Diagnosis of acute disease is by finding A. marginale bodies at the periphery of the RBCs. Once the acute stage is advanced, it can be difficult to find the organism. Late clinical
disease or the chronic carrier state has to be diagnosed with serologic tests.
In the past, the rapid card agglutination and complement-fixation (CF) tests most commonly were run. A new competitive enzyme-linked
immunosorbent assay (cELISA) appears to have better sensitivity. However, the sensitivity of the serologic tests depends on
the stage of infection. Early in the incubation phase, these tests might be negative.
On necropsy, tissues are pale, turning icteric in later stages. The blood is thin and watery. Splenomegaly occurs in most
Clinical signs usually don't occur until about 50 percent of the RBCs have been destroyed. However, because many of the remaining
cells already are infected and will also be destroyed, the clinical course is difficult to change, even with treatment. Oxytetracycline
is effective, but it works best in early or preclinical stages. Many times cattle aren't recognized as infected until they
are severely anemic, so treatment might not help. In fact, the stress can kill the animal, especially beef cattle. A blood
transfusion might be indicated for some animals.
Treatment of the remainder of the herd with long-acting formulations of oxytetracycline should be considered. In endemic areas,
the mortality rate is rarely very high, although subclinical disease (abortions, decreased milk production) might still warrant
For herds in non-endemic areas that are unlikely to have any immunity, mortality rates can be high. Therefore, once the disease
is diagnosed, treatment of the entire exposed group should be considered.
Prevention and control
In non-endemic areas, the goal is to prevent the disease from coming into a herd. Herd additions should be screened with the
cELISA or purchased from test-negative herds. Because of the potential for possible false negatives on the cELISA during the
incubation phase and the potential for wildlife reservoirs, the disease still can sneak in. Therefore, a new needle should
be used for all animals and equipment should be washed/disinfected between animals.
If available, vaccination, especially of the most valuable animals, should also be considered. The only vaccine available
is produced by University Products and is offered in 14 states via a U.S. Department of Agriculture experimental approval.
However, work is ongoing to allow availability nationwide. Visit
http://www.anaplasmosis.com/ for more information about the vaccine.
In infected herds in non-endemic areas, or in herds in endemic areas, the goal is to minimize losses. Because of potential
in utero infections, vector transmission, wildlife reservoirs and false-negative serologic tests in the incubation phase,
once a herd is infected it is difficult to eliminate the disease, even in non-endemic areas.
For infected herds in non-endemic areas, periodic testing with the cELISA to continually remove carrier animals and proper
needle and equipment use might help decrease losses. Vaccination or tetracycline in feed or mineral supplements during the
vector season can help. Neither will prevent animals from becoming carriers but will decrease chances of abortions, milk-production
losses and clinical disease.
Supplements containing tetracycline must be labeled for anaplasmosis prevention to contain high enough levels to be effective.
Due to the continual risk of introduction of anaplasmosis in endemic areas, and because carrier cattle are resistant to the
disease, it is not recommended that herds in endemic areas remove carriers. And, because young animals are resistant to disease,
control of transmission is concentrated on older, more susceptible animals.
A new needle should be used for each pregnant replacement heifer and any animal 2 years of age and older to prevent spread
to animals most likely to abort or have acute infections. Tetracycline in feed or mineral supplements might also help. However,
intake of these products is variable from animal to animal and, with the high exposure pressure in endemic areas, some animals
might still be vulnerable.
If available, vaccination probably is the most effective means of preventing losses in herds in endemic areas. Vector control
and good biosecurity practices are important in decreasing spread of anaplasmosis in all herds.
Introducing cattle from non-endemic areas to endemic areas should be done carefully. If possible, introduce new animals during
the non-vector season (if there is one). Vaccination on arrival is indicated. If vaccination is not available, consider treatment
of the new animals with long-acting oxytetracycline two weeks after arrival if introduced during the vector season.
Anaplasmosis can be devastating for producers and costs the American cattle industry millions of dollars each year. There
is still much that is not understood about the disease, including the importance of wildlife reservoirs. However, with the
availability of more modern research techniques, these questions can start to be answered. In the near future we hope to gain
a greater understanding of the epidemiology of this disease and have better methods of controlling it.
Dr. Navarre works as an extension veterinarian with Louisiana State University's Department of Veterinary Science.