Ljungan virus : a novel pathogen : effects on the pregnant female and her offspring

Abstract

Ljungan virus (LV) was isolated in the mid 1990s from a native rodent, the bank vole (Myodes glareolus), trapped in the Ljungan valley of central Sweden. It is a member of the parecho virus genus in the picorna virus family. A consistent characteristic feature of the LV is that it is difficult to cultivate in tissue culture and therefore difficult to isolate and diagnose. The virus grows to only low titers in both tissue culture and organs infected under controlled conditions. These features in combination with a poor humeral immune response constitute a diagnostic challenge.

Interest in LV heightened when it was found that the virus is associated with diabetes, neurological diseases and reproductive disorders in its natural reservoirs – small wild rodents.

This research explores the influence of LV infection in laboratory mice through several experimental approaches. Male mice infected during the first postnatal days were persistently infected during the entire five month (174 days) long follow-up period, as determined by real time RT-PCR (Paper IV). When dams were infected early in pregnancy, the litter experienced a high frequency of perinatal death. In addition, malformations were observed in some of the offspring (Paper II). Females infected intraperitoneally, and females infected in utero, were followed over time with the problems remaining through three consecutive pregnancies (Paper V). In addition, it was observed that the offspring of infected mice had delayed reproduction. This phenomenon would, in wild rodents, have major consequences for their population dynamics (Paper II). Another effect produced by LV infection was that surviving offspring of infected females developed diabetes at 10-15 weeks of age (Paper I). These animals had intact β-cells, increased abdominal fat, hyperinsulinemia and impaired glucose tolerance, suggestive of a type-2 diabetes-like state. The earlier in pregnancy the infection took place, the more severe the diabetes in the mature offspring. Mild stress was necessary for LV to induce the diseases. Infection or stress alone did not cause any disease, but animals subjected to the combination of stress and LV infection developed the reproductive disorders and diabetes (Paper I and II).

The association between LV and disease in the wild reservoirs, and the fact that these diseases can be reproduced in a mouse model under controlled conditions, engendered interest in the possibility of whether the virus might be responsible for these same conditions in humans. An earlier epidemiological study had seen associations between human type-1 diabetes, Guillain-Barré syndrome, lethal myocarditis and the abundance of the animal reservoir of LV. Our follow-up study then found a striking association between IUFD in humans and the abundance of wild rodents. This finding led to an investigation of the presence of LV in human IUFD cases. In a small pilot study, LV was found in approximately half of the cases investigated using immunohistochemistry (Paper III).These findings were later confirmed by a recently developed real-time RT-PCR assay (Paper VI).

This work probes entirely new ground on a novel infectious agent. The first animal model of pathogenesis from this agent has been published. While the excitement of the results must be tempered somewhat due to the ongoing development of new diagnostic methods, the potential importance of the pathogen is already evident.