– William Shulaw, Extension Veterinarian, Ohio State University
Anaplasmosis is a disease that does not get a lot of attention in Ohio although I have gotten calls about it about every 2-3 years since I have been in Veterinary Extension (1988). One of my colleagues indicated to me several years ago that he had diagnosed it in at least one Ohio herd as far back as the mid-1970s. We know very little about the prevalence or natural history of the disease in this state, however, it occurs in many states in the southeastern US, the Gulf Coast states, and some regions of the West.
Anaplasmosis is a disease affecting the red blood cells of cattle and is caused by a rickettsial parasite called Anaplasma marginale. Parasitized red blood cells are removed from the circulation and destroyed by the spleen and liver. When high levels of parasitized cells occur, usually shortly after a cow is first infected, severe anemia can result; sometimes resulting in deaths or abortions. If a cow recovers from the infection, she becomes a carrier of the disease for the rest of her life. Calves that become infected before about a year of age seldom show much in the way of clinical signs except for perhaps a fever lasting just a few days. They too become carriers for life.
The disease can be mechanically transmitted by biting flies and blood-contaminated inanimate objects such as hypodermic needles, some tagging instruments, surgical instruments, nose tongs, and possibly tattoo equipment. Fortunately, the organisms in a drop of blood don’t survive on insect mouth parts or equipment for very long and cleaning and disinfection of equipment reliably destroys them. There is recent evidence that contaminated injection equipment may be a more reliable way to spread the disease than we once thought. Certain species of ticks can also spread the disease, and the male tick in some of those species is now thought to be most important because it can become a persistent carrier in which the organism actually multiplies. Ticks do not appear to be important transmitters of the disease in some areas. The incubation period from exposure to clinical signs varies from about 10-60 days.
Animals that develop the clinical signs of 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 anemia 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. In one recent Ohio outbreak, two abortions preceded the first adult cow’s death and a total of four of 50 cows died over a period of about two weeks.
Treatment with injectable tetracyclines will usually halt an outbreak in a herd and may prevent the development of severe anemia in an animal already incubating the disease. However, one or two injections will not prevent newly infected animals from becoming carriers. In fact, there are no products currently available that are labeled for the elimination of the carrier state. Elimination of the carrier state with injectable or oral tetracycline antibiotics may be successful with prolonged treatment under the direction of a veterinarian. These carrier animals represent a continual source of infection for their herdmates and potentially for other herds in the area.
In our state, the disease is usually reported in the late summer and early fall following periods when biting flies are most active. However, it may occur any time of the year if contaminated needles or other instruments transfer blood from a carrier animal to susceptible animals. It seems to be most frequently reported in beef cattle, but dairy cattle are also susceptible and the disease has occurred in at least one dairy farm in Ohio in the past. Carrier animals are responsible for moving the disease to uninfected herds and regions. If farmers believe that their herds are not infected, it would be well to consider testing for this disease in purchased animals.