All findings in fatteners with impaired mobility on this farm led to the assumption that M. hyosynoviae as an infectious factor was involved in disease pathogenesis in combination with additional factors. Infection of the joints with M. hyosynoviae can be asymptomatic, but the joints can also be filled macroscopically with a yellow/brownish viscous fluid, which often has an enlarged volume, whereby in chronic cases the joint capsule can be extended [1]. In the present study, a mild to moderate lymphoplasma-histiocytic inflammation as well as a fibrino-suppurative synovialitis were detected by histology. A slight increase in synovia volume was found in two of the pigs and in two pigs a subcutaneous edema of the joints was observed. Infectious arthritis caused by M. hyosynoviae often results in decreased profitability for the farmers due to higher medication costs and time-consuming measures that have to be taken, such as segregating diseased pigs in recovery pens. This infectious agent also further impairs skeletal health in fatteners, which is an important welfare issue [2]. In this case, antibiotic treatment with amoxicillin in combination with an anti-inflammatory substance was successful in individual pigs, although mycoplasma species are intrinsically resistant to β-lactam antibiotics [39]. In contrast, MIC values for tiamulin are low for M. hyosynoviae (< 0.25 µg/mL) [39], but treatment with this substance was not successful on the farm in the case report. With high probability, the anti-inflammatory parenteral treatment, which was not performed in combination with the in-feed-treatment with a pleuromutilin, was responsible for the improvement. If this was the case, infection with M. hyosynoviae might not be the primary cause of the clinical signs, but instead pain either caused by inflammation or by the degenerative joint alterations. Due to the generally differing outcome of M.-hyosynoviae-infection, identifying further influencing factors in affected swine is of high importance. On the farm in question, claw lesions and osteochondropathy were identified as additional factors. During nursery, pigs were kept on plastic flooring, which is characterized by a different hardness compared to concrete flooring in the fattening stable. These differences between both materials in hardness but also surface roughness are risk factors for the development of claw lesions as a consequence of mismatching in horn quality and underground [40]. Piglets experience a sudden change in underground after moving to the fattening unit without an adaptation period. The observed claw lesions were indicative of abrasive injuries (sole erosions), but can also be the consequence of extended resting periods due to painfulness while moving (skin lesions at the coronary band region). In general, additional factors can support the development of claw lesions, e.g., genetically determined asymmetries in inner and outer claws and abnormal toe angles, but also nutrient deficiency, e.g., biotin [41, 42]. While anatomical claw abnormalities were not obvious in the examined pigs, biotin was not recorded in feed declarations. Biotin is important for hardness and compressive strength of sidewall regions on pig hooves [42]. As a supportive treatment to improve horn quality on this farm, biotin supplementation in feed could be taken into account. Claw lesions can be the consequence of degenerative joint diseases but also a risk factor for the development thereof such as OC [43]. The progress of OC might also be triggered by a change in flooring conditions especially in heavyweight pigs, because joint structures are not adapted to the new pressure forces. Finally, some authors consider OC to be a predisposing factor for M.-hyosynoviae-related disease [44]. Cartilage pre-damage due to other reasons might also contribute to the adhesion of M. hyosynoviae. In puppies, a negative impact of P deficiency on the musculoskeletal system has been described. Affected puppies showed a loss of muscular activity, deviation of the limb axis and hyperflexion of joints, indicating the demand for P also for the connective tissue [45]. Several studies have shown that dietary P deficiency can cause degenerative but also other pathologic skeletal alterations. It is known that Ca and P accretion in the skeleton in later life in mammals depends on the supply in the early stages of growth and bone development [25]. During skeletal development growth, cartilage in the physis plate is responsible for longitudinal growth, while the articular–epiphyseal cartilage shapes the long bone ends. Within a sequential progress including matrix mineralization, enchondral ossification is achieved by parallel continuous production of cartilage and its replacement by bone mineral [12]. The rate and direction of growth are assumed to be affected also by nutritional and metabolic factors [12].
Blood parameters shown in Table 5 varied within the reference range. A hypothesized very low current dietary P intake would have been indicated by low P concentrations in the blood, indicating acute P deficiency. Pre-analytical treatment of blood samples is critical for P analysis because hemolysis leads to an artificial increase in P concentrations. For this reason, hemoglobin was measured in the serum samples as a quality control. In this case, the result of bone marker determination hinted at a stimulated bone resorption maybe as a consequence of marginal P supply of individuals with higher daily weight gain. Serum osteocalcine has been found to be a more accurate indicator of bone mineralization in pigs than alkaline phosphatase in serum [46]. Disadvantages are that osteocalcine is relatively unstable and serum samples should therefore be processed and frozen within one hour after collection. Suppressive effects might occur when the animal is pretreated with corticosteroids, but this was not the case in the three pigs examined here [47]. Since diagnostic imaging or invasive procedures such as a bone biopsy are still difficult to perform in swine, diagnostic methods easy to implement and providing meaningful results are of high practical impact. While it is usually not feasible to perform radiography in several pigs in a herd, taking several blood samples and pooling them for bone marker analysis is much more practical.
Nevertheless, the hypothesis of an additional nutritional impact on disease development could only be supported by bone composition in one pig in this case. The comparably low bone mineral content in combination with the bone marker result might suggest that bone formation with respect to mineral accretion was reduced. Bone ashing in pigs is a further diagnostic approach to verify the suspicion of impaired mineralization. Standardization of the method has been improved in recent years so that preliminary reference values could be elaborated for the femur [36]. Bone ash diagnostic was found to be appropriate for diagnostic evaluation of marginal supply with minerals lasting at least three weeks [36]. Of high importance is the removal of adjacent tissue from the bone before starting the diagnostic procedure. In growth periods with insufficient Ca and P supply, at first, the total bone mass is reduced, while bone formation and composition might be maintained. A lower ratio of the diameter of the long bones to body mass can be the consequence [36]. The pressure on the end of bones covered by cartilage depends on body mass and the area of contact within the joint. It could be that also in young pigs – as observed in growing dogs- the dietary P supply affects the strength of muscles that keep the bones in the right position. With an insufficient P intake and impaired muscle tonus, there is a predisposing effect for alterations of the cartilage especially in pigs with high growth rates.
Analysis of diet composition and feeding anamnesis are fundamental in the diagnostic procedure. The authors assume that a marginal mineral supply during a specific juvenile phase of life with high growth rates might be predisposing factors of disease development in later life. The three pigs examined in this study were slightly heavier (approximately 3 kg) than the average pigs in the respective age-group, so that high daily weight gain can be assumed. In individuals with high growth rates (~900 g average daily weight gain), the uptake of digestible P might have been insufficient at the end of nursery/beginning of fattening, especially when the phytase content in the final diet is reduced by a relatively high proportion of phytase-free fermented ingredients. To diagnose a marginal mineral supply in critical growth phases, not only the demand of the pigs with respect to feed intake and growth rate, but also for bone mineralization should be considered.
A ratio of digestible Ca to digestible P for adequate mineralization of bones and optimal growth performance was found to be approximately 1.23:1 in cases when digestible P met the requirements [48]. Excess Ca in combination with P concentrations below recommended requirements led to decreased growth rates [48]. Both requirements were fulfilled in this case report with low P concentrations and a relatively wide Ca:P ratio in the final diet. Estimates for Ca requirements of Ca in growing pigs range from 6.3-4.2 g/kg DM in pigs with 20-80 kg bw [19, 20]. Different batches of compound feed vary in P and Ca concentrations, which can lead to deficiencies in short periods depending on batch size. Labeled diet compositions are only based on an analysis in the first charge. In general, digestibility of dietary P is markedly improved by fermenting the liquid diet before offering it to the animals and by adding phytase to the compound feed [49-51]. Both strategies were applied on this farm.
In the three examined pigs in this study, histologic findings were only indicative of osteochondropathy and not mineral deficiency. Deficiencies in Ca, P or vitamin D lead to an impaired bone growth, modeling or remodeling characterized by distinct morphologic entities as trabecular bone rarefaction, enlarged thickness of osteoid tissue, failure of newly formed osteoid to mineralize, thin trabeculae and increased osteoclasts [52, 53]. Metabolic bone diseases are usually reversible; the cause is detected early and no lasting effects cause pathologic fractures or a disruption of growth plates in young animals. At the time of necropsy in these animals, the morphologic findings were not consistent with the aforementioned metabolic bone diseases. In case any histologic changes typical of mineral deficiency had been present at an earlier point in time, they would most likely have been superimposed by the degenerative processes at the time of examination.
Dyschondroplasia in physeal and epiphyseal locations [54, 55] is accompanied by a premature regression of blood supply. Microscopic focal lesions or necrotic chondrocytes below the interface of articular or within the epiphyseal cartilage growth, which were replaced by fibrous connective tissue, can undergo calcification. Lesion development can already start at the age of four weeks of life with a widening of growth plate parts. Resulting bone deformation with incongruity in cartilage surfaces can result in osteoarthritis. Hereditary risk factors are of importance for the development of osteochondrosis, while the impact of rapid growth with early excess weight and nutritional factors are controversially discussed. In the study by Faba et al. (2019), reduction in weight gain in combination with supplementation of minerals and amino acids did not influence the prevalence of lameness [56]. In another study, mineral supplementation alone did not improve locomotion scores, but in combination with female-only rearing, a significant beneficial effect on bone mineralization and joint lesions was shown [57]. Both studies support the multifactorial pathogenesis of osteochondropathy and the lack of evidence for an interaction with mineral deficiency. Under experimental conditions, hypophosphataemia was found to cause focal cartilage lesions, which were different from cartilage degeneration typical of osteochondrosis [58].
At the beginning of being involved in this clinical case, the primary hypothesis was that the development of the disease was a multifactorial process starting with an impaired bone mineralization and triggering the development of osteochondropathy of the joints. Pigs with high growth rates develop high pressure on cartilage surfaces of the long bones. In the case that muscular forces are weakened due to a marginal P supply, deviations in limb axes with the consequence of improper biomechanical stress on joint cartilage surfaces, aseptic inflammation and subsequent pain during movement might occur.
Although the authors suggest that a triad of three factors such as infection with M. hyosynoviae, degenerative joint alterations and marginal dietary P supply might be responsible for the spectrum of symptoms, so far, no connection between the development of OC and an undersupply of calcium or P has been proven [2]. Whether cartilage alterations as shown in these pigs are predisposing factors for colonization and infection with M. hyosynoviae remains hypothetical. Some authors have considered that OC could be a predisposing factor for joints to be infected with M. hyosynoviae [44]. However, these assumptions were refuted by authors who did not find M. hyosynoviae any more frequently in individuals at the slaughterhouse with OC than in individuals without OC [3].
So far, the potential triad involved in disease development can be covered by the most straightforward diagnostic steps reported in veterinary practice; namely, i) comparison of recommended dietary P levels with those analyzed in the investigated case, during specific growth phases with expected high growth rates based on available farm-specific production and feed data, ii) bacteriological diagnostic of articular samples and iii) histologic examination of articular tissue. The new blood markers for diagnosis of mineral deficiency should be validated in the context of more practical cases, because they might be a promising new diagnostic tool for assessing skeletal health. Bone marker determination should be recommended especially in pigs with high daily weight gains showing stiff walking, tripping and reluctancy to stand up, in order to assess the relation between bone catabolism and bone mineralization. In addition, we recommend in cases of sudden increased lameness to look at recent findings without neglecting the period before the episodes, including the previous dietary supply with P, especially of individuals with higher daily weight gains.
Finally, the total P level in the diet should be analyzed. Even at optimal conditions, the digestibility of P is not higher than 70-80 %. A comparison can be made between the required level of digestible P and the needed total P in the diet, assuming a usual feed intake at a common energy density in the diet.
Finally, the question concerning the degree of impact of either P deficiency or osteochondropathy as well as claw lesions on the disease and especially on disease pathogenesis in M. hyosynoviae infection cannot be answered in this case. Thus, experimental studies under standardized conditions, also taking various genetic backgrounds into account, would be necessary.