A pest is an organism characterized by humans as injurious or unwanted. The term is sometimes reserved for animals also, but many people use it for all kinds of harmful and unwanted organisms, including plants, fungi and viruses.  

One person's pest can be someone else's pet and it is possible for an animal or other organism to be a pest in one setting and highly beneficial in another. The Black Rat (Rattus rattus) is for instance shunned as a pest in many agricultural societies since it breaks into grain storages and homes to eat food intended for human consumption. It is also a carrier of the rat flea and therefore indirectly responsible for spreading the bacterium Yersinia pestis which causes bubonic, pneumonic and septicemic plague. The Black Rat can also carry a long row of other diseases, including typhus, toxoplasmosis and trichinosis.

All these characteristics makes the Black rat a typical pest, but that doesn’t mean that everyone regard the animal as a pest. The Karni Mata temple at Deshnoke in India is for instance colloquially known as the Temple of Rats since all rats living in the temple are treated as sacred and given food and protection. The Black Rat is also sometimes kept as a pet throughout the world, although domesticated forms of the Brown rat (Rattus norvegicus) are more common in pet shops.

Some pests cause problems for humans directly by spreading human diseases, while others affect us indirectly, e.g. by killing, injuring or spreading disease to livestock or pets, by feeding on crops in the field, or by seeking out and damaging food supplies. In addition to this, some animals feed on us humans, e.g. tics, lice and fleas. Some animals are shunned as pests since they can deliver a harmful sting or bite, such as wasps and venomous snakes. Pests can also damage clothes, building materials, fishnets, forests, and etcetera. Sometimes invasive species are regarded as pest, when disturbing existing eco-system.

Example of a pest that carry disease to humans
The feared malaria disease is caused by a protozoan parasite that cannot spread directly from human to human through air or skin contact. Instead, it is spread by mosquitoes from the genus Anopheles. Some species of Anopheles can also transmit canine heartworm (Dirofilaria immiti), elephantiasis, and o’nyong’nyong fever.

Example of a pest that harms domesticated animals
Certain species of the family Oestridae, commonly known as bot flies, are a problem for horses and cattle since they live as parasites on mammals. On horses, bot flies usually lay their eggs on the inside of the front legs, on the cannon bone and knees, and sometimes also on throat and nose. When the horse rubs its nose among the eggs, the eggs travel from nose and mouth to the intestines where the larvae hatch and migrate back to the skin. When a larva is ready to emerge, a painful thumbnail-sized lump will form on the skin through which the animal crawls out. The emergence of larvae can make the horse to tender to ride and migrating larvae within the body are known to cause stomach ulcers and colic. The whole where the larva escaped can become infected with bacteria.   

Example of a pest that lives in our food
The Indianmeal Moth (Plodia interpunctella) is a small moth with reddish brown coppery forewings. When the eggs hatch, the emerging larvae are ready to attack a wide range of food products, such cornmeal, flour, rice, cereals, dried fruit and vegetables, and candy. The larva spins a web as it grows larger and you may be able to see silken threads all over infested cupboards. A fully grown Plodia interpunctella larva is roughly 1.25 cm long.


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