Somewhat contrary to expectations, our results did not implicate a single species of animal as being the sylvan source of MPXV for the U.S. outbreak. We found evidence suggestive of intermittent exposure to OPXVs among various species across a broad geographic span. Species of animal showing evidence of OPXV exposure (i.e., Cricetomys
spp.) mirrored those implicated in the U.S. outbreak,11
with the exception of the Xerus
spp., which had not previously been associated with OPXV exposure. It should be emphasized that our study was not intended to constitute a comprehensive survey of the sylvan fauna of Ghana for evidence of OPXV infection. Only rodent species were evaluated. Species implicated in the U.S. outbreak were preferentially collected. Many of the species were sampled below the threshold needed to determine whether their OPXV nucleic acid and seroprevalence rates were comparable to those observed for Graphiurus
spp., the most well sampled species (for which nucleic acid and antibody prevalence rates of 6.2% and 10%, respectively, were observed).
If we assume that rodents, like humans, will sustain life-long detectable OPXV antibody titers after infection, the relatively low seroprevalence rates observed for two of well sampled species of interest, Graphiurus
spp. and Cricetomys
spp. (2.6% and 10%, respectively), might indicate that these species are unlikely to serve as natural sylvan reservoirs for OPXVs. Members of the genus Funisciurus
, which showed a seroprevalence of 40%, might be good candidates. The only isolation of MPXV from an animal in the wild came from a Furnisciurus anerythrus
further indicating the relevance of this species as a putative maintenance source of the virus in nature. However, studies of other, better-characterized, rodent-borne zoonotic viruses have documented similarly low point seroprevalence rates.17
Additionally, ecologic investigations of OPXVs from Europe performed in the United Kingdom, which investigated the influence of cowpoxvirus infection on population cycles in the reservoir host (the field vole Microtus agrestis
), have shown that population seroprevalence rates can vary temporally, thus limiting the informational value of single cross-sectional measurements to implicate, or rule out, virus maintenance hosts.18
In one such study,19
sharp decreases in seroprevalence were observed in bank vole populations that had experienced recent demographic crashes. The investigators noted that in some instances population seroprevalence decreased to undetectable levels after approximately two-fold decreases in density during the period six months prior, and conversely, reached as high as approximately 90% in the post-rebound period (in delayed density-dependent fashion, with a lag occurring between population rebound and increased seroprevalence). Therefore, understanding the interplay between MPXV and the population dynamics of various candidate reservoir species will be crucial to arriving at an accurate interpretation of OPXV serologic and virologic findings.
Animal infection experiments that illuminate features of virus transmission dynamics and pathobiology in various candidate reservoir species can also be helpful in identifying the reservoir host, and differentiating it from other species that are merely permissive of virus replication. The interpretation of human serologic findings is similarly nuanced. Also, contrary to expectations, the results of our study did not show enhanced risk of OPXV exposure among persons directly involved in the exotic animal trade. Instead, we observed IgG against OPXV among a large proportion of those persons living in rural communities near the original animal trapping sites, albeit no persons had serologic evidence suggesting recent infection. Smallpox vaccination continued in Africa until 1980. Therefore, a substantial proportion of persons greater than 23 years of age at the time that this study was performed would be anticipated to retain cross-reactive antibodies because of vaccination. The presence of antibodies against OPXV in persons born after cessation of vaccination is of somewhat greater interest. In this study, 36% of young non-vaccinated persons had IgG against OPXV. Within this group, mean values for absolute IgG (0.17, 95% CI = 0.12–0.22; derived from a single-dilution ELISA OD values) decreased between those observed for OPXV-negative persons (–0.05, 95% CI = –0.04 to –0.06) and those anticipated for persons one year after having clinically defined MPXV infection (0.80, 95% CI = 0.93–1.25) ().
These comparison groups (OPXV-negative persons and persons with clinically defined MPXV infection) are derived from investigations performed during the U.S. MPX outbreak.14,20
The constituent population for this study is different in many ways from that that found in the United States, but in theory members of both groups, those exposed to MPXV-infected animals in the United States and Ghanaians living near the animal trapping sites, would have had opportunities for exposure to the same strain of MPXV. The question of whether the intermediate levels of seropositivity observed in many young or otherwise unvaccinated persons in this study might be a consequence of asymptomatic or sub-clinical infections with this strain of MPXV is worthy of consideration. Asymptomatic MPXV infections in members of young age groups have been previously hypothesized,21
and three persons affected during the U.S. outbreak are presumed to have had sub-clinical MPXV infections.22
However, all three of these persons had had prior smallpox vaccination and each had a robust IgG titer against OPXV.
Another possibility to account for intermediate levels of seropositivity is that perhaps beginning at a young age, persons living in the areas surveyed may be exposed to other, as yet uncharacterized, OPXVs that are non-pathogenic in humans. Demonstration of this phenomenon emerged in the 1970s when Baxby and others23
demonstrated that young camels, which were fully susceptible to infection with camelpox virus, could not be productively infected with variola virus. However, exposure to variola virus by scarification resulted in production of intermediate-level titers of antibodies against OPXV in these animals,23
which were sufficient to provide protection against subsequent challenge with camelpox virus. Taterapox virus is one example of an OPXV that has been identified in west Africa that has an unknown pathogenic capacity in humans.24
In general, a broad comparison between the age-specific seroprevalences seen in this study from Ghana versus one performed in an MPXV-endemic setting in the Democratic Republic of the Congo in 198121
showed similar progressive increases in age-specific seroprevalence through successive older age classes, but the overall proportion of seropositive persons at each age class was considerably elevated in this study as opposed to the former study. Similar to the study in the Democratic Republic of the Congo, children in this study from communities surrounded by dense forest (Eastern region) had a significantly higher probability of being seropositive than did children from villages surrounded by mosaic forest-savannah habitat. The habitats in these different areas tend to support varied abundances of several common animal species, including those species of rodent implicated in the MPXV importation event to the United States; Funisciurus
spp. and Cricetomys
spp. were more commonly trapped in the Eastern locations and Graphiurus
sp. was more commonly trapped in the Volta Region. The potential for differing intensities of human exposure to rodent species harboring MPXV exists between the two areas, but has yet to be demonstrated. Cultivating crops in forested areas was also significantly associated with the presence of antibodies against OPXV (in non-vaccinated persons greater than nine years of age). This finding could also be associated with enhanced opportunities for contact with sylvan rodents (crop pests). Alternatively, cultivation activities could merely signal more frequent and longer durations of time spent by persons in a forested habitat.
The lack of an appropriate urban Ghanaian population sample against which to compare human serologic findings and risk factor data limits our ability to investigate absolute risks for OPXV exposure attributable to living in rural communities. Also, the cross-sectional nature of our study precluded our ability to fully explore the possibility that some OPXV-seropositive persons may have had sub-clinical OPXV infections, a question better addressed in a prospective investigation. Other limitations preventing broader interpretation of our study findings include the fact that not all questions were answered by all participants, and that our survey investigation tool was insufficient to capture reliable (non-biased) individual responses to questions about animal exposure.
In summary, the accumulated findings from this study lead us to conclude that no one species of animal and neither collection site could be strongly implicated, nor dismissed, as being the source for the MPXV imported into the United States in 2003. Rather, our results are suggestive of geographically widespread exposure to OPX among rodents found at both sites, near both communities surveyed, leading us to postulate that persons living in these communities may have enduring, but potentially low-intensity or indirect, exposure to OPXV- infected animals resulting in sub-clinical (i.e., asymptomatic) infection concomitant with seroconversion. Possibilities for exposure are likely to begin at a young age, with additional risks present for children living in heavily forested areas. The presence of substantial clinical illness associated with MPXV infection among persons in the United States might be explained by the virus having passed through an amplification host, namely prairie dogs. Contact with MPXV-infected prairie dogs was identified in each instance of human infection, whereas no persons coming into contact with infected African rodents became infected.22
We cannot discount the possibility that other uncharacterized OPXVs are also circulating in southern Ghana. This possibility should also be thoroughly explored.