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

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.

Fig 1. Dorsal and ventral views of Gambian pouched rats (Cricetomys gambianus) infected intradermally with MPXV/Luc+.

Gambian pouched rats that were infected intradermally with Monkeypox virus that expresses firefly luciferase (MPXV/Luc+) show luminescence, indicative of virus replication, at the initial site of inoculation, over the dorsal scapulae, and then at distant sites. The intensity of light produced is used to estimate the quantity of replicating MPXV/Luc+. By day 7, luminescence is visible in the oronasal area of all three infected rats. This is corroborated by tests for viral shedding; all three animals had detectable virus in oral and nasal swabs. Viral replication continues in GPR1, especially in the location of numerous abdominal skin lesions. GPR 5 developed systemic infection, with a large amount of luminescence detected in the abdominal and thoracic regions by day 14.

Fig 2. Dorsal and ventral views of Gambian pouched rats (Cricetomys gambianus) infected intranasally with MPXV/Luc+.

Gambian pouched rats infected intranasally with Monkeypox virus that expresses firefly luciferase (MPXV/Luc+) show luminescence (viral replication) at the initial site of inoculation, in the nasal area. In two animals (GPR 6 and GPR 14), there is luminescence in the region of the submandibular lymph nodes, a common site of secondary replication for Orthopoxviruses. In GPR 14, luminescence is evident in distant sites from day 8 to day 22 post-infection. Despite evidence of viral replication, none of these animals showed any grossly visible skin lesions.

Fig 3. Luminescence of Gambian Pouched Rats (Cricetomys gambianus) infected with Monkeypox/Luc+ by intradermal (A) and intranasal (B) routes.

Total luminescence (p/s/cm³/str), an estimate of viral load, peaks in all animals between days 8 and 14 and returned to background levels by day 21 presumably in relation to viral load. The total luminescence of the negative control (GPR 51), taken at day 0, is shown as a dashed line (A), to estimate background luminescence of Gambian pouched rats. The total luminescence of the sentinel animal (GPR 7) is shown in B. It remained similar to that of the negative control.

Fig 4. Gross lesions of Gambian pouched rats (Cricetomys gambianus) infected with MPXV/Luc+.

A. GPR 5, 14 days after infection with Monkeypox Virus expressing luciferase (MPXV/Luc+) with a vesicle on the tongue (red circle), necrotic gingivitis (white arrow) and a vesicle that has opened and crusted (black arrow). B. The left eye of GPR 1 with white-yellow corneal opacities (black arrow) and ocular discharge (white arrow). C The abdomen of GPR 5, 14 days after infection, with prominent ribs and folds of skin which show severe weight loss, as well as many skin vesicles on the abdomen. Luminescence (fig 3) is visible in all of these areas and there were high levels of viral shedding in the conjunctival/corneal swabs taken from GPR 1, as well as in oral swabs taken from GPR 5.

Fig 5. Luminescence is present in the absence of gross lesions of monkeypox disease.

In this photograph (left) of the right lateral leg of GPR1, 14 days after it was infected with Monkeypox virus that expresses luciferase, no skin lesions are apparent. However, on the right, the luminescent image of the same animal, on the same day, shows abundant viral replication in the leg and foot of this animal. This is clear evidence that viral replication can occur in the absence of visible skin lesions.

Fig 6. Shedding of Monkeypox virus in oral, nasal, rectal, and ocular swabs from intradermally infected Gambian pouched rats (Cricetomys gambianus).

Viral shedding was detected by TCID50 assay from swabs of oral, nasal, rectal, and ocular mucosal surfaces. Oral shedding was as high as 10⁶ pfu, which was the dose used to infect the animals. All three infected animals shed in oral, nasal, rectal, and ocular swabs.

Fig 7. Shedding of Monkeypox virus in oral, nasal, rectal and ocular swabs from intranasally infected Gambian pouched rats (Cricetomys gambianus).

Viral shedding was detected by TCID50 assay from swabs of oral, nasal, rectal, and ocular mucosal surfaces. Shedding began slightly earlier than in intradermally infected rats (fig 8), but was lower in titer.

Fig 8. Anti-Orthopox virus antibody titers of animals infected intradermally (A) and intranasally (B) with Monkeypox virus expressing luciferase (MPXV/Luc+).

Antibody titers indicate that animals develop a strong humoral immune response around 3 weeks, which coincides with the time at which luminescence and viral shedding begins to cease. Orthopox antibodies are strongly cross-reactive, so antibody responses to Vaccinia virus, as in this method, very closely approximate titers to Monkeypox virus.