Inger K Damon. Vaccine. Volume 29, December 2011.
Initial identification
Monkeypox, a vesiculo-pustular rash illness, was initially discovered to cause human infection in 1970 through the World Health Organization (WHO)-sponsored efforts of the Commission to Certify Smallpox Eradication in Western Africa and the Congo Basin. From 1969 to 1971, of 1177 specimens sent to the WHO-Collaborating Centers in Atlanta or Moscow, 182 were positive for variola, and 9 were positive for monkeypox virus. Of the monkeypox virus positive specimens, from Zaire, Nigeria, Cote d’Ivoire, Sierra Leone, and Liberia, all were single cases or felt to be co-primaries (from a common non-human animal source). Most of the studies, through 1987, on this virus and associated disease have been summarized in a monograph co-authored by Jezek and Fenner. Monkeypox virus was previously identified as a member of the Orthopoxvirus genus in 1958, at a time when there was an increased use of monkeys and their tissues for the safety testing and initial growth of both the inactivated and live attenuated poliovirus vaccines. Investigation of an outbreak of generalized vesiculo-pustular rash illness within a captive cynomologous monkey colony at the State Serum Institute, Copenhagen, led to identification of a virus within the rash lesions. Laboratory analyses, including cross-neutralization with sera from convalescent animals, biologically distinct virus growth on choriolallantoic membrane (white pocks at 2 days, hemorrhagic at 3 days), genome restriction mapping and later sequence analysis of single genes, and whole genome, identified this virus as a distinct member species of the Orthopoxvirus genus. Eight outbreaks in primate colonies were characterized between 1958 and 1968, and one outbreak in a European zoo was also identified. In the latter outbreak, multiple species were infected and became ill, including a giant anteater. No human infections were identified to be associated with any of these primate colony or zoo outbreaks across Europe and North America.
54 cases of human disease were identified between 1970 and 1979 in the forested habitats of Western and Central Africa; 44 (80%) occurred in DRC. In DRC, 5 isolated possible instances of secondary human to human transmission were reported, but no 3rd generation cases were reported. No deaths were reported in cases from West Africa (Liberia, Nigeria, Ivory Coast, Sierra Leone), but 8/38 (21%) of those cases initially reported from DRC resulted in death during acute illness.
Virus and disease studies: Congo Basin 1981-1986
Subsequent to declaration of smallpox eradication in 1980 by the World Health Assembly, WHO sponsored enhanced human monkeypox surveillance efforts in the central regions of DRC and some limited animal/human ecologic studies were undertaken. Through these efforts, between 1981 and 1986, 338 cases were identified in DRC; 67% of these suspected cases were confirmed by viral culture. From this work, and investigations associated with this enhanced surveillance campaign, a great deal of clinical, epidemiologic and animal ecology information has accrued on zoonotic monkeypox disease in the Congo Basin.
Human illness
Clinical disease was described as presenting similar to discrete, ordinary smallpox. Lymphadenopathy, however, was a prominent feature of monkeypox disease. Nevertheless, the typical time course and manifestations of disease were similar to those of human smallpox. The primary routes of infection were surmised to be large respiratory droplet (virus was present in oropharyngeal secretions at the onset of rash) or direct percutaneous or mucosal contact. Apparent transplacental exposure and infection was also anecdotally documented. As with smallpox those patients requiring hospitalization had more severe disease courses, and a higher case fatality rate, than outpatients.
Incubation period
Careful case studies, and studies of case-contacts, from the 1980 surveillance program measured the time interval from exposure to fever onset. These intervals ranged from 10 to 14 days, and the time interval from exposure to rash onset ranged from 12 to 16 days. Fever was accompanied by headache, backache, malaise, and prostration. 84% developed rash 1-3 days post fever onset, 4% developed rash at fever onset, and 12% developed rash >3 days post fever onset. 32.6% of previously vaccinated individuals developed rash within 48 h of fever onset, among unvaccinated patients 19.6% developed rash within 48 h of fever onset. The fever to rash onset interval appeared to be of a longer duration than that observed in unvaccinated individuals. The interval, from infectious exposure to onset of fever, as with smallpox, was an asymptomatic period.
Rash
Most patients had a rash presentation and evolution consistent with discrete or semi-confluent smallpox (intact skin visible between rash lesions on face, or not, respectively). Similar to smallpox, rash progressed through characteristic macular, papular, vesicular and pustular phases. This feature is dissimilar from most other vesiculopustular rash illnesses and, thus, is a useful clinical sign to differentiate from other rash illnesses. The differentiation of monkeypox from smallpox disease, however, cannot easily be done on clinical criteria alone, and requires the use of laboratory diagnostics, in addition to urgent epidemiologic investigation. In monkeypox, as with smallpox, the number of rash lesions was less in previously smallpox-vaccinated case patients than in unvaccinated case patients. With onset of rash, fever fell in 46% cases; 43% or 27% continued with fever into days 2 and 3 of rash, respectively. At the onset of pustulation of rash 37% developed a secondary fever, lasting 2-3 days, and this finding was associated with a more severe disease course.
Oropharynx, upper respiratory, and ocular symptoms
Oropharyngeal lesions were seen in 76% unvaccinated and 47% previously smallpox-vaccinated individuals with monkeypox disease. Oral ulcers and tonsillitis could be observed. Cough was observed in 39% of unvaccinated, and in 14% of previously vaccinated patients. Conjunctivitis and blepharitis were seen in 30% of unvaccinated, and in 7% of previously smallpox vaccinated patients.
Lymphadenopathy
Lymphadenopathy was reported in 86% of unvaccinated patients, and in 54% of previously vaccinated patients. Usually this sign of disease occurred early, 1 or 2 days post fever, and occasionally within 1-2 days post rash onset. Lymph nodes could range from 1 to 4 cm diameter, and were firm, tender, and occasionally painful. Enlarged cervical or submaxillary lymph nodes were common, and generalized lymphadenopathy was also observed. The presence of lymphadenopathy in monkeypox may indicate the human immune system has a more effective immune recognition of and response to monkeypox infection than variola infection.
Convalescence/severity of illness
Illness lasted 2-4 weeks. Convalescent periods were roughly equivalent in unvaccinated (70% convalesced after 3 weeks) and in previously vaccinated (81% convalesced after 3 weeks) case patients. The interval from previous smallpox vaccination to monkeypox disease presentation ranged from 3 to 19 years in this cohort. Illness was graded by rash burden, incapacitation, and need for nursing care were the indices to describe and to stratify disease morbidity. Overall, 11.2% (38/338) had mild illness (<25 skin lesions), were not incapacitated, and did not require nursing care. 19.3% (65/338) had moderate illness (25 < pock count > 99), required some nursing care, and were incapable of physical activity. 235/338 (69.5%) had severe disease (>100 pocks), and required more intensive nursing care. Severity of disease correlated with young age, previous vaccination, nutritional, and immunologic/concurrent disease status.
Complications
Secondary bacterial infections were observed. Corneal ulceration seen in 4% of unvaccinated, and 1% of previously smallpox-vaccinated case patients. Bronchopneumonia and pulmonary distress were observed late in disease (and was presumed to be of bacterial origin); 19 of 34 unvaccinated patients with this complication died. Bronchopneumonia occurred in 34/270 unvaccinated case patients, and in 2/43 previously smallpox-vaccinated case patients. Other complications included vomiting and diarrhea leading to dehydration.
Case fatality rate
Overall the case fatality rate (CFR) was 33/338 (9.8%). All deaths occurred in unvaccinated children of 3 months-8 years of age (33/295 (11.2%)). In stratifying the mortality associated with disease acquired from non-human animal exposure, vs. human exposure, the CFR of primary/co-primary cases was 11.8% and was apparently no different from that of secondary cases (9.6%), but the absolute number of “secondary’ cases is small. The specific etiologies of and pathologies leading to death have not been ascertained.
Sequelae
The major disease sequelae in disease survivors were disfiguring scarring of the skin and blindness (similar to that seen in smallpox). No complications were observed in >90% survivors regardless of vaccination status.
As with human smallpox, previous smallpox vaccination provided a protective benefit against disease severity, as well as disease acquisition. In the WHO-sponsored 1980s studies of monkeypox, previous smallpox vaccination, administered 3-19 years previously, and verified by the presence of a scar over the left deltoid, appeared to provide over 85% protection against disease acquisition in studies of close contacts of cases.
Two, or three, clinical-epidemiologic forms of human smallpox have been described: major, Alastrim minor, and some investigators described an intermediate form of disease. These forms were discriminated by different, age-adjusted, case fatality rates. Genotypic analyses support phylogenetic differences between variola viruses isolated from outbreaks where some of these clinical-epidemiologic forms of disease were predominantly described, although viruses attributed to “African minor” disease outbreaks do not form a genetic group discrete from “Variola major” isolates. Similarly, different forms of monkeypox disease have been described, are associated with different virus clades, appear to be geographically distinct, and are described later in this paper.
Laboratory studies
Virus characterization
The growth of virus on embryonated hen eggs, with characteristic pock formation, and restriction endonuclease digestion of whole genome were used to differentiate the virus species monkeypox from other Orthopoxvirus species. Later, single gene sequence and whole genome sequence analyses were used to further characterize monkeypox virus isolates and to differentiate strains of monkeypox. Monkeypox virus isolates from captive monkeys (Copenhagen 58, McConnell 61, Washington 61, Paris 68, Utrecht 65) associate separately from the Zairean monkeypox isolates (v79-I-005, v96-I-96, Congo 70, Zaire 77, Zaire 79), and appear to associate more closely with the early West African monkeypox isolates from human disease (Liberia 70, Sierra Leone 70).
Electron microscopic analysis of specimens from monkeypox rash reveals characteristic large brick shaped particles. In vesicular fluids, the M (mulberry) form is more predominant, with mulberry protrusions evident, and lack of phosphotungstic acid (PTA) stain penetration through the particle. In crust specimens, the C (capsule) form appears to predominate and PTA penetrates the particle. Both of these negative stained electron microscopic characterized particles represent mature virions.
Population serosurveys and associated methods
The 1980s investigations used serologic cross adsorption tests (to remove seroreactivity to the orthopoxviruses vaccinia and variola) in order to detect the specific presence of monkeypox virus serologic reactivity. Another more specific monkeypox serologic test was the use of a modified RIAA. In the absence of virus containing specimens, RIAA became one of the more specific methods for identification of infection. The specific additional use of 125 I staph protein A enhanced the specificity for monkeypox, as opposed to the previous detection use of a hyperimmune gamma globulin generated vs. the specific orthopoxvirus in question. These tests largely look for IgG serologic reactivity vs. these viruses, and the IgG response may be long-lived. Recently, an anti-orthopoxvirus IgM capture assay has been developed at the CDC; potentially this will have benefit in distinguishing recent from remote infections and in determining disease incidence.
Epidemiology
An early observation was the zoonotic nature of disease, but identification of specific reservoir host(s) remained, and continues to remain, elusive. Epidemiologic studies were undertaken to understand disease prevalence, incidence and the etiology of transmission to, and between, humans. Studies in the 1980s stratified cases into primary (if the exposure was from a non-human animal), and secondary (if it was linked (only) to exposure to an ill human).
Prevalence studies
Serosurveys of individuals without vaccination scars in Congo and Zaire, Cote d’Ivoire and Sierra Leone (areas of Central and Western Africa where monkeypox had been identified in the 1970s) were conducted in 1981. By hemagluttination inhibition testing (HAI), an orthopoxvirus generic serologic test, sampling on DRC (then Zaire) 663/3460 (19.2%) were seropositive. Of those HAI positive sera, 178 were tested using a more monkeypox-specific serologic assay, the RIAA test. Of the 178 sera tested, 27 (15%) tested were positive for “monkeypox”. In evaluating seroprevalence in studies from Cote d’Ivoire and Sierra Leone, slightly different results were found. In Cote d’Ivoire, 369/2840 (13.0%) were positive by HAI testing, 93 of the 369 HAI positive sera were tested using the monkeypox RIAA test and, of those, 20 (21%) were positive. In Sierra Leone, 320/2567 were positive by HAI, 71 of the 320 were tested using the monkeypox RIAA test and, of those, 27 (38%) tested positive for monkeypox. It is not clear whether there was a sampling bias used to select those sera which were then tested using the more specific serologic assays. Subsequent follow up suggested some of those included in these studies indeed had vaccination scars. Others reported no antecedent rash illness, leading to the consideration that asymptomatic illness may occur.
Incidence
Although previous observations through 1979 suggested that human disease incidence occurred in the dry seasons, the studies from 1981 to 1986, which involved the use of monetary incentives to report cases, did not identify this precise seasonality. Instead, in these longitudinal studies conducted largely in the Kole/central area of DRC, cases appeared to occur throughout the year, with increased case reporting occurring in June-August.
Transmission studies/patterns of transmission
Between 1980 and 1986, 338 cases were identified; 245 (72%) were considered to be primary or coprimary cases. Of these, 52% were between the ages of 0 and 4; 37% were between the ages of 5 and 9; 96% of these children never vaccinated. 93 cases (28%) were characterized as resulting from possible person-to-person transmission. Investigations of contacts of cases were conducted to evaluate interhuman transmission. In the evaluation of 723 unvaccinated contacts of these 338 cases, 431 (60%) were household contacts. 40 of these unvaccinated contacts developed disease. The secondary attack rate in unvaccinated household members was 9.3% – far less than that observed in human smallpox. The secondary attack rate in vaccinated household contacts was 1.3%. This led to the descriptions that prior smallpox vaccination resulted in >85% protection (vaccination was 3-19 years prior) from disease acquisition. The specific consideration of interhuman transmission was further evaluated by looking for extended chains of interhuman transmission. The longest uninterrupted chain of interhuman transmission was suspected to be four generations.
Case-control studies were undertaken to understand the sources and types of animal exposures that may lead to disease. Ultimately they were confounded by the fact that cases and contacts had multiple animal exposures. In questioning confirmed primary cases, animal contact 2-3 weeks prior to rash onset included hunting, skinning, trapping, cooking, playing with carcasses of or eating members of the following animal families: non-human primates (30%), terrestrial rodents (20%), antelopes and gazelles (13%) and tree squirrels (7%).
Ecology
Trapping studies, conducted in areas where human cases had been identified, characterized anti-orthopoxvirus, or “monkeypox-virus specific” antibodies in the following rodent and non human primate genera: seropositive rodents genera included Funisciuris (rope squirrels), Heliosciuris (sun squirrels), Lemniscomys, Lophuromys, Thamnomys, Oenomys, Praomys, and seropositive non human primates included Cerocebus, Cercopithicus, Colobus, Allenopithecus. Virus was isolated, however, from only one wild-caught African squirrel, a Funisciurus anerythrus which was moribund, and with sparse rash lesions on its capture. The greater degree of seroprevalence in squirrel populations, the lack of terrestrial rodents with significant seropositivity, and the abundance of squirrel genera in the secondary forests around villages in the rural areas of Northwestern DRC led to the hypothesis that Funisciurus was the most probable reservoir host.
Summary – disease predictions
Modeling studies, based on the human epidemiology, along with the limited ecologic studies were used to assess whether monkeypox might transmit efficiently between humans, and replace smallpox as a serious, interhuman transmissible pathogen. These studies were also influential in the decision to not reinstitute “smallpox” vaccination. Using data from the 1981-1986 investigations, the interhuman transmission reproductive rate ( R 0 ) was calculated to be 0.8; so as less than 1, disease would not be predicted to sustain human infections without repeated zoonotic introductions.
Following these studies, monkeypox was rarely reported from Zaire/DRC or West Africa. In 1996, however, reports of an outbreak of disease in the Kasai Orientale region were made, and once again disease was investigated by an international team.
1996-1997-1998
During a period of civil unrest in Zaire/DRC, monkeypox was again suspected to cause human disease, this time in the central Katako-Kombe health zone. Because of the practical difficulties encountered because of civil unrest, confirmation of cases via viral specimen isolation was often impossible, and detailed investigations were limited in their scope. Thus, few cases were identified by direct virus isolation, and the majority were laboratory-identified by a combination of serologic tests which are generic for orthopoxvirus infection (inclusive of vaccinia (smallpox) vaccination). 88 suspect cases were identified in a semi-retrospective investigation. Concurrent community-wide varicella infections were noted; the overall case fatality rate was 3.7%. Epidemiologically, the three limited investigations in Katako-Kombe, Sankuro and Kasai Oriental from 1996 to 1998 were quite provocative. Overall, mild clinical illness was reported, and absent viral confirmations, varicella cases were likely included in the case count. Only 22% cases were identified as primary or coprimary (from animal exposure), the remainder were secondary or tertiary transmission events between humans. Thus, interhuman transmission appeared to be of greater incidence, and this was largely attributed to the waning population immunity due to cessation of smallpox vaccination in 1980. However, the probable inclusion of varicella cases within the overall case count confounded the definitive analysis of monkeypox interhuman transmission.
Ecologic studies of animal species from convenience samplings of fauna around areas where human disease had been suspected in the Katako-Kombe confirmed anti-orthopoxvirus seroprevalence in some of the same species previously identified in the 1980s investigations. The 1990s study additionally identified anti-orthopoxvirus seroreactivity in one domestic pig sampled, and also in terrestrial rodent species of Cricetomys (Giant pouched Gambian rat). Monkeypox continues to occur in the DRC, and this characterization will be the subject of the Rimoin paper in this supplement.
2003
U.S.A.
Monkeypox emerged as a human pathogen, outside the African continent, in 2003. Disease importation was traced back to a consignment of rodents from the West African country Ghana shipped to Texas and destined for the exotic pet trade. Some rodents from the shipment died in transit, others soon after arrival, and others were shipped across the United States, some species even being shipped internationally to Japan. Within the United states, apparent mixing or interaction of West African infected rodents occurred with North and South American rodents (also destined for the exotic pet industry), leading to their infection. Some animals appeared relatively asymptomatic, others had more protracted illnesses. North American prairie dogs, purchased as household pets however, appeared susceptible to infection and ensuing illness, and were the key component of the outbreak to transmit disease to humans.
Ultimately, 37 human cases were laboratory confirmed, and an additional 10 were classified as probable cases. The case definition is on the CDC website. There was an occupational predilection for disease. Whereas human health care worker infection was a concern in the WHO-sponsored Congo studies, and in smallpox transmission, no transmission to U.S. health care workers (HCWs) was noted despite sometimes lax use of personal protective equipment, and some HCWs without prior smallpox vaccination. Veterinary clinic and pet store employees appeared to have the occupational risk for disease acquisition; these were the frontline personnel to care for or to treat the ill prairie dogs.
Clinically, human disease appeared to be less severe than that reported in DRC in the 1980s. An extensive comparison with the more recent 1990s DRC data has not been attempted due to the incomplete nature of that information. After controlling for age, and vaccination status, illness in the U.S. appeared to be less severe based on both burden of rash, and case fatality rates than that described in the 1980s data collection. No deaths resulted from the U.S. disease, and only two severely ill cases were reported – one with encephalitis, the other with severe mucosal/oropharyngeal edema requiring intubation for airway protection. One individual had serious ocular sequelae, resulting in the need for a corneal transplant. As well, disease in the U.S. was less interhuman transmissible – no cases in the U.S. were associated with human exposure absent an ill prairie dog exposure.
Subsequent investigations demonstrated that the route of infection was associated with illness manifestation in the onset of fever and rash; complex exposures – believed to be via both respiratory and contact were associated with a more rapid progression of disease and more severe disease. Behaviors associated with increased risk for infection were direct exposure (handling) or cleaning the prairie dog cage; this would be consistent with previous considerations for African-acquired infection – animal handling or exposure to peridomestic pests or their excreta. A number of studies assessed the protection provided by remote (>25 years prior to exposure) smallpox vaccination. Three of the four studies concluded there was no benefit either on mitigation of illness severity, or preventing disease acquisition.
Laboratory studies
The rapid ability to confirm the reports of suspect disease was facilitated by work on the smallpox research agenda – intensified in 2000 to aid with growing concerns that smallpox could now be used as a bioweapon. A battery of recently developed rapid, high throughput orthopoxvirus, smallpox and monkeypox real time PCR assays were used to rapidly identify (within 8 h of receipt of specimens) the presence of monkeypox virus and orthopoxvirus nucleic acid signatures in the material. Subsequent single gene sequence was used to confirm the virus was monkeypox, and genetically associated with previous West African isolates.
The outbreak allowed validation of the IgM anti-orthopoxvirus assay, also developed as part of smallpox preparedness efforts. In confirmed monkeypox cases, IgM was positive between days 4 and 56 post rash onset. This analysis allowed use of the IgM to classify cases as “probable” if lesion material was not available to “confirm” a case.
Response
Coordination of local, state and federal academic and public health efforts proceeded through an iterative process to enable eventual control of the outbreak. Initial cases were reported in May 2003; the last suspect cases were reported in August 2003. A series of communications – health alert network reports, and more comprehensive MMWRs were used to disseminate information about the outbreak and response efforts locally, nationally, and internationally. It is difficult to determine the individual impact of the multiple outbreak response efforts employed: cessation of importation of African rodents; use of standardized epidemiologic and laboratory based case definitions; contact tracing, evaluation of contacts, and vaccination; voluntary quarantine/euthanasia of animals associated with the distribution of the shipment of animals implicated in the importation of virus.
Republic of Congo
Although serosurveys from the 1980s had suggested anti-orthopoxvirus seroprevalence in 16% of the population unlikely to have been previously vaccinated in the Pool and Sangha districts of Republic of Congo (“Congo-Brazzaville”), no human monkeypox had been reported from Republic of Congo, despite habitats similar to that in DRC. In July of 2003, an outbreak of disease on the grounds of a community hospital was reported in Impfondo, within the Likuola district, and cases were confirmed by laboratory testing at CDC. Ultimately 11 probable or confirmed cases were identified, with one death in a child. Sequela in one child consisted of blindness in one eye from corneal opacification. The remarkable finding in this outbreak was the documentation, with laboratory confirmation, of 6 consecutive, uninterrupted chains of interhuman transmission. The original source of infection remains uncertain.
Comparison of U.S./West African and Congo Basin viruses and attributable disease
Complete genomic sequencing of West African derived and Congo Basin derived monkeypox viruses has allowed further specificity in defining genetic differences which may affect the apparent differences in human virulence associated with viruses from each of these clades. Animal model studies are being conducted in a number of laboratories to develop the systems to address the roles of hypothetical virulence and transmission determinants. Ironically, use of the North American prairie dog has led to the development of a model which manifests disease with a temporal semblance to human systemic orthopoxvirus disease (smallpox or monkeypox), and differentiates West African and Congo Basin clade disease. Various small animal models of systemic orthopoxvirus disease have recently been reviewed.
Disease prevalence studies
Following the disease outbreaks in Republic of Congo, and the United States, studies were undertaken to evaluate native wildlife, and human populations near the outbreak, or the potential source of the outbreak to understand what the virus/disease prevalence might be in these regions. In the Likuola region of Republic of Congo, an overall anti-orthopoxvirus IgG seroprevalence of 59% was surmised from a convenience sample of the population living along major roads, or in larger villages/towns. In those unlikely to have been previously vaccinated, i.e. <25 years of age, the anti-orthopoxvirus IgG seroprevalence was 49%. Recent orthopoxvirus infection, implied by anti-IgM orthopoxvirus seroreactivity, was documented in 1.7% of the population sampled. Investigations in Ghana, in villages adjacent areas where the animals imported to the United States had originally been collected revealed similar seroprevalence estimates. In 3 villages, two from the Eastern region, the third in the Volta region, 33, 85 and 92% of the population was surveyed. Of those <23 (and greater than nine) years of age, unlikely to have been previously vaccinated, those living in two villages in the Eastern region (a forested habitat) were significantly more likely to have anti-orthopoxvirus IgG seroprevalence (44.2%) than the same age cohort living in one village in the Volta region (savannah habitat) (15.8%). Human IgG anti-orthopoxvirus seropositivity was significantly associated with cultivation in the forest, but not firewood collection nor work as a professional animal worker, in this age group. Although no monkeypox virus was isolated from any animals collected during this study, orthopoxvirus nucleic acid signatures were detected in Xerus and Cricetomys species collected from the Eastern region, and from Graphiurus species in the Volta region. The latter two were involved as infected species in the U.S. importation of monkeypox virus. Anti-orthopoxvirus ELISA seroprevalence was only robustly detected in Cricetomys and Funisciuris species collected in the Eastern region, and in one Graphiurus species from the Volta region which was also orthopoxvirus-nucleic-acid signature positive.
Sudan 2005
Suspicious vesiculo-pustular rash illness continues to be recognized in areas outside the “range” for monkeypox. In southern Sudan’s Unity state in 2005, the delay between initial recognition of a child with systemic pustular rash illness, obtaining diagnostic specimens and shipment for testing illustrates the continued need for education and training in recognition and diagnosis of systemic orthopoxvirus illness. Enhancements of surveillance, laboratory based surveillance, and community outreach efforts will improve our global ability to respond quickly to contain these outbreaks. Recent community and healthcare worker disease recognition and awareness efforts in Africa include work by the International Conservation and Education Fund (INCEF), in collaboration with CDC scientists, to improve the ability to recognize and respond to disease occurrences, and to facilitate safer behaviors in the face of this emerging public health threat ( http://www.incef.org/node/63 ).
Summary thoughts
Currently, few laboratories are well-trained and proficient to perform definitive analyses for monkeypox, or rule in more common causes of systemic rash illness, including chickenpox. Additionally, issues of transportation and safe handling of specimens need to be addressed to facilitate shipment of suspect and confirmed material which may be infectious. At the time of this writing, the United States considers monkeypox to be a “select agent” and additional security and safety measures are applied to the study of infectious materials. The majority of diagnostic assays rely on the detection of small regions of genomic DNA which appear to be specific for monkeypox (as opposed to other orthopoxviruses, and other genomes); these have been recently reviewed. Novel approaches include a pan poxvirus diagnostic assay, which will identify genetic material from most all poxviruses in specimen material, and then speciates via sequencing. Other assays use standard or real time PCR methods; real time PCR assays offer the advantage of being rapid, high throughput, and are highly sensitive. However, these assays are also more prone to false positives, due to cross contamination resulting from non-optimal laboratory procedures. Recent diagnostic advances include assays able to differentiate West African clade and Congo Basin clade variants of monkeypox. The use of these may be especially important in utilizing and evaluating different measures to contain outbreaks of and prevent disease which have variable interhuman transmission dynamics (i.e. West African vs. Congo Basin clade disease).
Is the distribution of monkeypox static? This perhaps is a rhetorical question, but is worth exploring. The experiences in the United States in 2003, and in Sudan in 2005 certainly indicate that monkeypox can effectively exploit new ecologic habitats/animal species to perpetuate. The ability of this virus to replicate in, and transmit from, various species in the U.S. outbreak was especially remarkable. Response and control measures that contributed to the end of the outbreak included measures to cease importation of suspect carrier species, limit the movement of these species within the U.S., quarantine or cull potentially infected non-human species, vaccination of human contacts, and guidance on personal protection for healthcare and veterinary/pet-store care workers. Studies since the U.S. outbreak have applied various ecologic modeling and remote sensing approaches to evaluate the potential suitable habitats for the virus based on human disease occurrence data, and for potential transmitting, amplifying or reservoir species based on trapping data. At least one of these studies has identified a suitable habitat for one suspect reservoir-host species, the Gambian giant pouched rat, Cricetomys genus, outside Africa. As these studies become further refined, they will become more informative, and potentially predictive.
In conclusion, monkeypox virus causes disease in humans clinically similar to that caused by variola virus but it is an evolutionarily distinct virus, which we are seeing as a “living model” for the evolution and emergence of human pathogenic orthopoxviruses. Presently, monkeypox virus largely causes epizootic human disease. Although variola virus likely emerged from a zoonotic disease causing virus ancestor, the evolutionary changes in monkeypox virus are ones which we have the ability to, and need to characterize and observe in the present. Studies elucidating the factors which affect transmission, virus virulence, acquired or host genetic factors which predispose to disease susceptibility/virulence need to be better understood. Reservoir, amplifying and transmitting hosts need to be characterized in tandem with characterization of human disease—“inducing” behaviors. In turn, this knowledge will provide the information necessary to apply appropriate disease control and prevention measures – whether education, behavior modification, or vaccination and/or therapeutics utilization. The use of our current smallpox vaccine has been debated, as the uncertainty of HIV/AIDS prevalence in some areas may be a barrier to widespread, safe use of this approach. However, as less-reactogenic smallpox vaccines have been further developed, as part of smallpox preparedness research efforts, the use of these measures may be of benefit in reducing the burden of monkeypox disease. Similarly, the use of antiviral agents which have been developed as part of the smallpox research agenda, may have additional benefit in treating monkeypox disease.