Recently I have visited several cattle producers with cases of anaplasmosis in their herd. Anaplasmosis is an infectious disease of cattle that causes destruction of red blood cells. The disease is caused by a minute parasite, Anaplasma marginale, found in the red blood cells of infected cattle. It can be transmitted from infected animals to healthy animals by insects or by surgical instruments. Anaplasmosis outbreaks are related to the lack of a control program, the ratio between anaplasmosis carriers and susceptible animals in the herd, and the amount of vector transmission. Animals with anaplasmosis become weak, abort, reject exercise and potentially die. Anaplasmosis is spread by the transfer of blood from an infected animal to a susceptible one. Primarily, the transmission is “mechanical”—that is, it is transmitted by the mouth part s of biting insects contaminated with A. marginale-infected blood or by contaminated instruments used by human beings. Three biting insects (horse flies, stable flies, and mosquitoes) are known to transmit anaplasmosis mechanically by carrying A. marginale-infected RBCs from diseased cattle to susceptible cattle. In general, if more than 5 minutes elapse between the time when an insect bites a diseased animal and the time it bites a susceptible animal, anaplasmosis is not transmitted. The disease is more likely to be transmitted by insects when cattle gather together, making it easier for insects to bite several animals in a short period of time. Anaplasmosis spreads easily between herds when neighboring cattle congregate under shade trees along fence lines. Control of biting insects, especially the large biting flies, can often be frustrating and is generally not a practical, reliable method for totally preventing transmission of anaplasmosis. However, applications of insecticides that reduce the biting insect population will substantially reduce the number of clinical anaplasmosis cases occurring in a herd. Periodic spraying and dipping, as well as forced use of dust bags and back rubbers, are common methods of insecticide application for beef cattle. The elimination of carriers of anaplasmosis requires the use of antibiotics inject or by mouth. Consult a large animal veterinarian for diagnostic, treatment and prevention of anaplasmosis in your herd.