Why Mast Cells matter for the itchy horse
A change of grazing several weeks back meant we waved goodbye to our wonderful summer track and the horses returned to rested fields.
Starved of water over the summer these were looking pretty poor (not ‘too’ risky I thought), but with a sudden dump of rain, warm weather and intermittent sunshine - we’ve hit a new danger point. The grass has been shocked into responding - it’s growing rapidly and has higher levels of potassium, nitrogen and protein - all of which put strain on a horses metabolism.
Aerial view of paddocks - 7 September 2025 - Rain since then has made everything VERY green!
The return of the ‘itch’
This week we’ve had a LOT of rain - and as my horses are all out in their winter field from 5pm - 8.30am, they’re eating a LOT. They do come into a sparse day paddock for some time out from the green stuff and have a modest amount of dust extracted hay and low sugar (tested) haylage to munch on, on top of their bucket feed (chaff, malle mash and supplements).
Due to other allergies (C is allergic to dust biting insects and develops respiratory issues when stabled) - we really need to live out full time, so it’s important to come up with a management plan that works for our needs.
Four days into the new routine, C is on the verge of breaking fencing - rubbing his chest against the rails and really putting his body weight behind it. It’s hard to pull him away. You can also see his body twitching when he’s unrugged for work and he’s definitely a little more spooky and reactive.
Once the rug is removed he immediately wants to scratch - either his chest, flank or back. It’s not delicate, and it’s stressful - both for him and for me!
So what’s going on?
Mast cells are go!
The sudden increase in potassium, nitrogen and protein in the rapidly growing (green) grass has triggered Cs mast cells to activate.
What are mast cells?
Mast cells are highly specialised immune cells that serve as the body's "sentinel guards," and they play a particularly crucial role in histamine production in horses. In fact, they’re complete powerhouses: storage units packed with granules containing preformed histamine, along with other inflammatory mediators like leukotrienes and cytokines.
Helpfully, they're concentrated in tissues that interface with the external environment, such as the skin, respiratory tract, and digestive system. This allows them to rapidly respond to potential threats. Lucky us!
When activated, mast cells undergo ‘degranulation’, explosively releasing their contents within seconds. A single mast cell can contain thousands of histamine-filled granules, making their response both immediate and potent. Unlike other immune cells that need time to produce inflammatory substances, mast cells have their chemical arsenal pre-loaded and ready for instant deployment.
The Itch response mechanism
When histamine is released from mast cells, it binds to histamine receptors (primarily H1 receptors) in the skin and other tissues. This binding initiates a cascade of inflammatory responses that directly stimulate nerve endings, creating the sensation of itching. Histamine also increases blood vessel permeability, causing the characteristic swelling and redness associated with allergic reactions.
The itch sensation serves an evolutionary purpose - it's designed to make the animal remove the perceived irritant through scratching or rubbing. But for us, this scratching often causes additional tissue damage and could potentially trigger more mast cell degranulation, creating a self-perpetuating cycle of inflammation and itching.
Reducing reactivity
In an ideal world, I’d remove access to the green grass: we already know this is causing the reaction so eliminating it from his diet would, in theory, solve the problem. But with our summer fields out of action, stabling causing additional problems (particularly when his body is already reacting), I need to find another way to help him…
Heree’s what I’m going to try…
Supporting gut health through the addition of prebiotics and probiotics: a healthy digestive system is better able to process potential allergens without triggering immune responses. A compromised gut barrier can allow larger protein molecules to enter the bloodstream, increasing the likelihood of allergic reactions.
Using natural anti-inflammatory supplements such as omega-3 fatty acids, which can help stabilize mast cell membranes and reduce their tendency to degranulate. We already use omega-3 but one anti-inflammatory that I haven’t considered before, is Quercetin: an anti-oxident which can help stabilize mast cells when used consistently. This is available in a wide range of food types, including apples (with skin on) and green tea - which I already have in my ‘horse herb emporium! It’s also, to a lesser extent, in bananas, so I’m going to test this out by trialling Equinutrive’s ‘B-Complex’ which consists of dried green bananas - which offers the added benefit of providing lami-friendly gut support (see point 1). My only worry is the added potassium levels so I’m going to tread carefully.
As a temporary measure and to get us over this little ‘blip’, I’m also going to up C’s antihistamines (cetirizine) and chamomile intake just while he’s struggling, and I’ll review the dose of milk thistle I’m giving him to ensure his liver function is supported when it’s particularly under pressure. The liver is crucial to the detoxification and metabolism of carbohydrates, fats, and proteins, breaking down toxins in the body for elimination.
Note: Milk thistle seeds and leaves contain ‘silymarin’ - a flavonolignan with antioxidant, anti-inflammatory and immunomodulating properties. Mad Barn have a really useful article all about this. Their recommended dose is 4g per day for a 500kg horse, but many equine herb/supplement brands recommend between 20 and 40g per day - with doses at the higher range recommended for periods of stress. I’ll be taking my scales to the yard tonight to check!
