A zoonotic disease is a disease that originates in wildlife and which has a reservoir in a wild population of animals, but which leaps the species barriers and can be transferred to humans.
Unlike diseases that reside only in humans, diseases that reside in both humans and wild animal populations are just about impossible to eradicate. The bad news is that there are a tremendous number of such diseases, and the number of them is growing every year.
Malaria -- an old zoonotic disease -- still kills 2 million people a year, while HIV/AIDS is a newer pathogen that has killed 25 million people so far, with another 55 million people (or more) infected worldwide.
The Bubonic plague is an old zoonotic disease, while a newer one is Mad Cow disease, to which can be added SARS, West Nile, and Ebola hemorrhagic fever.
When I was a kid there was no such thing as Lyme Disease in either humans or dogs, but now Lyme disease is one of the most commonly diagnosed diseases here in the Eastern United States.
Meanwhile, it seems a day cannot go by when a rabid fox or raccoon story does not appear in the press somewhere in the United States.
The total number of zoonotic diseases is unknown, but of the 1,415 known human pathogens in 2001, 62% were of zoonotic origin. A quick summary of the vectors for some of the more common zoonotic diseases:
- Bubonic Plague is caused by a bacterium transmitted to humans from black rat fleas. A possible epidemic of bubonic plague is described in the First Book of Samuel, and the so-called Black Death which emerged in the 14th century killed approximately one third of Europe's population. The prairie dog towns of the Western U.S. became infected with the bubonic plague after Chinese immigrants brought the disease to the U.S. after 1900.
- Rabies was described among hunting dogs in Mesopotamia as early as 2,300 BC, and accounts of the disease can be found in early Chinese, Egyptian, Greek, and Roman records. Any mammal can have rabies, but in the U.S. it is most common in feral cats, skunks, bats, fox and raccoons. That said, a groundhog was recently found in Pennsylvania with rabies.
- West Nile Virus is a kind of encephalitis and may have been what killed Alexander the Great, as crows were reported to have died at his feet. In 1999, West Nile virus was introduced into the US, thanks to the introduction of the tiger mosquito which came to the U.S. in a load of tires from Asia. West Nile now strikes rare birds in zoos (especially perching western hemisphere birds such as parrots, eagles, hawks and cormorants) as well as bird feeder birds, horses and humans. The disease is named after a section of Uganda where it was first found.
- Tularemia is transmitted by skin contact with an infested, diseased, or dead hare, rabbit or rodent.
- Hantavirus is spread from rodents to humans by aerosols composed of dust from rodent scat. It is rare, but a concern in the driest parts of the American southwest, especially after wet (or relatively wet) winters and springs.
- Salmonella is generally transmitted to humans through contaminated water reserves and can come from a wide variety of animals, but especially turtles and other reptiles and amphibians.
- Leptospirosis ("rat catcher's jaundice") is generally transmitted to humans who drink water into which a rat has urinated. It can also be transmitted to dogs, pigs and other farm stock.
- Rift Valley Fever is spread by mosquitoes, as is equine encephalitis, and Japanese encephalitis.
- Leishmania is spread by sand-flies.
- Typhimurium occurs endemically in wild birds, causing sporadic cases and small outbreaks in humans.
- Anthrax is primarily a disease of herbivores, but it can be transmitted from wildlife to humans by contaminated meat or water, or by the spread of spores by flies, vultures, and other scavengers.
- Lyme disease comes from ticks that feed on rodents and deer.
- Bovine Tuberculosis can be spread from cattle to humans, and frequently finds a refuge in badgers, possums, wild boar, feral pig, and deer.
- Echinococcosis comes from tapeworms, and moves easily from canids to humans. Fox are particularly susceptible to this parasite as small rodents are the intermediate host.
- Monkeypox is a disease caused by a pox virus that typically occurs in Africa where the African squirrel is the natural host. The disease now occurs among prairie dogs in the United States, thanks to the importation of African rodents for the pet trade.
- Severe acute respiratory syndrome (SARS) is a viral respiratory illness that is believed to have originated in palm civet cats, raccoon dogs, and rats. It has not yet shown up in wild animal populations in the U.S.
- Influenza (the flu) has been reported in a variety of animal species, including birds, humans, pigs, horses, and sea mammals, but its main reservoir seems to be wild waterfowl, especially ducks.
- AIDS is caused by the HIV virus, which appears to be a variety of Simian Immune-Virus (SIV) which may have spread to humans from nonhuman primates in West Africa -- perhaps due to consumption of meat, or infection from monkey blood during slaughter.
- The common cold is a virus that scientists now believe originated in horses.
Humans are not the only species impacted by emerging diseases leaping the species barrier.
Black-footed ferrets have been decimated by distemper that has crossed over from wild dog populations, as have seals and sea lions.
Frogs and many other amphibians around the globe have been decimated by diseases brought into new habitats -- often by scientists coming to study the creatures.
Lowland gorilla populations are being hammered by Ebola hemorrhagic fever, while elk and deer in the western U.S. are falling to chronic wasting disease very similar to Mad Cow disease.