Anaplasma: How This Tick-Borne Bacterium Infects the Body

Anaplasma is a genus of bacteria best known for causing anaplasmosis, a tick-borne disease that infects white blood cells in humans, dogs, horses, and cattle. The species most relevant to human health, Anaplasma phagocytophilum, has a remarkable and unsettling trick: it specifically targets and multiplies inside neutrophils, the very immune cells your body sends to kill invading bacteria. The result is a disease that can range from a mild flu-like illness to a life-threatening emergency, and one that shares geography and tick vectors with Lyme disease, making coinfection a real concern in endemic areas.

How Anaplasma Gets Into You

In North America, A. phagocytophilum is transmitted primarily by blacklegged ticks (Ixodes scapularis) in the eastern and central United States and by the western blacklegged tick (Ixodes pacificus) on the Pacific coast. In Europe, the castor bean tick (Ixodes ricinus) fills that role. Temperature and precipitation patterns strongly influence where these ticks thrive, and modeling studies suggest that climate change is expanding suitable habitat for several key tick vectors worldwide.

A common question is how long a tick has to be attached before transmission occurs. In a study using nymphal I. scapularis ticks, some mice became infected even when ticks were removed within the first 20 hours of feeding, but infection rates were low, around 10 to 30 percent. When ticks remained attached for 36 hours or longer, transmission jumped to 70 percent or higher.¹ The reason for this delay is that the bacteria need time to replicate and reactivate inside the feeding tick’s salivary glands before enough organisms are injected to establish a sustainable infection. So while the old advice to check for ticks promptly still holds, Anaplasma can potentially be transmitted faster than some people assume.

The two North American tick species also differ in how efficiently they transmit the pathogen. Research comparing I. pacificus and I. scapularis found that the western tick actually showed higher vector competence for A. phagocytophilum. Interestingly, geographic variation in the susceptibility of different animal hosts to different bacterial strains may matter more for how the disease spreads than differences in tick efficiency alone.²

A Bacterium That Hijacks Your Immune Cells

What makes A. phagocytophilum so unusual among pathogens is where it lives. Most bacteria that encounter a neutrophil are quickly engulfed and destroyed. Anaplasma, instead, infects neutrophils, eosinophils, and monocytes and actively grows inside them by dismantling several of their killing mechanisms.³ The bacterium blocks the fusion of the compartment it sits in with the cell’s destructive lysosomes, suppresses the burst of toxic oxygen molecules that neutrophils normally unleash on bacteria, and delays the programmed death of the infected cell so it has more time to replicate.⁴

Studies using gene-expression profiling of infected human neutrophils shed light on just how stealthy this process is. When researchers compared neutrophil responses to A. phagocytophilum against responses to a common bacterial pathogen, Staphylococcus aureus, Anaplasma triggered far fewer changes in the host cell’s gene activity. The normal programmed-death sequence that neutrophils launch after eating a bacterium simply did not fire. Instead, inflammatory genes were turned on gradually and slowly, a pattern more like delayed activation than the immediate defensive burst triggered by other microbes.⁵ The bacterium essentially keeps its host cell alive and docile long enough to reproduce.

To accomplish all of this, Anaplasma uses a molecular syringe called a type IV secretion system to inject proteins directly into the host cell’s interior. Two of these injected proteins, called AnkA and Ats-1, contain structural features that mimic host-cell proteins, letting them interact with and redirect normal cell processes.⁶ AnkA, for instance, gets injected into the cytoplasm, binds to a host adaptor protein, and activates a signaling pathway that the bacterium needs for successful infection. Blocking that pathway with targeted drugs or antibodies inhibits infection in laboratory experiments, which has raised interest in the idea of treating anaplasmosis by targeting host cell pathways rather than the bacterium directly.⁷

What Human Anaplasmosis Looks Like

Human granulocytic anaplasmosis (HGA) typically begins one to two weeks after a tick bite. The hallmark symptoms are fever, headache, muscle aches, and fatigue, a picture easily mistaken for a summer flu. In a case series from Massachusetts, 85 percent of patients reported fever. Notably, 76 percent had at least one respiratory or gastrointestinal symptom, which can further muddy the diagnosis. Rash, the feature most people associate with tick-borne diseases, occurred in only 6 percent of cases. The most reliable early clue on blood work is a drop in platelets and white blood cells, which was present in 86 percent of patients at the time they were first seen.⁸

Most people recover fully, especially with prompt treatment, but complications are not uncommon. A systematic review of published case reports found that roughly 40 percent of patients with HGA experienced at least one complication. The fatality rate was about 3 percent among those with available outcome data, and a small number of survivors had lingering health effects afterward.⁹ Severe disease tends to occur in older adults and people with weakened immune systems, though healthy individuals can occasionally become seriously ill.

Diagnosing Anaplasmosis at the Right Time

One of the trickiest aspects of anaplasmosis is that the best diagnostic test changes depending on when you are tested relative to when you got sick. In the first few days of illness, PCR (a test that detects bacterial DNA in your blood) is the most sensitive and specific option. A French prospective study found PCR had perfect specificity and a sensitivity of 0.74, making it the best single test in early disease.¹⁰ Another study confirmed that PCR positivity peaks in the first zero-to-four days of symptoms and drops off after that, while blood smear examination (looking for clumps of bacteria inside white blood cells under a microscope) has an even narrower detection window, maxing out around 14 days.¹¹

Antibody testing (serology) works in the opposite direction: it is unreliable early on, since the immune system has not yet mounted a measurable response, but it becomes the better tool during recovery. Because patients are usually antibody-negative in the initial phase, and because cross-reactions between different Anaplasma species occur once antibodies do appear, direct detection methods like PCR are generally recommended as the first-line approach for confirming a case during acute illness.¹² In practice, clinicians in endemic areas often start treatment based on symptoms and blood-count abnormalities before any confirmatory test comes back.

Anaplasmosis Alongside Lyme Disease

Because A. phagocytophilum and Borrelia burgdorferi (the Lyme disease agent) share the same tick vector, coinfection is a genuine possibility in the northeastern and upper midwestern United States. Among 311 patients presenting with the classic Lyme rash (erythema migrans) in one study, the estimated coinfection rate with anaplasmosis ranged from about 2 to 10 percent depending on which diagnostic definition was used. Only one of four coinfection-definition groups had significantly more symptoms than patients with Lyme disease alone, and high fever and low blood counts were actually less common in coinfected patients than in those with anaplasmosis by itself.¹³ That counterintuitive finding suggests that the presence of both pathogens does not simply add up to a worse illness. The practical takeaway is that doxycycline, the first-line antibiotic for both diseases, covers both infections simultaneously, so a missed anaplasmosis coinfection during Lyme treatment is not as dangerous as it might sound.

How Tissue Damage Actually Happens

A common assumption is that the bacteria themselves are what destroy tissue during an anaplasmosis infection. The picture is more complicated. Studies in mice have shown that much of the liver injury seen during A. phagocytophilum infection is caused by the host’s own inflammatory response rather than direct bacterial damage. Innate immune pathways involving reactive oxygen and nitrogen species, certain signaling molecules, and the protein TNF all contribute to hepatic injury even while the bacterium is actively evading those same defenses.¹⁴

Further research found that the severity of liver inflammation peaked early, around day two of infection in mice, coinciding with a spike in interferon-gamma levels. Natural killer cells and related immune cells appear to be major drivers of the early inflammatory damage. Strains that produced more severe disease in mice did so not because they replicated to higher numbers, but because they triggered a more intense immune reaction.¹⁵ This is an important distinction: in anaplasmosis, a vigorous immune response can be part of the problem, not just the solution.

Anaplasmosis in Dogs, Horses, and Cattle

Humans are far from the only species affected. In dogs, A. phagocytophilum causes canine granulocytic anaplasmosis with an incubation period of one to two weeks. The most common signs are lethargy and fever, reported in roughly 90 percent of clinically ill dogs, along with loss of appetite in about half to nearly all cases. Lameness and reluctance to move are also frequent. As in humans, thrombocytopenia occurs in about 90 percent of affected dogs and serves as a key diagnostic clue.¹⁶ A course of doxycycline is highly effective: in one study, all 16 naturally infected dogs cleared the bacteria from their blood by 30 days after treatment began, and clinical symptoms resolved.¹⁷

Horses develop a similar clinical picture. A systematic review covering 189 equine cases found fever in 90 percent, limb swelling in about half, and loss of appetite in 42 percent. Blood work showed thrombocytopenia in 90 percent and anemia in 75 percent.¹⁸

In the cattle world, the problem species is different: Anaplasma marginale. This organism infects red blood cells rather than white blood cells, causing severe anemia, weight loss, decreased milk production, and death. It is transmitted by ticks in the genus Rhipicephalus and also mechanically by biting flies and contaminated needles. Bovine anaplasmosis is found on every continent where cattle are raised and represents a major economic burden for producers, particularly in tropical and subtropical regions.¹⁹

Transmission Without a Tick

Though ticks are the primary route, Anaplasma can occasionally be transmitted through blood transfusions and organ transplants. A review of U.S. cases between 1997 and 2020 identified 132 cases of ehrlichiosis or anaplasmosis linked to transfusions or transplants, including 12 transfusion-associated cases.²⁰ The bacterium can survive inside refrigerated blood products for weeks. One fatal case in New York documented bacterial survival in a stored red-blood-cell unit for 22 days. Leukoreduction, a standard filtering step used on donated blood, reduces the bacterial load dramatically but does not eliminate it entirely.²¹ There is currently no routine screening of the U.S. blood supply for A. phagocytophilum, though the risk remains very low relative to the millions of transfusions performed each year.

Wildlife Reservoirs and Strain Diversity

Figuring out which animals maintain Anaplasma in nature is more complex than it first appears. White-tailed deer, which are the dominant host for I. scapularis ticks in much of eastern North America, carry A. phagocytophilum, but research suggests they harbor a variant strain that is not associated with human infection.²² In other words, deer feed huge numbers of ticks but may not be a meaningful source of the strains that make people sick. Small mammals like rodents appear to play a role as reservoirs for human-pathogenic strains, though the relative importance of different rodent species varies geographically. In the Italian Alps, for example, bank voles were found to be a more important reservoir host than yellow-necked mice.²³

Genetic analyses have confirmed that distinct A. phagocytophilum strains tend to cluster by host species. Sequencing of the ankA gene across 194 strains from humans and various animals showed that host species correlate with specific genetic clusters, supporting the idea that not all Anaplasma circulating in wildlife poses the same risk to people.²⁴ This strain diversity complicates surveillance efforts and helps explain why tick infection rates do not always predict human disease rates in a given area.

A Climate-Sensitive Disease

Anaplasmosis is expanding geographically, and climate change is one reason. Modeling of the cattle tick Rhipicephalus microplus, a major vector of A. marginale, projects increases in suitable habitat worldwide under both moderate and high emissions scenarios through 2050 and 2070, with the biggest gains in major cattle-producing regions.²⁵ For A. phagocytophilum specifically, global distribution models identify temperature, precipitation, and tick habitat-suitability indices as the most significant factors governing where the pathogen shows up.²⁶ In the United States, reported cases of anaplasmosis have increased sharply over the past two decades, a trend driven by both genuine range expansion and improved awareness and testing.

A Bacterium That Helps Its Own Vector Survive Winter

One of the stranger discoveries about A. phagocytophilum is that it does not merely hitch a ride in ticks; it actively helps them. Researchers found that ticks carrying the bacterium survive cold conditions better than uninfected ticks. The mechanism involves an antifreeze glycoprotein in I. scapularis whose production the bacterium switches on. When this gene was silenced experimentally, tick cold tolerance dropped. When the protein was expressed in yeast cells, they too gained improved survival at low temperatures.²⁷ This is a textbook example of a pathogen manipulating its vector for mutual benefit: the tick gets better winter survival, and the bacterium gets a vector that persists through harsh conditions. It also raises a troubling possibility in the context of warming climates, where tick populations that would previously have been thinned by cold winters may now thrive year-round.

The Vaccine Problem

No approved vaccine exists for human anaplasmosis. For bovine anaplasmosis, vaccine development has been underway for decades, with mixed progress. Early attempts using antibodies from vaccinated cattle failed to prevent A. marginale development and transmission inside ticks.²⁸ More recent strategies have shifted toward targeting the tick-pathogen interface. Researchers have identified an A. marginale surface protein (MSP1a) that, when used as a vaccine antigen, reduced clinical disease in cattle and lowered infection levels in ticks that fed on immunized animals. A separate tick-specific antigen called subolesin showed promise in reducing both tick infestations and the ticks’ ability to acquire and transmit the bacterium.²⁹ The dual-target approach, hitting both the pathogen and the tick, is an area of active research but remains experimental.

The lack of a human vaccine means prevention still depends on avoiding tick bites: wearing treated clothing, using repellents, checking for ticks after time outdoors, and keeping yards mowed and leaf litter cleared. For dogs, tick-prevention products (topical treatments, collars, or oral medications) are the main line of defense. For cattle producers, managing pasture rotation, controlling tick habitat, and monitoring herds for early signs of anemia remain the primary tools alongside targeted antibiotic treatment when cases arise.