Experimental Rickettsia typhi Infection in Monodelphis domestica: Implications for Opossums as an Amplifying Host in the Suburban Cycle of Murine Typhus

Abstract

Murine typhus is an acute undifferentiated febrile illness caused by Rickettsia typhi. In the United States, its reemergence appears to be driven by a shift from the classic rat-rat flea cycle of transmission to one involving opossums (Didelphis virginiana) and cat fleas. Little is known of the ability of opossums to act as a reservoir and amplifying host for R. typhi. Here, we use Monodelphis domestica (the laboratory opossum) as a surrogate for D. virginiana. Opossums were inoculated via the intraperitoneal (IP) or intradermal (ID) route with 1 × 106 viable R. typhi. Blood and tissues were collected on days 6, 13, 20, and 27 or if moribund. Although one ID-infected opossum died, the remainder did not appear ill, whereas half of the IP-inoculated animals succumbed to infection. Rickettsemia was demonstrated in all animals through week 2 of infection and sporadically in weeks 3 and 4. Rickettsia typhi DNA was detected in all tissues, with most animals demonstrating the presence of bacteria into weeks 3 and 4. Histopathology and immunohistochemistry demonstrated typical findings of rickettsial infection. Akin to infection in rats, the demonstration of disseminated infection, typical inflammation, and prolonged rickettsemia with relatively few clinical effects (especially in the more natural route of ID inoculation) supports the potential of opossums to act as a competent mammalian reservoir and component of the zoonotic maintenance cycle of R. typhi. Understanding the dynamics of infection within opossums may have implications for the prevention and control of murine typhus.

Figure 1.

Temperature and weight change curves of opossums during the first 2 weeks after infection with Rickettsia typhi.

Figure 2.

Survival curve of opossums after infection with Rickettsia typhi. Plot shows animals euthanized because of moribund status. Animals killed at planned scientific endpoints are censored from the plot at the time of euthanasia.

Figure 3.

Mean bacterial loads in the blood of Rickettsia typhi–infected opossums, inoculated via different routes, collected through time.

Figure 4.

Mean bacterial loads in various tissues of Rickettsia typhi–infected opossums, inoculated via different routes, collected through time. Placement of an asterisk (*) indicates the two groups at a particular timepoint are statistically different (P < 0.05) as determined by Mann-Whitney U test.

Figure 5.

Geometric mean of reciprocal immunofluorescence assay titers for opossums infected intraperitoneally and intradermally by week the sera were collected (at scientific endpoints or when moribund).

Figure 6.

Photomicrographs of Monodelphis domestica tissues collected at necropsy. (A) The liver showed randomly distributed lobular and periportal foci (arrows) of lymphohistiocytic infiltrates (hematoxylin and eosin stain; ×100). (B) The kidney demonstrated multifocal intertubular pericapillary nephritis (arrow) (hematoxylin and eosin stain; ×200), and (C) immunohistochemical detection of Rickettsia typhi demonstrated the organism (red) within renal intertubular capillaries (×400). (D) A characteristic typhus nodule in the brain (arrow) demonstrating a mononuclear cellular infiltrate around a small blood vessel (×200). (E) Immunohistochemistry showing R. typhi (red) within a longitudinal section of endothelial cells in the brain (arrows) (×400). (F) Immunohistochemical staining of skin adjacent to the intradermal inoculation site of an opossum, 10 days after infection, showing abundant diffuse intracellular infection (red) with scant inflammatory response (hematoxylin and eosin stain; ×200). This figure appears in color at www.ajtmh.org.