Opportunistic Infection

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2021-01-18

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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

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
Aging
Leukopenia (i.e. neutropenia and lymphocytopenia)
Burns

The lack of or the disruption of normal vaginal microbiota allows the proliferation of opportunistic microorganisms and will cause the opportunistic infection - bacterial vaginosis.

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.

Restoration of immune system

In patients with HIV, starting antiretroviral therapy is especially important for restoration of the immune system and reducing the incidence rate of opportunistic infections
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.

Avoidance of infectious exposure

The following may be avoided as a preventative measure to reduce risk of infection:

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)
Any oral exposure to feces.
Contact with farm animals, especially those with diarrhea: source of Toxoplasma gondii, Cryptosporidium parvum
Cat feces (e.g. cat litter): source of Toxoplasma gondii, Bartonella spp.
Soil/dust in areas where there is known histoplasmosis, coccidiomycosis
Reptiles, chicks, and ducklings that are a common source of Salmonella.
Unprotected sexual intercourse with individuals with known sexually transmitted infections.

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:

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.

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