Investigation of an mpox outbreak affecting many vaccinated persons in Chicago, IL-March 2023-June 2023

BACKGROUND

After months of few mpox cases, an increased number of cases were reported in Chicago during May 2023; predominantly among fully vaccinated patients. We investigated the outbreak scope, differences between vaccinated and unvaccinated patients, and hypotheses for monkeypox virus (MPXV) infection after vaccination.

METHODS

We interviewed patients and reviewed medical records to assess demographic, behavioral, and clinical characteristics, mpox vaccine status, and vaccine administration routes. We evaluated serum antibody levels after infection and compared patient viral genomes with MPXV sequences in available databases. We discussed potential vaccine compromise with partners who manufactured, handled, and administered vaccine associated with breakthrough infections.

RESULTS

During March 18-June 27, 2023, we identified 49 mpox cases; 57% of these mpox patients were fully vaccinated (FV). FV patients received both JYNNEOS doses subcutaneously (57%), intradermally (7%), or via heterologous administration (36%). FV patients had more median sex partners (3, IQR=1-4) versus not fully vaccinated (NFV) patients (1, IQR=1-2). Thirty-six of 37 sequenced specimens belonged to lineage B.1.20 of clade IIb MPXV, which did not demonstrate any amino acid changes relative to B.1, the predominant lineage from May 2022. Vaccinated patients demonstrated expected humoral antibody responses; none were hospitalized. No vaccine storage excursions were identified. Approximately 63% of people at risk for mpox in Chicago were FV during this period.

CONCLUSIONS

Our investigation indicated cases were likely due to frequent behaviors associated with mpox transmission, even with relatively high vaccine effectiveness and vaccine coverage. Cases after vaccination might occur in similar populations.

Pathology and Monkeypox virus Localization in Tissues From Immunocompromised Patients With Severe or Fatal Mpox

BACKGROUND

Pathology and Monkeypox virus (MPXV) tissue tropism in severe and fatal human mpox is not thoroughly described but can help elucidate the disease pathogenesis and the role of coinfections in immunocompromised patients.

METHODS

We analyzed biopsy and autopsy tissues from 22 patients with severe or fatal outcomes to characterize pathology and viral antigen and DNA distribution in tissues by immunohistochemistry and in situ hybridization. Tissue-based testing for coinfections was also performed.

RESULTS

Mucocutaneous lesions showed necrotizing and proliferative epithelial changes. Deceased patients with autopsy tissues evaluated had digestive tract lesions, and half had systemic tissue necrosis with thrombotic vasculopathy in lymphoid tissues, lung, or other solid organs. Half also had bronchopneumonia, and one-third had acute lung injury. All cases had MPXV antigen and DNA detected in tissues. Coinfections were identified in 5 of 16 (31%) biopsy and 4 of 6 (67%) autopsy cases.

CONCLUSIONS

Severe mpox in immunocompromised patients is characterized by extensive viral infection of tissues and viremic dissemination that can progress despite available therapeutics. Digestive tract and lung involvement are common and associated with prominent histopathological and clinical manifestations. Coinfections may complicate mpox diagnosis and treatment. Significant viral DNA (likely correlating to infectious virus) in tissues necessitates enhanced biosafety measures in healthcare and autopsy settings.

How the Orthodox Features of Orthopoxviruses Led to an Unorthodox Mpox Outbreak: What We've Learned, and What We Still Need to Understand

Orthopoxviruses have repeatedly confounded expectations in terms of the clinical illness they cause and their patterns of spread. Monkeypox virus (MPXV), originally characterized in the late 1950s during outbreaks among captive primates, has been recognized since the 1970s to cause human disease (mpox) in West and Central Africa, where interhuman transmission has largely been associated with nonsexual, close physical contact. In May 2022, a focus of MPXV transmission was detected, spreading among international networks of gay, bisexual, and other men who have sex with men. The outbreak grew in both size and geographic scope, testing the strength of preparedness tools and public health science alike. In this article we consider what was known about mpox before the 2022 outbreak, what we learned about mpox during the outbreak, and what continued research is needed to ensure that the global public health community can detect, and halt further spread of this disease threat.

Tecovirimat Resistance in Mpox Patients, United States, 2022-2023

During the 2022 multinational outbreak of monkeypox virus (MPXV) infection, the antiviral drug tecovirimat (TPOXX; SIGA Technologies, Inc., https://www.siga.com) was deployed in the United States on a large scale for the first time. The MPXV F13L gene homologue encodes the target of tecovirimat, and single amino acid changes in F13 are known to cause resistance to tecovirimat. Genomic sequencing identified 11 mutations previously reported to cause resistance, along with 13 novel mutations. Resistant phenotype was determined using a viral cytopathic effect assay. We tested 124 isolates from 68 patients; 96 isolates from 46 patients were found to have a resistant phenotype. Most resistant isolates were associated with severely immunocompromised mpox patients on multiple courses of tecovirimat treatment, whereas most isolates identified by routine surveillance of patients not treated with tecovirimat remained sensitive. The frequency of resistant viruses remains relatively low (<1%) compared with the total number of patients treated with tecovirimat.

Prevalence of Undiagnosed Monkeypox Virus Infections during Global Mpox Outbreak, United States, June-September 2022

Since May 2022, mpox has been identified in 108 countries without endemic disease; most cases have been in gay, bisexual, or other men who have sex with men. To determine number of missed cases, we conducted 2 studies during June-September 2022: a prospective serologic survey detecting orthopoxvirus antibodies among men who have sex with men in San Francisco, California, and a retrospective monkeypox virus PCR testing of swab specimens submitted for other infectious disease testing among all patients across the United States. The serosurvey of 225 participants (median age 34 years) detected 18 (8.0%) who were orthopoxvirus IgG positive and 3 (1.3%) who were also orthopoxvirus IgM positive. The retrospective PCR study of 1,196 patients (median age 30 years; 54.8% male) detected 67 (5.6%) specimens positive for monkeypox virus. There are likely few undiagnosed cases of mpox in regions where sexual healthcare is accessible and patient and clinician awareness about mpox is increased.

Evidence of Mpox Virus Infection Among Persons Without Characteristic Lesions or Rash Presenting for First Dose of JYNNEOS Vaccine-District of Columbia, August 2022

We assessed mpox virus prevalence in blood, pharyngeal, and rectal specimens among persons without characteristic rash presenting for JYNNEOS vaccine. Our data indicate that the utility of risk-based screening for mpox in persons without skin lesions or rash via pharyngeal swabs, rectal swabs, and/or blood is likely limited.

The CDC Domestic Mpox Response - United States, 2022-2023

Monkeypox (mpox) is a serious viral zoonosis endemic in west and central Africa. An unprecedented global outbreak was first detected in May 2022. CDC activated its emergency outbreak response on May 23, 2022, and the outbreak was declared a Public Health Emergency of International Concern on July 23, 2022, by the World Health Organization (WHO),* and a U.S. Public Health Emergency on August 4, 2022, by the U.S. Department of Health and Human Services.^† A U.S. government response was initiated, and CDC coordinated activities with the White House, the U.S. Department of Health and Human Services, and many other federal, state, and local partners. CDC quickly adapted surveillance systems, diagnostic tests, vaccines, therapeutics, grants, and communication systems originally developed for U.S. smallpox preparedness and other infectious diseases to fit the unique needs of the outbreak. In 1 year, more than 30,000 U.S. mpox cases were reported, more than 140,000 specimens were tested, >1.2 million doses of vaccine were administered, and more than 6,900 patients were treated with tecovirimat, an antiviral medication with activity against orthopoxviruses such as Variola virus and Monkeypox virus. Non-Hispanic Black (Black) and Hispanic or Latino (Hispanic) persons represented 33% and 31% of mpox cases, respectively; 87% of 42 fatal cases occurred in Black persons. Sexual contact among gay, bisexual, and other men who have sex with men (MSM) was rapidly identified as the primary risk for infection, resulting in profound changes in our scientific understanding of mpox clinical presentation, pathogenesis, and transmission dynamics. This report provides an overview of the first year of the response to the U.S. mpox outbreak by CDC, reviews lessons learned to improve response and future readiness, and previews continued mpox response and prevention activities as local viral transmission continues in multiple U.S. jurisdictions (Figure).

Mpox respiratory transmission: the state of the evidence

The relative contribution of the respiratory route to transmission of mpox (formerly known as monkeypox) is unclear. We review the evidence for respiratory transmission of monkeypox virus (MPXV), examining key works from animal models, human outbreaks and case reports, and environmental studies. Laboratory experiments have initiated MPXV infection in animals via respiratory routes. Some animal-to-animal respiratory transmission has been shown in controlled studies, and environmental sampling studies have detected airborne MPXV. Reports from real-life outbreaks demonstrate that transmission is associated with close contact, and although it is difficult to infer the route of MPXV acquisition in individual case reports, so far respiratory transmission has not been specifically implicated. Based on the available evidence, the likelihood of human-to-human MPXV respiratory transmission appears to be low; however, studies should continue to assess this possibility.

Epidemiology of Human Mpox - Worldwide, 2018-2021

Monkeypox (mpox) is a zoonotic disease caused by Monkeypox virus (MPXV), an Orthopoxvirus; the wild mammalian reservoir species is not known. There are two genetic clades of MPXV: clade I and clade II (historically found in central and west Africa, respectively), with only Cameroon reporting both clades (1). Human cases have historically been reported from 1) mostly rural, forested areas in some central and west African countries; 2) countries reporting cases related to population migration or travel of infected persons; and 3) exposure to imported infected mammals (2). The annual number of cases in Africa has risen since 2014 and cumulatively surpassed reports from the previous 40 years for most countries. This reemergence of mpox might be due to a combination of environmental and ecological changes, animal or human movement, the cessation of routine smallpox vaccination since its eradication in 1980, improvements in disease detection and diagnosis, and genetic changes in the virus (2). This report describes the epidemiology of mpox since 1970 and during 2018-2021, using data from national surveillance programs, World Health Organization (WHO) bulletins, and case reports, and addresses current diagnostic and treatment challenges in countries with endemic disease. During 2018-2021, human cases were recognized and confirmed in six African countries, with most detected in the Democratic Republic of the Congo (DRC) and Nigeria. The reemergence and increase in cases resulted in its being listed in 2019 as a priority disease for immediate and routine reporting through the Integrated Disease Surveillance and Response strategy in the WHO African region.* In eight instances, patients with mpox were identified in four countries outside of Africa after travel from Nigeria. Since 2018, introductory and intermediate training courses on prevention and control of mpox for public health and health care providers have been available online at OpenWHO.^†,§ The global outbreak that began in May 2022^¶ has further highlighted the need for improvements in laboratory-based surveillance and access to treatments and vaccines to prevent and contain the infection, including in areas of Africa with endemic mpox.

Multiple lineages of monkeypox virus detected in the United States, 2021-2022

Monkeypox is a viral zoonotic disease endemic in Central and West Africa. In May 2022, dozens of non-endemic countries reported hundreds of monkeypox cases, most with no epidemiological link to Africa. We identified two lineages of monkeypox virus (MPXV) among two 2021 and seven 2022 US monkeypox cases: the major 2022 outbreak variant called B.1 and a minor contemporaneously sampled variant called A.2. Analyses of mutations among these two variants revealed an extreme preference for GA-to-AA mutations indicative of human APOBEC3 cytosine deaminase activity among Clade IIb MPXV (previously West African, Nigeria) sampled since 2017. Such mutations were not enriched within other MPXV clades. These findings suggest that APOBEC3 editing may be a recurrent and a dominant driver of MPXV evolution within the current outbreak.

Rapid Diagnostic Testing for Response to the Monkeypox Outbreak - Laboratory Response Network, United States, May 17-June 30, 2022

As part of public health preparedness for infectious disease threats, CDC collaborates with other U.S. public health officials to ensure that the Laboratory Response Network (LRN) has diagnostic tools to detect Orthopoxviruses, the genus that includes Variola virus, the causative agent of smallpox. LRN is a network of state and local public health, federal, U.S. Department of Defense (DOD), veterinary, food, and environmental testing laboratories. CDC developed, and the Food and Drug Administration (FDA) granted 510(k) clearance* for the Non-variola Orthopoxvirus Real-time PCR Primer and Probe Set (non-variola Orthopoxvirus [NVO] assay), a polymerase chain reaction (PCR) diagnostic test to detect NVO. On May 17, 2022, CDC was contacted by the Massachusetts Department of Public Health (DPH) regarding a suspected case of monkeypox, a disease caused by the Orthopoxvirus Monkeypox virus. Specimens were collected and tested by the Massachusetts DPH public health laboratory with LRN testing capability using the NVO assay. Nationwide, 68 LRN laboratories had capacity to test approximately 8,000 NVO tests per week during June. During May 17-June 30, LRN laboratories tested 2,009 specimens from suspected monkeypox cases. Among those, 730 (36.3%) specimens from 395 patients were positive for NVO. NVO-positive specimens from 159 persons were confirmed by CDC to be monkeypox; final characterization is pending for 236. Prompt identification of persons with infection allowed rapid response to the outbreak, including isolation and treatment of patients, administration of vaccines, and other public health action. To further facilitate access to testing and increase convenience for providers and patients by using existing provider-laboratory relationships, CDC and LRN are supporting five large commercial laboratories with a national footprint (Aegis Science, LabCorp, Mayo Clinic Laboratories, Quest Diagnostics, and Sonic Healthcare) to establish NVO testing capacity of 10,000 specimens per week per laboratory. On July 6, 2022, the first commercial laboratory began accepting specimens for NVO testing based on clinician orders.

Teaching a new mouse old tricks: Humanized mice as an infection model for Variola virus

Smallpox, caused by the solely human pathogen Variola virus (VARV), was declared eradicated in 1980. While known VARV stocks are secure, smallpox remains a bioterrorist threat agent. Recent U.S. Food and Drug Administration approval of the first smallpox anti-viral (tecovirimat) therapeutic was a successful step forward in smallpox preparedness; however, orthopoxviruses can become resistant to treatment, suggesting a multi-therapeutic approach is necessary. Animal models are required for testing medical countermeasures (MCMs) and ideally MCMs are tested directly against the pathogen of interest. Since VARV only infects humans, a representative animal model for testing therapeutics directly against VARV remains a challenge. Here we show that three different humanized mice strains are highly susceptible to VARV infection, establishing the first small animal model using VARV. In comparison, the non-humanized, immunosuppressed background mouse was not susceptible to systemic VARV infection. Following an intranasal VARV challenge that mimics the natural route for human smallpox transmission, the virus spread systemically within the humanized mouse before mortality (~ 13 days post infection), similar to the time from exposure to symptom onset for ordinary human smallpox. Our identification of a permissive/representative VARV animal model can facilitate testing of MCMs in a manner consistent with their intended use.

Lyssavirus Vaccine with a Chimeric Glycoprotein Protects across Phylogroups

Rabies is nearly 100% lethal in the absence of treatment, killing an estimated 59,000 people annually. Vaccines and biologics are highly efficacious when administered properly. Sixteen rabies-related viruses (lyssaviruses) are similarly lethal, but some are divergent enough to evade protection from current vaccines and biologics, which are based only on the classical rabies virus (RABV). Here we present the development and characterization of LyssaVax, a vaccine featuring a structurally designed, functional chimeric glycoprotein (G) containing immunologically important domains from both RABV G and the highly divergent Mokola virus (MOKV) G. LyssaVax elicits high titers of antibodies specific to both RABV and MOKV Gs in mice. Immune sera also neutralize a range of wild-type lyssaviruses across the major phylogroups. LyssaVax-immunized mice are protected against challenge with recombinant RABV and MOKV. Altogether, LyssaVax demonstrates the utility of structural modeling in vaccine design and constitutes a broadened lyssavirus vaccine candidate.

Magnitude and diversity of immune response to vaccinia virus is dependent on route of administration

Historic observations suggest that survivors of smallpox maintained lifelong immunity and protection to subsequent infection compared to vaccinated individuals. Although protective immunity by vaccination using a related virus (vaccinia virus (VACV) strains) was the key for smallpox eradication, it does not uniformly provide long term, or lifelong protective immunity (Heiner et al., 1971). To determine differences in humoral immune responses, mice were inoculated with VACV either systemically, using intranasal inoculation (IN), or locally by an intradermal (ID) route. We hypothesized that sub-lethal IN infections may mimic systemic or naturally occurring infection and lead to an immunodominance reaction, in contrast to localized ID immunization. The results demonstrated systemic immunization through an IN route led to enhanced adaptive immunity to VACV-expressed protein targets both in magnitude and in diversity when compared to an ID route using a VACV protein microarray. In addition, cytokine responses, assessed using a Luminex® mouse cytokine multiplex kit, following IN infection was greater than that stemming from ID infection. Overall, the results suggest that the route of immunization (or infection) influences antibody responses. The greater magnitude and diversity of response in systemic infection provides indirect evidence for anecdotal observations made during the smallpox era that survivors maintain lifelong protection. These findings also suggest that systemic or disseminated host immune induction may result in a superior response, that may influence the magnitude of, as well as duration of protective responses.

Antiviral Ranpirnase TMR-001 Inhibits Rabies Virus Release and Cell-to-Cell Infection In Vitro

Currently, no rabies virus-specific antiviral drugs are available. Ranpirnase has strong antitumor and antiviral properties associated with its ribonuclease activity. TMR-001, a proprietary bulk drug substance solution of ranpirnase, was evaluated against rabies virus in three cell types: mouse neuroblastoma, BSR (baby hamster kidney cells), and bat primary fibroblast cells. When TMR-001 was added to cell monolayers 24 h preinfection, rabies virus release was inhibited for all cell types at three time points postinfection. TMR-001 treatment simultaneous with infection and 24 h postinfection effectively inhibited rabies virus release in the supernatant and cell-to-cell spread with 50% inhibitory concentrations of 0.2–2 nM and 20–600 nM, respectively. TMR-001 was administered at 0.1 mg/kg via intraperitoneal, intramuscular, or intravenous routes to Syrian hamsters beginning 24 h before a lethal rabies virus challenge and continuing once per day for up to 10 days. TMR-001 at this dose, formulation, and route of delivery did not prevent rabies virus transit from the periphery to the central nervous system in this model (n = 32). Further aspects of local controlled delivery of other active formulations or dose concentrations of TMR-001 or ribonuclease analogues should be investigated for this class of drugs as a rabies antiviral therapeutic.

Use of live Variola virus to determine whether CAST/EiJ mice are a suitable surrogate animal model for human smallpox

Numerous animal models of systemic orthopoxvirus disease have been developed to evaluate therapeutics against variola virus (VARV), the causative agent of smallpox. These animal models do not resemble the disease presentation in human smallpox and most used surrogate Orthopoxviruses. A rodent model using VARV has a multitude of advantages, and previous investigations identified the CAST/EiJ mouse as highly susceptible to monkeypox virus infection, making it of interest to determine if these rodents are also susceptible to VARV infection. In this study, we inoculated CAST/EiJ mice with a range of VARV doses (10²-10⁶ plaque forming units). Some animals had detectable viable VARV from the oropharynx between days 3 and 12 post inoculation. Despite evidence of disease, the CAST/EiJ mouse does not provide a model for clinical smallpox due to mild signs of morbidity and limited skin lesions. However, in contrast to previous rodent models using VARV challenge (i.e. prairie dogs and SCID mice), a robust immune response was observed in the CAST/EiJ mice (measured by Immunoglobulin G enzyme-linked immunosorbent assay). This is an advantage of this model for the study of VARV and presents a unique potential for the study of the immunomodulatory pathways following VARV infection.

Characterization of Monkeypox virus dissemination in the black-tailed prairie dog (Cynomys ludovicianus) through in vivo bioluminescent imaging

Monkeypox virus (MPXV) is a member of the genus Orthopoxvirus, endemic in Central and West Africa. This viral zoonosis was introduced into the United States in 2003 via African rodents imported for the pet trade and caused 37 human cases, all linked to exposure to MPXV-infected black-tailed prairie dogs (Cynomys ludovicianus). Prairie dogs have since become a useful model of MPXV disease, utilized for testing of potential medical countermeasures. In this study, we used recombinant MPXV containing the firefly luciferase gene (luc) and in vivo imaging technology to characterize MPXV pathogenesis in the black-tailed prairie dog in real time. West African (WA) MPXV could be visualized using in vivo imaging in the nose, lymph nodes, intestines, heart, lung, kidneys, and liver as early as day 6 post infection (p.i.). By day 9 p.i., lesions became visible on the skin and in some cases in the spleen. After day 9 p.i., luminescent signal representing MPXV replication either increased, indicating a progression to what would be a fatal infection, or decreased as infection was resolved. Use of recombinant luc+ MPXV allowed for a greater understanding of how MPXV disseminates throughout the body in prairie dogs during the course of infection. This technology will be used to reduce the number of animals required in future pathogenesis studies as well as aid in determining the effectiveness of potential medical countermeasures.

Analgesia during Monkeypox Virus Experimental Challenge Studies in Prairie Dogs (Cynomys ludovicianus)

Because human patients with monkeypox virus (MPXV) infection report painful symptoms, it is reasonable to assume that animals infected with MPXV experience some degree of pain. Understanding whether and how analgesics affect MPXV disease progression is crucial when planning in vivo challenge experiments. In the current study, we challenged prairie dogs with a low dose (4 ×10³ pfu) of MPXV and treated with meloxicam (NSAID) or buprenorphine (opioid); control animals did not receive analgesia or received analgesia without MPXV challenge. Subsets of animals from each group were serially euthanized during the course of the study. Disease progression and viral kinetics were similar between groups, but MXPVinfected, meloxicam-treated animals showed increasing trends of morbidity and mortality compared with other groups. Differences between no-analgesia MPXV-infected control animals and MPXV-infected animals treated with buprenorphine were minimal. The findings in the current study allow more informed decisions concerning the use of analgesics during experimental MPXV challenge studies, thereby improving animal welfare. In light of these findings, we have modified our pain scale for this animal model to include the use of buprenorphine for pain relief when warranted after MPXV challenge.

Revisiting rabies virus neutralizing antibodies through infecting BALB/c mice with live rabies virus

This study investigates the production of rabies virus (RABV) neutralizing antibody after virus infection through a mouse model. The BALB/c mice from different age groups (three, five, seven week old) were intramuscularly inoculated with live rabies virus (TX coyote 323R). Without pre-exposure or post-exposure prophylaxis (PEP), we found there is a decreased fatality with increased age of animals, the mortalities are 60%, 50%, and 30%, respectively. Interestingly, through assay of rapid fluorescent focus inhibition test (RFFIT), direct fluorescent antibody (DFA) and quantitative Polymerase Chain Reaction (qPCR), the results showed that all the animals that succumbed to rabies challenge, except one, developed circulating neutralizing antibodies, and all the healthy animals, except two, did not generate virus neutralizing antibodies (VNA). Our animal study suggests that the induction of VNA was an indicator of infection progression in the central nervous system (CNS) and speculate that RABV neutralizing antibodies did not cross the blood-brain barrier of the CNS for those diseased animals. We hypothesize that early release of viral antigens from damaged nerve tissue might potentially be a benefit for survivors, and we also discuss several other aspects of the interaction of RABV and its neutralizing antibodies.

Pharmacokinetic Profiles of Meloxicam and Sustained-release Buprenorphine in Prairie Dogs (Cynomys ludovicianus)

In this study, we evaluated the pharmacokinetic profiles of meloxicam and sustained-release (SR) buprenorphine in prairie dogs. The 4 treatment groups were: low-dose meloxicam (0.2 mg/kg SC), high-dose meloxicam (4 mg/kg SC), low-dose buprenorphine SR (0.9 mg/kg SC), and high-dose buprenorphine SR (1.2 mg/kg SC). The highest plasma concentrations occurred within 4 h of administration for both meloxicam treatment groups. The therapeutic range of meloxicam in prairie dogs is currently unknown. However, as compared with the therapeutic range documented in other species (0.39 - 0.91 μg/mL), the mean plasma concentration of meloxicam fell below the minimal therapeutic range prior to 24 h in the low-dose group but remained above therapeutic levels for more than 72 h in the high-dose group. These findings suggest that the current meloxicam dosing guidelines may be subtherapeutic for prairie dogs. The highest mean plasma concentration for buprenorphine SR occurred at the 24-h time point (0.0098 μg/mL) in the low-dose group and at the 8-h time point (0.015 μg/mL) for the high-dose group. Both dosages of buprenorphine SR maintained likely plasma therapeutic levels (0.001 μg/mL, based on previous rodent studies) beyond 72 h. Given the small scale of the study and sample size, statistical analysis was not performed. The only adverse reactions in this study were mild erythematous reactions at injection sites for buprenorphine SR.

Further Assessment of Monkeypox Virus Infection in Gambian Pouched Rats (Cricetomys gambianus) Using In Vivo Bioluminescent Imaging

Monkeypox is a zoonosis clinically similar to smallpox in humans. Recent evidence has shown a potential risk of increased incidence in central Africa. Despite attempts to isolate the virus from wild rodents and other small mammals, no reservoir host has been identified. In 2003, Monkeypox virus (MPXV) was accidentally introduced into the U.S. via the pet trade and was associated with the Gambian pouched rat (Cricetomys gambianus). Therefore, we investigated the potential reservoir competence of the Gambian pouched rat for MPXV by utilizing a combination of in vivo and in vitro methods. We inoculated three animals by the intradermal route and three animals by the intranasal route, with one mock-infected control for each route. Bioluminescent imaging (BLI) was used to track replicating virus in infected animals and virological assays (e.g. real time PCR, cell culture) were used to determine viral load in blood, urine, ocular, nasal, oral, and rectal swabs. Intradermal inoculation resulted in clinical signs of monkeypox infection in two of three animals. One severely ill animal was euthanized and the other affected animal recovered. In contrast, intranasal inoculation resulted in subclinical infection in all three animals. All animals, regardless of apparent or inapparent infection, shed virus in oral and nasal secretions. Additionally, BLI identified viral replication in the skin without grossly visible lesions. These results suggest that Gambian pouched rats may play an important role in transmission of the virus to humans, as they are hunted for consumption and it is possible for MPXV-infected pouched rats to shed infectious virus without displaying overt clinical signs.

Laboratory Investigations of African Pouched Rats (Cricetomys gambianus) as a Potential Reservoir Host Species for Monkeypox Virus

Monkeypox is a zoonotic disease endemic to central and western Africa, where it is a major public health concern. Although Monkeypox virus (MPXV) and monkeypox disease in humans have been well characterized, little is known about its natural history, or its maintenance in animal populations of sylvatic reservoir(s). In 2003, several species of rodents imported from Ghana were involved in a monkeypox outbreak in the United States with individuals of three African rodent genera (Cricetomys, Graphiurus, Funisciurus) shown to be infected with MPXV. Here, we examine the course of MPXV infection in Cricetomys gambianus (pouched Gambian rats) and this rodent species' competence as a host for the virus. We obtained ten Gambian rats from an introduced colony in Grassy Key, Florida and infected eight of these via scarification with a challenge dose of 4X104 plaque forming units (pfu) from either of the two primary clades of MPXV: Congo Basin (C-MPXV: n = 4) or West African (W-MPXV: n = 4); an additional 2 animals served as PBS controls. Viral shedding and the effect of infection on activity and physiological aspects of the animals were measured. MPXV challenged animals had significantly higher core body temperatures, reduced activity and increased weight loss than PBS controls. Viable virus was found in samples taken from animals in both experimental groups (C-MPXV and W-MPXV) between 3 and 27 days post infection (p.i.) (up to 1X108 pfu/ml), with viral DNA found until day 56 p.i. The results from this work show that Cricetomys gambianus (and by inference, probably the closely related species, Cricetomys emini) can be infected with MPXV and shed viable virus particles; thus suggesting that these animals may be involved in the maintenance of MPXV in wildlife mammalian populations. More research is needed to elucidate the epidemiology of MPXV and the role of Gambian rats and other species.

Comparison of Monkeypox Virus Clade Kinetics and Pathology within the Prairie Dog Animal Model Using a Serial Sacrifice Study Design

Monkeypox virus (MPXV) infection of the prairie dog is valuable to studying systemic orthopoxvirus disease. To further characterize differences in MPXV clade pathogenesis, groups of prairie dogs were intranasally infected (8 × 10(3) p.f.u.) with Congo Basin (CB) or West African (WA) MPXV, and 28 tissues were harvested on days 2, 4, 6, 9, 12, 17, and 24 postinfection. Samples were evaluated for the presence of virus and gross and microscopic lesions. Virus was recovered from nasal mucosa, oropharyngeal lymph nodes, and spleen earlier in CB challenged animals (day 4) than WA challenged animals (day 6). For both groups, primary viremia (indicated by viral DNA) was seen on days 6-9 through day 17. CB MPXV spread more rapidly, accumulated to greater levels, and caused greater morbidity in animals compared to WA MPXV. Histopathology and immunohistochemistry (IHC) findings, however, were similar. Two animals that succumbed to disease demonstrated abundant viral antigen in all organs tested, except for brain. Dual-IHC staining of select liver and spleen sections showed that apoptotic cells (identified by TUNEL) tended to colocalize with poxvirus antigen. Interestingly splenocytes were labelled positive for apoptosis more often than hepatocytes in both MPXV groups. These findings allow for further characterization of differences between MPXV clade pathogenesis, including identifying sites that are important during early viral replication and cellular response to viral infection.

Identification of Giardia duodenalis and Enterocytozoon bieneusi in an epizoological investigation of a laboratory colony of prairie dogs, Cynomys ludovicianus

Since 2005, black-tailed prairie dogs (Cynomys ludovicianus) have been collected for use as research animals from field sites in Kansas, Colorado, and Texas. In January of 2012, Giardia trophozoites were identified by histology, thin-section electron microscopy, and immunofluorescent staining in the lumen of the small intestine and colon of a prairie dog euthanized because of extreme weight loss. With giardiasis suspected as the cause of weight loss, a survey of Giardia duodenalis in the laboratory colony of prairie dogs was initiated. Direct immunofluorescent testing of feces revealed active shedding of Giardia cysts in 40% (n=60) of animals held in the vivarium. All tested fecal samples (n=29) from animals in another holding facility where the index case originated were PCR positive for G. duodenalis with assemblages A and B identified from sequencing triosephosphate isomerase (tpi), glutamate dehydrogenase (gdh), and β-giardin (bg) genes. Both assemblages are considered zoonotic, thus the parasites in prairie dogs are potential human pathogens and indicate prairie dogs as a possible wildlife reservoir or the victims of pathogen spill-over. Molecular testing for other protozoan gastrointestinal parasites revealed no Cryptosporidium infections but identified a host-adapted Enterocytozoon bieneusi genotype group.

Transmissibility of the monkeypox virus clades via respiratory transmission: investigation using the prairie dog-monkeypox virus challenge system

Monkeypox virus (MPXV) is endemic within Africa where it sporadically is reported to cause outbreaks of human disease. In 2003, an outbreak of human MPXV occurred in the US after the importation of infected African rodents. Since the eradication of smallpox (caused by an orthopoxvirus (OPXV) related to MPXV) and cessation of routine smallpox vaccination (with the live OPXV vaccinia), there is an increasing population of people susceptible to OPXV diseases. Previous studies have shown that the prairie dog MPXV model is a functional animal model for the study of systemic human OPXV illness. Studies with this model have demonstrated that infected animals are able to transmit the virus to naive animals through multiple routes of exposure causing subsequent infection, but were not able to prove that infected animals could transmit the virus exclusively via the respiratory route. Herein we used the model system to evaluate the hypothesis that the Congo Basin clade of MPXV is more easily transmitted, via respiratory route, than the West African clade. Using a small number of test animals, we show that transmission of viruses from each of the MPXV clade was minimal via respiratory transmission. However, transmissibility of the Congo Basin clade was slightly greater than West African MXPV clade (16.7% and 0% respectively). Based on these findings, respiratory transmission appears to be less efficient than those of previous studies assessing contact as a mechanism of transmission within the prairie dog MPXV animal model.

Monkeypox disease transmission in an experimental setting: prairie dog animal model

Monkeypox virus (MPXV) is considered the most significant human public health threat in the genus Orthopoxvirus since the eradication of variola virus (the causative agent of smallpox). MPXV is a zoonotic agent endemic to forested areas of Central and Western Africa. In 2003, MPXV caused an outbreak in the United States due to the importation of infected African rodents, and subsequent sequential infection of North American prairie dogs (Cynomys ludovicianus) and humans. In previous studies, the prairie dog MPXV model has successfully shown to be very useful for understanding MPXV since the model emulates key characteristics of human monkeypox disease. In humans, percutaneous exposure to animals has been documented but the primary method of human-to-human MPXV transmission is postulated to be by respiratory route. Only a few animal model studies of MPXV transmission have been reported. Herein, we show that MPXV infected prairie dogs are able to transmit the virus to naive animals through multiple transmission routes. All secondarily exposed animals were infected with MPXV during the course of the study. Notably, animals secondarily exposed appeared to manifest more severe disease; however, the disease course was very similar to those of experimentally challenged animals including inappetence leading to weight loss, development of lesions, production of orthopoxvirus antibodies and shedding of similar levels or in some instances higher levels of MPXV from the oral cavity. Disease was transmitted via exposure to contaminated bedding, co-housing, or respiratory secretions/nasal mucous (we could not definitively say that transmission occurred via respiratory route exclusively). Future use of the model will allow us to evaluate infection control measures, vaccines and antiviral strategies to decrease disease transmission.

Monkeypox virus infections in small animal models for evaluation of anti-poxvirus agents

An ideal animal model for the study of a human disease is one which utilizes a route of infection that mimics the natural transmission of the pathogen; the ability to obtain disease with an infectious dose equivalent to that causing disease in humans; as well having a disease course, morbidity and mortality similar to that seen with human disease. Additionally, the animal model should have a mode(s) of transmission that mimics human cases. The development of small animal models for the study of monkeypox virus (MPXV) has been quite extensive for the relatively short period of time this pathogen has been known, although only a few of these models have been used to study anti-poxvirus agents. We will review those MPXV small animal models that have been developed thus far for the study of therapeutic agents.

Dosage comparison of Congo Basin and West African strains of monkeypox virus using a prairie dog animal model of systemic orthopoxvirus disease

The prairie dog is valuable for the study of monkeypox virus (MPXV) virulence and closely resembles human systemic orthopoxvirus disease. Herein, we utilize a variable dose intranasal challenge with approximately 10(3), 10(4), 10(5), and 10(6)PFU for each clade to further characterize virulence differences between the two MPXV clades. A trend of increased morbidity and mortality as well as greater viral shedding was observed with increasing viral challenge dose. Additionally, there appeared to be a delay in onset of disease for animals challenged with lower dosages of virus. Mathematical calculations were used to determine LD(50) values and based on these calculations, Congo Basin MPXV had approximately a hundred times lower LD(50) value than the West African clade (5.9x10(3) and 1.29x10(5) respectively); reinforcing previous findings that Congo Basin MPXV is more virulent.

Comparison of West African and Congo Basin monkeypox viruses in BALB/c and C57BL/6 mice

Although monkeypox virus (MPXV) studies in wild rodents and non-human primates have generated important knowledge regarding MPXV pathogenesis and inferences about disease transmission, it might be easier to dissect the importance of virulence factors and correlates of protection to MPXV in an inbred mouse model. Herein, we compared the two clades of MPXV via two routes of infection in the BALB/c and C57BL/6 inbred mice strains. Our studies show that similar to previous animal studies, the Congo Basin strain of MPXV was more virulent than West African MPXV in both mouse strains as evidenced by clinical signs. Although animals did not develop lesions as seen in human MPX infections, localized signs were apparent with the foot pad route of inoculation, primarily in the form of edema at the site of inoculation; while the Congo Basin intranasal route of infection led to generalized symptoms, primarily weight loss. We have determined that future studies with MPXV and laboratory mice would be very beneficial in understanding the pathogenesis of MPXV, in particular if used in in vivo imaging studies. Although this mouse model may not suffice as a model of human MPX disease, with an appropriate inbred mouse model, we can unravel many unknown aspects of MPX pathogenesis, including virulence factors, disease progression in rodent hosts, and viral shedding from infected animals. In addition, such a model can be utilized to test antivirals and the next generation of orthopoxvirus vaccines for their ability to alter the course of disease.

A prairie dog animal model of systemic orthopoxvirus disease using West African and Congo Basin strains of monkeypox virus

Multiple monkeypox virus (MPXV) animal models have been discussed in previous studies, but no small animal models, nor most non-human primate models, demonstrated the protracted asymptomatic incubation phase seen in systemic human orthopoxvirus illness. Herein, we characterize a black-tailed prairie dog (PD) (Cynomys ludovicianus) model of infection, via intranasal and intradermal exposures, with the two MPXV clades. Daily observations of the animals were made (food consumption, general symptoms, disease presentation), while weights and virus evaluations (ocular, nasal, oropharyngeal, faeces, blood) were obtained/made every third day. Generalized rash became apparent 9-12 days post-infection for all animals. Individual animals demonstrated a range of symptoms consistent with human monkeypox disease. Measurable viraemias and excretas were similar for both clade-representative strains and persisted until at least day 21. Greater morbidity was observed in Congo Basin strain-challenged animals and mortality was observed only in the Congo Basin strain-challenged animals. The PD model is valuable for the study of strain-dependent differences in MPXV. Additionally, the model closely mimics human systemic orthopoxvirus disease and may serve as a valuable non-human surrogate for investigations of antivirals and next generation orthopoxvirus vaccines.

Monkeypox zoonotic associations: insights from laboratory evaluation of animals associated with the multi-state US outbreak

At the onset of the 2003 US monkeypox outbreak, virologic data were unavailable regarding which animal species were involved with virus importation and/or subsequent transmission to humans and whether there was a risk for establishment of zoonotic monkeypox in North America. Similarly, it was unclear which specimens would be best for virus testing. Monkeypox DNA was detected in at least 33 animals, and virus was cultured from 22. Virus-positive animals included three African species associated with the importation event (giant pouched rats, Cricetomys spp.; rope squirrels, Funisciuris sp.; and dormice, Graphiuris sp.). Virologic evidence from North American prairie dogs (Cynomys sp.) was concordant with their suspected roles as vectors for human monkeypox. Multiple tissues were found suitable for DNA detection and/or virus isolation. These data extend the potential host range for monkeypox virus infection and supports concern regarding the potential for establishment in novel reservoir species and ecosystems.

Two cases of hantavirus pulmonary syndrome in Randolph County, West Virginia: a coincidence of time and place?

Hantavirus pulmonary syndrome (HPS) is caused by an infection with viruses of the genus Hantavirus in the western hemisphere. Rodent hosts of hantaviruses are present throughout the United States. In July 2004, two HPS case-patients were identified in Randolph County, WV: a wildlife science graduate student working locally and a Randolph County resident. We interviewed family members and colleagues, reviewed medical records, and conducted environmental studies at likely exposure sites. Small mammals were trapped, and blood, urine, and tissue samples were submitted to the Centers for Disease Control and Prevention for laboratory analyses. These analyses confirmed that both patients were infected with Monongahela virus, a Sin Nombre hantavirus variant hosted by the Cloudland deer mouse, Peromyscus maniculatus nubiterrae. Other than one retrospectively diagnosed case in 1981, these are the first HPS cases reported in West Virginia. These cases emphasize the need to educate the public throughout the United States regarding risks and prevention measures for hantavirus infection.

Smallpox vaccine stability after maintenance at temperatures not recommended for shipping

Two distinct smallpox vaccine formulations were exposed to temperatures beyond the ranges specified by the manufacturers for vaccine maintenance and shipping. Under the conditions investigated, titers of both Dryvax smallpox vaccine and Aventis Pasteur smallpox vaccine remained at or above the titers recommended for successful vaccination. From these data it can be inferred that vaccine efficacy would not be expected to be adversely affected by unintended fluctuations of temperature, within the ranges studied, for a 4-day period.