Anaplasmosis is a tick-borne blood disease in cattle caused by a microscopic parasite called Anaplasma marginale. The parasite invades red blood cells, multiplies inside them, and triggers severe anemia that can kill adult cattle if untreated. It’s one of the most economically significant cattle diseases in the United States, with clinical cases costing an estimated $660 per head. The disease is endemic across much of the southern and central U.S., and evidence suggests it’s becoming more widespread.
How the Parasite Attacks Red Blood Cells
Anaplasma marginale is a type of rickettsia, a tiny bacterium that can only survive inside living cells. In cattle, the only place it replicates is inside red blood cells. Once the parasite enters a red blood cell, it creates a protective bubble made from the cell’s own membrane and begins multiplying. The number of infected red blood cells increases exponentially until the animal’s immune system catches up.
When the immune system does respond, the spleen begins filtering out and destroying infected red blood cells. This is what causes the anemia. Unlike some blood parasites that burst cells open, anaplasmosis destroys red blood cells through the immune system’s own cleanup process. That’s why affected cattle don’t produce red or dark urine, a detail that helps distinguish anaplasmosis from other blood diseases like babesiosis.
How Cattle Get Infected
Ticks are the primary biological carriers. In North America, two species of Dermacentor ticks do the heavy lifting: the Rocky Mountain wood tick, found from British Columbia through south-central Saskatchewan and parts of the western U.S., and the American dog tick, which has a broader range across the continent. These ticks pick up the parasite from an infected animal and pass it along when they feed on the next one.
Biting flies, particularly horse flies and deer flies, serve as mechanical carriers. They don’t harbor the parasite the way ticks do. Instead, they transfer infected blood cells on their mouthparts when they move from one animal to another. This type of transmission is less efficient but still significant, especially during fly season when large numbers of flies are feeding on a herd.
The third route is often overlooked: contaminated equipment. Needles, dehorning tools, castration instruments, ear taggers, and even OB sleeves can carry infected blood between animals. A single contaminated needle used across a group during routine vaccinations can spread the parasite through an entire herd in one afternoon.
Symptoms by Age
One of the most important things to understand about anaplasmosis is that severity tracks directly with age. Calves under six months rarely show signs at all. Between six months and a year, infection is typically mild and subclinical, meaning the animal is infected but appears healthy. Cattle between one and two years old develop acute disease but rarely die from it.
The real danger starts at two years of age and older. Mature cattle are the ones that get hit hardest. Early signs include going off feed, constipation, and pale mucous membranes. As the disease progresses, you may notice labored breathing, exercise intolerance, and sometimes marked aggression. Mucous membranes and the whites of the eyes can turn yellow or golden as the liver becomes overwhelmed. Blood drawn from sick animals is often thin and watery.
In cattle over three years old, 30 to 50 percent of clinical cases are fatal without treatment. Many producers first encounter the disease as “sudden death” in apparently healthy adult cattle, particularly after a stressful event like being moved or worked through a chute. Forced movement and excitement can kill an anemic animal because its blood simply can’t carry enough oxygen.
Disease Stages and Timeline
The incubation period averages three to eight weeks after infection, though wider ranges have been documented. During this phase, cattle look completely normal. The developmental stage follows, lasting four to nine days, and this is when clinical signs appear and escalate rapidly. Death, when it occurs, typically happens in the late developmental stage or early convalescence.
Cattle that survive the acute phase enter a long convalescent period lasting two to three months. During recovery, they lose significant weight and pregnant cows commonly abort. The body ramps up red blood cell production to replace what was lost, but the process is slow.
After recovery comes the carrier stage, and this is where anaplasmosis becomes a persistent herd problem. Unless specifically treated to clear the infection, recovered cattle remain carriers for life. They show no outward signs of disease but harbor the parasite in their blood, serving as a reservoir that ticks and flies can draw from and spread to other animals.
Diagnosing Anaplasmosis
A blood smear examined under a microscope can reveal clusters of the parasite inside red blood cells, but this method isn’t sensitive enough to rely on alone. It works best during peak infection when large numbers of cells are affected, and it can miss early or low-level infections entirely.
PCR testing on whole blood is the most reliable method for confirming active infection, especially in the first week of illness. Its sensitivity drops significantly within 48 hours of starting antibiotics, so timing matters. If you suspect anaplasmosis, collecting a blood sample before treatment begins gives the best chance of a definitive diagnosis.
Antibody testing is useful for identifying carriers and for herd-level screening. It works by detecting the immune response rather than the parasite itself, so it typically comes back negative during the first week of illness. Paired blood samples, one taken during acute illness and another two to ten weeks later, provide the most reliable serological diagnosis. A single elevated antibody titer can support a diagnosis but isn’t conclusive on its own.
Treatment Options
Tetracycline antibiotics are currently the only drugs used in the United States for treating acute anaplasmosis. Both oxytetracycline and chlortetracycline are approved options. Treatment during the acute phase can reduce mortality, but it comes with an important caveat: standard therapeutic doses typically do not clear the infection entirely. Treated animals often survive but become lifelong carriers.
Supportive care matters as much as antibiotics in acute cases. Minimizing stress is critical. Handling sick cattle as little as possible, avoiding forced movement, and keeping them in a calm environment can mean the difference between survival and death for a severely anemic animal.
Prevention and Herd Management
Tick control is the first line of defense. Pour-on treatments, ear tags, and pasture management that reduces tick habitat all play a role. Fly control during peak season helps reduce mechanical transmission. Neither approach eliminates risk entirely, but both lower the odds of exposure.
Equipment hygiene is a simple, high-impact practice. The USDA recommends against using needles, OB sleeves, or other instruments on multiple animals without thorough cleaning and sanitizing between each one. Changing needles between animals during vaccinations and deworming is one of the cheapest and most effective ways to prevent iatrogenic spread.
Vaccination is available but imperfect. One inactivated commercial vaccine is currently produced in the United States. It doesn’t prevent infection, but the manufacturer reports it induces enough immunity to protect cattle against severe clinical disease. In some countries, a live vaccine using a related but less harmful species (Anaplasma centrale) is used to reduce the severity of infection, though it doesn’t prevent it either, and occasional vaccine failures and even fatalities have been documented. No commercially available vaccine worldwide provides complete, reliable protection.
For a 100-head herd, preventive vaccination costs roughly $1,120, while feed-based antibiotic prevention runs about $1,400. Post-infection antibiotic treatment is considerably more expensive at around $3,890 per 100 head. Diagnostic testing for the whole herd costs approximately $700, making it the least expensive tool in the management toolkit and a practical starting point for understanding your herd’s status.
Where Anaplasmosis Is Most Common
The disease is found throughout much of the United States, but the central and southern states carry the heaviest burden. Missouri has an estimated statewide seroprevalence of 46.5%, meaning nearly half of cattle tested show evidence of exposure. In Kansas, over 50% of beef herds are infected, and modeling work has shown the disease increasing there since 2005. The geographic range appears to be expanding, likely driven by shifts in tick populations and cattle movement patterns.
The economic toll is substantial. A confirmed clinical case costs an estimated $660 per head when accounting for weight loss, reduced productivity, treatment costs, and death loss. If death loss can be held to 10% through early detection and treatment, that figure drops to about $285 per head. But in herds where the disease goes unrecognized until animals start dying, losses accumulate fast, especially among mature, high-value breeding stock that are most vulnerable to fatal outcomes.