Читать книгу Parasitology - Alan Gunn - Страница 29

A zoonotic infection (zoonosis) is one that is freely transmissible between humans and other vertebrate animals. The transfer of Plasmodium falciparum malaria between two people by a mosquito is therefore not zoonosis because a mosquito is not a vertebrate and P. falciparum only infects humans. By contrast, a mosquito transmitting Plasmodium knowlesi from a monkey to a human would be an example of a zoonosis because the P. knowlesi infects both monkeys and humans and we are both vertebrates. A disease that is only transmitted between humans is called an anthroponosis and a good example would be P. falciparum.

Many of the most important parasites in human and veterinary medicine are zoonotic infections. For example, pigs are the normal intermediate host of the pork tapeworm Taenia solium, and we are its definitive host. Therefore, pigs infect humans, and we infect pigs. Sometimes, humans are just one additional host within a parasite’s life cycle. For example, the blood fluke Schistosoma japonicum has many definitive hosts apart from humans, including dogs, cattle, pigs, and rats. Consequently, all these definitive hosts can shed eggs that will infect the snail intermediate hosts, and the resultant cercariae can infect all of them.

The transmission of zoonotic parasites is usually heavily influenced by the nature of human: animal relationships. Therefore, they can be both simultaneously theoretically simple and recalcitrant to control. This is because their control often depends upon changing human behaviour, and this depends upon a complex mix of culture, religion, tradition, economics, personality, and politics. For example, theoretically, many zoonotic infections might be halted by simple acts of basic hygiene or the cooking of food. However, people are often unable or unwilling to change the way they live their life for all sorts of reasons. Zoonotic infections should not always be considered from the risks that they pose to us. Sometimes, wild animal populations can be threatened by the diseases that we transmit to them. We will consider specific instances of this throughout the book.