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  Discussion on Research Summary: Testing for Culicoides Hypersensitivity
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DrO

Posted on Thursday, May 8, 2008 - 9:52 am:

Allergic skin disease including summer eczema caused by Culicoides Hypersensitivity (CH) common in areas where there is standing water. A accurate test for the allergy has not been commercially available however. This has not stopped the selling of intradermal allergy tests and serological testing purported to be useful in identifying the disease. Experiments showing such tests as being useful are not available. Here a novel test called the Histamine Release Test (HRT) was shown to be useful in a small number of cases when culicoides saliva extract was used. HRT using saliva extract was 100% accurate at identifying effected and non-effected horses based on a diagnosis of whether there was a history consistent with CH. Here is a description of the HRT:

Histamine Release Test
Histamine is the chemical that drives the initial acute allergic reaction causing itching, swelling and congestion. Sometimes in allergy, histamine is released from Basophils and Mast Cells by non-IgE controlled mechanisms and our conventional IgE measuring allergy tests are of no diagnostic use. It would therefore be ideal to directly measure histamine released in an allergic reaction. A method to measure histamine released from Mast Cells and Basophils has now been developed. The Histamine Release test can directly measure Histamine release from Basophils in "In Vitro". Hence we can measure allergy to any number of agents by mixing these agents with the patients blood Basophils in the laboratory. The resultant Histamine released will accurately and positively identify if the patient is allergic to that allergen by using a Radio-immune Assay (RIA) which measures the amount of Histamine released. The test only requires a small amount of blood for each allergen. Unfortunately the blood must be freshly drawn, the test is expensive and not widely available yet. Occasionally other histamine like substances (vasoactive amines) may cross-react in the blood specimen, resulting in a false positive test. As a result this test is still undergoing evaluation and is not yet commercially available.

And here is the summary of the results of the test. Just as important to note is that serological testing using Elisa was not a accurate way to diagnose the disease.
DrO


Vet Immunol Immunopathol. 2008 Mar 15;122(1-2):126-37.
Comparison of cellular and humoral immunoassays for the assessment of summer eczema in horses.

Langner KF, Darpel KE, DrOlet BS, Fischer A, Hampel S, Heselhaus JE, Mellor PS, Mertens PP, Leibold W.

University of Veterinary Medicine, Bischofsholer Damm 15, 30173 Hannover, Germany. kathrin.langner@tiho-hannover.de

The objective of this study was to compare and analyze three common diagnostic methods for summer eczema (SE) in horses, an allergic dermatitis caused by bites of Culicoides spp. Nine horses with a medical history of SE and nine control animals were intradermally challenged with whole body extracts (WBE) and the saliva of a native (C. nubeculosus) and exotic (C. sonorensis) Culicoides species. Blood and serum samples of the horses were examined for basophil reactivity by a histamine release test (HRT) and for Culicoides-specific serum immunoglobulin E (IgE) and G (IgG) by enzyme-linked immunosorbent assay (ELISA). The results of intradermal testing (IDT) at 30min (immediate reactivity) and 4h (late-phase reactivity) post challenge with most insect preparations revealed significant differences between horses with and without SE. Overall, the HRT showed the most accurate results with a sensitivity of 1.00 for all Culicoides preparations and specificities of 0.78 (WBE) and 1.00 (saliva). By contrast, delayed reactions of the IDT (24h), and levels of Culicoides-specific IgE and IgG in the native serum showed little or no distinction between allergic and non-allergic horses. However, the use of purified serum IgE and IgG indicated the possibility for elevated titers of insect-specific serum immunoglobulins in horses with SE. The IDT and HRT did not reveal obvious differences in onset and intensity of positive reactions for the native verses exotic Culicoides species, whereas the ELISA showed slightly higher numbers of positive reactions for serum IgG with the indigenous species. Saliva, as compared to WBE, was found to have improved sensitivity and/or specificity for the HRT and for the late-phase immune reactions as measured by the IDT. Overall, the results indicate that allergy tests utilizing effector cells (mast cells, basophils) are more accurate in diagnosing SE in horses than serological analysis by ELISA.
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