Opportunistic infections: connect immune risk to the affected organ
Use immune status, symptoms and tissue findings to distinguish HIV and transplant opportunistic disease, select treatment and prescribe targeted prevention.
A CD4 count estimates vulnerability; it does not name the infection. A person with a count of 40 can have pneumococcal pneumonia, tuberculosis, several opportunistic infections together or a noninfectious illness. Start with the affected organ, then use immune status, exposure and preventive therapy to decide what evidence you need.
Use immune risk without turning it into a diagnosis
Loss of CD4 T-cell function weakens coordination of macrophage and other antimicrobial responses. Latent organisms can reactivate, normally controlled environmental organisms can disseminate, and some viruses produce end-organ disease. Yet the depth and duration of immune dysfunction matter alongside the current count. Effective antiretroviral therapy, viral suppression, prior infections and prophylaxis all change risk.
Read across the risk bands, then return to the patient’s syndrome
At any count
Consider bacterial infections and tuberculosis. Thrush and Kaposi sarcoma are not confined to one numerical interval.
Below about 200
PCP becomes especially important. Esophageal candidiasis is a marker of advanced disease. Exposure can add endemic fungal risk.
Below about 100
Toxoplasma reactivation in a seropositive person and cryptococcosis enter the foreground. Chronic protozoal diarrhea becomes more severe.
Below about 50
CMV retinitis and disseminated MAC become major considerations, particularly without effective ART.
These bands overlap. They describe epidemiology and selected prevention thresholds, not permission to exclude an organism above a boundary. [1][2][4][5]
Ask whether the immune defect is actually HIV-related. Neutropenia increases concern for invasive molds; corticosteroids can permit Strongyloides hyperinfection; transplantation combines drug effects, donor and recipient exposures, and surgical complications. A single HIV CD4 chart cannot substitute for those distinct host assessments.
In adults, a CD4 count below 200 cells per microliter or an AIDS-defining condition can establish stage 3 HIV disease. This classification does not imply that all immune function is absent. Herpes zoster can occur across CD4 counts, with more extensive disease and complications at lower counts. [17][18]
Separate a lung syndrome from disseminated infection
PCP commonly produces progressive exertional dyspnea, dry cough, fever and diffuse ground-glass lung opacities over days to weeks in people with advanced HIV. Hypoxemia may be substantial despite limited auscultatory findings. Increased lactate dehydrogenase is nonspecific. Pneumocystis jirovecii is a fungus, but its treatment differs from routine azole therapy. Organism demonstration or molecular testing on an induced sputum or bronchoalveolar specimen supports diagnosis; a PCR result still needs interpretation for colonization versus disease. Routine fungal culture is not the diagnostic method. [1]
TMP-SMX treats PCP, generally for 21 days in HIV. Adjunctive corticosteroids are indicated for moderate or severe oxygenation impairment, defined by room-air arterial PaO2 below 70 mm Hg or an alveolar-arterial gradient of at least 35 mm Hg. Give them early, ideally within 72 hours of treatment. A pulse oximetry number alone is not the guideline’s exact criterion. Begin ART within two weeks of PCP diagnosis when feasible. [1]
Tuberculosis can occur at any CD4 count. With advanced immunosuppression, upper-lobe cavitation may disappear from the presentation; diffuse infiltrates, adenopathy, dissemination or even a normal chest radiograph can occur. Obtain site-appropriate molecular testing and cultures when active TB is suspected. A negative IGRA or skin test cannot exclude active disease in an anergic host. Rifamycin interactions with ART influence treatment selection. [10]
Disseminated MAC often causes persistent fever, weight loss, anemia, diarrhea, lymphadenopathy and increased alkaline phosphatase in advanced HIV. Mycobacterial blood cultures can establish disseminated disease. Treatment uses at least a macrolide plus ethambutol, with additional drugs for selected severe or high-risk disease. Macrolide monotherapy risks resistance. Treatment usually continues at least 12 months and until sustained immune recovery criteria are met. [4]
Exposure matters for endemic fungi. Histoplasma can disseminate to marrow and reticuloendothelial organs, producing fever, cytopenias and hepatosplenomegaly; urine or serum antigen is useful in advanced HIV. Small intracellular yeast support the diagnosis, but antigen cross-reactions require interpretation. Severe disease generally receives liposomal amphotericin B followed by itraconazole. Coccidioides exposure in endemic arid regions can lead to pulmonary or disseminated disease, with tissue spherules rather than budding yeast. Neither diagnosis follows automatically from a count near 150. [11][12]
Locate the neurologic lesion before naming it
Three different anatomic problems
Meninges and CSF suggest cryptococcosis when headache evolves over weeks with raised pressure. Focal enhancing brain lesions raise toxoplasmosis, lymphoma and other infections. White matter dysfunction without much mass effect raises progressive multifocal leukoencephalopathy. Imaging is a comparison tool, not a pathology result.
Toxoplasma encephalitis often represents reactivation in a person with positive Toxoplasma IgG and a CD4 count below 100. Multiple enhancing lesions, frequently involving the basal ganglia, support an empiric diagnosis in a compatible setting. A single lesion does not prove lymphoma, and multiple lesions do not prove toxoplasmosis. Preferred treatments include pyrimethamine plus sulfadiazine plus leucovorin or treatment-dose TMP-SMX. Leucovorin limits pyrimethamine-associated marrow toxicity. TMP-SMX is not merely a preventive drug. [2]
Follow the neurologic examination and imaging response. Failure to improve within about 10 to 14 days, deterioration or an atypical presentation warrants reconsideration and often biopsy. CSF EBV DNA can raise suspicion for primary CNS lymphoma but does not independently establish histology. PML due to JC virus commonly causes progressively focal deficits with nonenhancing white matter lesions and little mass effect; inflammatory changes can appear during immune reconstitution. Immune restoration is central to its treatment. [2][16]
For suspected cryptococcal meningitis, obtain CSF cryptococcal antigen and measure opening pressure when lumbar puncture is safe. India ink is less sensitive. A minimally inflammatory CSF does not exclude infection in profound immunosuppression. Positive CSF antigen requires CNS treatment even if headache is absent. Common resource-rich induction uses liposomal amphotericin B and flucytosine, followed by fluconazole consolidation and maintenance. Therapeutic punctures may be needed for symptomatic raised pressure. [3]
ART timing is particularly consequential here. For cryptococcal meningitis, it is generally deferred four to six weeks after antifungals begin. That delay should not be copied onto PCP or disseminated MAC, where earlier immune restoration is recommended. The relevant question is how the organism and its location interact with the recovering immune response.
Let the involved tissue distinguish look-alikes
New floaters, blind spots or reduced vision in advanced HIV need urgent dilated retinal examination. CMV retinitis can cause necrotizing white retinal lesions with hemorrhage and can threaten vision with little pain. Oral valganciclovir or intravenous ganciclovir supplies systemic treatment. Immediately sight-threatening lesions may also require intravitreal ganciclovir or foscarnet. Local ocular treatment alone does not protect the other eye or treat extraocular CMV. Ganciclovir can suppress marrow; foscarnet can injure kidneys and disturb electrolytes. It is an important alternative for selected ganciclovir-resistant infection, guided by susceptibility and specialist assessment. [5]
Positive blood CMV PCR does not by itself establish retinitis, esophagitis or colitis. End-organ disease needs the corresponding examination or tissue evidence. CMV esophageal ulcers can be large and linear, while HSV often produces smaller ulcers; overlap makes biopsy important when the diagnosis is uncertain. For CMV end-organ disease, ART is generally started within one to two weeks after anti-CMV treatment, with individualized attention to neurologic disease and inflammation. [5]
Oropharyngeal candidiasis often forms white plaques that can be scraped away. Oral hairy leukoplakia is EBV-associated and typically has adherent corrugated plaques on the lateral tongue. Candida can produce yeast and filamentous forms, but this is not the environmental thermal dimorphism of Histoplasma. Odynophagia or dysphagia raises esophageal candidiasis, which needs systemic treatment such as fluconazole rather than topical mouth therapy alone. Esophageal candidiasis is AIDS-defining; oral thrush alone is not. [6]
Useful specimen distinctionTiny spores using dedicated stains or molecular methods
Treatment directionART and species-directed therapy
Cryptosporidium can also involve the biliary tract and cause cholangitis. TMP-SMX is not its standard treatment. Nitazoxanide may be considered with ART, but sustained immune restoration is the essential intervention in advanced HIV. Albendazole treats some microsporidial species, especially Encephalitozoon, but is unreliable for Enterocytozoon bieneusi. Do not prescribe a single drug merely because the stool report says parasite. [7][8][9]
Violaceous lesions with spindle cells and irregular vascular spaces suggest HHV-8-associated Kaposi sarcoma. Tissue establishes the diagnosis. Visceral disease can occur without obvious skin lesions, and management combines effective ART with oncology-directed therapy when indicated. A low count increases concern but is not a required diagnostic criterion. [13]
Prescribe prevention for the actual ART situation
Current NIH PCP guidance recommends primary prophylaxis for people not taking ART or starting ART with CD4 counts below 200. For people already on ART, the restart criteria distinguish a count below 100 regardless of viral load from a count of 100 to 200 with detectable HIV RNA. TMP-SMX is preferred. After immune recovery, prophylaxis can generally stop at a count of at least 200 sustained for three months; selected virologically suppressed patients between 100 and 200 may qualify after three to six months. A single improved count is insufficient. [1]
Toxoplasma primary prophylaxis is indicated for IgG-positive people with a count below 100. TMP-SMX can protect against both toxoplasmosis and PCP when used in the appropriate regimen. An asymptomatic IgG-positive patient does not need routine brain MRI solely for the antibody result. An IgG-negative patient needs exposure prevention and reassessment rather than treatment for presumed latent encephalitis. When TMP-SMX cannot be used, a combined dapsone, pyrimethamine and leucovorin regimen is one alternative that can cover PCP and toxoplasmosis; assess G6PD status before dapsone. Dapsone alone does not provide equivalent toxoplasma protection. [2]
MAC primary prophylaxis is not automatic below 50. It is indicated when effective ART is not being started promptly, viremia persists on ART or no suppressive regimen is available. Exclude active disseminated MAC before giving a macrolide alone. A symptomatic patient may need combination treatment rather than prophylaxis. [4]
In HIV, a tuberculin skin reaction of at least 5 mm is positive. Before latent TB treatment, assess symptoms, imaging and microbiology as indicated to exclude active disease. Short rifamycin-based preventive regimens are useful when compatible with ART; isoniazid remains an alternative in appropriate circumstances. A normal radiograph alone cannot erase a concerning active TB syndrome. [10]
After transplantation, interpret time through immunosuppression
In the first month, surgical sites, catheters, donor-derived infections, HSV reactivation and hospital exposures dominate much of the differential. During roughly months one through six, CMV, PCP and other opportunistic infections become more prominent when not prevented. Later, community infections predominate in stable recipients with lower net immunosuppression. Rejection treatment or cessation of prophylaxis can shift these patterns. CMV disease at three months is plausible; that date alone does not diagnose it. [14]
Posttransplant lymphoproliferative disorder may present with fever, lymphadenopathy, organ dysfunction or extranodal masses. Many cases are EBV-associated, but timing and a positive viral load do not establish the tissue diagnosis. Histology, disease extent and graft considerations guide treatment. Coordinated reduction of immunosuppression is often part of care; CD20-directed rituximab and other therapies may also be required. Avoid interpreting every posttransplant fever as either infection alone or a condition cured by simply stopping all transplant drugs. [15]
Practice syndrome-based decisions
Case 1
Show answer and explanations for case 1
A. Routine fungal culture of sputum alone (Why this does not fit)
Pneumocystis is not diagnosed through routine fungal culture.
B. Serum LDH alone (Why this does not fit)
LDH can increase but is nonspecific and cannot establish the organism.
C. Toxoplasma IgG alone (Why this does not fit)
That serology assesses exposure relevant to toxoplasmosis, not the cause of this diffuse pneumonia.
D. Pneumocystis testing on induced sputum or bronchoalveolar lavage (Best answer)
A respiratory specimen can demonstrate organisms or DNA in a compatible pulmonary syndrome.
Takeaway: Use organism-directed respiratory testing rather than a nonspecific marker.
