Horse owners know this disease only too well: the summer seasonal recurrent dermatitis (SSRD, or sweet itch). It is a scourge, and it’s hard to get rid of. The summer is now well installed, and we thought it would be a good idea to give you some little reminders about this disease.
#1 – “Culicoides, the jury finds you guilty!” 👉🏻
The cause of the sweet itch is a small insect (1 to 3 mm) but a nasty one. It’s a diptera of the Culicoide type. To put it simply, it’s a kind of little fly.
Here it is. It doesn’t look like much, yet… 👿
To tell you everything, we know of about 1000 species but only a few of them seem like they are involved with sweet itch . Only the females bite (always the girls fault! 😝🤷♀️). Their bloody meals are spaced by a few days and they live 1 to 2 months. During this time they reproduce by laying eggs in humid areas (ponds, mud, shores, etc). Larvae develop and become adults who usually stray away from the reproduction areas. But not too far away! A hundred meters at most .
Culicoides are active for almost the whole year but they only multiply around March-April and get rarer at the end of the year. This explains why there are cases of summer dermatitis… in winter!
#2 – “Help, it itches!”
Indeed, sweet itch is a highly pruritic disease for horses. It’s the medical term to say it itches!
In France, most of the rashes can be observed at the base of the mane, on the croup and at the base of the tail. Then, progressively, these rashes spread out onto the neck and reach the withers, the back, and the face (sometimes even the ears). Some Culicoides can also induce rashes on the belly and the chest.
It started with an itch…
The disease starts with small papules which irritates the horse a lot. What does it do? Of course, it scratches itself! And this only makes it worse. This way, we can start to see broken hair (which ends up falling out), erythema and even scabs. The hair is tangled and falls out. The tail starts to look like a “rat’s tail”. The horse can seem nervous and be difficult to ride.
Try to imagine how you’d feel if you were covered in mosquito bites for months: you’d go crazy as well! 😨 The repeated friction from the horse desperately trying to scratch itself to get a bit of relief create weeping nodules, sometimes even bloody.
It’s around this period that the opportunistic bacterias can cause infections in the rashes and create ulcers.
… and it gets worse
After a few years, the skin thickens (we call that the lichenification) and the hair stops to grow in these areas. Important folds can form because of the keratinisation (the skin gets full of keratin). We can even observe a weight loss, as the horse is so stressed it spends more time scratching than feeding itself.
You understood it: it’s not a disease to be taken lightly. And unfortunately it can lead to euthanasia in more severe cases.
#3 – “It came back again and it’s even worse!”
Indeed, it is unfortunately a chronic disease. As soon as the horse gets bitten the next spring, the disease comes back. The reason is simple: it’s actually a hypersensitivity, also known as allergy. We don’t know the exact mechanism. Scientists assume that there is an immediate allergic reaction (20 minutes after the bite), the most violent one, and then several delayed allergic reactions (between 6h to 48h after the bite).
Let’s summarize the mechanism
To put it shortly, here’s what happens. The Culicoide bites the horse and releases a little saliva in the blood when it drinks. This saliva is an allergen. This means it can cause an allergic reaction on hypersensitive subjects. It contains what we call antigens. These are molecules capable of triggering an immune reaction in the horse’s organism. The immune system is activated and antibodies will position themselves on saliva antigens.
So far, so good. But for allergic subjects, the immune reaction will be exaggerated and abnormal. Some mechanisms too complex and long to explain here will take place. In the end, chemical transmitters are released and will induce abnormal reactions. These are the symptoms of the allergy. The small pruritic papules in the case of the sweet itch.
It starts as a seasonal illness
At first, the disease is seasonal. It appears at spring, intensifies during summer and gradually regress at fall and the horse seems fine for the winter. Then, as seasons go and if nothing is done, the disease tends to get worse. The crisis last for 6 months. It’s a true vicious circle. Lastly, the horse won’t have any remission phases and the disease will take place for the whole year. Spontaneous regression (which means the disease disappears) is possible but extremely rare and we don’t know how it happens.
#4 – “Help, how do I know if my horse is susceptible to this disease?”
There is no sure way to know if your horse is more susceptible to get this disease than another horse. Hereditary transmission is strongly suspected but unless you know your horse’s family background, you can’t know for sure how affected it’ll be by Culicoides .
As for racial susceptibility, the opinions diverge. Some believe there isn’t one, and others believe the following breeds are more susceptible: Thoroughbred, Arabian, Friesian, Shire, Connemara, Shetland pony, Welsh. Icelandic horses seem particularly affected by the disease. Male and female are equally affected and the disease often starts when the horse is between 1 to 3 year old .
Several factors could also increase the susceptibility, like high protein based food, lack of exercise or a thin skin . But all of these are suppositions.
What’s important to remember is that the sweet itch is a multifactorial disease which regroups inherited and environmental factors. Like a lot of diseases, it’s a case of “a genome and an environment walk into a bar”…
#5 – “Is there a cure for my horse?”
Unfortunately there is no treatment for the sweet itch. By this I means that you can’t completely cure you horse. There is however a preventive treatment which will allow you to act before the first troubles at spring and a symptomatic treatment which will treat the symptoms of the disease (but not its cause!).
Preventive treatment: [1,2,3]
If you know your horse has this disease, it’s important that you take preventive measures at spring, every year. You’ll see, it’s not that easy! The goal is to keep the horse away from what triggers the disease (the Culicoides) as much as possible. By the way, keep in mind that you need 3 weeks without a new bite to see the symptoms disappear  :
- Firstly, the horse needs to stay in its stall as much as possible (it’s not the best, I know). Try to only let it out before 5pm or after 11pm (5pm-11pm corresponds to the peak activity time for the Culicoides) and disinfect regularly the stables.
- In the field, give them an open and shady shelter.
- Cover your horse with a mosquito net rug with a tight mesh.
- Apply an insecticide lotion on the horse’s hair.
It’s important to note that the action of the insecticides is brief because it gets diluted by the horse’s sweat. It’s important to re-apply regularly. Be mindful of the dilution indications! Every year, horses get intoxicated with insecticides.
- Limit your horse’s access to the water and the humid grass.
- In the field, change your horse’s water every day (if it’s not an automatic drinking trough)
- Limit your horse’s access to a surface it can scratch itself on.
- Regularly use insectifuges and insecticides (oil based are better)
Check beforehand that your horse isn’t allergic to it (put a bit of the product on your horse’s chest for instance and check if it has any reaction in the next 24h)
- Lastly, decrease the excess of protein in your horse’s feed (that counts not only for sweet itch, but for any dermatologic disease)
For the last 2 points, you’ll have to ask your veterinary for help to adapt the treatment for your horse’s personal case. Unfortunately, I want to remind you that if you do no preventive treatment, the lesions can get worse and worse as the years go and the disease can become chronic.
Symptomatic treatment: [1,2,3]
It’s important to quickly start treating the lesions and also to control the itch. Once again, your veterinary is the best suited to advise you on products that will help your horse.
- Apply calming shampoos or solutions
- Disinfect the wounds
Your veterinary can decide to start a corticoids treatment. Firstly a loading dose, and then a degressive dose. It will prevent the horse from scratching. However this treatment isn’t without risks: there are important side effects (e. g. immunosuppression or risks of laminitis).
Desensitisation via increasing allergen doses injected regularly is difficult because this allergen is difficult to get. Also, clinical studies give mixed results as to the efficiency of this treatment . Maybe the scientific progress will one day allow us to get a more efficient treatment.
#6 – “What if I still want my horse to breed?”
Slow down there cowboy! Indeed, since this disease is in part considered hereditary, it’s better to set aside the horses affected. It’s often easy to convince the owner of the males. But, as it goes for a lot of health issues, owners have a tendency to make a mare with sweet itch (unable to work) reproduce. Unfortunately, the foals have high risks to be affected with the disease as well. Check with your veterinary about the risks of reproduction with this disease.
Read more: 5 Things to know about the mare’s heat cycle
See you soon for another article