Pneumococcal osteomyelitis of the rib in a vaccinated infant: An exceptional case

Osteomyelitis mainly affects the long bones of the lower limbs. Rib osteomyelitis is rare (<1%). In the ribs Staphylococcus aureus is the cause of the majority of cases, followed by Mycobacterium tuberculosis.¹ There are three cases in the literature caused by microorganisms of the genus Streptococcus, and only one by Streptococcus pneumoniae.²

This was a six-month-old infant who came to the emergency department with a swelling in his left rib cage, with no history of trauma, with a low-grade fever of 37.5 °C 48 h previously, although apyrexial at the time of the hospital visit. No relevant family or personal history. No allergies. Vaccination schedule up to date, including three doses of 13 V pneumococcal vaccine (first dose batch G64205, second G75546), the third dose of pneumococcal vaccine had been administered ten days before (batch G92049). No previous infectious symptoms except for an isolated fever peak of 38.6 °C without other symptoms 24 h after the pneumococcal vaccine. The swelling was located in the 6th left costal arch, and was hard and painful on palpation.

Blood showed only leucocytosis of 20,600 mm^–3 (40% neutrophils) and CRP 15 mg/l. Admitted for further tests.

Chest X-ray showed a bone lesion with insufflation in the anterior arch of the left 6th rib; rib ultrasound showed insufflation of the 6th rib with interruption of the cortex and hypoechoic content; and bone series had similar findings without additional abnormalities. With suspicion centring on a rib tumour or osteomyelitis, antibiotic treatment was started with cloxacillin 150 mg/kg/day and cefotaxime 200 mg/kg/day.

CT of chest with contrast was performed due to its better visualisation of bone, and its availability, despite the superiority of magnetic resonance imaging in this disease, and an expansive lytic lesion was identified in the anterior 1/3 of the left 6th rib, with destruction of the cortex, compatible with abscessed osteomyelitis. A cancerous tumour was ruled out by Tc-99 scintigraphy. A fine-needle biopsy was taken of the swelling, isolating sensitive Streptococcus pneumoniae. Intravenous cefotaxime was continued for eight days, with amoxicillin 100 mg/kg/day at discharge for four more weeks, leading to complete resolution.

Pneumococcal serotyping was performed, finding serotype 1 (vaccine). An immunological study was therefore carried out: immunoglobulins, C3, C4, CH50, AP50, lymphocyte populations and a study of the response to mitogen, burst test, IL-6 response after stimulation as a check of normality of the Toll-like receptor pathway; all within normal limits. Peripheral blood smear without Howell-Jolly bodies. Spleen visualised on ultrasound at admission. At two years of age, response to a polysaccharide vaccine was studied, using purified pneumococcus from Merck®, with the titres of specific IgG against pneumococcus increasing from 8.91 mg/dl pre-vaccination to >27 mg/dl post-vaccination, and specific IgG2 from 0.95 mg/dl to >2.4 mg/dl. The child is currently asymptomatic.

The diagnosis was made from both the symptoms and the radiological images, with culture of the purulent discharge being fundamental, and ruling out other similar diseases such as fibrous dysplasia and histiocytosis. The finding of S. pneumoniae type 1 in a child previously immunised with three doses of 13-valent vaccine makes this case even more important, as the pathogenesis is not at all clear.

It was strongly suspected that it could be an immunodeficiency (slight elevation of acute phase reactants, low-grade fever), but no abnormalities were found in the tests performed.

The low pneumococcal antibody titres at two years of age was an interesting finding, perhaps being due to the lack of “immunological memory”, considered a variant of normality if the subsequent production of antibodies increases after stimulation; also considering that at two years of age titres are lower than at later ages.^3,4 A three-fold increase in titres is adequate, even more so considering the low pre-vaccination levels in which a greater post-vaccination response is described.⁵

Vaccination failure could be an option, despite being uncommon, and even rarer with serotype 1. With the immunisation received, three doses of 13-valent vaccine, IgG titres are obtained against serotype 1 in 96.1%.^6,7 Vaccination failure for the 13-valent vaccine occurs in 0.66 cases/100,000 vaccinations/year.^8,9 The most common serotypes that appear after vaccination failure are 19F, 19A, 6B and 3.^8,10

Another hypothetical but plausible option could be bacteraemia prior to the third dose of conjugate vaccine, with haematogenous spread and subsequent rib osteomyelitis.