(Foggie 1949) Dumler et al. 2001
|Rickettsia phagocytophila ovis
Anaplasma phagocytophilum (formerly Ehrlichia phagocytophilum) is a Gram-negative bacterium that is unusual in its tropism to neutrophils. It causes anaplasmosis in sheep and cattle, also known as tick-borne fever and pasture fever, and also causes the zoonotic disease human granulocytic anaplasmosis.
Anaplasma phagocytophilum is currently regarded as a single species. However, molecular studies indicate that it can be subdivided into ecotypes, each with distinct but overlapping transmission cycle. Here, we evaluate the interactions between and within clusters of haplotypes of the bacterium isolated from vertebrates and ticks, using phylogenetic and network-based methods.
Patients suffering from HGA undergo doxycycline therapy, 100 mg twice daily until the patient’s fever subsides for at least 3 days. This drug has been the most beneficial to those patients infected with the bacteria. Some other tetracycline drugs are also effective. In general, patients with symptoms of HGA and unexplained fever after a tick exposure should receive empiric doxycycline therapy while their diagnostic tests are pending, especially if they experience leukopenia and/or thrombocytopenia. In animals, antibiotics such as oxytetracycline, sulphamethazine, sulphadimidine, doxycycline, and trimethoprim-sulphonamides have been used.
- phagocytophilumis a small, obligate, intracellular bacterium with a Gram-negative cell wall. It is 0.2–1.0 μm and lacks a lipopolysaccharide biosynthetic machinery. The bacterium first resides in an early endosome, where it acquires nutrients for binary fission and grows into small groups called morulae. This bacterium prefers to grow within myeloid or granulocytic cells.
Role in human disease:
- phagocytophilumcauses human granulocytic anaplasmosis (HGA). This disease was first identified in 1990, although this pathogen was known to cause veterinary disease since 1932. Since 1990, incidence of HGA has increased, and it is now recognized in Europe. This disease was first identified due to a Wisconsin patient who died with a severe febrile illness two weeks after a tick bite. During the last stage of the infection, a group of small bacteria was seen within the neutrophils in the blood. Other symptoms include fever, headache, and absence of skin rash, leucopenia, thrombocytopenia, and mild injury to the liver.
Clinical signs in animals:
The disease is multisystemic, but the most severe changes are anaemia and leukopenia. This organism causes lameness, which can be confused with symptoms of Lyme disease, another tick-borne illness. It is a vector-borne zoonotic disease whose morula can be visualized within neutrophils (a type of white blood cell) from the peripheral blood and synovial fluid. It can cause lethargy, ataxia, loss of appetite, and weak or painful limbs.
- phagocytophilumbinds to fucosylated and sialylated scaffold proteins on neutrophil and granulocyte surfaces. A type IV secretion apparatus is known to help in the transfer of molecules between the bacterium and the host. The most studied ligand is PSGL-1 (CD162). The bacterium adheres to PSGL-1 (CD162) through the 44-kDa major surface protein-2 (Msp2). After the bacterium enters the cell, the endosome stops maturation and does not accumulate markers of late endosomes or phagolysosomes. Because of this, the vacuole does not become acidified or fused to lysosomes. A. phagocytophilumthen divides until cell lysis or when the bacteria leave to infect other cells.
This bacterium has the ability to affect neutrophils by altering their function. It can survive the first encounter with the host cell by detoxifying superoxide produced by neutrophil phagocyte oxidase assembly. It also disrupts normal neutrophil function, such as endothelial cell adhesion, transmigration, motility, degranulation, respiratory burst, and phagocytosis. It causes an increase in the secretion of IL-8, a chemoattractant that increases the phagocytosis of neutrophils. The purpose of this is to increase bacterial dissemination into the neutrophil.
The presence of A. phagocytophilum DNA was determined in ticks and vertebrate tissue samples. A fragment of the groEl gene was amplified and sequenced from qPCR-positive lysates. Additional groEl sequences from ticks and vertebrate reservoirs were obtained from GenBank and through literature searches, resulting in a dataset consisting of 1623 A. phagocytophilum field isolates. Phylogenetic analyses were used to infer clusters of haplotypes and to assess phylogenetic clustering of A. phagocytophilum in vertebrates or ticks. Network-based methods were used to resolve host-vector interactions and their relative importance in the segregating communities of haplotypes.
Phylogenetic analyses resulted in 199 haplotypes within eight network-derived clusters, which were allocated to four ecotypes. The interactions of haplotypes between ticks, vertebrates and geographical origin, were visualized and quantified from networks. A high number of haplotypes were recorded in the tick Ixodes ricinus. Communities of A. phagocytophilum recorded from Korea, Japan, Far Eastern Russia, as well as those associated with rodents had no links with the larger set of isolates associated with I. ricinus, suggesting different evolutionary pressures. Rodents appeared to have a range of haplotypes associated with either Ixodes trianguliceps or Ixodes persulcatus and Ixodes pavlovskyi. Haplotypes found in rodents in Russia had low similarities with those recorded in rodents in other regions and shaped separate communities.
The groEl gene fragment of A. phagocytophilum provides information about spatial segregation and associations of haplotypes to particular vector-host interactions. Further research is needed to understand the circulation of this bacterium in the gap between Europe and Asia before the overview of the speciation features of this bacterium is complete. Environmental traits may also play a role in the evolution of A. phagocytophilum in ecotypes through yet unknown relationships.
History of Anaplasma phagocytophilum:
Anaplasma phagocytophilum is the etiological agent of human granulocytic anaplasmosis (HGA), and tick-borne fever in domesticated animals. Although a wide range of wildlife species can be infected with A. phagocytophilum, the impact of these infections on wildlife health is unclear . The main vectors of A. phagocytophilum are ticks of the Ixodes ricinus complex: Ixodes ricinus in Europe, Ixodes persulcatus in eastern Europe and East Asia, and Ixodes scapularis and Ixodes pacificus in North America, although several other Ixodes species have been implicated in maintaining A. phagocytophilum in enzootic cycles as well . The transmission dynamics of A. phagocytophilum predominantly rely on horizontal transmission between ticks and vertebrate hosts and on transstadial transmission in its vectors. While its vertical transmission (transovarial) has only been documented for Dermacentor albipictus in laboratory conditions, no conclusive evidence of such a route has been reported in Ixodes ticks. Therefore, A. phagocytophilum is exposed to the evolutionary pressures of complex interactions among the vertebrate reservoirs and its vectors, which are instrumental in shaping the underlying tapestry of the genetic constellation of A. phagocytophilum.
- phagocytophilumincludes the newly discovered human pathogen whose original name was the human granulocytic ehrlichiosis (HGE) agent and previously known ruminant (Cytoecetes phagocytophila and Ehrlichia phagocytophila) and equine (Ehrlichia equi) pathogens A. phagocytophilum belongs to the family Anaplasmataceae, in the order Rickettsiales and the class Alphaproteobacteria . The family Anaplasmataceae includes five well-known genera, Ehrlichia, Anaplasma, Neorickettsia, Aegyptianella, and Wolbachia, and two less-well-studied genera, “Candidatus Neoehrlichia” and “Candidatus Xenohaliotis.” All of these genera infect specific invertebrate hosts (ticks, insects, trematodes, nematodes, or mollusks) that are abundant in nature. Unlike Neorickettsia and Wolbachia spp., which can be transmitted through generations of invertebrate hosts, Anaplasma and Ehrlichia cannot effectively pass from adult ticks to offspring (transovarial passage) All genera except Wolbachia and “Candidatus Xenohaliotis” are known to infect vertebrates (mammals or birds). Vertebrate infection can be acute or chronic and may result in fatality. The bacteria infect specific host cell types within vertebrates, usually cells of hematopoietic origin, such as neutrophils, monocytes/macrophages, platelets, red blood cells, or endothelial cells. Characteristics of members of the family Anaplasmataceae are summarized in next chapter A. phagocytophilum is one of four species belonging to the genus Anaplasma, which have different host cell specificities. For A. phagocytophilum, primary host cells are granulocytes, and endothelial cells are also infected.
- phagocytophilumis a small Gram-negative pleomorphic coccus enveloped by two membranes, as are other members of the family Anaplasmataceae. The bacterial size is generally 0.4 to 1.3 μm, but the bacteria can be as large as 2 μm. The outer membrane of the bacterium is often ruffled, which creates an irregular periplasmic space, and there is no capsule layer. Fine DNA strands and ribosomes are distinctly seen within the bacteria. Unlike members of the family Rickettsiaceae, which escape from phagosomes and replicate directly within the cytoplasm of eukaryotes, members of the family Anaplasmataceaereplicate in membrane-bound vacuoles (referred to as inclusions or parasitophorous vacuoles) within the cytoplasm of eukaryotic host cells. The bacteria may be tightly packed inside inclusions in part due to a loss of peptidoglycan and lipopolysaccharide (LPS). The loss permits the bacteria to squeeze within a limited intravacuolar space while maintaining the plasticity of the infected granulocytes that is required for capillary circulation.
Gram staining is not suitable to visualize intracellular bacteria because of a lack of contrast against the host cytoplasm. Romanowsky staining is generally used, usually with a quick method such as Diff-Quik. This approach stains the bacteria purple, which allows the visualization of characteristic mulberry-like bacterial clumps called morulae. (The term “morula” is derived from the Latin term “morus,” which means mulberry.) Morulae are usually 1.5 to 2.5 μm in diameter but can be as large as 6 μm.
Natural reservoir. A. phagocytophilum DNA has been detected in several species of Ixodes ticks (I. scapularis, I. pacificus, I. spinipalpis, I. ricinus, I. persulcatus, and I. ovatus) in the United States, Europe, and Asia. Naturally infected ticks were shown to transmit A. phagocytophilum to naïve mammals. Once ticks acquire the bacterium from infected mammals through a blood meal, the bacterium is maintained from the larva or nymph stage to adult stages of metamorphosis and is transmitted to mammals during the next blood meal. Since there is no evidence of transovarial (from adult ticks to eggs) transmission, larvae do not transmit the bacterium to mammals, but infected nymphs and adult ticks do. The mammalian reservoir for A. phagocytophilum infection within the United States includes white-footed mice (Peromyscus leucopus), raccoons (Procyon lotor), gray squirrels (Sciurus carolinensis), gray foxes (Urocyon cinereoargenteus), and redwood chipmunks . A variety of other wild animals are also implicated as reservoirs. Although Ixodes ticks often feed on white-tailed deer, the deer are infected with the Ap-Variant 1 strain of A. phagocytophilum, rather than with the human strain, in the United States. Diverse A. phagocytophilum strains are also found in animals and ticks in Europe, Japan, and Russia, where HGA has been rarely reported. These findings imply that the zoonosis potential of A. phagocytophilum depends not only on the transmissibility, habitats, and population density of ticks and infected mammals but also on the genetic variations of A. phagocytophilum. A primary natural tick-mammalian transmission cycle of A. phagocytophilum interlacing with bacterial strain diversity and host susceptibility is depicted in. The transovarial transmission of A. phagocytophilum variants occurs in Dermacentor albipictus . Thus, it is possible that there are atypical systems such as D. albipictus feeding into the normal Ixodes infection cycle.
Anaplasma phagocytophilum persists in nature by cycling between mammals and ticks. Human infection by the bite of an infected tick leads to a potentially fatal emerging disease called human granulocytic anaplasmosis. A. phagocytophilum is an obligatory intracellular bacterium that replicates inside mammalian granulocytes and the salivary gland and midgut cells of ticks. A. phagocytophilum evolved the remarkable ability to hijack the regulatory system of host cells. A. phagocytophilum alters vesicular traffic to create an intracellular membrane-bound compartment that allows replication in seclusion from lysosomes. The bacterium downregulates or actively inhibits a number of innate immune responses of mammalian host cells, and it upregulates cellular cholesterol uptake to acquire cholesterol for survival. It also upregulates several genes critical for the infection of ticks, and it prolongs tick survival at freezing temperatures. Several host factors that exacerbate infection have been identified, including interleukin-8 (IL-8) and cholesterol. Host factors that overcome infection include IL-12 and gamma interferon (IFN-γ). Two bacterial type IV secretion effectors and several bacterial proteins that associate with inclusion membranes have been identified. An understanding of the molecular mechanisms underlying A. phagocytophilum infection will foster the development of creative ideas to prevent or treat this emerging tick-borne disease.