Thank you for your inquiry.
The case you presented brings up two salient points. The first is related to when to discontinue venom immunotherapy, and the second concerns whether or not to add the new venom, wasp, to the treatment protocol.
The first issue is best answered by a quote from the 2011 Practice Parameters on Insect Sting Hypersensitivity. As you can see from this quote, which is copied below, there is great leeway as to when one might consider discontinuation of venom immunotherapy after three to five years of treatment. Thus, your strategy of continuing venom immunotherapy in this situation is certainly acceptable.
"Once initiated, VIT should usually be continued for at least 3 to 5 years.42,43 An increasing body of evidence suggests that despite the persistence of a positive skin test response, 80% to 90% of patients will not have a systemic reaction to an insect sting if VIT is stopped after 3 to 5 years.44-52 There are no specific tests to distinguish which patients will relapse after stopping VIT, but there is a higher risk in some patients than others. Relapse is less likely with 5 years than with 3 years of VIT.50,53 Although most patients can safely discontinue immunotherapy after this period of time, some patients with a history of severe anaphylaxis with shock or loss of consciousness still might be at continued risk for a systemic reaction if VIT is stopped, even after 5 years of immunotherapy.46,47,52
For this reason, some experts recommend an extended duration of immunotherapy, possibly indefinitely, in such patients. Other criteria suggested for stopping VIT include a decrease in serum venom-specific IgE to insignificant levels or conversion to a negative skin test response.54 Some patients have relapsed despite negative venom skin test responses. Repeat skin (or venom-specific IgE serum) testing is not required for consideration of discontinuing VIT. Measurements of venom-specific IgG antibodies have no predictive value when discontinuing VIT. The decision on stopping VIT requires a context-sensitive flexibility based on the available evidence".
(SOURCE: INSECT VENOM SENSITIVITY PARAMETER 2011)
The second issue is perhaps a little more difficult to deal with. However, I think the most important point to be considered is that expressed in an article by David Golden regarding the epidemiology of venom sensitivity; the abstract of this article is copied for you below. As you can see in this article, the presence of a positive skin test does in no way automatically assure that the patient will react to a particular venom upon a sting. As noted in this abstract, 26.5% of individuals had detectable IgE antibodies to venom even though the prevalence of sting reactions was only 3.3%; and moreover, after a recent sting, even in the absence of a systemic reaction, the incidence of a positive skin test could be as high as 35%. Thus, the demonstration of sensitivity alone, in the absence of a reaction to a sting, does not necessarily serve as a sufficient reason to initiate venom immunotherapy. It would seem that, since your patient has not experienced a second reaction, the detection of specific IgE to wasp venom more than likely represents sensitivity without reactivity (since the patient did not have a systemic reaction after the last sting). Thus, my own inclination would be not to treat your patient with wasp venom.
Thank you again for your inquiry and we hope this response is helpful to you.
Epidemiology of Insect Venom Sensitivity
David B. K. Golden, MD
The prevalence of insect sting reaction and of venom sensitization in adults is unknown. We report the results of intake evaluation of a stratified random sample of a large adult population previously studied for the determinants of atopic disease. In 269 subjects, the prevalence of systemic allergic sting reactions was 3.3% and 26.5% had IgE antibodies to venom demonstrated by skin test or radioallergosorbent test. Asymptomatic sensitization (positive venom skin test) was observed in 15% of subjects with no history of an allergic sting reaction. Positive venom skin tests were more frequent in men, in those with positive skin tests to inhalant allergens, and in subjects aged 20 through 29 years. A positive venom skin test or radioallergosorbent test was more frequent in subjects who had been stung within the previous 3 years (35%) than in those stung more than 3 years before (20%). We conclude that both systemic allergic reactions to insect stings and asymptomatic sensitivity to venom are common and that most affected persons never seek medical advice. The significance of asymptomatic venom sensitization is unknown
Phil Lieberman, M.D.