Animal diseases.

  • United States Department of Agriculture
Date:
[1956]
Catalogue details

Licence: Public Domain Mark

Credit: Animal diseases. Source: Wellcome Collection.

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    products, but insects have always plagued mankind and domestic ani- mals. Many of the insects that affect livestock also attack man. Several thousand species of arthro- pods are parasitic on animals. They are commonly referred to as external parasites, although some species, such as cattle grubs and horse bots, spend a part of their life in the body of the host. Mosquitoes, gnats, and flies feed on different species of animals. Cattle grubs, horse bots, lice, and some kinds of ticks and mites are so specialized in their mode of living that they live only on one kind of host. External parasites affect the health of animals in one or more ways. Their mere presence may cause severe an- noyance. Many kinds suck the blood of animals. Others destroy body tissues. Insects and ticks also carry important animal diseases, or they may serve as alternate hosts for internal parasites. The bloodsucking parasites are most numerous. They include mosquitoes, enats, flies, lice, and ticks. Some of them become so numerous they de- plete the blood supply of the host. The annoyance and pain caused by mosquitoes, blackflies, horseflies, and deerflies may keep animals from feed- ing at a time when the need for food to replenish lost blood is most acute. The parasites also produce a toxin— sometimes referred to as insect venom— which is injected into the host during the process of piercing the skin and sucking blood. The severe local reac- tion caused in people when they are bitten by insects, ticks, and mites gives some indication of damaging effects on livestock subjected to severe attack by certain bloodsucking parasites. The larvae of cattle grubs, horse bots, and sheep bots live within the host, although the flies that produce them do not remain or feed on the animal. The presence of heavy infestation of the grubs or larvae will weaken ani- mals, lower production, and make them susceptible to diseases. | Larvae of cattle grubs also cause losses to the leather industry by dam- aging the hides of cattle. The different kinds of botflies are serious pests be- cause the animals are extremely an- noyed while the flies lay their eggs or while they deposit living larvae, as does the sheep botfly. Animals react about the way you would expect if the insects were stinging the host. Actually, however, there is no pain associated with egg laying—the animals just have an instinctive fear of the fly—that is probably Nature’s way of helping them avoid excessive parasitism. The screwworm fly is about three times as large as the housefly. Eggs are deposited on wounds, and the tiny maggots that hatch feed on the flesh until they are about one-half inch long and as big around as a match. Infested wounds begin bleeding and attract more flies. Thousands of maggots may be feeding in the wound after a few days. The destruction of tissues and the loss of blood and lymph soon cause the death of the animal if it is not treated. This pest normally occurs only in the southern part of the United States, although local outbreaks occur else- where in the summer. Fleeceworms are caused by flies sim- ilar to the screwworm fly. They lay eggs on soiled wool of sheep. The maggots feed on the surface of the skin and cause severe annoyance. In- festations of fleeceworms are often as- sociated with screwworm infestations in sheep. Like the screwworm, the fleeceworms can kill infested animals unless infestations are treated promptly. Elimination of the cattle-fever tick has solved the most important arthro- pod disease carrier among livestock. There are, however, other diseases among farm animals that are trans- mitted by arthropods. Anaplasmosis, another important protozoan disease of cattle, may be transmitted in several ways, but ticks, horseflies, and other bloodsucking arthropods can transmit the causative agent. Equine encephal- omyelitis, a virus disease of horses, is transmitted by mosquitoes, the princi- pal vector being Culex tarsalis. Blue- tongue, a virus disease of sheep, is car-