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Dear Sir:
I have read with interest the retrospective study carried out by Garrote and others ("Seroepidemiology Study of Leishmania infantum Infection in Castilla-Leon, Spain")1 and I would like to make the following observations. First, Spain has a wide range of climatic regions and ecosystems. As the authors point out, the area of Castilla-Leon, one of the largest but least inhabited geographical communities in Europe, it has a continental climate, with very cold winters for this latitude. It is thus considered to be a region of low endemicity for Leishmania, with a very low canine prevalence and an annual incidence of human cases that falls below the mean for the country (0.3 cases of visceral leishmaniasis per 100,000 inhabitants).
In this context, I would like to draw attention to some of the data presented in this study. First, the seroprevalence found in the general population (4.4%, ranging from 3.5% to 6%) is comparable to that found in another study performed in Spain with blood donors from Ibiza (Balearic Islands, Spain). In that study, 7.6% of the population had anti-Leishmania antibodies (as determined by Western blot), 22.3% of the blood donors showed a positive leishmanin skin test, and Leishmania infantum DNA was detected in peripheral blood by nested polymerase chain reaction (PCR) in 22.2% (data that confirm the endemicity in the area studied).2
A second aspect of the study that is both particularly striking and at the same time alarming is the high seroprevalence (64%) found among asymptomatic human immunodeficiency virus (HIV+)-positive patients who were intravenous drug users (IVDUs). In the light of these results, we would like to make the following observations about the methodology used and the discussion of the results obtained. In any epidemiologic study, the techniques used and the conditions in which they are applied can have a strong influence on the percentage of seropositive subjects found. The definition of a cutoff in the enzyme-linked immunosorbent assay (ELISA), when this cutoff is established to improve the sensitivity of the technique, could entail a lack of specificity. The low humoral immunoresponse, characteristic of asymptomatic subjects, frequently places their antibody levels at the borderline of detectability. When antibody titers are low, positive results may be caused by artefacts and cross-reaction. So the results obtained by Garrote and others could be due to the cutoff used to discriminate sera with antibody from those without and other technical factors such as the lack of confirmation of initially reactive samples. The use of techniques that do not distinguish between antigen fractions (IFAT, ELISA) makes it difficult to differentiate between infection and other conditions. Our experience in these circumstances is that it is necessary to confirm seropositives with Western blot, which is a sensitive serological technique to identify specific antibodies. Western blot has been used successfully in epidemiologic surveys in endemic areas.2,3 If this test had been applied, it would have been possible to obtain important confirmation of the seropositives detected, especially in those patients who were HIV positive-IVDUs, considered a high-risk group for leishmaniasis.
Moreover, I was surprised at the explanation offered by the authors for the high prevalence of this infection. They state that these data confirm the existence of an anthroponotic cycle of leishmaniasis transmission through syringes (which is obviously not applicable to the group of healthy subjects), but no explanation is given for this statement. Other authors do not support this hypothesis, as the proportion of IVDU patients was not significantly different between HIV+ positive patients with or without visceral leishmaniasis.4 Furthermore, it is also surprising that such a high seroprevalence, in an area in which the annual incidence over the last 10 years has not surpassed 0.04 cases per 100,000 inhabitants, is not reflected in an increase in the number of cases of leishmaniasis, bearing in mind that visceral leishmaniosis appears in 75–90% of cases of coinfection and that only 5% of these subjects remain asymptomatic.5
I believe that this study is of value because it begins research in an area of Spain that has hitherto been considered as a nonendemic zone for leishmaniasis and where no other studies have been done, but I also feel that the results obtained and the methods used should be considered with great caution.
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