An opportunistic infection is an infection caused by pathogens (bacteria, viruses, fungi, or protozoa) that take advantage of an opportunity not normally available, such as a host with a weakened immune system, an altered microbiota (such as a disrupted gut microbiota), or breached integumentary barriers. Many of these pathogens do not cause disease in a healthy host that has a normal immune system. However, a compromised immune system, which is seriously debilitated and has lowered resistance to infection, a penetrating injury, or a lack of competition from normal commensals presents an opportunity for the pathogen to infect.
Types of infections[]
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A partial listing of opportunistic organisms includes:
- Aspergillus sp.
- Candida albicans
- Clostridium difficile
- Coccidioides immitis
- Cryptococcus neoformans
- Cryptosporidium
- Cytomegalovirus
- Geomyces destructans (bats)
- Histoplasma capsulatum
- Isospora belli
- Polyomavirus JC polyomavirus, the virus that causes Progressive multifocal leukoencephalopathy.
- Kaposi's Sarcoma caused by Human herpesvirus 8 (HHV8), also called Kaposi's sarcoma-associated herpesvirus (KSHV)
- Legionnaires' Disease (Legionella pneumophila)
- Microsporidium
- Mycobacterium avium complex (MAC) (Nontuberculosis Mycobacterium)
- Mycobacterium tuberculosis
- Pneumocystis jirovecii, previously known as Pneumocystis carinii f. hominis
- Pseudomonas aeruginosa
- Salmonella
- Staphylococcus aureus
- Streptococcus pneumoniae
- Streptococcus pyogenes
- Toxoplasma gondii
Causes[]
Immunodeficiency or immunosuppression can be caused by:
- Malnutrition
- Fatigue
- Recurrent infections
- Immunosuppressing agents for organ transplant recipients
- Advanced HIV infection
- Chemotherapy for cancer
- Genetic predisposition
- Skin damage
- Antibiotic treatment leading to disruption of the physiological microbiome, thus allowing some microorganisms to outcompete others and become pathogenic (e.g. disruption of intestinal microbiota may lead to Clostridium difficile infection
- Medical procedures
- Pregnancy Template:Further
- Ageing
- Leukopenia (i.e. neutropenia and lymphocytopenia)
The lack of or the disruption of normal vaginal microbiota allows the proliferation of opportunistic microorganisms and will cause the opportunistic infection - bacterial vaginosis.[1][2][3][4]
Prevention[]
Since opportunistic infections can cause severe disease, much emphasis is placed on measures to prevent infection. Such a strategy usually includes restoration of the immune system as soon as possible, avoiding exposures to infectious agents, and using antimicrobial medications ("prophylactic medications") directed against specific infections.[5]
Restoration of immune system[]
- In patients with HIV, starting antiretroviral therapy is especially important for restoration of the immune system and reduces the incidence of opportunistic infections[6][7]
- In patients undergoing chemotherapy, completion of and recovery from treatment is the primary method for immune system restoration. In a select subset of high risk patients, granulocyte colony stimulating factors (G-CSF) can be used to aid immune system recovery.[8][9]
Infectious exposures to avoid[]
- Cat feces (e.g. cat litter): source of Toxoplasma gondii, Bartonella spp.
- Eating undercooked meat or eggs, unpasteurized dairy products or juices
- Potential sources of tuberculosis (high risk healthcare facilities, regions with high rates of tuberculosis, patients with known tuberculosis)
- Contact with farm animals, especially those with diarrhea: source of Toxoplasma gondii, Cryptosporidium parvum
- Soil/dust in areas where there is known histoplasmosis, coccidiomycosis
- Reptiles, chicks, ducklings: source of Salmonella spp.
- Unprotected sexual intercourse with individuals with known sexually transmitted infections.
- Any oral exposure to feces.[10]
Prophylactic medications[]
Individuals at higher risk are often prescribed prophylactic medication to prevent an infection from occurring. A patient's risk level for developing an opportunistic infection is approximated using the patient's CD4 T-cell count and sometimes other markers of susceptibility. Common prophylaxis treatments include the following:[11]
| Infection | When to Give Prophylaxis | Agent |
|---|---|---|
| Pneumocystis jirovecii | CD4 < 200 cells/mm3 or oropharyngeal candidasis (thrush) | TMP-SMX |
| Toxoplasma gondii | CD4 < 100 cells/mm3 and positive Toxoplasma gondii IgG immunoassay | TMP-SMX |
| Mycobacterium avium complex | CD4 < 50 | Azithromycin |
Treatment[]
Treatment depends on the type of opportunistic infection, but usually involves different antibiotics.
Veterinary treatment[]
Opportunistic infections caused by feline leukemia virus and feline immunodeficiency virus retroviral infections can be treated with lymphocyte T-cell immunomodulator. Template:Commons category Template:Clear