Nerve cells in the firing line

Just like the virus responsible for bluetongue disease (BTV) (see article The fight against bluetongue disease), the Schmallenberg virus is transmitted from one animal to another through a vector: culicoides. In fact, researchers at University College London have shown that some of these midges are carriers of the new virus, especially in the samples of culicoides collected in autumn 2011... “What’s extraordinary”, Mutien-Marie Garigliany continues, “is that while having a very short viremia this virus has managed to infect a large majority of ruminants in Europe. This raises questions”, he stresses. “Are there perhaps other means of transmitting this virus? Several teams are currently studying this hypothesis”, the researcher continues. 

The vector isn’t the only point in common between the bluetongue virus and the Schmallenberg virus. Indeed, like its predecessor, the latter can cause nerve lesions in infected foetuses. “If the infection occurs during a certain window of the gestation period, sufficiently late so that the placenta is in place and sufficiently early in the development of the foetus, the virus can affect certain tissues and cause malformations", explains Mutien-Marie Garigliany.

Among other things, the Schmallenberg virus attacks nerve cells leading to their gradual destruction. The symptoms vary depending on the stage of development during which the foetus was infected. The earlier the foetus comes into contact with the virus during its development, the greater the risk of abortion because the malformations caused aren't compatible with the foetus’ survival. However, in the majority of cases, the young are born at term or slightly early and mainly have abnormalities in the encephalon, especially hydrocephalus. “Basically, the encephalon is comprised of ventricles, which are cavities containing cerebrospinal fluid, and a cortex of nerve tissue”, the scientist tells us. “Normally, the cortex is thick and the ventricles are small, but in the case of hydrocephalus, these proportions are reversed. The ventricles take up a lot of space and, in extreme cases, the cortex becomes nothing more than a fine membrane of nerve tissue. This is referred to as hydranencephaly”.    

Study of an atypical case

Mid-January 2012, Professor Daniel Desmecht, Mutien-Marie Garigliany and Calixte Bayrou, assistant in the Pathology Laboratory, were entrusted with a somewhat atypical case. “We had the chance to be able to study a living and viable calf despite the almost total absence of an encephalon”, reveals Mutien-Marie Garigliany. “This allowed us to observe the nerve signals associated with brain lesions in a living animal”.  The animal was then euthanized for ethical reasons. “We carried out tests to ensure the presence of the Schmallenberg virus in the lesions as well as tests allowing us to exclude the involvement of other viruses”, the researcher specifies. This work was published in the journal Emerging Infectious Diseases in June(1).