Which of the following is least likely to be a mode of transmission of Hansen disease?

Most leprosy infections are transmitted following prolonged contact with infected nasal secretions. This can be as aerosols containing M. leprae (droplet infection). Other possible modes of transmission are contact with desquamated infected epidermis, ingestion of infected breast milk, and bites of mosquitoes or other vectors. Transplacental transmission of infection is least likely to occur. Ref: Park's Textbook Of Preventive And Social Medicine, By K. Park, 19th Edition, Page 267, 268 ; Nelson Textbook of Pediatrics, By Kliegman, Stanton, St. Geme, Schor, Behrman, 19th Edition, Pa XVII, Section 7, Chapter 208, Pathogenesis