Sire risk factors for vertical transmission of Leishmania infantum by the dam

Leishmania (L.) infantum causes visceral leishmaniasis (VL) in humans across the Mediterranean basin and in Central and South America. VL is a zoonotic disease, with dogs as the predominant domestic reservoir. Traditionally, this disease is transmitted via sand flies of the genus Phlebotomus or Lutzomyia as vectors. Case reports of transmission in non-endemic areas have increased, where transmission is predominantly vertical, sexual, or bloodborne. L. infantum has been shown to be enzootic in hunting hounds within the United States (U.S.), with no indication of vector borne transmission. In this population, there is very high risk of parasite spread if the dam's diagnostic status is positive. In comparison, very little is known about the risk if the sire not the dam is positive for L. infantum. This is a retrospective cohort study of 24 U.S. hunting hound sires' L. infantum exposure between 2013 and 2022 and the resultant evidence of infection in their 183 pups over that time. Offspring from sires who tested serologically positive for L. infantum during the year of birth had 1.59 times the risk of becoming diagnostically positive via serology or molecular detection for L. infantum during their lifetime (RR: 1.59 95% CI: 1.15-2.20 p-value: 0.0046) when compared to outcomes in pups from diagnostically negative sires. The basic reproductive number (R0) for the risk of the sire to indirectly transmit L. infantum to the pup within this cohort was 3.71. These results demonstrate the impact of sire's infection on transmission of L. infantum to offspring. There is a need for control efforts that address non-vectorial transmission from both parents. Control efforts targeting vector borne transmission of canine leishmaniosis in endemic countries are also needed.