Opportunistic infection is defined as an infection that occurs more frequently or is more severe in individuals with poor immunologic status, e.g. HIV, immunosuppressant therapy, severe malnutrition, etc. These include:
- Infections due to ordinarily non-pathogenic bacteria or fungi
- Unusual clinical infections with common pathogens.
In Normal Host
Saprophytic microorganisms that form the indigenous flora of the host or are commonly associated with the neonatal period may turn opportunistic causing clinical infection in normal, healthy infants, and children.
Examples include:
- Bacteroides—abscesses, septicemia, peritonitis
- Bacillus subtilis—abscess, cellulitis, conjunctivitis, septicemia
- Diphtheroids—endocarditis, meningitis
- Lactobacillus—lung abscess.
In Susceptible Host
Derangement of the host defense as a result of an identifiable congenital, acquired, or environmental defect.
Host Compromised by Changes in the Skin or Mucous Membranes, or by Anatomic Defect
Here the barriers to infection are bypassed or compromised, producing conducive environment for opportunistic infections.
In various shunts [cerebrospinal fluid (CSF), renal dialysis], opportunistic organisms mostly isolated are
S. epidermidis, S. aureus, Bacillus species, and diphtheroids. Manifestations include fever, erythema around the tubing employed for shunt, and hypocomplementemic glomerulonephritis. Treatment, pending sensitivity report, should be with penicillin and chloramphenicol, or chloramphenicol and a cephalosporin, together with removal of the shunt in majority of the cases.
Intravenous catheterization, particularly for total parenteral nutrition, may cause local thrombophlebitis, bacteremia or fungemia by opportunistic organisms such as S. epidermidis, Bacteroids, Mimeae, Pseudomonas, Candida, and Cryptococcus. If clinical signs and positive cultures persist, suitable antibiotic therapy should be instituted.
Use of inhalation therapy equipment, especially in newborns, may lead to infection with opportunistic organisms such as Pseudomonas and Serratia. Urethral catheterization may predispose an individual to opportunistic infection of the urinary tract with Pseudomonas species, Serratia, Herellea, S. epidermidis, or Candida.
Burns may lead to opportunistic infection with Pseudomonas, Serratia, Staphylococcus, Candida, or Mucor. The possible causes include change in ecology of skin flora and physiochemical properties of skin, neutrophil dysfunction, abnormal responses to antigenic stimulation, impairment of delayed hypersensitivity, long-term administration of antibiotics, and prolonged intravenous or urethral catheterization.
Dermal sinus tracts that communicate with neural tissue or subarachnoid space may lead to meningitis with organisms such as S. epidermidis or diphtheroids. Cardiac defects (both congenital and acquired) predispose the damage tissue to act as nidus for opportunistic infection with Streptococcus viridans, Corynebacterium, Pseudomonas, or non-pathogenic Neisseria.
Surgery, especially cardiac surgery, predisposes to infection because of prophylactic use of antibiotics which alter the normal flora or nidus of infection provided by foreign bodies inserted. Staphylococcus epidermidis, Diphtheroids, Mimeae, Pseudomonas, Candida, and Aspergillus are the opportunistic organisms that may produce disease.
Host Compromised by Inherited/Acquired Defects Affecting Defense
B cell defects are frequently accompanied by recurrent infections, often due to opportunistic organisms such as bacterial pathogens and Pseudomonas. Treatment consists in giving gamma globulin 1.4 mL/kg [intramuscular (IM)], drainage of abscess if present, and antibiotic therapy depending upon the etiologic agent. Prevention consists in giving gamma globulin 0.7 mL/kg/month (IM), vigorous attention to postural drainage in chronic respiratory disease, and prophylactic use of penicillin or ampicillin in selected cases demonstrating recurrent middle ear or lung problem.
T cell defects also are often complicated by recurrent opportunistic infections with Mycobacterium, Listeria, Nocardia, cytomegalovirus, Varicella, Cryptococcus, Candida, Pneumocystis, and Strongyloids sterocalis. Treatment consists in giving a narrow-spectrum antimicrobial (depending on the responsible agent) application of topical and non-absorbable to gamma globulin, all the therapeutic and preventive measures required in T cell defects are indicated in these patients too.
In immunosuppression resulting from drug therapy, infection with aerobic Gram-negative organisms occurs more commonly and may cause significant morbidity and infection from Pseudomonas, Klebsiella, E. coli, Herellea, Serratia, herpes simplex, Varicella zoster, cytomegalovirus, Epstein-Barr virus (EBV), papovirus, hepatitis virus, Candida, Aspergillus, Mucor, and Cryptococcus.
Transplantation may per se predispose the host to infection and also through use of immunosuppressive therapy. Opportunistic organisms isolated usually include Staphylococcus, Pseudomonas, Klebsiella, Candida, Aspergillus, Nocardia, Pneumocystis, cytomegalovirus, hepatitis virus, herpes simplex , and varicella zoster.
antimicrobial agent and incision and drainage of abscess, if any. Prevention consists of prophylactic administration of cotrimoxazole for prevention of Pneumocystis carinii pneumonia, protective environments for certain patients, oral non-absorbable antimicrobial agents to lower concentration of GIT flora, and careful screening for tuberculosis. No live vaccine should be given to these patients.
Combined immunodeficiency syndromes are also vulnerable to opportunistic infections with organisms such as bacteria, fungi, viruses, and Pneumocystis. In addition Malignancy is often complicated by infection which may prove a terminal event. Opportunistic organisms in malignancy include Pseudomonas, Klebsiella, E. coli, Listeria, and Mycobacterium (Table 24.4). Probable mechanisms include granulocytopenia, reduced chemotaxis, reduced bacterial activity of neutrophils, lymphopenia, defective cellular response, and defective antigenic response to challenge.
Malnutrition renders the host vulnerable to opportunistic infection with organisms such as measles virus, herpes or varicella zoster virus, and Mycobacterium. The susceptibility is attributed to impaired T cell function, reduction in complement activity, impaired migration of phagocytes, and reduced bactericidal activity.
Collagen diseases are frequently complicated by infections with Candida, Mucor, Aspergillus, Pneumocystis, Diphtheroids, Listeria, Serratia, Staphylococcus, Nocardia, cytomegalovirus, herpes virus, Varicella zoster, etc. Host defense is reduced because of involvement of reticuloendothelial system and use of immunosuppressive agents.
