Infection lecture Dr. Ahmed Moyed HusseinBrucellosis
Brucellosis is an enzootic infection (i.e. endemic in animals). Although six species of Brucella Gram-negative bacilli are known, only four are important to humans: B. melitensis (goats, sheep and camels), B. abortus (cattle), B. suis (pigs) and B. canis (dogs). B. melitensis causes the most severe disease; B. suis is often associated with abscess formation.
Infected animals may excrete Brucella spp. in their milk for prolonged periods, and human infection is acquired by ingesting contaminated dairy products (especially unpasteurised milk), uncooked meat or offal. Animal urine, faeces, vaginal discharge and uterine products may act as sources of infection through abraded skin or via splashes and aerosols to the respiratory tract and conjunctiva.
Clinical features:Brucella spp. are intracellular organisms that survive for long periods within the reticulo-endothelial system. This explains many of the clinical features, including the chronicity of disease and tendency to relapse, even after antimicrobial therapy.
Acute illness is characterized by a high swinging temperature, rigors, lethargy, headache, joint and muscle pains, and scrotal pain. Occasionally, there is delirium, abdominal pain and constipation. Physical signs are non-specific, e.g. enlarged lymph nodes. Enlargement
of the spleen may lead to hypersplenism and thrombocytopenia.
Localized infection ( see Fig. below), which occurs in about 30% of patients, is more likely if diagnosis and treatment are delayed.
Fig: clinical features of BrucellosisDiagnosis:
Definitive diagnosis depends on the isolation of the organism.
Culture: Blood cultures are positive in 75–80% of infections caused by B. melitensis and 50% of those caused by B. abortus. Bone marrow culture may increase the diagnostic yield, particularly if antibiotics have been given before specimens are taken. CSF culture in neurobrucellosis is positive in about 30% of cases.
Serology (Brucella agglutination test) to detect antibody response to Brucella may also aid diagnosis. In endemic areas, a single high antibody titer of more than 1/320 or a fourfold rise in titer is needed to support a diagnosis of acute infection. The test usually takes several weeks to become positive but should eventually detect 95% of acute infections.
Management:Aminoglycosides show synergistic activity with tetracyclines against brucellae.
Typhoid and paratyphoid (enteric) feversTyphoid and paratyphoid fevers, which are transmitted by the faecal–oral route, are important causes of fever in India, sub-Saharan Africa and Latin America. Elsewhere, they are relatively rare. Enteric fevers are caused by infection with Salmonella typhi and S. paratyphi A and B.
After a few days of bacteraemia, the bacilli localize, mainly in the lymphoid tissue of the small intestine, resulting in typical lesions in the Peyer’s patches and follicles. These swell at first, then ulcerate and usually heal. After clinical recovery, about 5% of patients become chronic carriers (i.e. continue to excrete the bacteria after 1 year); the bacilli may live in the gallbladder for months or years and pass intermittently in the stool and, less commonly, in the urine.
Clinical featuresTyphoid fever:
The incubation period is typically about 10–14 days but can be longer, and the onset may be insidious. The temperature rises in a stepladder fashion for 4 or 5 days with malaise, increasing headache, drowsiness and aching in the limbs. Constipation may be caused by swelling of lymphoid tissue around the ileocaecal junction, although in children diarrhoea and vomiting may be prominent early in the illness. The pulse is often slower than would be expected from the height of the temperature, i.e. a relative bradycardia.
At the end of the first week, a rash may appear on the upper abdomen and on the back as sparse, slightly raised, rose-red spots, which fade on pressure. It is usually visible only on white skin. Cough and epistaxis occur.
Around the 7th–10th day, the spleen becomes palpable. Constipation is then succeeded by diarrhea and abdominal distension with tenderness. Bronchitis and delirium may develop. If untreated, by the end of the second week the patient may be profoundly ill.
Paratyphoid fever:The course tends to be shorter and milder than that of typhoid fever and the onset is often more abrupt with acute enteritis. The rash may be more abundant and the intestinal complications less frequent.
Complications:Haemorrhage from, or a perforation of, the ulcerated Peyer’s patches may occur at the end of the second week or during the third week of the illness. A drop in temperature to normal or subnormal levels may be falsely reassuring in patients with intestinal haemorrhage.
Additional complications may involve almost any viscus or system because of the septicaemia present during the first week. Bone and joint infection is common in children with sickle-cell disease.
Investigations:In the first week, diagnosis may be difficult because, in this invasive stage with bacteraemia, the symptoms are those of a generalized infection without localizing features.
A white blood count may be helpful, as there is typically a leucopenia. Blood culture is the most important diagnostic method. The faeces contain the organism more frequently in the second and third weeks.
Management:The fluoroquinolones are the drugs of choice (e.g. ciprofloxacin 500 mg twice daily) if the organism is susceptible. Extended-spectrum cephalosporins (ceftriaxone and cefotaxime) are useful alternatives. Azithromycin (500 mg once daily) is an alternative when fluoroquinolone
resistance is present but has not been validated in severe disease. Treatment should be continued for 14 days.
Pyrexia may persist for up to 5 days after the start of specific therapy. Even with effective chemotherapy, there is still a danger of complications, recrudescence of the disease and the development of a carrier state.
Chronic carriers were formerly treated for 4 weeks with ciprofloxacin but may require an alternative agent and duration, as guided by antimicrobial sensitivity testing. Cholecystectomy may be necessary.