Zoonoses are diseases which can be transmitted by animals to humans. They may be caused by bacteria, viruses, parasites, or fungi and can be passed on through food, the environment or direct contact with an animal. As the human population increases and more of the world is used for food production, contact with wild animals is increasing and zoonotic diseases may be becoming more common. Air travel and crowded living in cities help zoonoses to spread once they have been contracted. While some zoonoses, such as rabies, can be fatal, others cause milder illness, and many zoonoses may not cause any symptoms in the animals which carry them.
Some zoonoses are passed on to people from common domestic animals such as dogs and cats. Rabies is an infectious disease caused by a virus. It is typically transmitted to humans through a bite or scratch from an infected dog, and is usually deadly if left untreated. Flu-like symptoms progress until the infected person becomes paralyzed, falls into a coma and dies. The disease can be prevented, or successfully treated before symptoms develop, by cleaning the original wound and administering an anti-rabies vaccine.
Cat scratch fever is a zoonosis caused by bacteria and is passed to humans via a cat scratch or bite. Initially a rash develops around the infection site, followed by swelling of nearby glands, or lymph nodes. Sometimes infected people may experience fever and feel unwell, but many have no symptoms. The disease usually goes away on its own, although it can be more severe in children. Cat scratch fever may be treated with antibiotics.
The zoonoses known as swine flu and bird flu both cause forms of influenza which have the potential to become life-threatening in humans. Swine flu can be passed from pigs to humans and also from human to human. Bird flu cannot be transmitted between humans, and a person only becomes infected through direct contact with birds. Both diseases have symptoms which are similar to human flu, such as sore throat, fever and muscle aches, but complications such as pneumonia may occur and can cause death in some cases. Antiviral drugs are available to treat both bird flu and swine flu.
Perhaps one of the most famous zoonoses, the plague or Black Death, wiped out millions of people during the Middle Ages. The disease is caused by bacteria, which infect rodents and also the fleas which infest them. A human acquires the illness after being bitten by an infected flea. Symptoms typically affect the lungs, liver and spleen and, if not treated with antibiotics, the disease progresses into a pneumonia which is often fatal.
