Hives/Protein Lumps/Urticaria...
We have had queries about why these small, round, elevated lumps on the skin appear occasionally on our horses. Referred to as ‘protein lumps’ or ‘hives’.
These raised areas can persist for a few hours, or several days. They might be in one area or scattered over the body.
Sometimes they are itchy but usually not painful. In more severe cases the lumps may ooze a liquid (serum).
Over the years we have observed a strong correlation between the incidence of these lumps and the consumption of short or lush green grass especially when there is a high clover component or when the horse is also fed lucerne/alfalfa.
Such forage inevitably has a high nitrogen content. In horses, ‘hives’ have generally been associated with high-protein diets and are often observed as being ‘seasonal’, more common in spring and autumn where they can develop within 15 minutes to 24 hours of a change in the grass or with the proliferation of clover.
They don’t tend to occur when the grass dries off and conditions don’t favour clover.
When ‘Alto’ in the photos suddenly came out in these lumps, it was actually in June – mid-winter.
At the time he was grazing short, green grass which we immediately analysed. The results showed the Crude Protein was a very high 27% (should be <12%)and Nitrates were 359mg/kg – (too high for horses). Sugars were 5.4% and Dry Matter only 19% (far too low for horses).
The lumps resolved within a few days of removing him from the short green grass and feeding him more of his plain grass hay.
The hay by contrast showed: Crude Protein 4.2%, Nitrates undetectable, sugars 4.2%, and Dry Matter 85%. We had the hay analysed for comparison to identify the differences in the forage.
While similar bumps can be caused by insect bites or even reactions to injections – Alto’s ones were diet-related.*
The formation of the lumps is triggered by the immune system in order to protect the horse’s body from potentially harmful particles. This is achieved by the release of histamine from certain cells.
Histamine opens blood vessels and triggers break down of sub-cutaneous connective tissue, allowing them to fill with fluid and facilitate immune cells moving into the area. The harmful particles can then be moved out and into the Lymphatic system to be destroyed.
Once that unwelcome excess Non-Protein Nitrogen (NPN) is removed from the horse’s diet, the reaction calms down – hence the reasoning for removing the cause (green grass and legumes like clover or lucerne).
Treatments usually include administration of antihistamines or steroids but, rather than the latter, due to the risk of laminitis we would strongly recommend FIRST removing the cause. This is a much safer option.
*This is why ‘seed’ sources of protein are preferable to add to your horse's feed when needed rather than immature green grass or legumes like alfalfa / lucerne
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