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

Abstract

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.

Fig 1. Representative images of cutaneous lesions after experimental challenge with Congo Basin (A, C, E and G) or West African (B, D, F and H) clades of MPXV.

(A and B) Pictures of scarification site lesions, (C and D) secondary lesions C and D, (E and F) tongue lesions; and (G and H) eyelid lesions for animals in each experimental group.

Fig 2. Biotelemetry measurements of temperature (A and B) and activity (C and D); and weight (E and F) of Cricetomys challenged with West African or Congo Basin MPXV.

(A and B) Recorded core body temperature for experimental and control groups with higher temperatures for animals in C-MPX compared to controls. (C and D) Activity levels showing reduced activity for most experimental animals until day 9 p.i. (C-MPX) and day 12 p.i. (W-MPX), and becoming more active afterwards. (E and F) Weight loss was observed in experimental animals but not in the control group. W-MPX 4 died on day 13 p.i., thus, no information is available for this animal after this day.

Fig 3. Viable viral load (pfu/ml) for each experimental animal found in scarification site (A) and secondary lesion swabs (B) throughout the study.

(A) The maximum viral load from the scarification site was 1.05X10⁸ in an animal challenged with West African MPXV and 1.64X10⁷ for an animal challenged with Congo Basin MPXV. No viable virus was found in sample days after day 27 post infection (p.i.) (B) Lower loads of viable virus were recovered from secondary lesion swabs during a shorter period of time (day 3 p.i. to 21 p.i.); the maximum viral load was 3.88X10⁴ for an animal infected with Congo Basin MPXV and 6.27X10³ for an animal infected with West Africa MPXV.

Fig 4. Viable viral load (p.f.u./ml) for each experimental animal found in oral (A), nasal (B) and rectal swabs (C) throughout the study.

Higher viral loads were obtained from oral swabs than from rectal or nasal swabs. No viable virus was found in sample days after Day 27 post infection (p.i.).

Fig 5. Average and maximum viral loads from MPXV challenged Cricetomys.

(A) Average viral load per individual sample type and experimental group at each sampling day; no viable virus was found in sample days after Day 27 post infection (p.i.). (B) Maximum and average viral load for each sample type and experimental group throughout the entire study. Error bars represent one standard deviation.

Fig 6. Immune response in Cricetomys after MPXV challenge.

Absorbance values measured at 450nm from ELISA assays for both experimental groups: C-MPX (A) and W-MPX (B). No statistically significant difference was found between experimental groups (Wilcoxon signed test p-value = 0.3054). W-MPX 4 died on day 13 p.i., thus, no information is available for this animal after this day.