A zoonosis (pronounced /ˌzoʊəˈnoʊsɨs/) or zoonose is any infectious disease that is able to be transmitted (in some instances, by a vector) from other animals, both wild and domestic, to humans or from humans to animals (the latter is sometimes called reverse zoonosis). The word is derived from the Greek words zòon (animal) and nosos (ill). Many serious diseases fall under this category.
The simplest definition of a zoonosis is a disease that can be transmitted from other vertebrate animals to humans. A slightly more technical definition is a disease that normally exists in other animals, but also infects humans.
Historical development of zoonotic diseases
Most of human prehistory was spent as small bands of hunter-gatherers; these bands were rarely larger than 150 individuals, and were not in contact with other bands very often. Because of this, epidemic or pandemic diseases, which depend on a constant influx of humans who have not developed an immune response, tended to burn out after their first run through a population. To survive, a biological pathogen had to be a chronic infection, stay alive in the host for long whiles, or have a non-human reservoir in which to live while waiting for new hosts to pass by. In fact, for many ‘human’ diseases, the human is actually an accidental victim and a dead-end host. (This is the case with rabies, anthrax, tularemia, West Nile virus, and many others). Thus, much of human development has been in relation to zoonotic, not epidemic, diseases.
Many modern diseases, even epidemic diseases, started out as zoonotic diseases. It is hard to be certain which diseases jumped from other animals to humans, but there is good evidence that measles, smallpox, influenza, HIV, and diphtheria came to us this way. The common cold, and tuberculosis may also have started in other species.
In modern days, zoonoses are of practical interest because they are often previously unrecognized diseases or have increased virulence in populations lacking immunity. The West Nile virus appeared in the United States in 1999 in the New York City area, and moved through the country in the summer of 2002, causing much distress. Bubonic plague is a zoonotic disease, as are salmonella, Rocky Mountain spotted fever, and Lyme disease.
The major factor contributing to the appearance of new zoonotic pathogens in human populations is increased contact between humans and wildlife (Daszak et al.., 2001). This can be caused either by encroachment of human activity into wilderness areas or by movement of wild animals into areas of human activity due to anthropological or environmental disturbances. An example of this is the outbreak of Nipah virus in peninsular Malaysia in 1999, when intensive pig farming intruded into the natural habitat of fruit bats carrying the virus. Unidentified spillover events caused infection of the pig population which acted as an amplifier host, eventually transmitting the virus to farmers and resulting in 105 human deaths (Field et al.., 2001).
Similarly, in recent times avian influenza and West Nile virus have spilled over into human populations probably due to interactions between the carrier host and domestic animals. Highly mobile animals such as bats and birds may present a greater risk of zoonotic transmission than other animals due to the ease with which they can move into areas of human habitation.
Diseases like malaria, schistosomiasis, river blindness, and elephantiasis are not zoonotic, even though they may be transmitted by insects or use intermediate hosts vectors, because they depend on the human host for part of their life-cycle.
Outbreaks of zoonosis have been traced to human interaction with and exposure to animals at fairs, petting zoos, and in other settings. In 2005, the Centers for Disease Control and Prevention (CDC) issued an updated list of recommendations for preventing zoonosis transmission in public settings. The CDC recommendations, which were developed in conjunction with the National Association of State Public Health Veterinarians, include sections on the educational responsibilities of venue operators, managing public and animal contact, and animal care and management.
In 1988, a person became ill with Swine Influenza Virus (Swine Flu) and died after visiting the display area of the pig barn at a Wisconsin county fair. Three healthcare personnel treating the case patient also developed flu-like illness with laboratory evidence of Swine Influenza Virus infection. Investigators from the CDC indicated in their final report that the Swine Flu had been transmitted directly from pig to human host.
In 1994, seven cases of E. coli O157:H7 infection were traced to a farm in Leicestershire, United Kingdom. An epidemiological investigation into the outbreak revealed that the strain of E. coli O157:H7 isolated from nine animals on the farm was indistinguishable from the strain isolated from human samples. Investigators concluded that the most likely cause of this outbreak was direct human contact with animals.
In 1995, 43 children who had visited a rural farm in Wales became ill with Cryptosporidiosis. ‘’Cryptosporidium’’ was isolated from seven of the ill children. An epidemiological investigation indicated that the source of the children’s illness was contact with calves at the farm.
Also in 1995, at least 13 children became ill with Cryptosporidiosis after visiting a farm in Dublin, Ireland. In a case-control study, researchers compared the activities of the 13 ill children, or cases, to the activities of 52 out of 55 people who had visited the farm – the controls. The study revealed that illness was significantly associated with playing in the sand in a picnic area beside a stream where animals had access.
In 1997, an E. coli O157:H7 outbreak was identified among one child who lived on an open farm and two children who visited the farm during school parties. Two of the three children developed hemolytic-uremic syndrome (HUS). Isolates collected from the three children and from samples taken at the farm were indistinguishable, demonstrating evidence of the link between the farm and the children’s illness.
In 1999, what is believed to be the largest outbreak of waterborne E. coli O157:H7 illness in United States history occurred at the Washington County, New York fair. The New York State Department of Health identified 781 individuals who were suspected of being infected with either E. coli O157:H7 or Campylobacter jejuni. An investigation into the outbreak revealed that consumption of beverages purchased from vendors supplied with water drawn from an unchlorinated fairgrounds well was associated with illness. In all, 127 outbreak victims were confirmed ill with E. coli O157:H7 infections; 71 were hospitalized, 14 developed HUS, and two died.
In 2000, 51 people became ill with confirmed or suspected E. coli O157:H7 infections after visiting a dairy farm in Pennsylvania. Eight children developed HUS. A case-control study among visitors to the dairy was conducted jointly by the CDC, Pennsylvania Department of Health, and the Montgomery County Health Department. The study’s authors concluded that E. coli was transmitted to visitors as a result of contamination on animal hides and in the environment.
Also in 2000, 43 visitors to the Medina County fair in Ohio were confirmed ill with E. coli O157:H7 infections. An investigation into the outbreak suggested that the water system from which food vendors were supplied was the source of the E. coli outbreak. Several months later, five children became ill with E. coli infections after attending a “Carnival of Horrors” event held at the Medina County fairgrounds. PFGE analysis of the strains of E. coli isolated from members of both outbreaks revealed an indistinguishable pattern, and investigators from the Medina County Health Department and the CDC determined that the Medina County Fairgrounds water distribution system was the source of both E. coli outbreaks.
In 2001, an E. coli O157:H7 outbreak was traced to exposure in the Cow Palace at the Lorain County Fair in Ohio. CDC investigators identified 23 cases of E. coli infection associated with attendance at the Lorain County Fair, with additional secondary cases likely. Two people developed HUS. An environmental and site investigation revealed E. coli contamination on doorways, rails, bleachers, and sawdust. Investigators concluded that the Lorain County Fair was the source of the outbreak.
Wyandot County, Ohio, also reported an E. coli O157:H7 outbreak in 2001. Ninety-two E. coli infections were reported to the Wyandot County Health Department and the CDC, with 27 cases confirmed using laboratory analysis. Two cases developed HUS. Contact with infected cattle was believed to be the source of the outbreak; however, a specific cause was never identified.
In 2002, seven people became ill with E. coli O157:H7 infections after visiting a large agricultural fair in Ontario, Canada. Outbreak investigators conducted a case-control study, which indicated that goats and sheep from a petting zoo were the source of the E. coli among fair visitors. Other indications were that the fencing and environment surrounding the petting zoo could have been a source of transmission.
What is believed to be the largest E. coli O157:H7 outbreak in Oregon history occurred among attendees at the Lane County fair in 2002. An Oregon Department of Human Services – Health Services investigation led to the belief that the E. coli outbreak originated from exposure in the sheep and goat barn. In all, 79 people were confirmed ill with E. coli infections as part of the outbreak; 22 were hospitalized, and 12 suffered HUS.
In 2003, fair visitors and animal exhibitors at the Fort Bend County Fair in Texas became ill with E. coli O157:H7 infections. An outbreak investigation led to the determination that 25 people had become ill with E. coli infections after attending the Fort Bend County Fair; seven people were laboratory-confirmed with E. coli, and 5 developed HUS or TTP (Thrombotic thrombocytopenic purpura). Investigators isolated a strain of E. coli indistinguishable from the outbreak strain from four animal husbandry sites, and found high levels of E. coli contamination in both rodeo and animal exhibit areas.
In 2004, a large E. coli O157:H7 outbreak occurred among visitors at the 2004 North Carolina State Fair. During its investigation into the outbreak, the North Carolina Department of Health and Human Services (NCDHHS) received over 180 reports of illness, and documented 33 culture-confirmed cases of E. coli O157:H7 associated with attendance at the fair, with 15 children developing HUS. In its final investigation report, NCDHHS concluded that the North Carolina State Fair E. coli outbreak had originated at a petting zoo exhibit. The conclusion was supported by a case-control study, environmental sampling, and laboratory analysis of samples collected from the fair and members of the outbreak.
In 2005, a petting zoo that exhibited at two Florida fairs and a festival was traced as the source of an E. coli O157:H7 outbreak. Sixty-three people who had visited either the Florida State Fair, the Central Florida Fair, or the Florida Strawberry Festival reported illness to investigators for the Florida Department of Health, including 20 who were culture-confirmed and 7 with HUS. A case-control study revealed that illness was associated with exposure to a petting zoo exhibit present at all three events.
 Contribution of zoonotic pathogens to foodborne illness
In 2006, a conference held in Berlin was focusing on the issue of zoonotic pathogen effects on food safety, urging governments to intervene the problem, and the public to be vigilant towards the risks of catching food-borne diseases via the route of farm-to-dining table.