A. pleuropneumoniae negatively affects growth rate and feed efficiency 5 due to chronic lung lesions, like pleurisy and adherence together with a fibrino-haemorrhagic and necrotizing pleuropneumonia, as commonly seen at the abattoir 6.
Estimations on the economic burden of this disease is mainly based on the occurrence of acute outbreaks characterized by high mortality and medical costs; few reports on production efficacy parameters like average daily weight gain (ADG) and/or feed conversion ratio (FCR), and even less have measured losses due to subacute and/or subclinical pleuropneumonia 3. An analysis of five publications on 14 trials, found mean improvements due to antimicrobial intervention in ADG of 33.6% and FCR of 25.5%, with high variations 7. In a controlled long-term field study including a total of 33063 pigs, C-vaccinated (were compared to non-vaccinated for the control of acute pleuropneumonia, mortality was reduced by 28% (p = 0.011). However, the improvements on feed efficiency by 9.36 kg feed per pig produced (p = 0.023), and ADG by 40 per day or pen-efficacy by 6.3% (p < 0.001) will mainly be attributable to subclinical-subacute pleuropneumonia occurring earlier than the observed recurrent outbreaks 8.
The appearance of lung lesions, like pleurisy, at the abattoir, is often associated with A. pleuropneumoniae (OR = 8.75) 9, 10, and can be linked to decreased ADG, increased FCR, prolonged stay in finishing compartment, and reduced carcass weight 11, 12. Pleurisy present at slaughter reduced lifetime weight gain by 1.25 kg on post trimming carcass weight, equal to 1.66 kg of live weight, in average. Also, a 10%-increase of affected lung tissue was correlated of to a reduction of 3.3-4,6% on ADG 13.
Lung lesion scoring is considered highly relevant for estimating severity and losses of respiratory disease, such as caused by A. pleuropneumoniae, at the farm level 14, 15, 16, 17. To investigate pleuropneumonia in all its possible manifestations, pathological evaluation of lung lesions appears to be the least biased method. Performing this evaluation close to pneumonic infection would seem to reveal the most accurate validation of the degree of pleuropneumonic impact on the individual pig.
Lung lesion scoring as the endpoint of measuring on A. pleuropneumoniae induced disease is widely accepted 18, 19, 20 21, 22, 23, 24, 25, 26, 27, 28, 29, 30. A dose-response relation exists 31, 18, 22, 23. Depending on the challenge dose of the individual serovar one can achieve any stage of disease from absolute mortality 32, 33, 19, 20, 21, 22, even in bacterin vaccinated pigs 21, to subacute 18, 24 even to subclinical pleuropneumonia 18, 21, 24. However, due to high variation in disease inside A. pleuropneumoniae challenged groups 18, 21, 22, 23, 24, 27, 28, 29, 30, 31 a calculation of a dose-response will always produce a variation in lung lesions; least when mortality is the outcome. Here the lung lesions are severe and extensive affect 80–100% of the parenchyma 19, 20, 25, 27, 30, 31, 32, 33.
A. pleuropneumoniae is endemic world-wide and swine farms are often infected with more than one serovar. In some countries 80–90% of farms are estimated to be seropositive for A. pleuropneumoniae, with 1–6 out of 7 serogroups present at the same time: 93% having > = 2 and 80% having > = 3 simultaneous Ap-serogroups present 34, 35. The prevalence of serovars varies between countries, regions of countries, and by year of investigation 35, 36, 37, 38, 39, 40.
So far nineteen A. pleuropneumoniae serovars have been classified worldwide 41. However, as the difference between serovar 9 and 11 is only one amino acid in the complete CPS loci and they have identical toxin profiles (ApxI, ApxII), these two serovars can be considered as one: serovar 9/11 42. Strains belonging to different serovars are highly different in virulence and in some cases different strains of the same serovars can express different pathogenicity features; usually due to different Apx toxin profiles investigation 35, 36, 37, 38, 43. More extrinsic factors like general stress 3, 43, 44, poor air quality and climatic control, particularly high ambient temperature variations over the day, are associated with increased severity of pathological lesions caused by A. pleuropneumoniae, even in the case of what are considered low virulent A. pleuropneumoniae strains3, 6.
A. pleuropneumoniae has several virulence factors, some are well described, and several are under investigation. The three exotoxins: ApxI-III and lipopolysaccharide (LPS) have been proven responsible for lung lesions, but at the same time are both immunogenic and can induce protective immunity 1, 3, 46. Many A. pleuropneumoniae virulence factors have been described 46 including outer membrane proteins (OMPs), some of which are immunogenic and therefore potential vaccine candidates 47.
The variation in virulence between A. pleuropneumoniae serovars is mainly determined by the production of one or two of the ApxI-III toxins. These exotoxins providing nutrients for further growth and activity via lysis of the nearby cells in the lung tissue including neutrophils and macrophages 48, 49. LPS are both adhesion factors, allowing for colonisation and the production of exotoxins, and at the same time enhancing the cytotoxic effects of ApxI-III 50, 51, 52, 53.
Several commercial vaccines are available which differ in their composition and can be appointed into one of three A. pleuropneumoniae vaccine categories: 1) killed A. pleuropneumoniae whole-cell components only (bacterins); 2) subunit vaccines containing ApxI-III toxins only; and 3) a combination of these 47. With distinct differences in efficiency, they all reduce clinical signs, but none can fully prevent infection and colonization 54. Antibodies against ApxI-III are responsible for the serovar-independent protection against lung lesions3, 6, 46, 47. Due to limited cross protection between the serovars, bacterin vaccines lack efficacy compared to ApxI-III combined bacterin vaccines; pure toxoids vaccines lack in general protective capacity due to lack of LPS and other cell wall components 55, 56, 57, 58. A. pleuropneumoniae vaccine group 3 above is quite heterogenous, with a wide variety in composition, from a subunit vaccine containing the ApxI-III and only one of the cell wall OMPs, to the vaccine evaluated in this study based on whole-cell components of two serovars, together expressing all three of the ApxI-III toxins, and to vaccines containing whole-cell components of several A. pleuropneumoniae serovars together with some exotoxins.
A combination of the three exotoxins, ApxI-III with LPS, and likely more of the abundant cell-wall based antigens 56, 58, induces a strong and specific cell mediated immune response that can confer serovar independent protection 3. This is an effective design for an efficacious serovar-independent vaccine, feasible for A. pleuropneumoniae prophylaxis to: increase animal well-being, reduce antimicrobial use, and reduce losses due to pleuropneumonia in all its manifestations at any A. pleuropneumoniae-endemic farm at any time3, 6, 47.
The objective of this study was to evaluate the efficacy of a vaccine comprising whole cells of A. pleuropneumoniae serovars 1 and 2 which in combination express ApxI, ApxII and ApxIII to protect against pleuropneumonic lungs lesions following challenge with multiple prominent serovars of A. pleuropneumoniae in growing pigs.