Characterization of Lethal Zika Virus Infection in AG129 Mice

Abstract

Background: Mosquito-borne Zika virus (ZIKV) typically causes a mild and self-limiting illness known as Zika fever, which often is accompanied by maculopapular rash, headache, and myalgia. During the current outbreak in South America, ZIKV infection during pregnancy has been hypothesized to cause microcephaly and other diseases. The detection of ZIKV in fetal brain tissue supports this hypothesis. Because human infections with ZIKV historically have remained sporadic and, until recently, have been limited to small-scale epidemics, neither the disease caused by ZIKV nor the molecular determinants of virulence and/or pathogenicity have been well characterized. Here, we describe a small animal model for wild-type ZIKV of the Asian lineage.

Methodology/principal findings: Using mice deficient in interferon α/β and Ɣ receptors (AG129 mice), we report that these animals were highly susceptible to ZIKV infection and disease, succumbing within seven to eight days. Rapid viremic dissemination was observed in visceral organs and brain; but only was associated with severe pathologies in the brain and muscle. Finally, these results were consistent across challenge routes, age of mice, and inoculum doses. These data represent a mouse model for ZIKV that is not dependent on adapting ZIKV to intracerebral passage in mice.

Conclusions/significance: Foot pad injection of AG129 mice with ZIKV represents a biologically relevant model for studying ZIKV infection and disease development following wild-type virus inoculation without the requirement for adaptation of the virus or intracerebral delivery of the virus. This newly developed Zika disease model can be exploited to identify determinants of ZIKV virulence and reveal molecular mechanisms that control the virus-host interaction, providing a framework for rational design of acute phase therapeutics and for vaccine efficacy testing.

Fig 1. ZIKV causes mortality and morbidity in AG129 mice.

Kaplan-Meier curves illustrate the susceptibility of AG129 mice to ZIKV. Young mice were inoculated f.p. with several doses (n = 3 for doses 10⁵−10³ PFU and n = 6 for doses 10 and 1 PFU; 10² PFU was done as two separate independent replicates with n = 3 for each) of ZIKV (A). All mice challenged with 10⁴−1 PFU succumbed 8 d PI. Young (n = 4) and adult mice (n = 6) were inoculated f.p. with 10⁵ PFU of ZIKV (B). Mice were monitored until day 10 PI. Changes in weight were calculated daily for ZIKV- and mock-infected mice (C). Error bars represent standard error of the mean.

Fig 2. Mice have high serum viremia early during infection.

(A) Viral titers in the blood of young vs. adult mice receiving 10⁵ PFU of ZIKV at days two, four, and six PI. (B) Viral titers in the blood of mice receiving different doses of ZIKV at days two, four, and six PI. Symbols indicate the mean value between individual mice (n = 3 for young mice and n = 6 for adult mice) ± standard deviation.

Fig 3. Mice have high tissue viral loads 7 days post infection.

Mice were infected with 10⁵ PFU of ZIKV, euthanized at day seven (young) or day eight (adult), and tissue viral loads were determined by qRT-PCR as described in the text. Closed symbols indicate young mice (n = 3), open symbols indicate adult mice (n = 4), and the dotted line represents the limit of detection for the assay.

Fig 4. Comparative histological imaging of skeletal muscle and brain after mock infection and infection with ZIKV.

Musculature from the posterior rear limb of a ZIKV-infected mouse revealing nuclear rowing as well as degenerate muscle fibers and infiltrating inflammatory cells (A). Hippocampal section from a ZIKV-infected mouse revealing neutrophilic infiltration (B). Musculature from the same site in a mock-infected mouse (C) A section of hippocampus in the brain of mock-infected mouse (D). Scale bar, 20 μm. Data are representative of two independent experiments (n = 4 and 5).

Fig 5. Brain histopathology after ZIKV infection.

Cortical tissue revealing meningeal infiltration by a mixture of neutrophils and mononuclear cells. A small amount of a similar infiltrate is seen surrounding an adjacent vessel (A). Cerebral neuropil section will cellular pyknosis, scattered neutrophils, and perivascular neutrophilic infiltration (B). Apparent necrosis and neutrophil invasion of primordial germ cell region (C). Scale bar, 20 μm. Data are representative of two independent experiments (n = 4 and 5).