Introduction

Monkeypox virus (MPXV), a double-helix DNA virus, causes zoonotic monkeypox disease that belongs to the family of Poxviridae, a subfamily of Chordopoxvirinae and Orthopoxvirus genera [1, 2]. This genu induces cutaneous manifestation in diseases, such as smallpox (caused by variola virus), cowpox (caused by cowpox virus), camelpox (caused by camelpox virus), and monkeypox in humans [3, 4]. The most common symptoms of monkeypox infection in humans are fever, swollen lymph nodes, exhaustion, chills, back pain, and skin rashes [5, 6]. Furthermore, some neurological complications, such as headache, malaise, myalgia, anorexia, and altered consciousness have been sporadically reported in MPXV patients [7,8,9]. These neurological manifestations may be related to the invasive potential of MPXV to the brain tissue that has been elucidated in some infected animals [10,11,12]. There are very few studies in the literature addressing the neuroinvasive potential of MPVX. It is thus crucial to have a mechanistic view on this topic based on the current evidence. Therefore, we highlighted the range of neurological complications associated with MPXV infection in humans. Furthermore, we determined some possible neuroinvasive properties of MPXV from experimental data.

A brief history of geographical aspects of the monkeypox virus

The MPX was named for the first time when the virus was discovered in monkeys at the State Serum Institute of Copenhagen in 1958 [13]. In August 1970, the first human case of MPX infection was identified in a 9-year-old boy that was admitted with fever to Basankusu Hospital in the Democratic Republic of Congo [14]. Wild animals including primates (mangabey monkeys), Gambian pouched rats, and squirrels are considered as the main reservoirs for MPXV [15]. Direct contact with infected animals' body fluids, touch, bite, scratch, hunting, and cooking are major transmission ways of MPXV from animal to human [16, 17]. Up to now, two clades of MPXV were phylogenetically identified. One strain originated from Central Africa–Congo Basin [strain Zaire-96 (ZAI-96)] that is more virulent than other strains derived from Western Africa (SL-V70, COP-58, and WRAIR-61) [18, 19]. Between 1970 and 1986 about 404 new cases of MPX had been reported in Central Africa and Western African countries, especially in tropical rainforest areas like Zaire, where cases had direct contact with infected animals [20]. The first outbreak of MPXV in a country outside of Africa was reported in the Midwestern United States in 2003 [9, 21, 22]. In this outbreak, the reservoirs were changed from monkeys and squirrels to prairie dogs which are native animals of the United States (Fig. 1). The source of MPXV in the prairie dogs was imported infected Gambian pouched rats from Ghana into the United States [10, 23]. During the next outbreak, the first case of MPX was confirmed on May 7, 2022, in a British person who travelled to Nigeria and then returned to the United Kingdom [24]. As of 22 June 2022, 3413 confirmed cases of MPX have been reported from 50 countries. From 17 to 22 June, about 1310 new confirmed cases have been added. The United Kingdom, Germany, Spain, and Portugal with totally of 793, 521, 520, and 317 cases, respectively, had more confirmed cases of MPX worldwide until 22 June 2022 [25].

Fig. 1
figure 1

Schematic overview of initiation, spreading, and distribution of MPXV. MPXV appears mainly in tropical rainforest areas of Central and Western Africa where MPXV reservoirs, such as monkeys, squirrels, and other rodents have a habit. Prairie dogs have been recognized as the main animal reservoirs for transmitting MPV to human during the 2003 outbreak in the U.S.A. Recently, the first confirmed case of MPX was identified in a British person who travelled to Nigeria and returned to the United Kingdom (UK). MPXV: Monkeypox virus; WHO: World Health Organization

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Detection of MPXV at a glance

Poxviruses have a large linear double-stranded DNA that encodes all enzymes to replicate and assembly, but they use the host cell's ribosomes to conduct translation process in the cytoplasm of infected cells. Morphologically in MPXV, the virions are ovoid or brick-shaped that are covered by a lipoprotein outer membrane about 200 by 250 nm in size in electron micrograph examinations [26, 27]. MPXV is resistant to heat and cold as heating until 40 °C had no strong effect on its infectivity [28], while some chemical agents, such as chloroform, methanol, formaldehyde, and phenol significantly inactivate its pathogenesis [13, 28]. Host infection is mediated through the interaction of viral proteins to host glycosaminoglycans that subsequently initiate the cellular endocytosis process and virus entry. In most poxviruses infection, two types of morphologically different virions, such as intracellular mature virus and extracellular-enveloped virus spread infectious into the host [26]. MPXV transmits from animals to human and human to human. Animal to human transmission (zoonotic transmission) mainly takes place in direct contact with body fluids, touch, bite and scratch of infected live/dead animals [29, 30], just like what happened in the initiation of MPX endemic in Central and Western Africa in 1970. The second form of transmission is human to human which maybe the route responsible for raising the cases of MPXV worldwide. The main human-to-human transmission routes are close contact with a MPX positive or symptomatic person, especially in health workers, sexual partners, and household members. Furthermore, respiratory droplets, contaminated materials, mouth ulcers and lesions as well as other mucosal secretions are considered transmission routes for human-to-human spread [31,32,33,34]. MPXV is diagnosed using a set of genetic, phenotypic, immunologic, and electron microscopic methods [26]. For example, a real-time polymerase chain reaction (RT-PCR) test is used to detect the MPX-specific DNA in the skin biopsy specimens [35]. Moreover, immunohistochemistry is applied to reveal the virus antigens in specific tissues and enzyme-linked immunosorbent assay (ELISA) is utilized to detect IgG and IgM antibodies against MPXV in the blood samples. In some cases, electron microscopy may be used to identify poxvirus virions in tissue specimens [36]. It should be noted that MPXV can be identified if the characteristic skin lesions are present and there is a history of exposure with suspected cases of MPXV [37]. Despite the precise detection of MPXV, the invasiveness and complications of MPXV on the second organs, such as CNS are crucial for basic scientists and clinicians. The CNS infection of MPXV not only can cause long-lasting brain injury but also can induce other neurological manifestations as we reported for SARS-CoV-2. Therefore, presenting current evidence of the neuroinvasive property of MPXV can inform us to focus on this aspect of the virus and make a better decision on clinical management. In the next outlines, we present evidence of neurological manifestations and neuroinvasive properties of MPXV.

Neurological manifestations of MPXV

The preliminary data showed a wide range of neurological manifestations from less serious and nonspecific symptoms including headache, myalgia and fatigue to more severe complications like seizure and encephalitis. The symptoms varied broadly, but the most prevalent were headache, myalgia, fatigue, photophobia, pain and fewer cases of encephalitis and seizure (Table 1). Data for psychiatric symptoms (i.e., depression, anxiety, and suicide) were not included in the current study. Here are brief reviews of published reports on neurological manifestations after MPXV. For example, a severe case of MPXV, who suffered from headache, and myalgia was reported in The United States of America (USA). She was the third reported child there who exposed to an infected prairie dog before admission [38]. In addition, headache, fatigue and myalgia were frequent neurological symptoms in seven other confirmed MPXV patients in the Western hemisphere [9]. A family cluster with MPXV was also reported in the Midwestern USA that had been exposed with prairie dogs. Of three family members infected, two showed mild skin rash only and one presented with severe encephalitis which required hospitalization. The two milder forms had been previously vaccinated with smallpox vaccine. A wide range of neurological manifestations including headache, fatigue, myalgia, confusion and seizure were seen in the severe case. Neurological examination revealed decreased level of consciousness, pupillary dilatation, optic disc edema, loss of corneal reflexes and reduced deep tendon reflexes. Additionally, magnetic resonance imaging (MRI) confirmed hyperintensity in thalamus, brainstem and right posterior parietal cortex consistent with mixed cytotoxic and vasogenic brain edema. Pleocytosis was also detected in the analysis of the cerebrospinal fluid (CSF) [7]. In another study, seizure and confusion accompanied other neurological manifestations such as headache and myalgia in the USA [6]. Furthermore, a variety of neurological complications such as headache, myalgia, pain and photophobia were reported in MPXV confirmed cases in the Bayelsa State of Nigeria in a cross-sectional study [39]. Taken together all previous reports we can conclude that currently, the development of CNS and peripheral nervous system complications in MPX patients has not been well established, but there are at least scattered case reports of patients with neurological features and those with severe complications, in particular, require emergent treatments. It is thus imperative to start treatment as quickly as possible while the pathogen spread is mitigated at the population level. To this point, we highlighted available evidence of the neuroinvasive potential of MPXV from experimental data.

Table 1 Main reported neurological manifestations of patients infected with MPXV
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Neuroinvasive propensity of MPXV

The neurotropic feature of MPXV on human subjects has not been fully understood, while data from animal studies (i.e., on rodents) have revealed that MPXV can cross the blood–brain barrier (BBB) and showed neuroinvasive capacity as summarized in Table 2. Intranasal and intraperitoneal administration of MPXV strain of 2003 is a well-established disease and caused animal death in ground squirrels after about 1 week. Necropsy findings indicated high titers of MPXV, using the number of plaque-forming units (PFU) per millimeter of homogenized tissue, in the brain tissues of animals [52]. Furthermore, during the 2003 outbreak, results of MPXV-specific PCR assay of four MPXV suspected rodents (i.e., a prairie dog, a hamster, and two gerbils) showed that the virus penetrated into the brain tissue [53]. Similar findings were reported in rope squirrel, pouched rat, dormice, and again prairie dogs in which MPXV DNA was detected in the brain tissues [10]. As can be seen from Table 2, MPXV can reach the brain parenchyma in animal models; however, considerably more work will need to be done to detect the neuroinvasive and neurotropism of MPXV in human subjects. It is also important to figure out the transmission routes of MPXV to the CNS. Currently, the exact transmission routes of MPXV are not clearly defined; however, previous studies on animal subjects have suggested two probable different routes: (i) olfactory epithelium route and (ii) infected monocytes/macrophages transmission way (Fig. 2).

Table 2 Evidence for neuroinvasive property of MPXV in different infected animal species
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Fig. 2
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Probable transmission routes for entry of MPXV into the brain tissue. MPXV can reach the brain tissue through two probable routes, such as olfactory epithelium and infected monocytes/macrophages. After intranasal inoculation, the virus rapidly is replicated in the nasal septum and mucosa and detected in the brain parenchyma. Data also showed that infected monocytes in circulation may cross the blood–brain barrier (BBB) and reach the brain tissue. MPXV can rapidly replicate into the macrophages and release from these cells into the brain. MPXV Monkeypox virus

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For instance, the accumulation of MPXV was significantly increased in the nasal septum mucosa and brain tissue after intranasal inoculation of Congo Basin MPXV strain in ground squirrels [59]. Furthermore, intranasal inoculation of MPXV showed that the virus had more replication in the intranasal and brain areas of animals assessed by bioluminescence imaging [60]. Coupled with, the main organs for viral load were the brain, nasal septum, and nasal mucosa after intranasal inoculation of MPXV in mice. These findings represented that the nasal cavity and olfactory epithelium may act as a major route for transmission of MPXV into the brain parenchyma (Fig. 2). MPXV may gain access to the CNS by infecting circulating leukocytes, e.g., monocytes/macrophages as a second route (Fig. 2). As an example, specific antigens of MPXV-Zaire 79 were identified in circulatory monocytes of macaques after intravenous injection of the virus [62]. Additionally, subcutaneously injection of MPXV in cynomolgus monkeys strongly increased viral particles in the alveolar and mediastinal lymph node macrophages detected by electron microscopy, indicating a high replication process in these cells [63, 64]. Also, the viral particles have been obviously detected after intranasal inoculation of MPXV in activated alveolar macrophages of ground squirrels, mice, and prairie dogs as shown by electron microscopy images [16, 59, 61]. It should be mentioned that the current data suggest the possible neuroinvasive potential of MPXV; however, more broadly, research is also needed to determine the exact invasion routes.

Conclusion remarks and future perspectives

To sum up, MPXV may enter the brain parenchyma and show neuroinvasive propensity. This potential can be mediated via two suggested routes, such as olfactory epithelium and hematogenous penetration through infected monocytes/macrophages. In light of the MPXV neuroinvasiveness, it can be explained the onset of neurological manifestations and brain damage in MPX patients. A special focus on the long-term effects of MPXV on CNS using in vitro, in vivo, and postmortem analyses can be helpful to reveal the exact mechanisms of neuroinvasion and even neurotropism induced by MPXV. Finally, neurologists and physicians should be aware of the possible incidence of neurological complications caused by MPXV.