Zoonotic Diseases

Zoonotic diseases are an overarching category of infections transmissible from animals to humans. Causative organisms span bacteria, viruses, fungi and parasites.

There are numerous specific zoonotic conditions, including rabies, Australian bat lyssavirus infection, Q fever, brucellosis, leptospirosis, cat scratch disease, plague, and various tick borne diseases.

This entry provides the general framework and clinical principles for approaching zoonotic disease as a category. It is particularly relevant to structuring the assessment of a patient with a compatible illness and a relevant animal exposure history where the specific causative organism is not yet apparent.

Classification and transmission routes

• Direct transmission occurs through bites, scratches, or direct contact with infected animal tissue, blood or body fluids. Examples are rabies, Australian bat lyssavirus infection, cat scratch disease, and various bacterial infections including Pasteurella multocida following cat or dog bites.
• Vector borne transmission is mediated by an arthropod intermediate host (tick, flea, mosquito, or other insect vector). The vector acquires the organism from an infected animal reservoir before transmitting it to humans.
• Vector borne transmission accounts for a substantial proportion of clinically significant zoonotic disease, including Lyme disease, the various rickettsial and tick borne infections, plague, and numerous mosquito borne viral infections.
• Environmental or indirect transmission occurs through contact with contaminated animal products, urine, faeces or aerosols, without requiring a direct bite or vector intermediate.
• Examples of environmental transmission are Q fever (inhalation of contaminated aerosols), leptospirosis (contact with water contaminated by infected animal urine), and brucellosis (consumption of unpasteurised dairy products or direct contact with infected livestock).
• Foodborne zoonotic transmission occurs through consumption of contaminated animal products. It overlaps substantially with several conditions addressed under infectious gastroenteritis and food poisoning, including Salmonella, Campylobacter and specific Escherichia coli pathotypes.
• Many of these organisms have a significant animal reservoir contributing to human transmission risk.

• The specific clinical presentation is entirely dependent on the causative organism. It ranges from localised cutaneous or soft tissue infection following a direct bite or scratch injury, through systemic febrile illness of varying severity, to organ specific disease reflecting the particular tissue tropism of the pathogen. Individual presentations depend on the specific condition.
• A detailed, specific animal exposure history is the single most valuable element in evaluating any patient with a suspected zoonotic infection. It should cover the type of animal involved (domestic, wild, or livestock) and the nature of contact (direct bite or scratch, indirect environmental exposure, occupational contact, or dietary exposure to animal products).
• The exposure history should also cover the geographical location and any relevant seasonal factors. Record the timing of exposure relative to symptom onset, given the highly variable incubation periods across different zoonotic pathogens.
• Explore occupational and recreational risk factors systematically, given their direct relevance to the range of zoonotic pathogens to consider. These include farming, veterinary work, wildlife handling, hunting, and specific recreational activities such as freshwater swimming (relevant to leptospirosis risk) or outdoor activity in tick endemic areas.

• The diagnostic approach is directed by the specific clinical presentation and, critically, the exposure history obtained. This guides selection of the appropriate targeted investigation (serology, culture, polymerase chain reaction testing, or specific microscopic examination) for the zoonotic pathogens plausible given the particular animal exposure and geographical context.
• Maintain a broad differential diagnosis encompassing the zoonotic pathogens relevant to the specific documented exposure. Several zoonotic infections can produce overlapping, non specific febrile presentations.
• The exposure history therefore frequently provides more diagnostic guidance than the clinical presentation alone in narrowing the differential diagnosis.
• Specialist infectious disease, tropical medicine or, where relevant, veterinary public health input is valuable in complex or diagnostically uncertain presentations. This applies particularly where an unusual or less common zoonotic pathogen is being considered based on a specific, unusual exposure history.

• Management is entirely specific to the identified or strongly suspected causative organism, with no generic management approach applicable across this broad and heterogeneous category of disease.
• Consider post exposure prophylaxis promptly, where available and indicated for the specific suspected exposure. This applies most notably to rabies and Australian bat lyssavirus infection following a relevant animal bite, and to specific bacterial pathogens following certain high risk exposures.
• Where the specific exposure risk warrants it, initiate prophylaxis without delay, given the time critical nature of effective prophylaxis for several conditions within this category.
• Wound care is an important general component of managing any zoonotic exposure involving a direct bite or scratch injury. It includes thorough cleaning of the wound and tetanus prophylaxis assessment.
• Give appropriate antibiotic prophylaxis where clinically indicated based on the specific wound characteristics and animal involved. This is particularly relevant following cat and dog bites, given the recognised risk of Pasteurella multocida and other relevant organisms.
• Notification to relevant public health authorities is required for numerous specific zoonotic infections, given their significant public health surveillance implications, potential for outbreak, or specific biosecurity considerations. Requirements vary by jurisdiction and specific pathogen.
• Prevention advice is an important component of full management. Tailor it to the specific exposure risks identified for the individual patient: occupational protective measures, vector avoidance strategies, food safety practices, appropriate animal handling precautions, and, where available and indicated, specific pre-exposure vaccination.
• It is particularly relevant for patients with ongoing or recurrent occupational or recreational exposure risk to the relevant zoonotic disease reservoir.