Maternal Leishmania infantum infection status has significant impact on leishmaniasis in offspring

Visceral Leishmaniasis is a deadly disease caused by Leishmania infantum, endemic in more than 98 countries across the globe. Although the most common means of transmission is via a sand fly vector, there is growing evidence that vertical transmission may be critical for maintaining L. infantum infection within the reservoir, canine, population. Vertical transmission is also an important cause of infant morbidity and mortality particularly in sub-Saharan Africa. While vertical transmission of visceralizing species of Leishmania has been reported around the globe, risk factors associated with this unique means of Leishmania transmission have not been identified therefore interventions regarding this means of transmission have been virtually non-existent. Furthermore, the basic reproductive number, (R-0), or number of new L. infantum infections that one infected mother or dam can cause has not been established for vertical transmission, also hampering the ability to assess the impact of this means of transmission within reservoir of human hosts. Canine Leishmaniosis (CanL) is enzootic within a U.S. hunting dog population. CanL is transmitted within this population via transplacental transmission with no reported vector transmission, despite many repeated attempts to find infected sand flies associated with these dogs and kennels. This population with predominantly, if not solely, vertical transmission of L. infantum was used to evaluate the critical risk factors for vertical transmission of Leishmania and establish the R-0 of vertical L. infantum infection. Evaluation of 124 animals born to eighteen dams diagnostically positive for infection with L. infantum showed that there was a 13.84x greater chance of being positive for L. infantum within their lifetime if the mother was also positive within her lifetime (RR: 13.84, 95% CI: 3.54-54.20, p-value: <0.0001). The basic reproductive number for vertically transmitted L. infantum within this cohort was 4.12. These results underscore that there is a high risk of L. infantum infection to transmit from mother to offspring. Targeted public health interventions and control efforts that address vertical transmission of L. infantum are necessary in endemic countries to eliminate visceral leishmaniasis. Author summary Canine leishmaniosis (CanL) is a deadly disease caused by Leishmania infantum parasite, it is found in animal populations, including people, in more than 98 countries across the globe. CanL was first identified within the US in hunting dogs 1980 and then again in 1999 when a large outbreak in a kennel in New York occurred. As the US is usually not considered a tropical country, there was much debate about how this neglected, vector borne, tropical disease had made its way into these dogs. We found that within the U.S. hunting dog population CanL is transmitted from mom to pup with no reported sand fly transmission in the population, despite multiple attempts to find infected sand flies associated with these dogs. While vertical transmission of this disease has been reported in case reports around the globe, risk factors associated with this unique means of Leishmania transmission are not known. Furthermore, the basic reproductive number, (R-0), or number of new infections that one infected animal can cause has not been reported for vertical transmission of L. infantum. It is important to know the R-0 as it helps identify how infectious a route of transmission can be and therefore how easy it might be to control this infection. A cohort of 124 dogs from 18 dams was analyzed from 1999 to 2016 for factors related to vertical transmission. Offspring from dams ever diagnostically positive for infection with L. infantum were 13.84x more likely to become positive for L. infantum themselves within their lifetime (RR: 13.84 95% CI: 3.54-54.20 p-value: <0.0001). The basic reproductive number for vertically transmitted L. infantum within this cohort was 4.12. These results underscore that an infected mom is highly likely to infect her offspring if treatment is not started to prevent transmission. There is a need for any public health prevention and control efforts to address vertical as well as vector transmission of canine leishmaniosis in endemic countries.