24th July 2023

Gastrointestinal Infections

Gastroenteritis

Gastroenteritis (colloquially known as “stomach flu”) refers to enteric infections which are usually transient and self-limiting, characterised by an acute diarrhoeal illness, which may be associated with vomiting, abdominal pain and fever.

Food poisoning is a subset of gastroenteritis, in which the causative organism has been spread through the consumption of contaminated food or water.

Most cases of gastroenteritis are caused by viruses and generally resolve within 48-72 hours with no specific interventions. Bacterial infections may cause more significant illness and are more associated with systemic upset.

The most important causative organisms are summarised in the table below.

Table 4: Important causative organisms in gastroenteritis

Notifiable diseases

Notifiable diseases are infectious diseases with high transmission rates or a significant morbidity or mortality risk. Registered medical practitioners – including doctors – have a statutory duty to report suspected cases of notifiable diseases to the relevant local health authority (even before diagnosis is confirmed). Laboratories also share this duty if they process samples which identify notifiable organisms.

Certain organisms that cause gastroenteritis, food poisoning, as well as any cases of infectious bloody diarrhoea are notifiable diseases. A comprehensive list can be found on the GOV.UK website and this guidance can be found in the References section of this topic. It is an important consideration when managing a patient with gastroenteritis.

Other gastrointestinal infections

Helicobacter pylori infection

Helicobacter pylori (H. pylori) infection is commonly implicated in peptic ulcers – 95% of duodenal ulcers are caused by H. pylori infection, and between 70-80% of gastric ulcers. H. pylori is also implicated in the pathogenesis of gastritis, stomach cancer, and MALT lymphoma.

It is recommended that H. pylori is tested for in patients with uncomplicated dyspepsia, with no red flag features suggestive of malignancy, who do not respond to conservative measures to manage reflux or four weeks’ treatment with a PPI.

The hydrogen urea breath test or stool helicobacter antigen test are the first line investigations. A patient should be off PPI treatment for a minimum of two weeks before testing, as PPI use can lead to false negatives.

The goal of treatment is to eradicate the infection and hence reduce the risk of the aforementioned complications.

Treatment is triple therapy with a PPI, and two antimicrobials, with the choice of antibiotic affected by previous courses received.

First line treatment is a PPI with amoxicillin and one of clarithromycin or metronidazole for seven days. If this is not successful, then a further seven-day course of triple therapy can be given, with the alternative used for the secondary antibiotic (i.e. metronidazole if clarithromycin was first used). The alternative for a penicillin allergic patient.

If initial treatment is not successful, then there are other antibiotic combinations available, and retesting and further investigations into an alternative cause may be indicated.

Whipple’s disease

This is a rare infection caused by the Tropheryma whipplei bacteria, and causes a chronic illness which affects multiple body systems. The typical presentation is in middle-aged Caucasian men, with the classical clinical features being chronic diarrhoea, weight loss, fatigue and arthralgia.

Diagnosis is confirmed through biopsies of the duodenum. Characteristic histological findings include ‘foamy macrophages’ containing large amounts of periodic acid-Schiff (PAS) stain-positive particles. Treatment is with intravenous ceftriaxone for 14 days, with a maintenance course of oral trimethoprim or sulfamethoxazole for one year.

Small intestinal bacterial overgrowth

Small intestinal bacterial overgrowth, or SIBO, is a little known condition in which excessive growth of bacteria in the small bowel leads to a chronic gastrointestinal disorder. Every person has an amount of so-called ‘gut flora’, and in SIBO, this becomes overgrown. Prevalence is increasing due to the increased awareness of the disease, leading to more diagnoses being made. It is often co-morbid in people with irritable bowel syndrome, coeliac disease and inflammatory bowel disease.

The presentation of SIBO is variable, with some only having mild symptoms, whilst others suffer a syndrome of chronic diarrhoea, weight loss, and malabsorption similar to inflammatory bowel diseases. Diagnosis can be confirmed in an individual with suspected SIBO through a positive hydrogen breath test; although this is sensitive but not a specific investigation, as there are other causes of a positive test. Endoscopy may be required to rule out other pathologies.

Treatment includes targeting any contributing underlying diseases implicated in the aetiology, eradicating the bacteria, and managing any complications, such as nutritional deficiencies. The first line antibiotic is rifaximin, although this is off-licence.

Amoebiasis

Amoebiasis, or amoebic dysentery, is caused by the parasite Entamoeba histolytica. It may be asymptomatic but most commonly presents with abdominal pain, diarrhoea, or a dysenteric illness (i.e. profuse bloody diarrhoea). Amoebiasis may also cause disseminated disease, with extra-intestinal symptoms; the most common manifestation of which is a liver abscess, which will present with an acute right upper quadrant pain, and deranged liver function tests and inflammatory markers. Abscesses may also develop in the brain and lung, and colitis can occur, leading to a risk of bowel perforation and peritonitis.

The first line diagnostic investigation is of microscopy of stool samples, which show the characteristic trophozoite cysts created by the infection. If disease has become extra-intestinal, then stool tests may be negative, in which case serological tests may be used as an alternative.

This patient has Amoebic dysentery caused by e (7, 8). Given his right upper quadrant pain and raised ALT, this raises the possibility of amoebic liver abscess. Due to the presentation, it can mimic Ulcerative Colitis. Therefore it can be important to send stool PCR when starting treatment for UC.

Treatment of amoebic liver abscess is generally divided into a tissue agent and a luminal agent. Metronidazole or tinidazole is used as the tissue agent, while paromomycin, diiodohydroxyquin or diloxanide furoate can be used as a luminal agent, to eliminate intraluminal cysts.