Clinical signs of EI are highly variable, relating to infectious dose and immune status, the latter principally governed by vaccination status. In susceptible horses, clinical signs include fever and a harsh dry cough followed by a nasal discharge, in what can become ‘explosive’ outbreaks, rapidly spreading between individuals, with a short incubation period of 1-3 days. Depression, loss of appetite, muscular pain and weakness can occur. Epidemic coughing is always a worrying sign as this is not so commonly seen in association with the other, more common, upper respiratory viral infections of horses.
Clinical signs usually abate within a few days, but complications associated with secondary opportunist bacterial infections (e.g. pharyngitis, sinusitis, pneumonia) sometimes occur. Rarely, some cases progress to pleuropneumonia and may die or require euthanasia on humane grounds. Young foals appear most susceptible to fatal pulmonary complications, showing severe respiratory distress and failure. Foals have died during the 2019 outbreak in UK. Most infected adult horses recover in seven days to two weeks but in others, especially if not rested, recovery may be prolonged to as much as six months, before they regain their normal health and fitness.
Many fully vaccinated horses that are challenged by field infection show mild or no clinical signs at all. Infected vaccinated horses that do show signs are usually less severely affected than unvaccinated horses and recover more quickly and shed less infectious virus into the environment. Secondary pulmonary complications occur less commonly in challenged vaccinated horses. Clinical experience from outbreaks and evidence from scientific research shows that horses, especially younger ones that receive 6-monthly booster vaccinations, are better protected than those being revaccinated at longer intervals.