Is There A Cure For Sweet Itch?

At this time there is no actual cure for Sweet Itch. Once the lesions have appeared the only remedy for this affliction is one of prevention (keeping midges away from your horse) treatment to alleviate the itching and discomfort.

Here is an interesting article by British vet Dr Doug Wilson.
Source: http://www.thisisbristol.co.uk


Bristol Vet Continues Scratching His Head Over Tricky Sweet Itch Problem

May 28, 2010
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"Why can't you vets do something about this terrible sweet itch?" This question was put to me one afternoon by a lady at Pony Club when I arrived to pick up my children.

The story was a familiar one of a young pony that had been purchased in the winter months then suddenly developed the characteristic rubbing and weeping sores of sweet itch the following summer. Of course, the vendor was certain that the pony had shown no signs of the disease the previous year, which may well be true as the peak age of onset for sweet itch is about four years and it can first appear in much older horses.

Source: horsetalk.co.nz

Initially I was rather taken aback by this abrupt demand to "do something", yet on reflection I could only agree that it was time something was done about this distressing condition which affects about three per cent of horses and ponies in Britain.

Furthermore, as a vet with an interest in immunology I was in an undeniably good position to take on this task.

Sweet itch is most often an allergic reaction of horses to the bites of midges. After alighting on a horse, midges crawl down the hair shafts to the skin surface. Their mouth parts are too short to probe for a blood vessel like their larger cousins, the mosquitoes, so they have to chew their way through the tough outer layers of skin. To assist their efforts, they secrete saliva containing a mixture of enzymes that digest and soften the skin tissue as well as agents to encourage extra blood to flow to the site of the bite and several factors that will prevent the blood clotting. A small pool of blood forms just under the skin surface and is then sucked up by the midges.

The whole process takes about 15-20 minutes and, over the course of an evening, a horse may be bitten by hundreds or even thousands of midges, each one injecting a small amount of saliva containing foreign proteins into the horse.

When an animal is injected with something foreign like midge saliva, its immune system responds by making antibodies that can bind to the foreign proteins. Antibodies are made by specialised cells called B-cells, which are individually programmed to make a unique antibody. When a B-cell encounters a foreign substance that is matched to its unique antibody, it is stimulated to grow and divide so that in a few days there are thousands of them and lots of antibody can be made. So, if the immune system of horses with sweet itch is reacting to the saliva of midges, the horses should have the correct antibodies that will specifically target the proteins in midge saliva.

However, we found that all British horses had antibodies to midge saliva; perhaps this is not surprising as all horses living in Britain will be bitten by midges. Fortunately, we were able to get some serum from horses living in Iceland – one of the few places in the world where there are no midges – and we were able to confirm that their serum contained no antibodies to midge saliva.

But if all British horses have antibodies to midges, why do only some develop sweet itch? The answer lies in understanding more about the immune system and about antibodies.
The kind of antibody that is important in allergies like sweet itch is called Immunoglobulin E, or IgE for short. Its role is in immune responses to parasites like worms.

To help protect against worms, the immune system has to use one of its most powerful weapons, known as the "mast cell". These mast cells coat themselves in IgE antibodies and lie in wait just under the skin or in the lining of the intestine. When the IgE hits its target, the mast cell releases a cocktail of chemicals that cause a severe inflammatory reaction and attract other immune cells which can injure or kill the parasite.

Unfortunately, like other weapons of mass destruction, mast cells can cause a lot of collateral damage. Allergies occur when the immune system makes a mistake and mounts an anti-parasite response to the wrong thing. In horses, the commonest allergy is to midge saliva.

Source:netzarim.co.il

When we looked for IgE antibodies that bound to midge saliva glands, we only found them in the serum taken from horses with sweet itch, confirming that this disease is an allergic response to midge bites.

The next task in our research was to identify all the different proteins in midge saliva. One way of doing this is to isolate the genes that contain the instructions for making the proteins in midge saliva. In animals, every cell contains DNA that carries the code for making an entire animal, but only those genes that are needed by a particular cell are switched on.

From midge saliva glands, we isolated the switched on genes for saliva proteins and put them into a special type of bacteria in the laboratory. When grown overnight on a dish of agar gel, each individual bacteria forms a colony that will contain only one extra gene from midge saliva. We can then pick each colony of bacteria, isolate the midge gene and read its coded message. By reading lots of coded messages, we can work out which ones are most common in midge saliva and are most likely to be the genes of the proteins that cause sweet itch.

The second approach is to look at the proteins themselves. Using a combination of methods, proteins can be separated according to their size and acidity. The individual proteins form spots and each spot is then broken down into fragments. The sizes of the fragments are then measured and a computer program compares the pattern of the fragment sizes with those that we would expect to find based on the genetic codes. After putting all this information together, we can work out what the commonest proteins in midge saliva are.

Once the genes or the relevant proteins have been isolated, we can put them into cultures of insect cells which will make the protein in an identical way to a midge saliva gland. Each culture can be over a litre in size – equivalent to the contents of several million midges' saliva glands.

We can in theory use it to re-programme the immune system of an allergic horse to act like that of healthy horses. But first we need to better understand why only some horses' immune systems react with an allergic response.

Although horses do not get sweet itch in Iceland, where there are no midges, when brought to mainland Europe, over one in four Icelandic horses may eventually develop the condition. So are Icelandic horses as a breed genetically more likely to get sweet itch? 

Scientists and vets from Iceland and European countries have looked at this in detail, and the answer is no. Although there are genes in some horses that make them more likely to get sweet itch, these are not more common in Icelandic horses compared to other breeds and Icelandic horses born on mainland Europe do not get sweet itch any more often than other horse breeds.

Other studies on the development of the foal's immune system have shown that they do not make IgE antibodies until they are about six months old. We think that when foals are exposed to midges before this age their immune system usually becomes programmed not to make IgE antibodies to midge bites. But Icelandic horses first exposed to midge bites as adults are very susceptible to developing sweet itch because as foals their immune system was not programmed to ignore midge bites.

In horses affected by sweet itch, the immune system "detects" enzymes the midge uses to break down the skin and interprets them as an invading parasite when in fact it is only a midge bite. Can we re-program the immune system of horses with sweet itch to be like that of healthy horses?

The term immunotherapy is used to describe treatments that can be used to re-program the immune system of people (or animals) with allergies; usually this involves repeatedly exposing the immune system to small amounts of the allergen. Originally this was done by daily injections, but it is unlikely that this method would be suitable for use in horses. There are several newer ways that re-programming could be attempted, but it will take some time to work out how this can be done.

For example, would mixing the midge proteins with some parts of bacteria fool the immune system's chain of command into giving the "correct" orders? Or could feeding midge protein convince the immune systems that this is really a harmless food? Should all foals be inoculated with midge proteins at an early age?

We don't yet know the answer. It has taken almost ten years to get this far but we are making progress. In research, there are always new surprises that await, but one thing is certain in science – it always takes longer that you think. Yet if everyone works together, one day we will indeed be able to "do something" and develop an effective cure.

  For more information, contact Dr Doug Wilson on Doug.Wilson@bristol.ac.uk.