Opportunistic infection

Opportunistic infection
Classification and external resources
Specialty
Patient UK	Opportunistic infection
MeSH	D009894

An opportunistic infection is an infection caused by bacterial, viral, fungal, or protozoan pathogens that take advantage of a host with a weakened immune system or an altered microbiota (such as a disrupted gut flora). Many of these pathogens do not cause disease in a healthy host that has a normal immune system. A compromised immune system, however, presents an "opportunity" for the pathogen to infect.

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 flora may lead to Clostridium difficile infection
• Medical procedures
• Pregnancy
  Further information: Susceptibility and severity of infections in pregnancy
• Ageing
• Leukopenia (i.e. neutropenia and lymphocytopenia)

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

Types of infections

Further information: Immunodeficiency

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

Prophylaxis (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 reduces the incidence of all 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.

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 sex practice that might result in oral exposure to feces.

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 Immune Modulator.