Emerging arthropod-borne viral diseases: Equine encephalosis

Equine encephalosis (EE) is a viral vector borne disease carried by hematophagous midges of the genus Culicoides sustained by an Orbivirus of the family Reoviridae. The first isolation of the virus (EEV) occurred in South Africa in 1967 from the blood of a mare affected by neurological symptoms. EEV can infect all equines and, rarely, elephants; in any case, horses are the most susceptible species. Several epizootics were subsequently reported in South Africa and Botswana followed by periods of absence of disease or sporadic outbreaks. The highest number of cases was reported in 1983. Seven non-cross reactive serotypes have been identified so far. EEV infection is frequently sub-clinical, but has been also associated with acute or hyperacute illness. The most common observed clinical signs are fever, anorexia accompanied by increased respiratory distress and congestion of mucosal membranes. Some pregnant mares may abort during the first period of gestation. Less frequently, affected horses can show facial swelling, epistaxis, congiuntival petechiae, and signs of chronic heart failure. Neurological signs, including ataxia, convulsions, hyperexcitability have been described in severe cases. At post-mortem examination it can be observed pulmonary edema, hydropericardium, hepatomegaly, splenomegaly, petechial haemorrhages on serous membranes, hyperemia of glandular stomach, brain edema and congestion. Microscopic examination of liver specimens shows hydropic and fatty degeneration of hepatocytes and in CNS perivascular infiltration of lymphocytes in the periventricular areas of the midbrain and thalamus can be observed. Clinical diagnosis is not easy, due to frequent subclinical course. For this reason, the disease is probably underdiagnosed. Between 2008 and 2009 EE was reported for the first time in Israel. The question of how and when the virus arrived in Israel still remains unanswered. This was the first time in which the virus had been reported outside Africa and this finding is a warning bell for other areas of the Mediterranean basin.