There’s good news in the fight against sweet itch—known to scientists as insect bite hypersensitivity (IBH). This seasonal skin condition results from an allergic reaction to the saliva of biting midges (Culicoides). While a fly sheet can help prevent bites and topical creams can help ease the itching, there’s still no way to stop sweet itch from happening.
But thanks to new research in Iceland, a vaccine could soon be available.
“In the next few years an experimental vaccine could be available,” said Sigríður Jónsdóttir, MSc, a biologist at the University of Iceland Institute for Experimental Pathology in Reykjavík. “Currently, no preventive immunotherapy is available, neither for humans (for similar allergic skin conditions) nor animals.”
For this study, Jónsdóttir said she and her fellow researchers benefitted from having access to Iceland’s native horses.
“In the development of a preventive treatment against an allergy, it is essential to treat before the individual meets the causative agent (the allergen) and gets sensitized (becomes allergic),” Jónsdóttir told The Horse. “Iceland is free of Culicoides; therefore, horses raised in Iceland and exported to Culicoides-endemic areas provide a unique model for studying preventive treatment for allergy.”
In their study, the researchers developed a vaccine with recombinant allergens (the molecules within the midge saliva that provoke the allergies). They tested two forms of the vaccine: with adjuvant aluminum hydroxide (Alum) alone, or combined with monophosphoryl lipid A (MPLA).
The vaccine isn’t injected into muscle tissue or skin, but directly into the lymph nodes, Jónsdóttir said. And this could have real benefits.
“Injecting allergens directly into the lymph nodes has been shown to strongly enhance allergen-specific immunotherapy in humans,” she said. “And it requires lower allergen doses, less treatment time, and fewer injections compared to the classical subcutaneous (under the skin) method.”
They administered the vaccine to 12 healthy Icelandic horses, three times each, using four recombinant Culicoides nubeculosus allergens, in one of the two combinations (Alum or Alum/MPLA).
They found that all the horses tolerated the injections well and that the Alum/MPLA showed the most promise as an effective vaccine. It induced high IgG antibody levels and Th1/Treg immune responses (strong signs of immunity) in the vaccinated horses, Jónsdóttir said.
However, at this stage, the researchers have not yet taken the next step, which is to expose those vaccinated horses to midges, she added.
They also need to investigate further to see how many allergens need to be included in the vaccine, as the four they used might not be enough—or might be more than necessary.
“What we need to do next is identify the most important causative allergens that are essential for the vaccine,” she said. “And then we need to do a challenge experiment, where horses are vaccinated in Iceland and exported along with unvaccinated (control) horses to a Culicoides-infested area, where we’ll monitor them for three years.”
The study, “A preventive immunization approach against insect bite hypersensitivity: Intralymphatic injection with recombinant allergens in Alum or Alum and monophosphoryl lipid A,” was published in Veterinary Immunology and Immunopathology.