Anaplasmosis is a tick-borne blood disease caused by the bacterium Anaplasma marginale, and it’s the most prevalent tick-borne cattle disease in the world. The organism infects red blood cells, triggering the animal’s immune system to destroy those cells en masse, which leads to severe anemia. It costs the U.S. beef industry an estimated $300 million annually through death losses, reduced productivity, and increased culling.
How Cattle Get Infected
Anaplasma marginale spreads through three main routes, all involving blood transfer from an infected animal to a healthy one. The primary biological vectors are Dermacentor ticks (the same genus that includes the American dog tick and the Rocky Mountain wood tick). These ticks pick up the organism while feeding on an infected animal and transmit it to the next animal they bite. Tabanid flies, commonly known as horse flies, serve as mechanical vectors, carrying infected blood on their mouthparts from one animal to another.
The third route is the one producers have the most control over: blood-contaminated equipment. Needles, dehorning instruments, ear taggers, castration tools, and tattoo equipment can all transfer infected blood between animals. A single contaminated needle used across a group during vaccination or treatment can spark an outbreak through an entire herd.
Why Age Determines Severity
One of the most important things to understand about anaplasmosis is how dramatically the disease changes with age. Calves under one year old can become infected, but they rarely show clinical signs. The disease is essentially invisible in young animals. In yearlings and two-year-olds, it becomes moderately severe. In mature cattle, particularly those over three years old, anaplasmosis is severe and often fatal.
This age pattern catches many producers off guard. A herd that has quietly harbored carrier animals for years can suddenly experience devastating losses when naive adult cattle are introduced, or when tick pressure increases and exposes previously uninfected older animals for the first time. In adult cattle that have never encountered the disease, mortality reaches 30 percent, calving success drops by 3.6 percent, and culling rates increase by 30 percent.
Signs to Watch For
Because the organism destroys red blood cells, the hallmark of anaplasmosis is progressive anemia. Infected cattle develop fever, weakness, and rapid weight loss. Milk production drops noticeably in dairy and beef cows nursing calves. You may notice constipation, labored breathing, and reluctance to move. Pregnant cows can abort.
The most visible diagnostic clue is the color of mucous membranes. As anemia worsens, the gums, inner eyelids, and vulvar membranes turn pale. As destroyed red blood cells release their contents into the bloodstream, those same membranes shift from pale to yellowish, a sign called icterus or jaundice. Increased aggression has also been reported in some infected animals, likely a result of oxygen deprivation to the brain as the anemia progresses.
The incubation period is long. After transmission, it takes two to six weeks before the infection even becomes detectable on a blood smear, and clinical signs may not appear until red blood cell destruction reaches a critical threshold. This delay makes it difficult to trace exactly when and where an animal was exposed.
How Anaplasmosis Is Diagnosed
The most common diagnostic method during active illness is a Giemsa-stained blood smear, where a veterinarian examines a drop of blood under a microscope for infected red blood cells. This works well when an animal is clinically sick and has a high level of infection, but it has real limitations. It can be difficult to distinguish between related Anaplasma species on a smear, and severe anemia often persists for weeks after the organisms have become nearly undetectable in the blood.
For identifying carrier animals that show no symptoms, PCR testing is far more sensitive. Standard PCR can detect infection when as few as 0.0001 percent of red blood cells are infected. A more advanced nested PCR method can identify as few as 30 infected red blood cells per milliliter of blood, well below the lowest levels found in carriers. Real-time PCR achieves similar sensitivity with better quality control. These molecular tests are particularly valuable when screening new animals before introducing them to a herd, or when trying to determine the carrier status of recovered animals.
The Carrier Problem
Once cattle recover from anaplasmosis, they remain latently infected for life. These carrier animals look perfectly healthy and perform normally, but they harbor low levels of the organism in their blood indefinitely. Every carrier is a potential source of infection for the rest of the herd, whether through tick feeding, fly bites, or contaminated equipment during routine herd work.
This lifelong carrier state is what makes anaplasmosis so persistent in a herd once it’s established. You can’t visually identify carriers, and standard blood smears won’t detect the low-level infections they maintain. Only sensitive molecular testing like PCR can reliably flag these animals. Producers managing endemic herds have to decide whether to test and cull carriers, manage tick populations aggressively, or accept endemic infection and focus on ensuring young animals develop immunity before they reach the age where disease becomes dangerous.
Treatment Options
Tetracycline antibiotics are the primary treatment for anaplasmosis in cattle. Oxytetracycline is the standard choice, administered by injection (intramuscular, subcutaneous, or intravenous depending on the situation). Early treatment during the acute phase can prevent death, but timing matters enormously. By the time an animal is visibly jaundiced and staggering, the red blood cell destruction may already be too advanced for treatment to save it. Handling severely anemic cattle roughly, including the stress of running them through a chute for treatment, can itself cause death from cardiac failure.
Some producers in endemic areas feed low-level tetracycline during peak tick season as a preventive measure, aiming to suppress infection before it reaches clinical levels. This approach requires careful planning around withdrawal periods for animals destined for slaughter.
Prevention and Biosecurity
Tick control is the foundation of anaplasmosis prevention. Pour-on insecticides, ear tags, and pasture management that reduces tick habitat all help lower transmission pressure. Controlling horse fly populations is more difficult but worth attempting in high-risk areas, particularly around water sources where these flies breed.
Equipment hygiene during herd work is straightforward but often neglected. Changing needles between animals is the single most impactful practice. Any instrument that contacts blood, from dehorning tools to castration knives, should be disinfected or replaced between animals. This is especially critical in herds with known carriers or in regions where anaplasmosis is endemic.
Testing new animals before they enter the herd prevents the most common way anaplasmosis arrives on a previously clean operation. A single purchased carrier, showing no symptoms whatsoever, can seed an outbreak that affects dozens of animals months later when tick activity peaks.
Vaccine Availability
Vaccination options for anaplasmosis remain limited. Killed vaccines made from infected red blood cells have been used for decades with inconsistent results. An experimental whole-cell killed vaccine is currently marketed in the United States, but published scientific evidence supporting its effectiveness is scarce. Recent research has demonstrated that whole-cell inactivated vaccines are ineffective against clinical anaplasmosis in controlled challenge studies. A genetically modified live vaccine has shown protective immunity against tick-transmitted infection in experimental settings, but it is not yet commercially available. For now, producers rely primarily on tick control, equipment hygiene, and strategic antibiotic use rather than vaccination to manage herd risk.