Date Published: October 30, 2015
Publisher: Public Library of Science
Author(s): Elizabeth A. Falendysz, Juan G. Lopera, Faye Lorenzsonn, Johanna S. Salzer, Christina L. Hutson, Jeffrey Doty, Nadia Gallardo-Romero, Darin S. Carroll, Jorge E. Osorio, Tonie E. Rocke, A. Desiree LaBeaud. http://doi.org/10.1371/journal.pntd.0004130
Abstract: 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.
Partial Text: Monkeypox (MPX) is an emerging disease caused by an Orthopoxvirus, Monkeypox virus (MPXV). It is closely related to smallpox, although it has a much lower mortality rate in humans, around 10% for the Congo Basin clade of MPX, compared to up to 40% mortality for smallpox [1,2]. Unlike smallpox, MPX is a zoonosis. Historically, MPXV has circulated in central and west Africa but was first discovered in 1958 in imported captive monkeys in Copenhagen . The prevalence and distribution of this disease first became of interest in the 1970s during the smallpox eradication campaign, as public health agencies needed a way to differentiate between cases of MPX and smallpox. Analysis of human clinical samples revealed that there are two geographically distinct clades of the virus, west African and central African . In west Africa, no human cases of MPX have been reported since the 1980s, although serological studies have demonstrated that anti-orthopox antibodies are still present in humans from this region [5,6]. In contrast, the number of cases of MPX continues to increase in central Africa . Historically the majority of MPX cases have been reported in Democratic Republic of Congo (DRC), but newer reports of MPX have emerged from the neighboring countries of Republic of Congo (ROC) and Sudan [1,8]. The reasons for the change in incidence and geographic distribution of MPX are unclear. Identification of the reservoir host(s) is necessary to build a more thorough understanding of the epidemiology of Monkeypox in Africa and to develop prevention strategies to reduce new human cases.
Most notably, this study demonstrated that Gambian pouched rats can be infected with and shed MPXV in the absence of clinical signs of disease. Infected Gambian pouched rats shed up to 106 pfu of virus, which is an infectious dose for many species, including non-human primates and other rodents [36,37,38]. The ID route of infection was more pathogenic than the IN route, although both routes resulted in shedding of live virus. Clinical characteristics of MPXV infection and shedding provide valuable information as to the potential role of this rodent species as a reservoir for MPXV. The ability to survive infection for a reasonable time period is an important trait for a reservoir host, as it must be able to transmit the virus to other individuals in order to maintain viral circulation. This study did not find evidence of aerosol transmission between Gambian pouched rats when animals were housed in individual cages within the same room. The sentinel animal was seronegative at the end of the study and no virus was detected in tissues, blood, or swabs from this animal. Direct contact or fomite transmission may be required. Pouched rats are solitary animals that live in burrows [39,40]. To easily transmit the virus they must be healthy enough to leave the burrow and come into contact with other animals or shed the virus into the environment. Our data indicate that an infected Gambian pouched rat is able to shed virus in oral, nasal, ocular, and rectal secretions, and most infected animals remain healthy enough to maintain normal foraging behavior. Viral titers in oral secretions were greater than 106 pfu, which has now been demonstrated to be an infectious dose for Gambian pouched rats. From other studies, we know this is also a lethal dose for cynomologous macaques, prairie dogs, and ground squirrels [36,37,38]. Thus, an infected pouched rat could transmit to other con-specifics via fomites, such as a shared food source, or via direct contact during mating or fighting.