Haematological measurements as an aid to early diagnosis and prognosis of respiratory viral infections in thoroughbred horses

Royal Hong Kong Jockey Club, Equine Hospital, Shatin.

In late November 1988 large numbers of thoroughbred horses in training in Hong Kong developed a transient pyrexia with, in some cases, the clinical signs of viral respiratory disease. Serial blood samples for haematological examination were taken from 10 of the horses which were stabled in six different blocks. They had developed a high temperature within three days of each other and subsequently seroconverted to equine herpes virus 1 (EHV1). The absolute monocyte count was more than 0.5 x 10(9)/litre in all 10 within the first five days, and nine of them had a high neutrophil/lymphocyte ratio on the first day, which then decreased and reversed within four or five days. Five of the horses had a high plasma viscosity, and a large difference between the viscosity of plasma and serum which in three of them returned to normal within 10 days. In the two and a half months after the initial infection six of nine of the horses, including the five which had a large difference between the viscosity of plasma and serum, developed visible mucopus by endoscopic examination. These haematological and endoscopic changes can be used to detect horses in the acute stages of EHV1 infection and monitor the progress of the disease, before it can be confirmed by isolation of the virus and, or, serology.