Enteric & Food/Water-Borne Infections

See also worm infections (helminths, a different mechanism to the protozoal/bacterial pathogens below).

Protozoal

Cryptosporidium

A protozoan causing watery diarrhoea, spread by the faecal–oral route and contaminated water (a notable cause of recreational-water/swimming-pool outbreaks — oocysts resist chlorine).

• Immunocompetent patients — usually an acute self-limiting gastroenteritis; no antimicrobial needed (fluids/electrolytes only).
• Immunocompromised patients (advanced HIV, transplant, hypogammaglobulinaemia, IgA deficiency, potent immunosuppression) — may be prolonged or severe.
  • Best response comes from improving immune status (e.g. ART with immune reconstitution in HIV). Trials have not shown clear benefit from antimicrobials.
  • Mainstay is fluid/electrolyte replacement ± antimotility drugs; for persistent severe diarrhoea consider nitazoxanide (500 mg BD for 3 days; weight-based in children).

Amoebiasis

Infection with Entamoeba histolytica (faecal–oral). It is morphologically identical on microscopy to the non-pathogenic E. dispar and E. moshkovskii — distinguished by PCR (or antigen testing).

Treatment:

• Asymptomatic carriage of E. histolytica — treat to reduce transmission and invasive-disease risk: paromomycin 500 mg TDS for 7 days (Special Access Scheme).
• Amoebic colitis (dysentery) — tinidazole 2 g daily for 3 days or metronidazole 600 mg TDS for 7 days.
• Severe colitis / amoebic liver abscess — higher-dose/longer tinidazole (5 days) or metronidazole (oral or IV, 7 days); add antibiotic cover for peritonitis if perforation/toxic megacolon.
• After any invasive treatment, follow with a luminal agent (paromomycin) to eradicate cysts and prevent relapse.

Giardiasis

Giardia intestinalis (a.k.a. G. lamblia / G. duodenalis) — the most common protozoal cause of chronic diarrhoea (bloating, malabsorption, foul-smelling stools).

Testing for GI protozoa: most chronic diarrhoea is non-infectious. Send faecal testing for parasites (microscopy/PCR) in patients with abdominal symptoms (pain, diarrhoea) lasting more than 2 weeks.

Treatment:

• Asymptomatic carriage in immunocompetent patients usually needs no treatment.
• Symptomatic: metronidazole 2 g daily for 3 days (or 400 mg TDS for 5 days). (Tinidazole was discontinued in Australia in Feb 2020.)
• Pregnancy: metronidazole (TDS regimen) or paromomycin.
• Recurrence may reflect post-infectious lactose intolerance, reinfection, or drug resistance. Metronidazole-resistant Giardia is cross-resistant to tinidazole — refer to ID; options include nitazoxanide or paromomycin (both via the Special Access Scheme), albendazole or mebendazole.

Bacterial

Cholera

Vibrio cholerae, ingested in contaminated food/water. Rare in Australia (mostly returned travellers); usual food/water safety precautions prevent it in most travellers.

• Presentation: sudden-onset, profuse large-volume watery (“rice-water”) diarrhoea → rapid severe dehydration.
• Diagnosis: stool culture.
• Management: aggressive rehydration is the mainstay (oral rehydration solution; IV if severe). Antibiotics shorten illness in moderate/severe cases — azithromycin 1 g stat or ciprofloxacin.

Vaccination: not routinely recommended for travellers (food safety suffices). Consider for high-risk travellers: poorly controlled diabetes, inflammatory bowel disease, HIV/immunocompromise, significant cardiovascular disease.

• Adults: 2 oral doses 1–6 weeks apart (repeat the primary course if >6 weeks elapse); protection ~2 years.
• Children: 3 doses 1–6 weeks apart; protection ~6 months.

Listeriosis

Listeria monocytogenes — a foodborne organism able to grow at refrigerator temperatures. Sources: unpasteurised milk/soft cheeses, processed meats (deli meats/pâté), smoked seafood, pre-prepared salads and fresh fruit (e.g. rockmelon).

At-risk groups: pregnant women, immunocompromised, frail/elderly, and neonates/the unborn.

Clinical features:

• Usually a self-limiting febrile gastroenteritis or mild flu-like illness in healthy adults.
• In at-risk groups: meningitis/encephalitis (neonates, elderly), sepsis, pneumonia. In pregnancy it can cause miscarriage, stillbirth or neonatal sepsis even with mild maternal illness — hence dietary avoidance advice in pregnancy.

Diagnosis and management:

• Blood culture (stool culture needs special media; ~5% asymptomatic carriage).
• The risk of infection after eating a contaminated product is small — test susceptible people only if they develop symptoms (e.g. a flu-like illness) within ~3 weeks. Invasive listeriosis is treated with IV amoxicillin/ benzylpenicillin (± gentamicin).

Typhoid fever

Enteric fever from Salmonella Typhi (and paratyphi), spread by the faecal–oral route — almost always acquired overseas (notably the Indian subcontinent). Some people become asymptomatic chronic carriers (Typhoid Mary).

Clinical features: after a 10–14 day incubation: gradual-onset stepwise fever, headache, arthralgia, pharyngitis and abdominal pain; relative bradycardia and rose spots may occur; diarrhoea (or constipation) develops later.

Diagnosis: blood culture (most sensitive early); stool and urine culture are not reliable for diagnosing acute infection (may just reflect convalescent or chronic carriage) and are not routinely used for that purpose.

Treatment: Azithromycin (e.g. 1 g daily for 5–7 days) is first-line for uncomplicated disease, reflecting widespread and increasing fluoroquinolone resistance (and emerging extensively drug-resistant strains); severe disease needs IV ceftriaxone. Typhoid vaccination is recommended for travellers to endemic areas.