Etiology
Anaplasmosis is an infectious, transmissible disease of cattle caused by a rickettsia organism, Anaplasma marginale.  Anaplasmosis is also called “yellow bag” or “yellow fever” as affected animals can develop a jaundiced appearance The organism invades (infects) and resides in red blood cells. The defense mechanism of the infected animal removes the infected red blood cells (RBCs) from circulation, thereby initiating the clinical signs of anemia.

Transmission
Anaplasma marginale can be transmitted two different ways. First, it can be transmitted mechanically when red blood cells infected with Anaplasma marginale are inoculated into susceptible cattle. This can occur through needles, dehorners, ear taggers, castrating knives or other surgical instruments, and tattoo instruments. Mechanical transmission can also occur through the mouthparts of biting insects, such as biting flies. Face flies, houseflies, and other non-biting insects do not transmit the disease. Horn flies, although they bite, typically do not go from animal to animal so they are not thought to spread Anaplasma. Mechanical transmission of infected red blood cells must occur within a few minutes of the blood leaving the infected animal, as the blood parasite does not survive more than a few minutes outside the animal.

 

anaplasmosis 

Outbreaks
Although many outbreaks of anaplasmosis occur in the spring and summer, they can occur at any time of the year. The many ways it can be transmitted and the potential for carrier animals makes the source of an outbreak confusing. If an outbreak occurs in spring or summer, it suggests that the source of the infection is from insect vectors. If the outbreak occurs 3 to 6 weeks after cattle are processed, that suggests Anaplasma was transferred from an infected animal to a susceptible one during processing. If an outbreak occurs at other times, new arrivals or increased stress should be considered as the source of the disease. When any outbreak occurs, it suggests that carrier animals are present either in your herd, or a neighboring herd, as carrier animals are an efficient source of infection. Carrier animals carry Anaplasma in their bodies, but do not show clinical signs and are able to infect other animals. Later, clinically ill animals can continue to spread the disease.

Incubation
The incubation or prepatent period is 3 to 8 weeks.  This is the time between inoculation (infection) and occurrence of A. marginale bodies in 1% of circulating RBCs.  The developmental stage is about 4 to 9 days and is the time when most of the characteristic signs of anaplasmosis appear.  During this phase from 10% to greater than 75% of the RBCs may be infected.

 

 anaplasmosis in RBC 

Clinical signs
Clinical signs of anaplasmosis are related to the age of the animal.  Calves under 6 months of age can become infected, rarely show clinical signs, and remain carriers.  Carrier animals seldom exhibit clinical signs when challenged with Anaplasma  marginale later in life.  A carrier animal may or may not give a positive reaction to serologic tests for Anaplasma marginale.  Anaplasma Marginale bodies may or may not be demonstrated in the stained blood smear of the carrier animal.  However, the carrier’s blood is infective for the susceptible animal.  Yearling to 18-month-old cattle may develop clinical signs of varying degrees of severity, usually recover, and become carriers.  Anaplasmosis in the adult is progressively more severe than the younger age groups; these animals will either die or become carriers.
Clinical signs in adult animals include depression; loss of appetite, fever up to 107 degrees F., anemia or paleness and/or yellow mucous membranes, rapid respiration, urine may be yellowish in color, occasional belligerence, dehydration and constipation, and sudden death.  Most deaths occur in either the later developmental stage or early convalescent stage.

Treatment
Chlortetracycline also known as CTC can reduce the risk of anaplasmosis. Chlortetracycline (CTC) consumed at the rate of 0.5 mg / lb. body weight daily during fly and tick season will help to prevent anaplasmosis. A consistent intake of the correct amount of mineral is crucial to a anaplasmosis prevention program. CTC is available in medicated feed, free choice salt-mineral mixes or medicated blocks. Be sure the product is labeled for anaplasmosis control and follow the label instructions exactly.