Clinical Features of Infection

EHV-1 and EHV-4 cause outbreaks of upper respiratory disease ('common cold') in young horses (EHV-4 also in adult horses) with no previous exposure to the viruses. Infection is characterized by fever (39-41°C) lasting 2—5 days, and the animal suffers from serous nasal discharge and congestion of the nasal mucosa and conjunctiva. Less frequently, one can detect a transitory period of anorexia, enlargement of the submandibular lymph nodes and edematous swelling of the subcutis and tendon sheaths of the legs. An initial leukopenia is followed by leukocytosis before the temperature falls. Recovery is usually uneventful and occurs within one week. Older horses show few or no clinical signs, although increased sensitivity is seen in stressed horses. Death from naturally acquired infection resulting in neurological disease is not uncommon.

Equine abortions induced by EHV-1 usually occur late in gestation (8-11 months). The foals are usually born dead, but if alive, often succumb to pneumonia within the first few days. Clinical signs of neurological disease caused by EHV-1 are highly variable and include paresis, ataxia and paralysis. EHV-1 and EHV-4 have been isolated from the central nervous system of infected horses; however, isolation of the viruses from the CNS is uncommon. The virus strains do not invade the extravascular nervous tissue and do not appear to be neurotropic. The virus spread from the respiratory tract to the CNS probably occurs via infected leukocytes. Some horses have fully recovered from neurological disease with no permanent neurological sequelae.

The role of EHV-2 and EHV-5 in clinical disease is virtually unknown. However, the viruses have been associated occasionally with chronic throat infections and have been isolated from the respiratory tract of horses with respiratory disease.

An incubation period as long as 10 days can be observed after natural infection with EHV-3. The initial lesions are small (1—2 mm), raised and reddened papules. The lesions then progress rapidly to the pustular form, and there is a general reddening of the vaginal mucosa in the mare. The number of lesions increases in the first few days, and by day 6, many of the lesions form ulcers up to 20 mm in diameter and 5 mm deep. Lesions can also be seen on the vulva and perineal skin, as well as the penis and prepuce of the stallion. The disease is usually mild such that the temperature, pulse, appetite and respiration remain close to normal. The severity of the disease can be increased by secondary bacterial infections; however, uneventful cases are usually cleared within 2 weeks. Lastly, EHV-3 is not abortigenic and does not lead to infertility.

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