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Introduction
Respiratory diseases are one of the major problems of porcine health worldwide. As the primary agent of enzootic pneumonia and one of the major pathogens of the porcine respiratory disease complex Mycoplasma hyopneumoniae is responsible for significant economic losses in the pig industry. 1 The latter are mainly due to increased treatment costs and reduced daily weight gain. 2 An increase in pneumonic lung lesions is often accompanied by a reduced average daily weight gain (ADWG) and mean carcass weight. 3 4 Even though vaccination against M hyopneumoniae may not be sufficient to eliminate the agent from infected herds, 5 it is an important measure to reduce the incidence of clinical disease, medication costs due to respiratory signs 2 and the extent of lung lesions caused by M hyopneumoniae under field conditions. 2 6 Field studies showed that a one-shot vaccination against M hyopneumoniae improved performance parameters such as ADWG and feed conversion ratio. 2 7 Apart from an implementation of a reliable vaccination scheme, the improvement of housing conditions and management practices are also important tools to control the disease. 1 The relevance of these control measures has greatly increased with the growing public interest in reducing antibiotic use in livestock.
Although needle stick and sharps injuries are a major occupational health hazard in veterinary practice, 8 most vaccines are still administered by injection with needle-syringe devices. Broken needles and needle fragments in pork carcasses also present a risk factor for physical hazard in the pig industry. 9 These risks could be eliminated by needle-free injection devices (NFID). Furthermore, needles can serve as mechanical vectors for the spread of haematogenous transferable pathogens like porcine reproductive and respiratory syndrome virus (PRRSV) 10 11 and Mycoplasma suis. 12 Although in general NFID diminish the probability of transmission of pathogens, 10 13 it was shown under experimental conditions that the repeated use of the same NFID did not entirely eliminate the risk of NFIDs acting as a vector for PRRSV. Previous studies evaluated the development of a protective immunity after an intradermal vaccination against bacterial and viral pathogens. 14 15 In contrast to an intramuscular route of administration, epidermal Langerhans cells and the dermal dendritic cells as antigen-presenting cells are primary immune cells activated following...