Anopportunistic infection is an infection that occurs most commonly in individuals with animmunodeficiency disorder and acts more severe on those with a weakened immune system. These types of infections are considered serious and can be caused by a variety of pathogens including viruses, bacteria, fungi, and parasites.[1] Under normal conditions, such as in humans with uncompromised immune systems, an opportunistic infection would be less likely to cause significant harm and would typically result in a mild infection or no effect at all. These opportunistic infections can stem from a variety of sources, such as aweakened immune system (caused byhuman immunodeficiency virus andacquired immunodeficiency syndrome), when being treated withimmunosuppressive drugs (as incancer treatment),[2] when amicrobiome is altered (such as a disruption ingut microbiota), or whenintegumentary barriers are breached (as inpenetrating trauma). Opportunistic infections can contribute toantimicrobial resistance in an individual making these infections more severe. Some pathogens that cause these infections possess intrinsic resistance (natural resistance) to many antibiotics while others acquire resistance over time through mutations orhorizontal gene transfer.[3] Many of these pathogens, such as the bacteriumClostridioides difficile (C. diff), can be present in hosts with uncompromised immune systems without generating any symptoms, and can, in some cases, act ascommensals until the balance of the immune system is disrupted.[4][5][6][7] WithC. diff and many other pathogens, the overuse or misuse of antibiotics can cause the disruption of normal microbiota and lead to an opportunistic infection caused by antibiotic resistant pathogens.[8] In some cases, opportunistic infections can be labeled as ahospital-acquired infection due to individuals contracting them within a healthcare/hospital setting.[9] In terms of history, there is not one individual that can be attributed for discovering opportunistic infections. Over time and through medical advancement, there have been many scientists that have contributed to the study and treatment options for patients affected by these infections.[10][11]
Opportunistic infections can be caused by a wide variety of different types of pathogens. These infections can be caused by viral, bacterial, fungal, as well as parasitic pathogens.[12]
A partial list of opportunistic pathogens and their associated effects are as follows:
Clostridioides difficile (formerly known asClostridium difficile) is a bacteria that is known to cause gastrointestinal infection and diarrhea. It is typically associated with being the most commonhospital acquired infection.[14][15]
Haemophilus influenzae is a bacterium implicated in causing illnesses such as meningitis, epiglottitis which can obstruct airways, pneumonia, otitis media affecting the ear, sinusitis involving the sinuses, and potentially leading to complications like mastoiditis, parameningeal abscess, and pericarditis.[18]
Klebsiella pneumoniae, a member of a group including pathogens, can be responsible for various infections in hospitalized individuals as it is frequently isolated from hospital admissions and the broader group (Enterobacteriaceae) is known to cause diverse infections.[19]
Salmonella is agenus of bacteria that is known to cause gastrointestinal infections causing an inflammatory response accompanied with fever and diarrhea.[27][28]
Stenotrophomonas maltophilia has emerged as a challenging nosocomial agent frequently associated with respiratory tract infections like pneumonia and exacerbations in individuals with COPD, and can also cause bacteremia, particularly linked to central lines in vulnerable patients.[31]
Aspergillus is a fungus, commonly associated with respiratory infection.[36][37]
Candida albicans is a species of fungus that is a part of the normal human microbiome. It acts as a commensal unless there is a change in concentrations. It can be associated with various conditions such asoral thrush and gastrointestinal infection.[38][39][40]
Cryptococcus gattii: This emerging fungal pathogen can cause severe and often fatal infections, manifesting as pulmonary disease and meningitis in both immunocompromised and immunocompetent individuals.[42]
Hyaline (non-pigmented) molds (e.g.,Acremonium,Paecilomyces,Scopulariopsis species): This group encompasses various molds, includingFusarium spp. causing hyalohyphomycosis, mycotic keratitis, and onychomycosis, as well as commonly causing pneumonia, sinusitis, and cutaneous lesions that may disseminate in neutropenic patients;Scedosporium spp. associated with sinusitis and pneumonia;Lomentospora prolificans causing a varied range of infections and disseminated disease;Scopulariopsis spp. andAcremonium spp. linked to sinopulmonary diseases;Paecilomyces variotii causing sinopulmonary disease; andPenicillium species implicated in necrotizing esophagitis and disseminated infections.[47]
Histoplasma capsulatum is a species of fungus known to causehistoplasmosis, which can present itself with an array of symptoms, but often involves respiratory infection.[48][49]
Rhodotorula species: These yeasts can causefungemia, often linked to central venous catheter use, as well as localized infections includingmeningitis, skin infections, ocular infections, peritonitis, and prosthetic joint infections.
Adenovirus is known to cause various illnesses, including lower respiratory tract infections, pneumonia, acute respiratory diseases, epidemickeratoconjunctivitis affecting the eyes, acute hemorrhagic cystitis of the bladder, andgastroenteritis affecting the digestive system.[60]
Cytomegalovirus is considered a member of the human herpesvirus family and is most frequently associated with respiratory infection.[61][62][63]
Hepatitis B Virus (HBV) can lead to acute liver infection manifesting as anicteric hepatitis, icteric hepatitis, or fulminant hepatitis, and chronic infection can progress to an asymptomatic carrier state, chronic hepatitis, cirrhosis of the liver, andhepatocellular carcinoma. Severe liver damage can result in complications likejaundice,hepatic encephalopathy,ascites, gastrointestinal bleeding, andcoagulopathy.[64]
Influenza Virus is responsible for seasonal flu epidemics and can be categorized into types affecting various hosts, such as avian (bird) flu, canine (dog) flu, swine (pig)/variant flu, and can cause pandemic flu, all generally resulting in respiratory illnesses.[65]
Human Metapneumovirus (HMPV) commonly causes upper and lower respiratory tract infections, with symptoms such as cough, mucous production, fever, and dyspnea, and can lead to more severe conditions like pneumonia andbronchiolitis.[66]
Human Papillomavirus (HPV) is the initiating force behind multiple conditions, including cutaneous and anogenitalwarts, which in some cases can progress to various carcinomas.[67]
Human T-cell leukemia virus type 1 (HTLV-1) causes a chronic lifelong infection that can lead to general immunosuppression,uveitis affecting the eyes, dermatitis of the skin, pneumonitis in the lungs, adult T-cell leukemia, and HTLV-1 associated myelopathy also known as tropical spastic paraparesis.[73]
Parainfluenza Virus commonly causes upper and lower respiratory illnesses with symptoms similar to the common cold, such as fever, runny nose, cough, sneezing, and sore throat, and can also cause more serious illnesses in children includingcroup, bronchitis, and bronchiolitis.[74]
SARS-CoV-2 is the virus that causesCoronavirus Disease 2019 (COVID-19), with symptoms that can include fever, cough, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, and diarrhea.[75]
Human Immunodeficiency Virus is a virus that targets theCD4 cells (a type of white blood cell) within the body's immune system. CD4 counts within a non-affected immune system would range anywhere from 500-1500 cells per cubic millimeter of blood, while an affected immune system would show cell counts below 200.[77] HIV infection can lead to progressively worsening immunodeficiency, a condition ideal for the development of opportunistic infection.[78][79] As HIV worsens over time, the term AIDS, or acquired immunodeficiency syndrome has been used to describe the condition and extensive damage to the immune system as well as the onset and susceptibility to other illnesses. The onset of AIDS leads to respiratory and central nervous system opportunistic infections, including but not limited topneumonia,tuberculosis andmeningitis.[80][81][82]Kaposi's sarcoma, a virally associated cancer, andnon-Hodgkin's lymphoma are two types of cancers that are generally defined as AIDS malignancies.[83] As immune function declines and HIV-infection progresses to AIDS, individuals are at an increased risk of opportunistic infections that their immune systems are no longer capable of responding properly to. Because of this, opportunistic infections are a leading cause of HIV/AIDS-related deaths.[84]
Immunodeficiency is characterized by the absence of or the disruption in components of the immune system such as white blood cells (e.g.lymphocytes,phagocytes, etc.). These disruptions cause a decrease in immune function and result in an overall reduction of immunity against pathogens.[2]
They can be caused by a variety of factors, including:
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.[105]
In patients with HIV, startingantiretroviral therapy is recommended for restoration of the immune system and reducing the incidence rate of opportunistic infections[106][107]
In patients undergoing chemotherapy, completion of, and recovery from treatment is the primary method for immune system restoration and to prevent infection occurrence. In a select subset of high-risk patients,granulocyte colony stimulating factors (G-CSF) can be used to aid immune system recovery and infection prevention.[108][109]
Ensure poultry is cooked to 165 °F (74 °C). Beef and pork cuts should reach an internal temperature of 145 °F (63 °C). Ground meat should be cooked to 160 °F (71 °C). Ensure juice, and dairy products are labeled as pasteurized. Eggs should be cooked until both yolks and whites are firm.[110]
Ensure proper steps such as regular handwashing, and use ofPPE are followed for the care of farm animals who are sick, specifically those experiencing diarrhea.[112][113]
Avoid soil/dust in areas where there is knownHistoplasma capsulatum present. If it cannot be avoided in an environment, ensure PPE is being used.[115]
Ensure reptiles and amphibians, their live food, and their waste are being handled properly and wash hands regularly after handling to prevent transmission of pathogens such asSalmonella.[116]
Individuals at higher risk for opportunistic infections are often prescribed prophylactic medication to prevent an infection from occurring. A person's risk level for developing an opportunistic infection is approximated using the person'sCD4 T-cell count and other indicators such as current medical treatments, age, and lifestyle choices. The table below provides information regarding the treatment management of common opportunistic infections.[118][119][120][121]
These current agents' doses/frequency will discontinue after two months. Depending on clinical presentation, maintenance agents will continue for at least four more months.
This current agent doses/frequency will discontinue after 21 days. Secondary prophylactic agent dose/frequency will continue until the CD4 count is above 200 cells/mm3 and the HIV viral load is undetectable for at least three months while takingantiretroviral therapy.
CD4 count is less than 100 cells/mm3 or less than 14%, and the person has a positive serology for Toxoplasma gondii.
Trimethoprim-sulfamethoxazole
This agent will discontinue after six weeks. Secondary prophylactic medications will continue until the CD4 count is above 200 cells/mm3 and HIV viral load is undetectable for at least six months while taking antiretroviral therapy.
Rifabutin may be added depending on clinical presentation.
These agent(s) will discontinue after 12 months only if the person does not have any symptoms that will be concerning for persistent Mycobacterium avium complex disease and their CD4 count is above 100 cells/mm3, and while their HIV viral load is undetectable for at least six months while taking antiretroviral therapy.
Alternative agents can be used instead of the preferred agents. These alternative agents may be used due to an individual's allergies, availability, or clinical presentation. The alternative agents are listed in the table below.[118][122][120]
Due to the prevention techniques used with HIV patients, such as prophylactic medications, opportunistic infections in HIV patients have decreased in number over the past few decades. In some circumstances, where individuals are not aware they have HIV and they develop an opportunistic infection, they may be prescribed,antivirals,antibiotics, orantifungals. After the infection has cleared, and to prevent it from coming back, they may be recommended to stay on that medication as well as it being coupled with another medication to ensure drug efficiency.[123]
^Lamas A, Miranda JM, Regal P, Vázquez B, Franco CM, Cepeda A (January 2018). "A comprehensive review of non-enterica subspecies of Salmonella enterica".Microbiological Research.206:60–73.doi:10.1016/j.micres.2017.09.010.PMID29146261.
^Mittal J, Ponce MG, Gendlina I, Nosanchuk JD (2018). "Histoplasma Capsulatum: Mechanisms for Pathogenesis". In Rodrigues ML (ed.).Fungal Physiology and Immunopathogenesis. Current Topics in Microbiology and Immunology. Vol. 422. Cham: Springer International Publishing. pp. 157–191.doi:10.1007/82_2018_114.ISBN978-3-030-30236-8.PMC7212190.PMID30043340.
^El Husseini, Nour; Carter, Jared A.; Lee, Vincent T. (2024-10-21). "Urinary tract infections and catheter-associated urinary tract infections caused by Pseudomonas aeruginosa".Microbiology and Molecular Biology Reviews.88 (4): e00066–22.doi:10.1128/mmbr.00066-22.PMC 11653733.PMID39431861.
^Kline, K., Bowdish, D., Hartland, E., & Richardson, A. (2016). Infection in an aging population.Current Opinion in Microbiology,29, 63–67. https://doi.org/10.1016/j.mib.2015.11.003
^Ledergerber B, Egger M, Erard V, Weber R, Hirschel B, Furrer H, et al. (December 1999). "AIDS-related opportunistic illnesses occurring after initiation of potent antiretroviral therapy: the Swiss HIV Cohort Study".JAMA.282 (23):2220–6.doi:10.1001/jama.282.23.2220.PMID10605973.
^Brooks JT, Kaplan JE, Holmes KK, Benson C, Pau A, Masur H (March 2009). "HIV-associated opportunistic infections--going, going, but not gone: the continued need for prevention and treatment guidelines".Clinical Infectious Diseases.48 (5):609–11.doi:10.1086/596756.PMID19191648.S2CID39742988.
^Freifeld AG, Bow EJ, Sepkowitz KA, Boeckh MJ, Ito JI, Mullen CA, et al. (February 2011). "Clinical practice guideline for the use of antimicrobial agents in neutropenic patients with cancer: 2010 update by the infectious diseases society of america".Clinical Infectious Diseases.52 (4): e56-93.doi:10.1093/cid/cir073.PMID21258094.
^abDyer, Mary; Kerr, Christine; McGowan, Joseph P.; Fine, Steven M.; Merrick, Samuel T.; Stevens, Lyn C.; Hoffmann, Christopher J.; Gonzalez, Charles J. (2021).Comprehensive Primary Care for Adults With HIV. New York State Department of Health AIDS Institute Clinical Guidelines. Baltimore (MD): Johns Hopkins University.PMID33625815.
