Zoonotic Infections

Abstract

The term zoonosis comes from the Greek: ζῷον (zoon) ‘animal’ and νόσος (nosos) ‘sickness’, and means an infection transmissible from animals to humans. Infected animals can be symptomatic or asymptomatic, and humans usually become accidental hosts through close contact with the reservoir animal. Six out of ten infections in humans globally are spread from animals, and 75% of emerging infections are zoonotic. Some occur worldwide e.g. E. coli O157:H7, whereas some are more restricted geographically, e.g. Ebola virus. The highest burden is in developing countries. There are various classifications of zoonoses. ● Causative pathogen: bacterial (anthrax, non-typhoidal Salmonelloses); viral (rabies, Yellow Fever, hantaviruses); parasitic (hookworm, Giardia, toxoplasmosis); fungal (dermatophytes, histoplasmosis); or prion (new-variant Creutzfeldt-Jakob disease). ● Mode of transmission (see Section 35.3 and Table 35.1 below) ● Distribution: endemic zoonoses are continually present in a population (e.g. leptospirosis, brucellosis); epidemic zoonoses occur intermittently (e.g. anthrax, Rift Valley Fever); emerging zoonoses are new infections, or existing infections that are increasing in incidence or geographical range (e.g. Nipah virus, Middle East Respiratory Syndrome coronavirus). ● Direct contact: infectious particles are present on an infected animal, in its body fluids, and in its excreta. Q fever, caused by Coxiella burnetii, and brucellosis may be acquired by direct contact with infected animals, particularly during parturition; cat-scratch disease caused by Bartonella henselae, and Pasteurella spp. may be acquired by bites or scratches from cats, and rabies from canine bites. Many zoonoses are also transmitted via indirect animal contact through exposure to soil or water contaminated by infectious material, e.g. leptospirosis may be acquired when water contaminated with infected rats’ urine comes into contact with broken skin or mucous membranes. ● Ingestion: infection occurs by ingesting contaminated food or water, e.g. unpasteurized milk, poorly processed or undercooked meat, or by eating/ drinking after handling animals without handwashing. Listeria, bovine tuberculosis, and brucellosis may be transmitted by unpasteurized milk and dairy produce; Hepatitis E through processed pork, and Ebola and Marburg through bushmeat. ● Vector-borne: infection is transmitted through a biting arthropod vector. Examples include West Nile Virus and Japanese encephalitis from mosquitoes, Lyme disease, tick-borne encephalitis, and Rocky Mountain Spotted Fever from ticks, and Rickettsia typhi from rat fleas.