Insect-Transmitted Disease: It’s That Time of Year Again

William Shulaw, Extension Veterinarian, The Ohio State University

West Nile virus and eastern equine encephalomyelitis virus (EEE) have been killing horses and birds for several weeks now across the USA. Last week we received reports of a new outbreak of EHD virus in deer in Montana that was killing deer “in droves.”(1) This is reminiscent of the 2007 outbreak of that disease in Ohio deer. And last Saturday I saw my first horse fly of the season in Hocking County.

These observations serve to remind us that the fall season of insect-transmitted disease is in full swing. West Nile virus and EEE virus are transmitted by mosquitoes; EHD is transmitted by a biting midge or gnat called Culicoides; and biting flies like horse flies can transmit anaplasmosis to cattle. Last fall several outbreaks of anaplasmosis were documented in herds in Ohio.(2) Anaplasmosis is transmitted by biting flies and mechanically by blood-contaminated objects such as hypodermic needles, some kinds of tagging instruments, surgical instruments, nose tongs, and possibly tattoo equipment. In some regions of the country, ticks are an important means of spread of the disease and may also serve as a reservoir for the anaplasma organisms. We don’t have very much information about the natural history of anaplasmosis in Ohio, but because the majority of outbreaks are in the months of August through October, transmission by horse flies is strongly suspected. There is no evidence at this time of tick transmission in Ohio, and deer are not thought to be important reservoirs for cattle.(3)

Animals infected as calves usually do not get sick, but they remain carriers for life unless effectively treated. Animals that recover from the disease also remain carriers unless effectively treated. These carrier animals serve as a reservoir for the infectious agent and are perhaps the most common way the disease is spread within and between herds. It is believed that biting flies, like the horse fly, are not highly efficient transmitters of the disease and for transmission to occur between herds, very close contact between cattle of different herds would have to occur as the anaplasma organisms in blood on fly mouth parts are believed to be viable for only a short time. Recently published work suggests that spread by contaminated needles, such as might occur when vaccinating a group of animals, may be more efficient than previously thought.(4)

Signs of disease appear about 10-60 days after infection. Animals that develop clinical anaplasmosis are usually older than a year, and the signs usually begin with a fever of about 104 degrees F. or above. The red blood count can fall very rapidly and animals can become severely anemic in just a few days. As the anemia progresses the animal gets weak, reduces or refuses feed intake, and becomes lethargic. Their gait may become wobbly. Lack of oxygen to the brain resulting from sludging of blood cells in the blood vessels may cause them to act aggressively or behave abnormally. Cows in advanced pregnancy may abort. Often the farmer doesn’t notice signs of the disease until he sees a very weak cow or finds one that has died. Cows in advanced stages of the disease may die when they are handled for diagnosis or treatment.

If you suspect anaplasmosis in your herd, call your veterinarian right away. Prompt treatment can salvage many animals and prevent further deaths. Prevention and control with chlortetracycline in feed is useful. This disease is not as common in cattle as some diseases, but it is becoming more common. The carrier state is not easily treated, but detecting carriers is relatively easy. Farmers should use caution in purchasing animals and would be well-advised to test animals before or soon after purchase to prevent introducing the disease to their herd with carrier animals.

1. Hanback, M. accessed 8-23-11
2. Shulaw, W. Issue #705, September 29, 2010 & November 28, 2012,BEEF Cattle Letter
3. Keel MK. et al. An assessment of the role of white-tailed deer in the epizootiology of anaplasmosis in the southeastern United States. J Wildl Dis. 1995 Jul;31(3):378-85.
4. Reinbold JB. et al. Comparison of iatrogenic transmission of Anaplasma marginale in Holstein steers via needle and needle-free injection techniques. Am J Vet Res. 2010 Oct;71(10):1178-88.