Choice of inbred rat strain impacts lethality and disease course after respiratory infection with Rift Valley Fever Virus

Abstract

Humans infected with Rift Valley Fever Virus (RVFV) generally recover after a febrile illness; however, a proportion of patients progress to a more severe clinical outcome such as hemorrhagic fever or meningoencephalitis. RVFV is naturally transmitted to livestock and humans by mosquito bites, but it is also infectious through inhalational exposure, making it a potential bioterror weapon. To better understand the disease caused by inhalation of RVFV, Wistar-Furth, ACI, or Lewis rats were exposed to experimental aerosols containing virulent RVFV. Wistar-Furth rats developed a rapidly progressing lethal hepatic disease after inhalational exposure; ACI rats were 100-fold less susceptible and developed fatal encephalitis after infection. Lewis rats, which do not succumb to parenteral inoculation with RVFV, developed fatal encephalitis after aerosol infection. RVFV was found in the liver, lung, spleen, heart, kidney and brain of Wistar Furth rats that succumbed after aerosol exposure. In contrast, RVFV was found only in the brains of ACI or Lewis rats that succumbed after aerosol exposure. Lewis rats that survived s.c. infection were not protected against subsequent re-challenge by aerosol exposure to the homologous virus. This is the first side-by-side comparison of the lethality and pathogenesis of RVFV in three rat strains after aerosol exposure and the first step toward developing a rodent model suitable for use under the FDA Animal Rule to test potential vaccines and therapeutics for aerosol exposure to RVFV.

Keywords: LD50; Rift Valley Fever Virus; aerosol exposure; inbred rat strain; respiratory infection.

Figure 1

Survival of three rat strains infected with aerosolized RVFV. Survival at each presented dose is shown. (A) Wistar-Furth, (B) ACI, and (C) Lewis. (D) Shows the probit curves for each rat strain, from which the LD₅₀ was determined.

Figure 2

Body weight of Wistar-Furth rats infected with aerosolized RVFV. Groups of six female Wistar-Furth rats were infected with RVFV by the aerosol route at four doses plus uninfected controls. (A) 0.4 pfu/rat; (B) 5 pfu/rat; (C) 40 pfu/rat; (D) 240 pfu/rat. The y-axis of each graph displays the change in weight of individual animals relative to the average weight of the uninfected control rats (n = 6). Rats that succumbed to infection are shown in red; rats that survived are shown in black.

Figure 3

Body temperature of Wistar-Furth rats infected with aerosolized RVFV. Groups of six female Wistar-Furth rats were infected with RVFV by the aerosol route at four doses plus uninfected controls. (A) 0.4 pfu/rat; (B) 5 pfu/rat; (C) 40 pfu/rat; (D) 240 pfu/rat; (E) uninfected control rats. The y-axis of each graph displays the change in body temperature of individual rats relative to their temperature on the day of infection. Rats that succumbed to infection are shown in red; rats that survived are shown in black.

Figure 4

Tissue viral loads in rats infected with aerosolized RVFV. Viral loads in tissues were measured by plaque assay and are expressed as pfu/g tissue. (A–F) show results from Wistar-Furth rats, with results grouped based on the presented dose. (G,H) show results of brain tissue from ACI and Lewis rats, respectively.

Figure 5

Body weight of ACI rats infected with aerosolized RVFV. Groups of six to eight female ACI rats were infected with RVFV by the aerosol route at five doses plus uninfected controls. (A) 0.4 pfu/rat; (B) 2 pfu/rat; (C) 20 pfu/rat; (D) 250 pfu/rat; (E) 3900 pfu/rat. The y-axis of each graph displays the change in weight of individual animals relative to the average weight of the uninfected control group (n = 4). Rats that succumbed to infection are shown in red; survivors are shown in black.

Figure 6

Body temperature of ACI rats infected with aerosolized RVFV. Groups of six to eight female ACI rats were infected with RVFV by the aerosol route at five doses plus uninfected controls. (A) 0.4 pfu/rat; (B) 2 pfu/rat; (C) 20 pfu/rat; (D) 250 pfu/rat; (E) 3900 pfu/rat; (F) Uninfected control rats. The y-axis of each graph displays the change in body temperature of individual animals relative to their temperature on the day of infection. Rats that succumbed to infection are shown in red; survivors are shown in black.

Figure 7

Example of porphyrin staining in ACI rats. Chromodacryorrhea (dried porphyrin) on the eye and paw of ACI rat H1 immediately prior to euthanasia (14 d.p.i.).

Figure 8

Body weight of Lewis rats infected with aerosolized RVFV. Groups of six to eight female Lewis rats were infected with RVFV by the aerosol route at four doses plus uninfected controls. (A) 1.5 pfu/rat; (B) 30 pfu/rat; (C) 350 pfu/rat; (D) 4400 pfu/rat. The y-axis of each graph displays the change in weight of individual animals relative to the average weight of the uninfected control group (n = 6). Rats that succumbed to infection are shown in red; survivors are shown in black.

Figure 9

Body temperature of Lewis rats infected with aerosolized RVFV. Groups of six to eight female Lewis rats were infected with RVFV by the aerosol route at four doses plus uninfected controls. (A) 1.5 pfu/rat; (B) 30 pfu/rat; (C) 350 pfu/rat; (D) 4400 pfu/rat; (E) Uninfected control rats. The y-axis of each graph displays the change in body temperature of individual animals relative to their temperature prior to infection. Rats that succumbed to infection are shown in red; survivors are shown in black.

Figure 10

Effect of infection route on survival of Lewis rats. (A) Shows survival of Lewis rats after either subcutaneous (blue line; n = 5) or aerosol exposure (red line; n = 6). The rats that survived s.c. exposure were re-exposed to RVFV by aerosol infection 28 days after the original exposure (green line; n = 5). (B,C) Change in body weight and body temperature of Lewis rats after exposure to RVFV as outlined in (A). The purple line shows the single rat that survived aerosol re-challenge.