|Overview of Human Anaplasmosis|
Overview of Human Anaplasmosis
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The pathogen causing human anaplasmosis was first observed as a tick-borne pathogen of sheep and cattle, in which it was found to cause transient fever (tick-borne fever), sometimes resulting in increased susceptibility to other pathogens, and also causing abortion in cattle (Woldehiwet, 2010). The observation that it occurs within circulating neutrophils initially resulted in the name Rickettsia phagocytophila subsequently changed to Cytoecetes phagocytophila and then to Ehrlichia phagocytophila (Stuen et al., 2013). Over the next few decades disease caused by this pathogen was also observed in dogs and horses, and in 1992 it emerged as a zoonotic organism in the USA (Chen et al., 1994), where it was referred to as Human Granulocytic Ehrlichiosis (HGE). The first European case was observed in 1996 (Petrovec et al., 1997). Following a review of taxonomy, especially concerning molecular markers, the pathogen was assigned to the genus Anaplasma (Dumler et al., 2001), and is now referred to as Anaplasma phagocytophilum, with the disease in humans known as human anaplasmosis or Human Granulocytic Anaplasmosis (HGA).
The incubation period is 7-10 days and patients present with high fever, myalgia, headache and malaise. These symptoms may be accompanied by arthralgias, nausea, anorexia and a non-productive cough. Occasionally (10% of patients) a non-specific rash may be present. In many cases the illness lasts for only a few days but some patients have remained ill for more than 60 days in the absence of antimicrobial therapy (Bakken & Dumler, 2000). In the USA clinical cases are both more severe and more frequent than in Europe. Whereas approximately 2000 cases occur each year in the USA, fewer than 100 cases have been reported in Europe since the first case was described by Petrovec et al., (1997) in 1996 (Dugat et al., 2015). The case fatality rate has been estimated as about 1% in the USA (Bakken & Dumler, 2015), but in Europe no fatalities have been reported so far. Coinfection with other tick-borne infections is possible and may cause some diagnostic confusion, but in contrast to the situation in domestic ruminants, there is no evidence to suggest that infection with A. phagocytophilum in humans can result in chronic disease or exacerbation of other infections.
Biology of the infectious agents
A. phagocytophilum is a tick-borne obligate intracellular bacterium that parasitises neutrophils in which it forms clusters of bacteria known as morulae. Characterisation of genotypes is in its infancy but it is now evident that considerable diversity exists, with two independent epidemiological cycles apparently occurring in Europe, one involving rodents as reservoir hosts, with Ixodes trianguliceps as the vector, and another with ruminants as reservoir hosts and I. ricinus as the vector (Dugat et al., 2015). Those found in deer differ from those in sheep, which differ again from those in horses and dogs (Stuen et al., 2013). Ruminants are probably the source of genotypes infective for humans in Europe, but in the USA rodents appear to be the main reservoir hosts of American zoonotic genotypes, which can also infect dogs and horses (Dugat et al., 2015).
Almost all human disease results from transmission by ticks, in which the pathogen persists transtadially but not transovarially. Transmission from vertebrate to vertebrate is therefore described as transstadial, with infection usually acquired by the larva and transmitted by the nymph or acquired by the nymph and transmitted by the adult female. Transfusion transmission occurs and so far one case has been reported in Europe (Jereb et al., 2016). At least eight cases have been reported in the USA and also one of transplacental transmission (Bakken & Dumler, 2015).
Biology of the vector
The main vectors of A. phagocytophilum in Europe are the ticks Ixodes ricinus in western, central and eastern Europe and I. persulcatus in parts of Eastern Europe. The distribution of the two species overlaps in the Baltic states and European Russia. The rodent tick I. trianguliceps has been identified as the vector of a genotype that is not thought to cause human disease. The non-parasitic (off-host) phases of I. ricinus and I. persulcatus require a high humidity at the base of the vegetation (RH >85%) and ideal conditions are to be found in temperate deciduous woodland with patches of dense vegetation and little air movement, coupled with high humidity. The need for questing ticks to maintain a stable water balance is an important factor in determining the location and duration of activity. In general, activity will begin in spring and early summer, with ticks being found on vegetation and animals from late March. In habitats where desiccation is high, such as open areas, periods of activity will be shortened to only a few weeks - as opposed to several months in dense woodlands. In some areas a second, less intense, phase of questing activity occurs in the autumn.
The tick ambushes its host from the vegetation and attaches to the skin with specialized mouthparts, feeding on blood for several days, the duration depending on the tick life cycle stage. A delay in transmission of A. phagocytophilum of up to 36 hours has been demonstrated experimentally in mice and it was concluded that the grace period for transmission is at least one day (Katavolos et al., 1998).
For further details of the biology of Ixodes ricinus see the Lyme borreliosis page, Biology, tick
Patients with HGA usually present with a non-specific febrile illness (see Clinical features above) and most have a history of tick-bite or access to tick-infested habitats. Leucopenia, thrombopenia, elevation of hepatic transaminase and C-reactive protein concentration are frequent findings, but although many patients report headache and/or stiff neck, spinal fluid analysis is unremarkable. Rashes rarely occur in HGA patients. In the USA laboratory confirmatory is possible at an early stage of infection by examination of Giemsa-stained blood smears for morulae within neutrophils (Fig 1), however in European cases morulae are rarely detected. Detection of pathogen DNA by PCR is more effective but accessibility to this test may be limited. Confirmation is most commonly achieved by analysis of acute and convalescent sera for IgG with an indirect immunofluorescence test (Bakken & Dumler, 2015).
Fig 1. A thin blood smear showing morulae (arrowed)
All symptomatic patients should receive antimicrobial treatment because even mild cases run the risk of complications. A. phagocytophilum is susceptible to tetracyclines and doxycycline is the antibiotic of choice. In addition to adults this applies to seriously ill children for whom doses should be divided and adjusted to the patient’s weight. Clinical improvement should occur in 24 – 48 hours, and in those cases that don’t respond, alternative diagnoses should be sought. The required duration of treatment has not been established, but for those considered to be at risk of Lyme borreliosis, 14 days is recommended and 7-10 days in children. If tetracyclines are contra-indicated, rifampin should be considered since it has excellent in vitro activity (Wormser et al., 2006; Bakken & Dumler, 2015).
The same preventative measures against most other pathogens transmitted by ticks of the I. ricinus species complex apply i.e. wearing of appropriate protective clothing, application of repellents, examination of skin as soon as possible after potential tick exposure, prompt removal of attached ticks. The main significance of the infection in Europe may be in providing an explanation for (acute) illness in cases thought to have occurred as a result of a tick bite.
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Wormser GP, Dattwyler RJ, Shapiro ED, Halperin JJ, Steere AC, Klempner MS, Krause PJ, Bakken JS, Strle F, Stanek G, Bockenstedt L, Fish D, Dumler JS, Nadelman RB. The clinical assessment, treatment, and prevention of Lyme disease, human granulocytic anaplasmosis, and babesiosis: clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2006 Nov 1;43(9):1089-134.
|Last Updated on Friday, 17 February 2017 16:10|