Methods of Disease Transmission. There are many insects that are the primary or intermediate hosts or carriers of human diseases. Pathogens that are capable of being transmitted by insects include protozoa, bacteria, viruses, and such helminths as tapeworms, flukes, and roundworms. There are two methods of transmission of a pathogen by insects: mechanical and biological.
Malaria. The foremost disease carried by insects is malaria, involving a Plasmodium protozoan that is transmitted by mosquitoes of the genus Anopheles. Malaria is the most deadly Arthropod-borne disease in the world, affecting some 250 million people in the world, with as many as 2 million deaths annually. In the United States, a few cases of malaria occur each year, but only in individuals who have traveled in diseased areas of foreign countries.
Arboviruses. These diseases are caused by viruses that are biologically transmitted by the bite of mosquitoes. There are about 28 viruses of major public health importance that are transmitted by a variety of mosquitoes. Dengue and Yellow Fever are transmitted by mosquitoes in the genus Aedes. There are several kinds of Encephalitis, and these are transmitted by mosquitoes in the genera Aedes and Culex.
Plague. Fleas are the vector for the plague (or black death), which infects man as well as rats and other rodents. There are three forms of plague that occur in humans: bubonic, pneumonic, and septicemic. The bubonic type, in the form of the bacterium, Yersina pestis, is transmitted by fleas. The disease is passed as fleas regurgitate plague bacilli when biting, when flea feces are scratched into the skin, or when the host ingests an infected flea. The plague has killed millions of people in history, especially in the 14th and 17th centuries. In 14th century Europe, the great pandemic resulted in twenty-five million deaths. The plague is still a problem to society, with some 5,000 cases annually.
Enteric diseases. There are many bacterial diseases that are transmitted by some form of fecal contamination of food or water, either directly or indirectly. House flies are a primary agent in transmitting these diseases, and do so mechanically. Typhoid Fever (Salmonella typhi) is a well-known enteric disease, and affects humans worldwide. Cholera is another enteric disease of great importance. Shigella, causing dysentery and diarrhea, and Escherichia coli, causing urogenital and intestinal infections, are widespread enteric diseases.
Lyme disease. This disease is caused by an arachnid, the deer tick, which carries a bacterium called Borrelia bugdorferi. When a person or warm-blooded mammal is bitten, the bacterium enters the bloodstream, and Lyme disease may occur. The disease was first described in New England and the mid-Atlantic states, but is now known from all of the Northern Hemisphere.
Sleeping Sickness. This disease is also known as African Trypanosomiasis. The disease is transmitted by the Tsetse Fly, in the genus Glossina. The causative agent of African trypanosomiasis is Trypanosoma brucei (two forms). The disease is known to have a high mortality rate, not only among people, but among cattle, which was one of the reasons that parts of Africa could not be settled. Wholesale destruction of habitat and reservoir hosts has had some positive impact on the distribution of the disease.
American Trypanosomiasis. This disease is also known as Chagas' Disease. Trvpanosoma cruzi, a protozoan and causative agent of Chagas' Disease, invades the muscle cells of the digestive tract and heart, and sometimes also the skeletal muscle. There the protozoa multiply. Adult trypanosomes may circulate in the blood, but they do not invade blood cells the way malaria parasites do. Transmission of the protozoa is by Conenose Bugs, also known as Kissing Bugs, and is by the bug's feces, not the bite. Conenose Bugs feed at night on their sleeping victims.
Selected References:
Faust, E. C., Beaver, P. C., & Jung, R. C. 1962. Animal Agents and Vectors of Human Disease. Lea and Fibiger, Philadelphia.
Horsfall, W. R. 1962. Medical Entomology. Arthropods and Human Disease. Ronald Press, New York.
James, M. T. & Harwood, R. F. 1969. Herms's Medical Entomology (6th ed.). MacMillan, New York.
Lane, R. P. and Crosskey, R. W., editors. 1993. Medical Insects and Arachnids. Chapman and Hall, London, etc.
Prepared by the Department of Systematic Biology, Entomology Section,
National Museum of Natural History, in cooperation with Public Inquiry Services,
Smithsonian Institution
Information Sheet Number 77, 5/99
