Anaplasma phagocytophilum

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  Anaplasma phagocytophilum cultured in human promyelocytic cell line HL-60
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  Doxycycline 100mg capsules

Anaplasma phagocytophilum

Anaplasma phagocytophilum is a Gram-negative bacterium that is an obligate intracellular pathogen. It is the causative agent of human granulocytic anaplasmosis (HGA), a tick-borne disease that affects humans and animals. This bacterium primarily infects neutrophils, a type of white blood cell, and is transmitted by Ixodes ticks, which are also vectors for other diseases such as Lyme disease.

Taxonomy

Anaplasma phagocytophilum belongs to the family Anaplasmataceae, within the order Rickettsiales. It was previously classified under the genus Ehrlichia but was reclassified based on genetic and phenotypic characteristics. The genus Anaplasma includes other species that infect animals, such as Anaplasma marginale and Anaplasma centrale.

Transmission

The primary vectors for Anaplasma phagocytophilum are ticks of the genus Ixodes, including Ixodes scapularis (the black-legged tick) in the eastern United States and Ixodes pacificus (the western black-legged tick) on the Pacific coast. These ticks acquire the bacterium by feeding on infected hosts, such as small mammals and birds, and can subsequently transmit it to humans and other animals during subsequent feedings.

Pathogenesis

Once transmitted to a human host, Anaplasma phagocytophilum targets neutrophils, where it resides within membrane-bound vacuoles. The bacterium evades the host's immune response by inhibiting the normal function of neutrophils, including their ability to undergo apoptosis and produce reactive oxygen species. This allows the bacterium to survive and replicate within the host cells.

Clinical Manifestations

Human granulocytic anaplasmosis typically presents with non-specific symptoms such as fever, headache, malaise, and muscle aches. Laboratory findings often include leukopenia, thrombocytopenia, and elevated liver enzymes. Severe cases can lead to complications such as respiratory distress, renal failure, and neurological symptoms, particularly in immunocompromised individuals.

Diagnosis

Diagnosis of HGA is primarily based on clinical suspicion and can be confirmed by laboratory tests. Polymerase chain reaction (PCR) assays are commonly used to detect the presence of Anaplasma phagocytophilum DNA in blood samples. Serological tests can also be employed to detect antibodies against the bacterium, although these may not be positive in the early stages of infection.

Treatment

The antibiotic doxycycline is the treatment of choice for human granulocytic anaplasmosis. Early initiation of treatment is crucial to prevent severe complications. In most cases, patients respond well to doxycycline, with symptoms resolving within a few days of starting therapy.

Prevention

Preventive measures focus on reducing exposure to ticks. These include using insect repellents, wearing protective clothing, and performing regular tick checks after spending time in areas where ticks are prevalent. Public health efforts also aim to educate the public about the risks of tick-borne diseases and the importance of early detection and treatment.

Related pages

• Human granulocytic anaplasmosis
• Ixodes scapularis
• Lyme disease
• Rickettsiales