Human granulocytic anaplasmosis (HGA) is a tick-borne, infectious disease caused by Anaplasma phagocytophilum, an obligate intracellular bacterium that is typically transmitted to humans by ticks of the Ixodes ricinus species complex, including Ixodes scapularis and Ixodes pacificus in North America. These ticks also transmit Lyme disease and other tick-borne diseases.
The bacteria infect white blood cells called neutrophils, causing changes in gene expression that prolong the life of these otherwise short-lived cells.
Signs and symptoms
Signs and symptoms may include: fever severe headache muscle aches (myalgia) chills and shaking, similar to the symptoms of influenza nausea vomiting loss of appetite unintentional weight loss abdominal pain cough diarrhea, aching joints sensitivity to light weakness fatigue change in mental status (extreme confusion, memory loss, inability to comprehend environment- interaction, reading, etc.) temporary loss of basic motor skills Symptoms may be minor, as evidenced by surveillance studies in high-risk areas. Gastrointestinal tract symptoms occur in less than half of patients and a skin rash is seen in less than 10% of patients. It is also characterized by a low number of platelets, a low number of white blood cells, and elevated serum transaminase levels in the majority of infected patients. Even though people of any age can get HGA, it is usually more severe in the aging or immune-compromised. Some severe complications may include respiratory failure, kidney failure, and secondary infections.
Cause
A. phagocytophilum is transmitted to humans by Ixodes ticks. These ticks are found in the US, Europe, and Asia. In the US, I. scapularis is the tick vector in the East and Midwest states, and I. pacificus in the Pacific Northwest. In Europe, the I. ricinus is the main tick vector, and I. persulcatus is the currently known tick vector in Asia.
The major mammalian reservoir for A. phagocytophilum in the eastern United States is the white-footed mouse, Peromyscus leucopus. Although white-tailed deer and other small mammals harbor A. phagocytophilum, evidence suggests that they are not a reservoir for the strains that cause HGA. A tick that has a blood meal from an infected reservoir becomes infected themselves. If an infected tick then latches onto a human the disease is then transmitted to the human host and A. phagocytophilum symptoms can arise.
Anaplasma phagocytophilum shares its tick vector with other human pathogens, and about 10% of patients with HGA show serologic evidence of coinfection with Lyme disease, babesiosis, or tick-borne meningoencephalitis.
While it is rare, it is possible for HGA to be transmitted human-to-human via a blood transfusion, in which case it is called Transfusion-Transmitted Anaplasmosis (TTA).
Major surface proteins
Many major surface proteins (MSPs) are found in Anaplasma and those which interact with Anaplasma can mainly be found in A. marginale and A. phagocytophilum. There are many different phenotypic traits that are associated with MSPs, because each MSP can only infect certain animals in certain conditions. A. phagocytophilum infects the most vast array of living things, including humans, and all around the world. A. marginale evolved to be more specific in infecting animals, such as deer and cattle in the subtropics and tropics. The main difference between these two MSPs is that the host cell for A. phagocytophilum is the granulocyte, while the host cell for A. marginale is erythrocytes. It is likely that these MSPs coevolved, because they had previously interacted via tick-pathogen interaction.
Anaplasma MSPs can not only cooperate with vertebrates, but also invertebrates, which make these phenotypes evolve faster than others, because they have a lot of selective forces acting on them.
Diagnosis
Clinically, HGA is essentially indistinguishable from human monocytic ehrlichiosis, the infection caused by Ehrlichia chaffeensis, and other tick-borne illnesses such as Lyme disease may be suspected. As Ehrlichia serologies can be negative in the acute period, PCR is very useful for diagnosis.
Prevention
Currently, there is no vaccine against human granulocytic anaplasmosis, so antibiotics are the only form of treatment. The best way to prevent HGA is to prevent getting tick bites.