Anaplasma marginale or Anaplasma caudatum are the infectious cause of the general disease termed Anaplasmosis, also known as gall sickness. Anaplasma species are rickettsial parasites or bacteria that spend some of their life cycle within an arthropod. This biologically transmissible disease is most commonly identified in tropical and subtropical areas such as the Gulf Coast, but has become a concern in the southern and western U.S. as well. The infective organisms, transmitted by ticks and biting flies, can be found in the affected animal’s red blood cells.
Mechanical vectors such as needles, dehorning instruments, and tattoo guns have also been found to spread the disease through contact of infected blood cells. Transmission through the placenta has been reported as well.
Young animals typically show less clinical signs of illness and become lifetime carriers of the microorganism. In endemic areas where young cattle are exposed early to the infective organisms, losses are minimal.
Cattle over 24 months of age reportedly have the highest mortality rate; often over 50 percent of infected animals succumb to disease. During the growth phase of illness, infective organisms can double in quantity every 24 hours.
The first sign of infection is the presence of organisms in the red blood cells—causing anemia due to the removal of infected cells by the host animal’s immune system. The initial incubation period can last from three to eight weeks without observable clinical signs until an animal demonstrates a raised body temperature (often 105 to 106 degrees) and anemia. Other signs of illness include anorexia, depression, bounding pulses, altered breathing patterns, and a slow gait. Mucous membranes will become very pale and may even turn yellow with severe illness.
Carrier animals are those that have survived either an acute or chronic infection and live with an undetectable parasitemia—the organism cannot be identified on a blood sample. Chronically infected animals often have decreased body condition scores, fertility, and production.
Diagnostic testing and treatment for Anaplasmosis can rapidly become an expensive endeavor. Necropsy will usually identify effusions, or fluid within body cavities, jaundiced tissues, and very thin, watery blood. Organisms may be identified on a blood smear as a bluish staining round lesion on the outer edge of the red blood cell. Chronically infected carriers may be identified with serologic testing using ELISA, complement fixation, or card agglutination tests. DNA-based detection methods are most useful as species and strain differentiation tests.
Antibiotics can help to reduce the number of organisms in the blood cells; however, this is most effective in animals with packed cell volumes over 15 percent. More severe infections may require blood transfusions in order to keep the animal’s organs functioning adequately. Antibiotic withholding periods may be lengthened as well.
In South Africa, Australia, South America and some of the Middle East, a vaccine containing live Anaplasma organisms has been used for partial protection. The U.S. does not advocate the live vaccine, but has previously had available a purified Anaplasma vaccine from infected bovine red blood cells. Many vaccine forms have been attempted and resulted in severe reactions; these are currently under investigation. A compounding factor is the variety of strains identified throughout the U.S.
As in many cases, the best scenario for your herd is to control and prevent disease. Protocols should be in place regarding insect and tick control, as well as disinfecting equipment between animals and utilizing singleuse needles. Maintaining a good history or where new animals have travelled from is also an important preventative measure. — Dr. Genevieve JM. Grammer
[Dr. Genevieve JM. Grammer is a veterinarian working out of the Pikes Peak region. Please address correspondence to drgigi19@ gmail.com.]