CLINICAL SIGNS
In affected horses, AHS elicits a wide range of clinical presentations which are
generally classified into four clinical forms. The ‘Dunkop’ or ‘Pulmonary’ Form
is the peracute form of the disease from which recovery is exceptional. The
incubation period is short, usually 5 to 6 days, followed by a rapid rise in
temperature, reaching a maximum of 40 - 41°C, however the deterioration of
the horse may be so fast that this fever is not realized. This form is
characterized by very marked and rapidly progressive respiratory failure and
the respiratory rate may exceed 50 breaths per minute. The animal tends to
stand with its forelegs spread apart, its head extended, and the nostrils
dilated. Expiration is frequently forced with the presence of abdominal heave
lines. Profuse sweating is common, and paroxysmal coughing may be
observed terminally often with frothy fluid exuding from the nostrils. The onset
of labored breathing is usually very sudden and death occurs within 30
minutes to a few hours of its appearance.
The ‘Dikkop’ or ‘Cardiac’ Form of AHS has an incubation period of 7 days or
more, followed by a fever of 39-41°C that persists for 3 to 4 days. The more
typical clinical signs often do not appear until the fever has begun to decline.
At first the supraorbital fossae fill as the underlying adipose tissue becomes
edematous and raises the skin well above the level of the zygomatic arch.
This can later extend to the eyelids, lips, cheeks, tongue, intermandibular
space, and laryngeal region. Subcutaneous edema may extend a variable
distance down the neck towards the chest, often obliterating the jugular
groove. However, ventral edema and edema of the lower limbs are not
observed. Terminally, small to large hemorrhages develop on the conjunctivae
and on the under surface of the tongue. The animal becomes very depressed
and may lie down frequently but for very short periods only. Occasionally,
signs of colic may develop, which has been reported to be unresponsive to
pain medication. Finally, the animal remains prostrate and dies from cardiac
failure about 4 to 8 days after the onset of the fever. In cases that recover,
swellings gradually subside over a period of 3 to 8 days, and the swelling may
move ventrally due to gravity.
The ‘Mixed’ Form of AHS is seen at necropsy in the majority of fatal cases of
AHS in horses and mules. Initial pulmonary signs of a mild nature that do not
progress may be followed by edematous swellings and effusions, and death
resulting from cardiac failure. In the majority of these mixed cases, however,
the SOF swelling of the cardiac form is suddenly followed by marked dyspnea
and other signs typical of the pulmonary form.
Horse sickness fever is the mildest form and is frequently overlooked in
natural outbreaks of AHS. There is an incubation period of up to 9 days after
which the temperature gradually rises over a period of 4 to 5 days to 40°C or
less. Apart from the fever, other clinical signs are rare and inconspicuous. The
conjunctivae may be slightly congested and the pulse rate may be increased.
This form of the disease is usually observed in previously immunized horses.
This form may be more prevalent as these mild symptoms are often not
observed. Owners may not report these findings to a veterinarian, as the
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
97