Rickettsial infections of the central nervous system

As a result of migrations and globalization, people may face a possible increase in the incidence of central nervous system rickettsial infections (CNS R). These diseases, caused by Rickettsia species and transmitted to humans by arthropod bites, are putatively lethal. However, the diagnosis of CNS R is challenging and often delayed due to their nonspecific clinical presentation and the strict intracellular nature of rickettsiae. Furthermore, transfer of rickettsiae to the brain parenchyma is not yet understood. The aim of this review is to analyze and summarize the features and correlated findings of CNS R in order to focus attention on these intriguing but frequently neglected illnesses. We also incorporated data on CNS infections caused by Rickettsia-related microorganisms.


Introduction
Recent reports have warned about the impact of global warming in facilitating the transmission of certain vector-borne infectious diseases. However, due consideration must be taken of the role played by other variables, such as the increase in international travel, migration, and trade, with the risk of importing parasites and vectors with the goods [1]. Rickettsioses are common tick-, flea-, or mite-borne bacterial illnesses with a clinical spectrum ranging from a mild febrile illness to potentially life-threatening complications [2,3]. Rickettsial infections can affect many organs, including the central nervous system (CNS) [4]. The most common neurological manifestations reported in rickettsial infections include meningitis, encephalitis, and acute disseminated encephalomyelitis [5]. However, unilateral facial nerve palsy [6], cerebral infarction [7], or visual loss [8] have also been diagnosed. Therefore, it is critical to enhance the awareness of physicians worldwide on CNS rickettsial infections (CNS R).
Few studies on the pathogenesis of rickettsiae for the CNS have been conducted. In neurons cultured in vitro, rickettsiae have been demonstrated to cause a profound morphological deterioration and intracellular decrease of ATP [9]. In addition, it was demonstrated that rickettsiae can persist and reappear after a relatively long time in the CNS in immunocompromised mice, causing a fatal neuroinflammation [10]. This suggests a potential neuropathic role of rickettsiae. However, data on rickettsial infections in a context of degenerative CNS diseases, which usually result from mixed neuropathologies and present with multiple symptoms, are a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 according to whether they break the integrity of BBB cells (which consist of microvascular endothelial cells of the brain, pericytes, and astrocytes) or interact with epithelial cells of the blood-choroid barrier. Mechanisms used by pathogens to enter the CNS are usually classified according to the involved cellular route [16,17]. These cellular entry routes are listed as intercellular or paracellular (passing between cells), transcellular (the intracellular mode), or leukocyte-facilitated in infected phagocytes (which is a "Trojan horse"-like mechanism) [18,19]. Rickettsial neuroinvasion occurs during the systemic phase of the disease and typically follows bacterial dissemination via the bloodstream. As other intracellular microorganisms, rickettsiae might use a transcellular approach, which is supported by the fact that bacterial replication takes place within endothelial cells. Other studies have demonstrated that bacterial pathogens (extra-and intracellular) can reach the BBB and are recognized by antigen-presenting cells through binding to Toll-like receptors (Fig 4). genes. Rickettsia species are distributed into 4 groups: SFG, TRG, TG, and AG. SFG rickettsiae are mostly associated with ticks; TG rickettsiae with human body lice (R. prowazekii) and rat fleas (R. typhi); TRG rickettsiae with ticks, cat fleas, or mites; and AG rickettsiae with ticks. Asterisks mark pathogenic species, and bold letters indicate species causing CNS infections. AG, ancestral group; CNS, central nervous system; SFG, spotted fever group; TG, typhus group; TRG, transitional group. https://doi.org/10.1371/journal.pntd.0007469.g001 They induce the activation of nuclear factor-kappa B-or mitogen-activated protein kinase pathways and subsequently up-regulate leukocyte populations and express numerous proteins involved in inflammation and the immune response [16,18]. However, the precise molecular mechanisms of rickettsial entry into the CNS are not yet described.

Treatment of rickettsioses
The most efficient antibiotics to cure rickettsioses are tetracycline derivatives, primarily doxycycline. The daily dose, 200 mg, is usually administered orally and is sufficient to cure rickettsioses. Treatment duration is 7 days or until 2 days after apyrexia is obtained [20,21]. In severe forms, intravenous doxycycline may be used. Alternatively, chloramphenicol and macrolides (azithromycin, clarithromycin, and roxytromycin) can be prescribed for treating rickettsial diseases [22,23]. As a consequence, empirical therapy for CNS infections most often does not include antibiotics active on rickettsiae.

Spotted fever group rickettsiae
In spotted fever group (SFG) rickettsioses, encephalitis is underdiagnosed due to underrecognition and low sensitivity of serologies at the time of early symptoms [24]. Therefore, it is Ticks are the main vectors and constitute a threat of rickettsial infection regardless of their development stage (larvae, nymphs, or adults). Many factors play a role in the epidemiology of tick-borne rickettsioses, including the prevalence and species diversity of rickettsiae in mammals [149][150][151][152][153] and the dispersion of infected ticks by migratory birds [154].
https://doi.org/10.1371/journal.pntd.0007469.g002 R. prowazekii is classified as a category B bioterrorism agent. It is stable in dried louse feces and can be transmitted through aerosols. Detection of the pathogen in body lice is crucial for monitoring the transmission risk to humans. However, Brill-Zinsser disease, a relapsing form of epidemic typhus that may develop as sporadic cases up to 40 years after the initial acute infection, is unrelated to louse infestation but to stress or a waning immune system that initiates the reactivation of an earlier and latent infection. Patients developing Brill-Zinsser disease may, in turn, be the source of new outbreaks when conditions facilitate louse infestation and transmission [155]. The mechanism of R. prowazekii latency has not been established. Brill-Zinsser disease should be considered as a possible diagnosis for acute fever in any patient who has lived in an area where epidemic typhus was endemic [156].
https://doi.org/10.1371/journal.pntd.0007469.g003 . These are scattered diffusely through the brain in close relation to small vessels. Vessels in the center of the lesions show severe degeneration. Endothelial cells are swollen, and the lumen may be occluded. Areas of focal necrosis are common as the result of thrombosis of small arteries. Some degree of perivascular infiltration without the presence of nodules may be seen in both the meninges and brain parenchyma [29].
To the best of our knowledge, there is just one report of experimental in vitro rickettsial infection of neuronal cells. Joshi and Kovacs [40] demonstrated that neurons are efficiently infected by R. rickettsii and that the infection causes significant neuronal apoptotic cell death. A double-immunofluorescent staining technique was employed to identify infected neurons, using anti-R. rickettsii serum and a neuron-specific (antineuronal β-III isoform of tubulin [anti-TuJ1]) antibody. The extent of apoptotic cell death of immunocytochemically identified neurons was determined by terminal deoxynucleotidyl transferase-mediated digoxigenin-dUTP nick-end labelling (TUNEL) assay. Primary cultures of rat cerebellar granular neurons were infected (approximately 65% of neurons) during a 6-hour exposure to R. rickettsii, and the infection induced widespread neuronal degeneration. To verify that the observed neuronal degeneration was R. rickettsii-induced, primary cultures of rat cerebellar granular neuron cultures were exposed to R. rickettsii in the presence of tetracycline. The authors also included R. rickettsii infection of endothelial cells as a control for infection and infection-induced apoptotic cell death [40].
The CNS is also a crucial target in other rickettsial diseases [41,42]. It has been demonstrated by a specific immunohistochemistry technique, during the development of mousemodel studies, that rickettsiae reach brain tissues when injected intravenously [43]. Such studies demonstrate that rickettsiae are able to cross the blood-brain barrier to reach brain tissues. The other rickettsial infections that may affect the nervous system exhibit pathological and clinical changes similar to those of RMSF [29]. In rickettsial meningitis, the cerebrospinal fluid shows pleocytosis with lymphocyte predominance, hypoglycorrhachia, and hyperproteinorrhachia.

Mediterranean spotted fever
Mediterranean spotted fever (MSF), also known as Boutonneuse fever and Marseilles fever, is caused by R. conorii. Although usually a benign and self-limiting exanthematous febrile illness
This syndrome is defined by the association of a tick bite and an inoculation eschar to the scalp, surrounded by a circular erythema, and painful regional lymphadenopathies [78]. The enlarged lymph nodes are located in the draining region of the tick bite, characteristically in the occipital region and/or behind the sternocleidomastoidal muscle. Although rare, the most frequent general symptoms are fever, nuchal lymphadenopathy, fatigue, dizziness, headache, sweat, myalgia, arthralgia, and loss of appetite [68,79]. Symptoms suggestive of acute encephalitis with febrile relapses and a persistence of neurasthenic disorders have also been reported [68,80].
Although R. slovaca was the first demonstrated agent of TIBOLA [76] and was isolated from a patient [81], other Rickettsia species were suspected to be involved in the disease in Spanish reports [71,82]. In 2008, R. raoultii DNA was detected in a tick taken from the scalp of a patient with TIBOLA [83]. In 2009, Parola and colleagues reported 6 more cases of R. raoultii infection [84]. To date, R. raoultii has been described as causing a milder form of TIBOLA than R. slovaca [84].

African tick bite fever
African tick bite fever (ATBF), caused by R. africae, is the most common rickettsiosis in travelers to sub-Saharan Africa and also the most important rickettsiosis worldwide in terms of numbers of cases per year [85][86][87]. The disease is most commonly mild and self-limiting without any sequellae. However, in 2006, Jensenius and colleagues reported 6 patients with evidence of long-lasting subacute neuropathy following ATBF contracted during a safari to southern Africa [88]. Of these, 3 developed radiating pain, paresthesia, and/or motor weakness of the extremities; 2 had hemifacial pain and paresthesia; and 1 developed unilateral sensorineural hearing loss.

R. helvetica infection
R. helvetica was first isolated from Ixodes ricinus ticks in Switzerland in 1979 [89], but its description as a distinct species of the SFG was confirmed only in 1993 [90]. Initially, the organism was considered as nonpathogenic. However, several patients with a mild and selflimiting disease associating fever, headache, and myalgia had serological evidence of R. helvetica infection [91]. Human cases have been reported in several European countries, e.g., Sweden [92] (proven by PCR and DNA sequencing), France [91], Switzerland [93], and Italy [94] (proven by serological findings). However, a more severe clinical disease has been reported and may be associated with severe symptoms such as CNS disorders [95,96] (verified by PCR in the cerebrospinal fluid, together with serologic evidence). R. helvetica was even cultured from a patient with subacute meningitis [97].

Typhus group rickettsiae
Epidemic typhus. R. prowazekii, the agent of epidemic typhus, causes frequent neurological manifestations. These include an agitated delirium that, when untreated, may progress to death. A severe headache is almost always present. Neurological complications include seizures, confusion, and coma. In severe cases, patients develop meningoencephalitis, with meningism, tinnitus, and hyperacusis, followed by deafness, dysphoria, and agitation. In Brill-Zinsser disease, the recurrent form of epidemic typhus that may occur months to years after the initial infection, symptoms are similar to the primary infection, although generally milder [29,98].
Murine typhus. R. typhi, the causal agent of murine or endemic typhus, is transmitted to humans by the rat flea Xenopsylla cheopis. It is most often a mild illness, but severe forms have been reported in refugee camps, and a fatal case in the United Kingdom was infected in Spain [99]. There is a considerable variability in the frequency of neurological manifestations of murine typhus, ranging from 2% to 20% of cases [100,101]. Headache is common, but meningitis and encephalitis are occasionally reported [102]. Mental confusion, seizures, stupor, and ataxia may occur infrequently [101,[103][104][105]. In some cases, systematic signs and symptoms may be minimal, and neurological manifestations may be the primary clinical manifestations of R. typhi infection. No neurologic sequellae have been reported in the past [29,42]. However, recently, an abducens nerve palsy and meningitis induced by R. typhi has been described [106].

Transitional group rickettsiae
Rickettsialpox. Rickettsialpox was first described in 1946 in New York City, New York [107]. The etiological agent is R. akari, transmitted by the mite Liponyssoides sanguineus [108]. Rickettsialpox is still occurring in the state of New York [109,110], and European cases of R. akari infection were reported in Ukraine [111], Croatia [112], and Northern Europe [113]. Clinical signs of R. akari infection develop about 7 to 10 days after an infected bite. At the mite inoculation site, a vesicle appears, which dries up, leaving a scar. High fever (�39.4˚C) occurs suddenly, along with severe headache and myalgias (especially in the back). A generalized papular rash develops, which can also be vesicular, resembling chickenpox. Other signs of the disease include regional lymphadenopathy, nausea, and neurological symptoms such as meningitis, photophobia, dizziness, eye movement, and neck stiffness. The disease is self-limiting, usually without complications, but myalgias and headache may persist for 2 more weeks after the rash [114].
Queensland tick typhus. R. australis, the causative agent of Queensland tick typhus (QTT), is transmitted by certain Ixodes spp. ticks, which are predominantly present along the eastern coast of Australia [115]. Originally described as a mild, self-limiting illness, recent case reports demonstrated the occurrence of more severe forms with complications. R. australis shares many clinical features with rickettsialpox. Acute infections often begin with high-grade fever, headache, myalgias, and a maculopapular rash after a short incubation period averaging 5 days [116,117]. Infection has the potential to progress to severe sepsis with multiorgan failure, requiring prolonged hospitalization and admission to intensive care [118]. Since R. australis was first isolated in 1946, there has been one reported fatal QTT case [119]. Less common manifestations of QTT include splenomegaly, abdominal pain, renal failure, dry cough, and conjunctivitis [120]. Some reports documented confusion, seizures, and hallucinations as features of the disease [116].
Flea-borne spotted fever. Flea-borne spotted fever is caused by R. felis. It is transmitted to humans by fleas, in particular Ctenocephalides felis fleas. Patients often present with elevated fever, headache, and myalgia. A cutaneous rash is inconstant. Other manifestations may include abdominal pain, nausea, vomiting, cough, eschar, and photophobia. Rare cases of neurological involvement have been reported, notably, hearing loss and subacute meningitis [121,122].

Scrub typhus
The genus Orientia contains two known species, O. tsutsugamushi and O. chuto [123], that cause scrub typhus, which is still regarded by some experts as a rickettsiosis, and are closely related to Rickettsia species [124,125]. Scrub typhus is a common cause of febrile illness in Asia. Fever and headache are the most common features of scrub typhus [126]. During World War 2, scrub typhus was a well-recognized cause of lethal meningitis in the Asia-Pacific region. Nowadays, only scarce data exist on the clinical burden of these pathogens in patients with CNS disease, e. g., in Laos and Thailand, where meningitis and meningoencephalitis occur as severe complications [127,128]. Isolated cases of acute encephalitis, abducens nerve palsy, and acute transverse myelitis due to O. tsutsugamushi were also recently described [129,130].

Rickettsia-related organisms
Even though we have focused our review on members of the genus Rickettsia, we have searched the literature for CNS infection caused by the related organisms Ehrlichia and Anaplasma species.
A. phagocytophilum appears to be a rare agent of CNS infections [145], but cases of cerebral infarction or brachial plexopathy associated to A. phagocytophilum infection were reported [146,147]. In addition, A. phagocytophilum-infected neutrophils were described to enhance transmigration of Borrelia burgdorferi across the human blood-brain barrier in vitro [148].

Conclusion
Through a literature review, we identified 13 rickettsioses, including scrub typhus, causing CNS infections ranging from simple headache to lethal meningoencephalitis. In addition, surviving patients may suffer incapacitating sequellae. To date, the precise mechanisms of rickettsial pathogenesis for the CNS are only partially known, but animal models have demonstrated their pro-apototic effect for neurons. However, these bacteria are constantly susceptible to tetracyclines that may efficiently be used to cure patients. The remaining challenge is to integrate rickettsioses more systematically in the differential diagnosis of CNS infections. Without this effort, the diagnosis and treatment may be delayed, increasing the risk of severe forms.

Key learning points
• Rickettsioses are arthropod-transmitted and zoonotic diseases with a worldwide distribution.
• Rickettsioses represent a frequent threat for travelers to rural areas but are often misdiagnosed.
• Eleven of the currently diagnosed rickettsioses cause CNS infections in humans.
• Rickettsial CNS infections range from meningitis to lethal encephalomyelitis.