Temporal analysis of Lassa virus infection and transmission in experimentally infected Mastomys natalensis

Abstract

Little is known about the temporal patterns of infection and transmission of Lassa virus (LASV) within its natural reservoir (Mastomys natalensis). Here, we characterize infection dynamics and transmissibility of a LASV isolate (Soromba-R) in adult lab-reared M. natalensis originating from Mali. The lab-reared M. natalenesis proved to be highly susceptible to LASV isolates from geographically distinct regions of West Africa via multiple routes of exposure, with 50% infectious doses of < 1 TCID₅₀. Postinoculation, LASV Soromba-R established a systemic infection with no signs of clinical disease. Viral RNA was detected in all nine tissues examined with peak concentrations detected between days 7 and 14 postinfection within most organs. There was an overall trend toward clearance of virus within 40 days of infection in most organs. The exception is lung specimens, which retained positivity throughout the course of the 85-day study. Direct (contact) and indirect (fomite) transmission experiments demonstrated 40% of experimentally infected M. natalensis were capable of transmitting LASV to naïve animals, with peak transmissibility occurring between 28 and 42 days post-inoculation. No differences in patterns of infection or transmission were noted between male and female experimentally infected rodents. Adult lab-reared M. natalensis are highly susceptible to genetically distinct LASV strains developing a temporary asymptomatic infection associated with virus shedding resulting in contact and fomite transmission within a cohort.

Fig. 1.

Temporal analysis of tissue specimens from LASV-infected M. natalensis. Groups of eight (four males and four females) M. natalensis was experimentally infected with 1 × 10³ TCID₅₀’s LASV strain Soromba-R via i.m. injection and euthanized from specimen collection at indicated time points. Total RNA was extracted, standardized, and analyzed in duplicate by real-time RT-qPCR methodologies. Data shown represent the average calculated infectious units/µg of tissue from four animals. Error bars represent the standard error of the mean. * show statistical significance between samples from male and female heart samples at days 3 (P = 0.0023) and 7 (P = 0.0168). Reproductive organs and GI tract specimens from female rodents at the 3 dpi time point were inadvertently not collected.

Fig. 2.

Direct tranmission of LASV. A single lab-reared M. natalensis was experimentally infected with 1 × 10³ TCID₅₀’s LASV strain Soromba-R via i.m. injection and immediately housed with three naïve cage mates. The rodents were cohoused for 45 days postinfection after which terminal serum samples were collected to assess seroconversion to the LASV nucleocapsid antigen. A total of 10 iterations of the experiment were conducted. Arrows represent the potential for direct contact between the infected rodent and its naïve cage mates, while waves represent hypothetical transmission of virus. * Original animal experimentally infected with LASV Soromba-R. Data shown are reciprocal titers based on ELISA serological analysis using a recombinant LASV nucleocapsid antigen.

Fig. 3.

Indirect transmission of LASV. (1) Male (M) and female (F) lab-reared M. natalensis were experimentally infected with 1 × 10³ TCID₅₀’s LASV strain Soromba-R via i.m. injection. A total of 7 days prior to schedule necropsy, infected rodents were individually housed. (2) After 7 days in clean cages, infected animals were removed for necropsy. Immediately following, a naive sentinel M. natalensis was placed in the potentially contaminated cage and maintained for another 7 days. (3) A total of 45 days later, the sentinel rodent was euthanized and assessed for exposure to LASV via seroconversion. Necropsy time point refers to the day postinfection for the experimentally infected rodent from the temporal kinetics study. Gender refers to the sex of the experimentally infected rodent. n.a. = not assessed.