Anaplasma phagocytophilum

by Geraldine Millan and Diana Suarez


Anaplasma phagocytophilum is a gram negative bacteria that can cause disease in a wide variety of mammals including cattle, domestic and wild animals. This bacteria has also shown to infect humans by tick bite resulting in the infectious disease: human granulocytic anaplasmosis (HGA). In the United States, this disease had an incidence of 6.1 cases per million persons in 2010.  Although the fatality cases are rare (less than 1 %) it can cause death if is not treated with antibiotics in an early stage of the infection. A. phagocytophilum has a small size of 0.2-1.0 μm and it infects neutrophils. Once the bacteria is inside white blood cells it replicates until it reaches a high population density to spread to other host cells.


A. phagocytophilum causes a zoonotic disease called human granulocytic anaplasmosis. HGA is an infectious disease transmitted by an infected thick which comes in contact with humans and share the bacteria through its saliva. HGA was first identified in a human subject during the 1990’s when a patient from Wisconsin, United States, died two weeks after a tick bite resulting in severe sickness. Human granulocytic anaplasmosis causes serious illness and even death in healthy children and adults because it is difficult to diagnose due to its similar signs and symptoms with viral infections such as the flu. Therefore, treatment is often neglected which leads to severe complications. The symptoms are detected within a week or two after the bite of an infected tick. It causes fever, headaches, fatigue, cough, chills, shaking and many other symptoms with the following being the most serious and life-threatening; leukopenia (a decrease in the number of white blood cells in the blood) and thrombocytopenia (low level of platelets in the blood). These physiological modifications cause individuals to be at a greater risk of infections and decrease their effectiveness of forming blood clots. A. phagocytophilum infects neutrophils which are cells that play an important role in the immune system response.  When neutrophils are infected their overall efficiency of fighting bacterial infections decreases which make the host more susceptible for getting disease.


Human beings are accidental hosts of A. phagocytophilum because the bacteria usually infects wild animals. Ticks will feed on wild animal’s blood and get infected at their turn. Once the ticks are infected, they can transmit the bacteria to other mammals through their saliva which makes them the main vector for human granulocytic anaplasmosis. The black-legged tick (Ixodes scapularis) and the Western black-legged tick (Ixodes pacificus) are the two species involved in the transmission of this disease.

Some cases have been reported concerning the transmission of the disease between humans by blood transfusion. According to the center for disease and prevention in the United States, the number of cases with tick-borne rickettsial disease has increased from 348 cases in 2000 to 1761 cases in 2010. In Canada, this disease has not been reported in humans, but animals have been known to be affected.

The signs and symptoms of HGA vary from person to person which makes it harder to detect and diagnose it at an early stage of the infection. The disease has been documented to be deadly in less than 1% of the infected patients. The risk of fatal cases is more significant in individuals with compromised immunity such as HIV positive patients, patients who have had their spleen removed, or patients undergoing immunosuppressive therapy (eg. cancer chemotherapy or organ transplant patients).

Virulence factors:

For this bacteria to be able to invade neutrophils, as it can be seen in figure 1,  it has developed a unique adaptation and pathogenic mechanism. A. phagocytophilum lacks lipopolysaccharide and peptidoglycan which are bacterial components easily identified by the host immunity that can induce an immune response. The absence of these components are an important factor for the avoidance of the host immune system by the bacteria . It also obtains cholesterol derived from host cells to maintain its membrane integrity and most importantly to look like a host cell. A. phagocytophilum’s entry mechanism into neutrophils is  successful due to the presence of a type IV secretion system (T4SS) that releases molecules into the host cells improving the effectiveness of colonization and infection. The virulence factors for adherence secreted by the T4SS are essential for the infection process because they enable the bacteria to adhere to the white blood cell’s surfaces and ultimately colonize them. The pathogen is phagocytosed, eaten,  by neutrophils and actively transported inside of the white blood cell. At the beginning of the bacterium’s life cycle, it populates an early endosome, a membrane-bound compartment inside of eukaryotic cells, where it has access to nutrients allowing for growth and division. It grows into small groups called morulae by using the process of binary fission which is the division of cells by asexual reproduction. This process can be seen in figure 2. It acquires nutrients by hijacking vesicles coming from organelles of nutrient-rich sources: the trans-Golgi and endoplasmic reticulum.

Figure 1: Morulae detected in a neutrophil on a peripheral blood smear, associated with A. phagocytophilum infection. Source:

Figure 2: Steps required for A. phagocytophilum to establish disease in humans. Source: Diana Suarez.

Normally during  phagocytosis, a bacteria is engulfed in an internal compartment called a phagosome and it is eventually  killed by the production of reactive oxygen species (ROS). A. phagocytophilum is protected against this deadly mechanism because it uses two strategies. The first strategy of the bacteria is to be an O2− scavenger which means that it can detoxify these reactive oxygen species. However the specificities of this strategy are yet to be understood. The second one is that the bacteria resides in a protective endosome that does not allow fusion with the lysosome an organelle containing enzymes that digest and disintegrate cells.

Other mechanisms that allow A. phagocytophilum to thrive inside neutrophils are the inhibition of  apoptosis for a period of 48 to 96 hours and the induction of autophagy.  Apoptosis is the process of programmed cell death. It is a response created by the innate immune system against bacterial infection. The inhibition of apoptosis allows the bacteria to replicate inside the cell for a longer period of time. Autophagy allows host cells to degrade and recycle cellular components. In this case, autophagy in neutrophils will create more space for the bacteria to grow as the host membrane is remodeled. In addition, there will be more nutrients available for replication and growth of the bacteria. “A. phagocytophilum subverts two important innate immune mechanisms of neutrophils, apoptosis and autophagy, by inhibiting and inducing, respectively, to keep the host cell alive and create a safe haven” (Rikihisa, 2011).


The treatment for human granulocytic anaplasmosis consists in the use of antibiotics. There is no vaccine against  A. phagocytophilum in humans and, therefore, the only method to prevent HGA is to avoid getting tick bites. Doxycycline is the most commonly prescribed antibiotic because it is effective in treating the disease. However, antibiotic treatment has to be taken as soon as possible when the disease-related symptoms are perceived. Pregnancy can also complicate the antibiotic treatment. The dosage for adults is usually 100 mg  every 12 hours and for children under 45 kg is 2.2 mg per kg of body weight administered twice a day. The treatment usually lasts 7 to 14 days, and symptoms such as fever will disappear within 24-72 hours after starting taking the antibiotics. Tetracycline antibiotic is also used to fight the bacterial infection. Tetracycline binds to the bacterial 30s ribosomal subunit and blocks synthesis of proteins preventing the spread of the bacteria.

Rikihisa Y. (2011). Mechanisms of Obligatory Intracellular Infection with Anaplasma phagocytophilum. Clinical Microbiology Reviews, 24(3):469-489. doi:10.1128/CMR.00064-10

Carlyon, J. A., Latif, D. A., Pypaert, M., Lacy, P., & Fikrig, E. (2004). Anaplasma phagocytophilum Utilizes Multiple Host Evasion Mechanisms To Thwart NADPH Oxidase-Mediated Killing during Neutrophil Infection. Infection and Immunity, 72(8), 4772-4783. doi:10.1128/iai.72.8.4772-4783.2004

Rikihisa, Y. (2010). Anaplasma phagocytophilum and Ehrlichia chaffeensis: subversive manipulators of host cells. Nature Reviews Microbiology 8, 328–339. doi:10.1038/ nrmicro2318

Truchan, H. K. (2014). Anaplasma phagocytophilum nutritional virulence mechanisms target the host cell secretory pathway. Virginia Commonwealth University Scholars Compass.

Center for Disease Control and Prevention. (2016). Anaplasmosis Symptoms, Diagnosis, and Treatment. National Center for Emerging and Zoonotic Infectious Diseases, Division of Vector-Borne Disease.

Minnesota Department of Health. (2017). Human Anaplasmosis Information for Health Professionals.

MOH Key Laboratory of Systems Biology of Pathogen.(2003-2017).Virulence Factors of Pathogenic Bacteria: Anaplasma. Institute of Pathogen Biology, CAMS & PUMC.

Huang, B., Hubber, A., Mcdonough, J., Roy, C., Scidmore, M., Carlyon, J. (2010). The Anaplasma phagocytophilum-occupied vacuole selectively recruits Rab-GTPases that are predominantly associated with recycling endosome. Cellular Microbiology. doi: 10.1111/j.1462-5822.2010.01468.x

Illustration references

Figure 1: Morulae detected in a neutrophil on a peripheral blood smear, associated with A. phagocytophilum infection. Source:

Figure 2: Steps required for A. phagocytophilum to establish disease in humans by Diana Suarez.




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