“A proper knowledge of animal parasites…and vigorous application of this knowledge will help to protect the nation’s livestock industry and save it millions of dollars annually.” 

E. M. Nighbert, Associate Veterinarian, Zoological Division, Bureau of Animal Industry, USDA, 1927


Roundworms of domestic swine

Animal parasites and their effect on agriculture

A parasite is an organism that lives in or on another and takes its nourishment from that other organism, or “host.” Parasites of animals and humans come in many forms, including helminths (worms), arthropods (lice, ticks, mosquitoes, etc.), and protozoa. There are over 1,000 species of parasites affecting domesticated animals throughout the world. They can be broadly classified as external or internal, depending on where they live on their host.

External parasites often annoy their hosts by biting, embedding, or otherwise irritating the skin. They can cause serious diseases, such as mange and scabies, which affect animals’ health and growth.

Internal parasites live in the blood or tissues inside an animal’s body. Some organisms enter an animal when it swallows contaminated food or water. Others burrow through the skin, reach the blood stream, and settle in a preferred location to mature and reproduce. Internal parasites often interfere with digestion and assimilation of food, causing poor growth, temporary or permanent injuries, or death. 

Both external and internal parasites may weaken an animal’s immune system and create conditions favorable to bacterial disease. In severe cases, these diseases can also be deadly.

Parasites have been responsible for economic losses ever since humans first undertook the domestication of animals. Farmers and ranchers whose herds are infected with parasites pay higher costs to raise sick animals and earn less because of lower production. Economic losses occur not only when animals die, but also when they are unable to perform their regular work, or when they produce inferior meat, milk, wool, hides, or eggs.


Andrews, John S. 1987. “Animal Parasitology in the United States Department of Agriculture, 1886-1984.” In 100 Years of Animal Health 1884-1984, edited by Vivian D. Wiser, Larry Mark, H. Graham Purchase, and Associates of the National Agricultural Library, 113–65. Beltsville, MD: Associates of the National Agricultural Library, Inc.

Nighbert, E. M. 1927. “Swine Sanitation in Colquit County, Georgia.” U.S. National Animal Parasite Collection Records. Box 135, Folder 7. Special Collections, National Agricultural Library.