Learn — Parasitology & mycology

2 notes · 11 questions on the topic

Entamoeba histolytica

Entamoeba histolytica is a pathogenic intestinal protozoan whose quadrinucleate cyst is swallowed in faecally contaminated water or food and whose trophozoite then invades the colonic mucosa to cut the classical flask-shaped ulcer. It matters at the bedside because one organism produces two very different presentations, amoebic dysentery with blood and mucus in the stool and amoebic liver abscess with fever and a tender liver, and because the cyst-passing carrier who feels perfectly well is the person who keeps the infection moving through a household, a hostel or a kitchen.

There are two forms: the trophozoite, which is the invasive motile form with a single nucleus, fine peripheral chromatin, a central karyosome and ingested red blood cells in its cytoplasm, and the cyst, which is the infective and transmissible form and is infective only when it has matured to four nuclei.
Transmission is faecal-oral through cysts in contaminated water, uncooked vegetables and salads, food handled by a cyst passer, and flies; the cyst survives for days to weeks outside the body and resists the chlorine concentrations used to treat drinking water, but boiling destroys it, so boiling and filtration are the household measures that work.
The incubation period is commonly 2 to 4 weeks but ranges from a few days to months, and a liver abscess may appear months after the diarrhoea has settled or in a patient who never had diarrhoea at all.
For stool diagnosis a fresh specimen must reach the laboratory within about 30 minutes and be kept warm, because the motile trophozoite containing ingested red cells is the diagnostic finding and it disintegrates quickly; formed stool shows cysts, three specimens on different days raise the yield, and because microscopy cannot separate E. histolytica from the harmless E. dispar, antigen detection by ELISA or a nucleic acid test is used where the distinction changes treatment.
Amoebic liver abscess is usually single and in the right lobe, with fever, tender hepatomegaly, intercostal tenderness and a raised right dome of the diaphragm; aspiration yields thick reddish-brown odourless anchovy-sauce pus that is sterile on bacterial culture and usually free of amoebae, since the organisms lie in the abscess wall, and serology is positive in the great majority of cases.
Treatment is a tissue amoebicide, metronidazole or tinidazole, which must always be followed by a luminal amoebicide such as diloxanide furoate or paromomycin to clear cysts from the gut; leaving out the luminal drug means relapse and continued transmission, and alcohol is avoided during and for a few days after metronidazole.
Aspiration or drainage of a liver abscess is reserved for the abscess that has not responded after about 72 hours of drug treatment, one that is large or in the left lobe and threatening the pericardium, or one that looks close to rupture.
Infection control is standard precautions, with contact precautions for the incontinent or diapered patient; hand hygiene with soap and running water, safe drinking water, sanitary disposal of faeces and exclusion of an infected food handler until treated are the measures that break transmission, and contaminated laboratory material is autoclaved at 121 degrees Celsius with a 15-minute holding time before disposal in the yellow bag under the Biomedical Waste Management Rules 2016.

Aspergillus

Aspergillus is a ubiquitous saprophytic mould whose spores are inhaled every day from soil, dust, damp walls, air-conditioning ducts and building work, and which is harmless to a healthy person but lethal to one with prolonged neutropenia. Aspergillus fumigatus causes most human disease, which takes three forms in ascending severity: allergic bronchopulmonary aspergillosis in an asthmatic or a patient with cystic fibrosis, aspergilloma growing inside an old tuberculous cavity, and invasive aspergillosis in the profoundly immunosuppressed.

In tissue and on a KOH mount it appears as septate hyaline hyphae branching regularly at an acute angle of about 45 degrees; the culture medium of choice is Sabouraud dextrose agar, on which a rapidly growing velvety colony appears within a few days, blue-green in A. fumigatus, black in A. niger and yellow-green in A. flavus, with the characteristic conidiophore ending in a swollen vesicle bearing chains of conidia.
Because the spores are everywhere, a single positive respiratory culture may be a contaminant and is read with the clinical picture; galactomannan in serum or bronchoalveolar lavage and beta-D-glucan support invasive disease, CT shows a halo sign early and an air-crescent sign later, and blood cultures are almost always negative.
It is not transmitted from person to person, so the patient is never isolated to protect the ward; the exposure is environmental, and it is the patient who has to be protected from the air around him.
Allergic bronchopulmonary aspergillosis presents as asthma that will not settle, with brown mucus plugs, blood eosinophilia, a total IgE well above 1000 IU/mL, Aspergillus-specific IgE, a positive skin prick test and central bronchiectasis; it is treated with oral corticosteroids, often with itraconazole, and not with an antibiotic.
Aspergilloma is a fungus ball lying free inside a pre-existing cavity, most often a healed tuberculous one, seen radiologically as a mass with an air crescent that shifts when the patient changes position; haemoptysis is its danger and can be massive, and treatment is bronchial artery embolisation or surgery, because antifungal drugs penetrate the ball poorly.
Invasive aspergillosis occurs in prolonged neutropenia, after stem-cell or solid-organ transplantation, on high-dose corticosteroids and in advanced HIV, presenting as fever that does not respond to broad-spectrum antibiotics, with pleuritic pain, dry cough and haemoptysis from angio-invasion; voriconazole is the first-line drug, with isavuconazole or liposomal amphotericin B as alternatives, and treatment is begun on suspicion rather than after culture because mortality is high.
Voriconazole needs its own nursing watch: transient visual disturbance, photosensitivity, hepatotoxicity and many drug interactions, so liver function and concurrent drugs are reviewed and the patient is warned about sun exposure.
A high-risk neutropenic or transplant patient is nursed in a protective environment, meaning a HEPA-filtered positive-pressure room with sealed windows, no potted plants, dried or fresh flowers or fruit baskets, a well-fitting mask whenever the patient is moved through the hospital, and construction or renovation dust sealed off, because building work beside a haematology unit is a documented cause of ward outbreaks.

Free today: the 10-question daily quiz · the NORCET 11 recall paper · what ₹999 unlocks — once, no auto-renewal.