Sunday, November 8, 2009

African Horse Sickness

African Horse Sickness (AHS) is an infectious non-contagious disease. This means that the virus can't be passed on from one horse to the other via contact (non-contagious). It is spread via midges, more specifically, Culicoides midge. If a horse is bitten by midge carrying the virus, it will get AHS. There are 9 different strains of the AHS virus. AHS will affect all breeds of horses, mules and donkeys, but the mortality rate is highest in horses (as high as (95%). Zebras appear to be resistant.

When are horses at the greatest risk of AHS?

As mentioned AHS is caused by Culicoides midge. Horses are at the greatest risk in warm, rainy seasons when midges are plentiful . Midges are also known to be most active at sunset and sunrise.

Different forms of AHS

There are 4 different forms of AHS: Pulmonary/ Lung ('dunkop') form, Cardiac ('dikkop') form, Mixed form and Fever form.

Pulmonary/ Lung form

This results in the lungs drowning with fluids. It has the highest death rate of 95%. It a preacute form of AHS, meaning it is the worst and quickest form of AHS. It has an incubation period of 3 to 4 days. There are no swelling of the head hence the name 'dunkop' (it means thin head). Symptomps are:

1. A very high temperature (T) up to 41 degrees
2. Laboured breathing (onset can be sudden)
3. Mouth open and head hanging down
4. Frothy discharge pouring from the nostrils.
5. Excessive sweating
6. Sudden onset of death

Cardiac form

This form affects the heart and is characterized by swelling of the head and neck hence the name 'dukkop' (it means thick head). It is a subacute form of AHS ie not as severe or sudden as the Lung form. It has a mortality rate of 50 % If horse survives it will often be left with complications such as Billiary fever and paralysis of oesophagus. Symptoms are:

1. Fever followed by swelling of head and eyes
2. Entire head swells in sever cases.
3. Loss of ability to swallow
4. Colic
5. Bleeding in the membranes of mouth and eyes.

Mixed form

It is he most common form of AHS. Often confused with Pulmonary and Cardiac form of AHS, because horse may show symptoms from both forms.. It can usually only be recognized after an autopsy.

Fever form

This is the mildest form of AHS. This usually occurs when the horse is immune to one or more strains of AHS, but comes infected by another strain. The horse's T will rise to 40 degrees for 8 to 12 days, but then returns to normal. The fever may be accompanied by loss of appetite and labored breathing.. All you need to do is feed the horse well, allow rest and provided good nursing. The horse will usually make a full recovery. Make sure to rest horse for 4 weeks before returning the horse to work.


Vaccination has helped to bring AHS under control. There are 2 vaccines that has to be given to the horse once a year. The vaccines should be given 3 weeks apart (3 week is the minimum). The first injection (AHS I) contains the strains 1,3,4, and 5 and the second injection (AHS II) contains strains 2,6,7 and 8. The horse must not be exerted for 3 weeks after each vaccination. Too much work will lead to the body T rising so much that it will cause the horse to catch AHS, because the current vaccine is a live vaccine. This means that if conditions are ideal the virus will start infecting the body. Unfortunately the current AHS vaccines aren't fool proof. Horses have been no to get AHS, despite regular vaccination. Thus, other precautions must be taken.

Additional Protection

1. Stable horses a few hours before sunset and sunrise.
2. Mesh screens on stable doors and windows are helpful in keeping midges out.
3. Turn of lights at night.
4. Fans in the stables (midges don't have wings strong enough to fly in a breaze/wind).

Competition horses

Competition horses must be vaccinated by a certified vet and each vaccination must be recorded in the horse's passport by the same vet. No horses may enter the AHS Control Area (in the Western Cape) less than 60 days after the second vaccination.

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