Over the past few weeks our clinic has seen a couple of cases of classic anaplasmosis. Anaplasmosis is a clinical condition primarily recognized in cattle caused by the organism anaplasma marginale. This organism acts as a parasite to the red blood cell of the animal resulting in destruction of circulating red blood cells and rapid onset of severe anemia. This condition can produce acute death in cattle and is a complex disease in regards to effective treatment and prevention strategies.
Anaplasma marginale survives mainly in the blood of the affected animal or in the biological vector, the tick. Certain cows within a herd can maintain very low levels of the organism in their blood resulting in a carrier state in which the animal shows no outward sign of illness but yet remains infected for life. These cows can then serve as a risk of infection to other animals in the herd. Ticks are primary vectors of the disease. When a tick feeds from an infected or carrier animal, anaplasma organisms are consumed and then multiply in the intestine of the tick. Other vectors include biting flies, mosquitoes and contaminated needles or instruments used for processing tasks such as dehorners and castrating knives.
Once infected, the animal goes through a 3-6 week incubation period and then may progress to acute clinical disease if no prior immunity is present. Cattle will generally spike a fever, go off feed and may show signs of labored breathing. Pale or icteric gum color may be observed as the animal becomes more anemic. In a matter of a few days the disease can result in death.
Oxytetracycline (LA 200) is the treatment of choice for clinically ill animals. Your vet may recommend other supportive treatments for more severely affected animals. For very valuable animals a blood transfusion can be utilized to provide additional circulating red blood cells.
After successful treatment with oxytetracycline most infected cattle will become chronic carriers. However, a chronic carrier state is not all bad as it allows the animal to have some level of natural immunity to the disease and makes the cow less likely to progress to clinical disease and death on future exposure. Many herds in southwest Missouri are endemic for the disease meaning that most cows in the herd have been exposed and many are chronic carriers. In endemic herds clinical cases are occasionally seen during the warmer months of the year corresponding to the vector season but large herd outbreaks are uncommon. The biggest risk to animal health comes with the introduction of a carrier animal to a naïve herd. Additionally, if a naïve bull, for example, is introduced into a herd of cows endemic for anaplasmosis, then the bull is at high risk for clinical disease.
Feeding chlortetracycline during the vector season is the best means by which to prevent clinical cases and herd outbreaks. Continuous feeding for 6-8 weeks at the higher recommended dosing level can provide herd clearance for those herds interested in achieving anaplasmosis free status. In my opinion, this should be reserved to seedstock producers or those producers looking to export cattle in which a disease free status is necessary. It is also important to remember that once the disease is eradicated from a herd the herd has no remaining natural immunity and extreme care should be taken not to inadvertently reintroduce the disease. Treatment, prevention and control strategies for anaplasmosis are detailed and complex and one in which each producer should closely involve their local veterinarian.
Darren Loula, DVM, is a large animal veterinarian at Fair Grove Vet Service in Fair Grove, Mo.
