Medically Important Zoonotic Bacteria

This section brings together the major zoonotic bacteria for consolidated review. Brucella, Francisella, Pasteurella, Leptospira, and Coxiella are summarised here in table form; Bartonella and Yersinia pestis are covered in full.

Reservoir. Bartonella henselae lives in domestic cats, particularly kittens, which typically remain asymptomatic carriers themselves. Bartonella quintana lives in humans, with the body louse as vector, an epidemiological pattern paralleling louse-borne relapsing fever and epidemic typhus, associated with similar conditions of crowding and poor hygiene, particularly relevant in populations experiencing homelessness.

Route of transmission. B. henselae is transmitted by a cat scratch or bite, the organism is present in cat saliva and on claws, likely via flea faeces contaminating the cat's fur and claws, though the flea itself is not thought to directly bite humans to transmit this particular organism. B. quintana is transmitted by body louse bite or faeces.

Major disease associations. B. henselae causes cat scratch disease, the most common clinical presentation, producing regional lymphadenopathy, often significant and sometimes suppurative, near the site of a cat scratch or bite, typically in an otherwise well child or young adult, generally self-limited.

In immunocompromised patients, notably advanced HIV infection, the same organism instead classically causes bacillary angiomatosis, a distinct vascular proliferative skin, and sometimes visceral, lesion that can mimic Kaposi sarcoma both clinically and, to some degree, histologically, an important differential distinction given the very different management implications of the two conditions.

Both B. henselae and B. quintana are also recognised, though less common, causes of culture-negative endocarditis, joining Coxiella burnetii on the standard differential list for this presentation.

B. quintana causes trench fever, historically significant from large World War-era epidemics among soldiers, hence the name, presenting with recurring fever, headache, and bone pain, classically shin pain, and a recognised cause of endocarditis and, in immunocompromised hosts, bacillary angiomatosis as well.

Diagnostic clues. Serology is the standard approach for cat scratch disease in an immunocompetent host with a compatible exposure history and clinical picture. Blood culture, requiring prolonged incubation given the organism's slow growth, or tissue biopsy with specific staining or PCR is used when endocarditis or bacillary angiomatosis is suspected, given the higher diagnostic stakes and lower specificity of serology alone in these more severe presentations.

Reservoir. Wild rodents, maintained in sylvatic, wildlife, rodent populations globally, with periodic spillover into domestic and peridomestic rodent populations that brings the organism into closer proximity to humans.

Route of transmission. Predominantly the bite of an infected flea, the classic vector, with the flea's foregut becoming blocked by a biofilm-like mass of the organism, causing the flea to bite repeatedly and regurgitate infected material in a failed feeding attempt, a specific and elegant transmission mechanism.

Also transmissible via direct contact with infected animal tissue, and, distinctively for the pneumonic form specifically, via respiratory droplets from another infected individual with pneumonic plague, making this the one presentation of this organism capable of genuine person-to-person transmission.

Major disease associations. Bubonic plague is the most common presentation, following a flea bite, producing a markedly swollen, exquisitely tender regional lymph node, the bubo, together with fever and systemic illness, and can progress to secondary septicaemic spread if untreated.

Septicaemic plague is bloodstream dissemination, either as a progression from bubonic disease or, less commonly, as a primary presentation, associated with disseminated intravascular coagulation and a high mortality rate if untreated, with the historically described, though now uncommon given modern treatment, peripheral tissue necrosis and gangrene, black death, reflecting this coagulopathic, vasculitic process.

Pneumonic plague is the most severe and most rapidly fatal presentation, occurring either as secondary spread from bubonic or septicaemic disease to the lungs, or as primary infection following inhalation of infected respiratory droplets from another pneumonic plague patient. It is distinctive for being the only form of this disease capable of direct person-to-person transmission, and for its potential for extremely rapid clinical deterioration, making it a disease of major biosecurity and outbreak-control concern.

Diagnostic clues. Clinical suspicion in an appropriate epidemiological context, known endemic area, recent flea exposure or contact with wild rodents, or, for pneumonic disease, contact with another plague case, is critical given the disease's potential for very rapid progression.

Gram stain of bubo aspirate or blood classically shows a distinctive safety pin bipolar staining pattern, the organism stains more densely at each pole than centrally, giving this specific and classically described appearance.

Culture confirms the diagnosis, though, as with several other zoonotic organisms, requires specific laboratory notification given the organism's biosafety and biosecurity significance, Yersinia pestis being a recognised historical agent of bioterrorism concern, alongside Bacillus anthracis and Francisella tularensis.