Opportunistic Infections in HIV
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Direct answer
Opportunistic infections in HIV are infections that appear as CD4 counts fall and cell-mediated immunity fails, and their pattern follows CD4 thresholds. Pneumocystis jirovecii pneumonia appears below about 200 cells per microlitre, while Cryptococcus neoformans and Toxoplasma gondii appear below about 100. Mycobacterium avium complex and cytomegalovirus retinitis appear at very low counts, and the CD4 count therefore guides both diagnosis and prophylaxis.
What you must remember
- CD4 thresholds: Toxoplasmosis and cryptococcal meningitis appear below about 100 cells per microlitre, Pneumocystis below 200, and MAC and CMV retinitis below about 50.
- Pneumocystis: Bilateral interstitial shadowing with raised LDH, diagnosed by silver stain or immunofluorescence on bronchoalveolar lavage, and treated and prevented with cotrimoxazole.
- Toxoplasma encephalitis: Multiple ring-enhancing lesions, classically in the basal ganglia, treated with pyrimethamine and sulfadiazine.
- Cryptococcal meningitis: India ink staining of CSF shows encapsulated yeasts, and serum cryptococcal antigen is a sensitive screen in advanced disease.
- Tuberculosis: The commonest opportunistic infection of HIV worldwide, treated under the national programme with isoniazid preventive therapy for latent infection.
- Mycobacterium avium complex: Disseminated disease with fever, weight loss, anaemia and diarrhoea, with prophylaxis historically given below a CD4 count of 50.
- Penicillium marneffei: Causes disseminated talaromycosis with skin papules in Southeast Asia and northeast India, treated with amphotericin B followed by itraconazole.
- Kaposi sarcoma: Linked to HHV-8, presenting with violaceous skin or mucosal lesions that often regress with antiretroviral therapy.
Mapping infections to CD4 thresholds
The CD4 count is the single most useful number in HIV opportunistic disease. Clinicians map each infection to a threshold, and each threshold guides both diagnosis and prophylaxis.
| CD4 count per microlitre | Typical infections | Preventive step |
|---|---|---|
| Below 200 | Pneumocystis pneumonia, oral candidiasis, tuberculosis | Cotrimoxazole prophylaxis |
| Below 100 | Toxoplasma encephalitis, cryptococcal meningitis | Cotrimoxazole if Toxoplasma IgG is positive, cryptococcal antigen screening |
| Below 50 | Mycobacterium avium complex, CMV retinitis | MAC prophylaxis where used, retinal screening |
| Any level | Tuberculosis | Isoniazid preventive therapy for latent infection |
The thresholds are approximate and overlap, so a patient above a threshold can still develop disease if antiretroviral therapy has been delayed. Effective antiretroviral therapy restores immunity and is the best long-term prevention against nearly all of these infections.
Screening at the threshold is what turns the table into practice. A Toxoplasma IgG test and a cryptococcal antigen test in a patient with CD4 below 100 identify those who need prophylaxis or early review. Tuberculosis screening belongs at every visit, because tuberculosis remains the leading cause of death among people living with HIV.
Where the exam tries to catch you
HIV opportunistic infection questions usually test the threshold and the investigation that matches it.
- Trap: Stopping cotrimoxazole prophylaxis as soon as the CD4 count rises slightly above 200. Correct: Prophylaxis is stopped only after sustained immune recovery on antiretroviral therapy, typically a CD4 count above 200 for at least three months.
- Trap: Starting fluconazole alone for cryptococcal meningitis. Correct: Amphotericin-based induction with flucytosine is standard, because fluconazole alone is inferior for meningitis.
- Trap: Assuming every lung infiltrate in HIV is tuberculosis. Correct: Pneumocystis gives diffuse bilateral interstitial shadowing with a low CD4 count and raised LDH, so the pattern and CD4 count should guide the workup.
- Trap: Reading the CD4 count alone and ignoring the viral load. Correct: Viral load and antiretroviral status shape management, and therapy is usually deferred for several weeks in cryptococcal meningitis.
Frequently asked questions
Which CD4 threshold triggers Pneumocystis prophylaxis in HIV?
Cotrimoxazole prophylaxis is classically started in HIV patients with a CD4 count below 200 cells per microlitre to prevent Pneumocystis pneumonia, although national programme criteria also use clinical stage and are broader. It is continued until immune recovery on antiretroviral therapy is sustained.
How is cryptococcal meningitis diagnosed in HIV?
Cryptococcal antigen testing on serum or CSF is a sensitive screen in advanced HIV, and India ink staining of CSF shows encapsulated yeasts. Fungal culture on Sabouraud agar confirms the diagnosis and allows species identification.
Why does toxoplasmosis cause ring-enhancing lesions in HIV?
Toxoplasma gondii reactivates from tissue cysts in the brain when CD4 counts fall below about 100 cells per microlitre. The resulting necrotic foci enhance as ring lesions on contrast CT or MRI, classically in the basal ganglia.
When can MAC prophylaxis be stopped in HIV?
Mycobacterium avium complex prophylaxis is stopped after sustained CD4 recovery on antiretroviral therapy, usually a CD4 count above 100 for more than three months. Where MAC prophylaxis is not used routinely, the same principle of immune recovery guides monitoring.
Which fungus causes disseminated infection in HIV patients in northeast India?
Penicillium marneffei causes disseminated talaromycosis in HIV patients in Southeast Asia and northeast India, with skin papules and fever as typical features. It is treated with amphotericin B in the acute phase, followed by itraconazole.