Scheduled to be published spring, 2004, in Eidfaxi
by Dr. Barbara Sollner-Webb, Professor, Johns Hopkins University School of Medicine, Baltimore, MD, who has four wonderful Icelandic horses in her backyard.

Sweet Itch (SE) is an allergic reaction to the bites of no-see-ums (technically called culicoides) and it provides a real-life lesson in basic immunology.

To better understand the affliction and to devise ways to mollify its severity and hopefully prevent its onset, lets start by reviewing a few key facts in immunology. Most fundamental, when an adult animal is exposed to a foreign protein or other biochemical ("antigen"), it generally elicits an immune response that involves developing antibodies which provide the animal with immunological protection against the invasion. However, depending on the antigen and especially on the magnitude of the initial doses, a very intense immunological response can ensue and create an allergic reaction on repeated exposure. This is the basis of SE in imported Icelandic horses.

Now, it would be disastrous if the developing immune system in a mammalian infant were to generate antibodies to molecules from its own body. So evolution has arranged that biochemicals which the baby contacts before birth and for a while after birth are recognized as "self" and do not elicit an immune response. [When this self-protection fails, it is called an "auto-immune disease".] Then for a while, until the baby’s immune system is fully developed, invading antigens elicit mild immune responses but generally not the intensity for an allergic reaction.

A familiar example is chicken pox, in humans. Babies who are exposed to this virus in their first few months generally develop no obvious reaction, but when later re-exposed, they are refractory to the allergic response that we call "chicken pox". Yet without an initial exposure (or immunization, as is now available for chicken pox), later exposure almost certainly causes the allergic reaction.

With this background, SE starts to makes sense. Foals born in SE areas get bitten by no-see-ums early in life, so generally do not develop an allergic reaction. Conversely, horses imported as adults from Iceland into SE areas will have had no such early protective exposure and thus are prone to the SE allergy. It develops due to the very large number of no-see-um bites that unprotected horses can get in a single day.

Because the intensity of the exposures is important in determining whether an antigen elicits an allergic reaction, it is important to minimize the number of no-see-um bites that a horse receives. That is why bug sprays which are effective against no-see-ums can help prevent SE, even though the horse still receives some bites. It also explains the virtue of bringing SE horses inside, under fans, at dusk and dawn, when no-see-ums are the most voracious.

With this information, you may well be wondering why horses born in the US, but in an area not afflicted by SE, frequently do not develop SE when later moved into a SE area. For this, one more fact about immunology is needed, namely, that an immune response raised against a particular antigen generally provides a partial response to closely related antigens. And the US has ever-so-many more kinds of bugs than does Iceland, including various ones related to SE-causing no-see-ums. Thus, horses born in areas where they get exposed to such bugs would be much less likely to develop an allergic reaction, when brought into a SE area, than horses brought from Iceland, that did not have such an early exposure.

Now indulge me with a bit more immunology: It is known that frequent very low doses of an antigen generally do not generate an allergic reaction, but rather elicit a protective immunity. This is the principle behind desensitization shots, such as people can get for poison ivy or bee stings. Certainly this logic should extend to no-see-ums and horses. That is why my recent (October, 2003) Eidfaxi article on SE suggested that newly imported horses should religiously be treated with a bug spray that has been proven effective at markedly reducing the number of bites from no-see-ums. It will be important to spray every single day that the horse might be exposed to those bugs, for a good long period, so that the horse never receives a large number of no-see-um bites until after its immune system has been desensitized to this antigen, which fortuitously should result from the limited number of bites that the horse will receive despite the bug spray. [I suggest using FlyGone 7000 in the AM and PM throughout their whole first spring/summer/fall in the US, and in their second year at least the spring and part of the summer, and then to use this spray as their forever insect repellent.] Should the new, long-lasting bug treatments prove to be similarly effective against no-see-ums, they should simplify this treatment considerably.

By considering the immunological basis of SE, we have already found a regime that largely alleviates the allergic symptoms in my first imported Icey, who had developed a terrible case of it during his first US summer. Hopefully the above-suggested desensitization protocol can help prevent the onset of this very nasty affliction in most imported Iceys.

Added note:
An exciting-looking product called "FLY_BAN HORSE SPOT-ON" was advertised in the spring 2004 "Country Supply" catalog that it "Kills and repels members of the Culicoidae and Simuliidae vectors that may cause Sweet Itch... especially useful on horses turned out to pasture." However, the manufacturer told me they have temporally pulled this product, to re-word their packaging and advertising, for tests have shown it is no more effective against SE than standard Permethrin-containing spot-ons.