An opportunistic infection is one that is caused by a bacterium, fungus, or virus that would normally be kept in check by the immune system. The advent of AIDS (acquired immunodeficiency syndrome) focused attention upon the problem of opportunistic infections. As the name suggests, AIDS results in weakened immunity, giving normally harmless microbes the opportunity to invade the body and cause infection.
Besides AIDS, other medical conditions associated with opportunistic infections include cancer, severe burns, malnutrition, and diabetes. Medical treatments such as cancer chemotherapy and the long-term immunosuppressant drug therapy needed after organ transplantation also undermine immunity. Among the most common of the opportunistic pathogens (disease-causing organisms) are Pneumocystis carinii (recently renamed P. jiroveci), Candida albicans, and cytomegalovirus. None of these pathogens would normally cause disease in a healthy person. Treatment of an opportunistic infection is therefore two-fold. The infection is treated with anti-biotics or other drug therapy, and then the underlying immune problem should also be addressed.
WORDS TO KNOW
COLONIZATION: Colonization is the process of occupation and increase in number of microorganisms at a specific site.
GRAM-NEGATIVE BACTERIA: All types of bacteria identified and classified as a group that does not retain crystal-violet dye during Gram's method of staining.
GRAM-POSITIVE BACTERIA: All types of bacteria identified and classified as a group that retains crystal-violet dye during Gram's method of staining.
HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.
IMMUNOCOMPROMISED: A reduction of the ability of the immune system to recognize and respond to the presence of foreign material.
MORTALITY: Mortality is the condition of being susceptible to death. The term “mortality” comes from the Latin word mors, which means “death.” Mortality can also refer to the rate of deaths caused by an illness or injury, i.e., “Rabies has a high mortality.”
PROPHYLAXIS: Treatment to prevent the onset or recurrence of disease.
The fungus Pneumocystis carinii infects nearly everyone at some point during their lives, but is normally completely harmless. The reporting of cases of P. carinii-associated pneumonia among previously healthy young men in the early 1980s was one of the early warning signs of the emergence of AIDS, as P. carinii pneumonia is normally a very rare occurrence.
The symptoms of an opportunistic infection depend upon the nature of the associated organism. Nearly all microbes—bacteria, fungi, viruses, and protozoa—can become pathogenic, given the right opportunity. However, certain organisms have a strong association with specific types of impaired immunity.
Gram-positive bacteria, such as Staphylococcus aureus, tend to invade the skin and implanted devices such as catheters. Gram-negative bacteria, including Pseudomonas aeruginosa are associated with a form of immune deficiency known as granulocytopenia. This refers to depletion of a type of white blood cell called a granulocyte, a condition that is found in leukemia and during chemotherapy for cancer. The terms Gram-positive and Gram-negative refer to a classification of bacteria according to how they react with Gram's stain, which is used in preparation of samples for microscopy.
Tuberculosis infection from Mycobacterium tuberculosis may be reactivated among those whose immune systems are impaired because of age or AIDS. The fungus C. albicans can infect blood or solid organs in cases of granulocytopenia. Finally, Toxoplasma gondii is another protozoan that commonly causes opportunistic infection among AIDS patients.
Transmission of an opportunistic infection depends upon the organism involved. Many of these organisms will normally be present on the skin or in the body in amounts less than necessary to cause infection. This is known as colonization. It is only because the defenses of the immune system are breached that they can take hold and cause infection.
Many different groups are at especial risk of opportunistic infections. The common factor is an abnormality or defect in the immune system or any related host defense system, such as the skin, which acts as a natural barrier.
Rarely, an immune deficiency can be present from birth. More commonly, immune deficiency is acquired, as in AIDS, where the human immunodeficiency virus attacks and destroys the immune system. Certain underlying diseases, including cancer, diabetes, cystic fibrosis, sickle cell anemia, and severe burns undermine immunity, making a person prone to opportunistic infection.
Various drug treatments impair immunity, such as steroids, immunosuppressants, cancer chemotherapy, and prolonged antibiotic therapy. Medical devices, including indwelling catheters and prosthetic heart valves also attract opportunistic infection. Finally, the very young and the very old tend to have weaker immunity, which puts them at higher risk of opportunistic infection.
Diagnosis of an opportunistic infection can be difficult, as most of the causative agents are normally benign (not harmful). Once the microbe has been identified, the treatment will generally consist of the appropriate antibiotic or antifungal drug, if the opportunistic infection is caused by a bacterium or fungus. Cytomegalovirus infection is generally treated with antiviral drugs like ganciclovir.
Prevention of opportunistic infections depends upon the organism involved and the medical condition of the patient. Sometimes, antibiotic and antiviral prophylaxis (preventative treatment) may be used. Scrupulous personal hygiene around patients with compromised immunity is always essential; for instance, hospital visitors and medical staff should wash their hands thoroughly and regularly before and after touching patients.
Opportunistic infections can be a challenge because they usually involve organisms that are normally not harmful. The incidence of opportunistic infections is likely to increase as the population ages, persons with HIV/AIDS live longer, more organ transplants are performed, and other populations of immunocompromised persons (those with weakened immune systems) increase.
In developing countries, malnutrition leaves millions of children with weakened immune systems and in turn, more vulnerable to infections. Acute infections such as diarrheal diseases, respiratory infections, measles, and malaria account for more than half of childhood mortality (deaths) in developing countries, and malnutrition is associated with over fifty percent of these deaths, according to the World Health Organization (WHO). When malnutrition is present, some health authorities broaden the definition of the term opportunistic infection to include a synergistic relationship between malnutrition and communicable disease: malnutrition weakens natural immunity, leading to increased susceptibility to infection, and more frequent, severe, and prolonged episodes of infection. The cycle is perpetuated when infection aggravates malnutrition by decreasing intake and increasing the body's metabolic needs. For example during a 2005 outbreak of measles in Somali refugees in Kenya, measles was considered an opportunistic infection due to malnutrition brought on by Somali crop failures after drought and conflict.
Tan, James S. Expert Guide to Infectious Diseases. Philadelphia: American College of Physicians, 2002.
Rice, Amy L., et al. “Malnutrition as an Underlying Cause of Childhood Deaths Associated with Infectious Diseases in Developing Countries.” Bulletin of the World Health Organization 78 (2000): 1207-1218.
Centers for Disease Control. “Guidelines for Preventing Opportunistic Infections Among HIV-Infected Persons.” June 14, 2002. <http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5108a1.htm> (accessed May 28, 2007).