Overwhelming post-splenectomy infection

An overwhelming post-splenectomy infection (OPSI) is a rare but rapidly fatal infection occurring in individuals following removal of the spleen. The infections are typically characterized by either meningitis or sepsis, and are caused by encapsulated organisms including Streptococcus pneumoniae.^[1]

The risk of OPSI is 0.23–0.42 percent per year, with a lifetime risk of 5 percent.^[2] Most infections occur in the first few years following splenectomy, but the risk of OPSI is lifelong.^[1][3] OPSI is nearly uniformly fatal without treatment, and modern treatment has decreased the mortality to approximately 40–70 percent.^[1][4] Individuals with OPSI are most commonly treated with antibiotics and supportive care.^[2] Measures to prevent OPSI include vaccination and prophylactic antibiotics.^[2][5][6]

Mechanism

The spleen contains many macrophages (part of the reticuloendothelial system), which are immune cells that phagocytose (eat) and destroy bacteria. In particular, these macrophages are activated when bacteria are bound by IgG antibodies (IgG1 or IgG3) or the complement component C3b. These types of antibodies and complements are immune substances called opsonizers, molecules that bind to the surface of bacteria to facilitate phagocytosis.

When the spleen is no longer present (asplenia), IgG and C3b are still bound to bacteria, but they cannot be removed from the blood circulation due to the loss of the splenic macrophages. Hence the bacteria are free to cause infection.

Patients without spleens often need immunizations against pathogens that normally require opsonization and phagocytosis by macrophages in the spleen. These include common human pathogens with bacterial capsules (Streptococcus pneumoniae, Salmonella typhi, Neisseria meningitidis, E. coli, Hemophilus influenzae, Streptococcus agalactiae, Klebsiella pneumoniae). Capsules made of polysaccharides (sugars) are an evolutionary development by bacteria to evade phagocytosis by macrophages alone, since only proteins are directly recognized by macrophages in phagocytosis. So humoral immunity in forms of IgG and complement proteins is the human immune system's response against bacterial capsules.

References

1. ^ ^{a b c} Waghorn DJ (March 2001). "Overwhelming infection in asplenic patients: current best practice preventive measures are not being followed". Journal of Clinical Pathology 54 (3): 214–8. doi:10.1136/jcp.54.3.214. PMC 1731383. PMID 11253134. http://jcp.bmj.com/cgi/pmidlookup?view=long&pmid=11253134. 
2. ^ ^{a b c} Davidson RN, Wall RA (December 2001). "Prevention and management of infections in patients without a spleen". Clinical Microbiology and Infection 7 (12): 657–60. doi:10.1046/j.1198-743x.2001.00355.x. PMID 11843905. http://www3.interscience.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=1198-743X&date=2001&volume=7&issue=12&spage=657. 
3. ^ Cullingford GL, Watkins DN, Watts AD, Mallon DF (June 1991). "Severe late postsplenectomy infection". The British Journal of Surgery 78 (6): 716–21. doi:10.1002/bjs.1800780626. PMID 2070242. 
4. ^ Schwartz PE, Sterioff S, Mucha P, Melton LJ, Offord KP (November 1982). "Postsplenectomy sepsis and mortality in adults". Journal of the American Medical Association 248 (18): 2279–83. doi:10.1001/jama.248.18.2279. PMID 7131680. 
5. ^ "Guidelines for the prevention and treatment of infection in patients with an absent or dysfunctional spleen. Working Party of the British Committee for Standards in Haematology Clinical Haematology Task Force". BMJ 312 (7028): 430–4. 1996. PMC 2350106. PMID 8601117. http://bmj.bmjjournals.com/cgi/content/full/312/7028/430. 
6. ^ J M Davies et al. (2001-06-02). "The Prevention And Treatment Of Infection In Patients With An Absent Or Dysfunctional Spleen - British Committee for Standards in Haematology Guideline up-date". BMJ. http://bmj.bmjjournals.com/cgi/eletters/312/7028/430#12088.