Refinement of a hematogenous localized osteomyelitis model in pigs

We have previously developed a model of localized osteomyelitis by injecting Staphylococcus aureus (S. aureus) unilaterally into the femoral artery of juvenile domestic pigs (Johansen et al., 2012; Nielsen et al., 2015). We used this model for the evaluation of bone-infection tracers applicable for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) (Nielsen et al., 2015). However, several of the 40 kg pigs were euthanized prior to PET and SPECT scanning due to lameness, shallow respiration, fever and anorexia; the last three clinical signs indicating dissemination of S. aureus to the lungs and other internal organs. We therefore decided to refine our model in order to improve the success rate. We speculated that younger pigs might respond differently to inoculations. A total of ten female domestic pigs were included in our study, three with a body weight of 40 kg (± 1 kg) and seven with a body weight of 20 kg (± 1 kg). The pigs were born in a SPF facility and were raised in the same herd. Pigs were scored daily for pain, including lameness. From the onset of the first clinical signs we treated seven of the pigs (one 40 kg and six 20 kg) with procaine benzylpenicillin to which the bacterial strain was susceptible. Further treatment (analgesia by buprenorphine) or euthanasia depended on disease severity and took place due to fever and shallow respiration in the 40 kg pig-group and due to lameness and pain in hind limbs in the 20 kg pig-group. After two weeks, the remaining pigs were euthanized; necropsy was performed on all ten pigs. One of the 40 kg and five of the 20 kg pigs were euthanized before day 14. The necropsy results showed osteomyelitis caused by S. aureus in six of the seven 20 kg pigs and additionally arthritis caused by S. aureus in three of the 20 kg pigs, whereas none of the 40 kg pigs had developed osteomyelitis or arthritis. Spreading of S. aureus to the lungs was seen in two of three 40 kg pigs but only in two of seven 20 kg pigs. As most of the 20 kg pigs were given penicillin, we cannot know whether the favorable results in the 20 kg pig-group were purely related to their body weight and younger age, or whether the pattern of infection also was modified by penicillin treatment. However, we are convinced that the health status and success rate can be improved by using 20 kg pigs treated with penicillin.