Fungal and Parasitic Infections
Core concepts
Fungal disease follows immune defect and iatrogenic risk: Candida in premature neonates and central-line carriers; Aspergillus in prolonged neutropenia and transplant recipients; Mucorales in diabetic ketoacidosis and iron overload; Pneumocystis jirovecii in T-cell deficiency and HIV with low CD4 counts; Cryptococcus in T-cell defects. Parasites to know: malaria (thick and thin smears, rapid antigen), Toxoplasma (ring-enhancing brain lesions), Giardia and Cryptosporidium (stool antigen or PCR), pinworm (cellophane tape test), scabies (clinical), and congenital Chagas disease.
Traps
- Candida isolated from urine may reflect colonization, especially with a catheter; interpret with pyuria and clinical signs.
- Blood cultures are usually negative in invasive aspergillosis; use galactomannan, beta-D-glucan, and imaging (halo sign).
- Galactomannan can be falsely positive with some beta-lactam antibiotics and infant formulas.
- Toxoplasma treatment is pyrimethamine plus sulfadiazine plus leucovorin; do not omit leucovorin.
- Severe malaria is treated with IV artesunate, not oral therapy.
Worked example
A 6-year-old with leukemia, absolute neutrophil count 80/microliter, fever, and nodular lung lesions with a halo sign: start an anti-pseudomonal beta-lactam plus voriconazole and obtain galactomannan and biopsy if feasible.
Rule of thumb: link the organism to the immune defect, then choose the least toxic regimen that covers it and remove the risk factor (catheter, immunosuppression) when possible.