Copy numbers of the S-adenosylmethionine synthetase (METK) gene in 14 dogs with active Leishmania infantum infection

Relapses of canine leishmaniasis during allopurinol treatment are common and complicate the course of disease. <i>S</i>-adenosylmethionine synthetase (<i>METK</i>) gene copy numbers (CN) < 3.0 have been demonstrated in allopurinol-resistant <i>Leishmania infantum</i> strains <i>in vitro</i>, but its clinical impact <i>in vivo</i> is still unclear. This study included 14 dogs divided into two cohorts (C): Cohort one (CI): nine dogs (64%) with signs of disease relapse under allopurinol treatment; Cohort two (CII): five dogs (36%) recently diagnosed with active leishmaniasis prior to treatment. <i>Leishmania infantum</i> infection was confirmed by positive PCR testing. <i>METK</i> gene CN was quantified by droplet digital PCR. Complete blood counts and biochemical profiles were performed where suitable samples were available. <i>METK</i> gene CN ranged from 0.7 to 3.4 [CN < 3.0 (<i>n</i> = 13), 93%; CN = 3.4 (<i>n</i> = 1), 7%; CI: CN = 1.2-3.4; CII: CN < 2.0 each]. Clinicopathological abnormalities consistent with active leishmaniasis were observed in all dogs. Allopurinol is used for long-term management of canine leishmaniasis, therefore identification of resistance to allopurinol is crucial, especially in cases of clinical relapses. Leishmaniasis poses a zoonotic risk to humans so that the spread of parasites due to resistance should be considered regarding the One Health aspect and the All Species approach. In dogs recently diagnosed with active leishmaniasis not receiving allopurinol yet, resistant <i>L. infantum</i> strains may most likely be transmitted by sand flies. The threshold of <i>METK</i> gene CN < 3.0 <i>in vivo</i> seems to be questionable in individual cases.