Fungal and parasitic infections

The phrase “immunocompromised host” hides three different defence systems, and each one can fail separately. The first is the physical barrier: intact skin and mucous membranes stop fungi and parasites from entering at all. The second is colonisation resistance, the competition from the normal bacterial flora that occupies nutrients and attachment sites. The third is cell-mediated immunity, the T-lymphocyte response that keeps intracellular organisms such as Candida, Cryptococcus and the protozoa of Chagas disease under control. When a patient’s defences are weakened, the same organism that a healthy person carries silently becomes a pathogen. HIV attacks CD4 T-lymphocytes, so cell-mediated immunity falls and opportunistic infections appear as the count declines — fungal pneumonia, oesophageal candidiasis, and parasitic disease of the gut and brain. Steroid and cancer treatments suppress the same arm of immunity. Long-term broad-spectrum antibiotics fail in a different way: they remove the competing bacteria while immunity is intact, which is why thrush can follow a simple antibiotic course in an otherwise healthy person. Nurses can name which defence is failing, and that naming changes the plan. A patient whose barrier is broken needs wound and skin care; a patient whose flora is disturbed needs the antibiotic reviewed; a patient whose cell-mediated immunity is suppressed needs prophylaxis and early reporting of fever.

Fungi are eukaryotes, like human cells, so finding a drug target that harms the fungus without harming the patient is difficult. The solution is to attack structures that human cells do not share. The fungal cell wall contains chitin and glucan, and the cell membrane uses ergosterol where human membranes use cholesterol. Azole antifungals such as fluconazole block the synthesis of ergosterol, so the fungal membrane becomes unstable and the fungus stops growing. Polyenes such as nystatin bind the ergosterol that is already there and punch holes in the membrane, which is why nystatin acts only where it is applied and is not absorbed from the gut. This explains a practical rule nurses use: nystatin is the right choice for oral thrush because it stays in the mouth and gut, while fluconazole is chosen when the infection has entered tissue that needs a drug carried in the blood. Resistance appears when fungi alter the target enzyme or pump the drug out, and it is commonest where antifungals are used for long periods or without a clear diagnosis. Antifungals are also not free of harm: azoles interact with many medicines through liver enzymes, and they can disturb the same liver function the nurse monitors. Knowing the mechanism explains the route, the duration and the monitoring, rather than treating the drug name as a fact to memorise.

A parasite that must pass from one host to another faces a problem: the outside world is hostile. Protozoa solve it with two forms. The trophozoite is the active, feeding, moving form that lives in the host and causes symptoms; the cyst is a resistant, dormant form with a tough wall that survives in water, soil and food and is the form that infects the next person. Giardia lamblia is swallowed as a cyst, excysts in the small intestine into trophozoites that attach to the gut wall and cause malabsorption and prolonged watery diarrhoea, then re-encysts and leaves in the stool. This cycle explains three practical facts. First, a patient can be infectious while feeling well, because cysts are shed before and after symptoms. Second, chlorine at the doses used in many supplies does not reliably kill Giardia cysts, so boiling or filtering is more certain in an outbreak. Third, the treatment of severe intestinal upset starts with oral rehydration, because the immediate danger is fluid and electrolyte loss while the anti-parasitic drug takes effect. Transmission routes differ by organism and change the control measure. Giardia needs contaminated water or food, so water treatment and hand hygiene are decisive. Malaria and Chagas disease need a vector: mosquitoes and blood-feeding triatomine bugs respectively inject the parasite, so nets, repellents and vector control matter more than water. The named drugs follow the same rule. Metronidazole is the anti-parasitic used for intestinal protozoa such as Giardia and amoebae, and IV artesunate is used for severe malaria because the parasite is in the blood and the patient may be too ill to take oral treatment, with quinine only as an alternative when artesunate is unavailable.