Moriuchi et al The Pediatric Infectious Disease Journal• Volume 32, Number 2, February 2013

176| www. pidj. com© 2013 Lippincott Williams & Wilkins the first to demonstrate transmission in breast milk (summarized in Hino et al9), a finding subsequently confirmed by other studies. The data supporting the importance of breast-milk transmission included 1) the demonstration of HTLV-1 antigen in breast milk derived from infected mothers; 2) oral administration of fresh human milk derived from HTLV-1-infected mothers to uninfected marmosets led to HTLV-1 infection; 3) a significantly increased HTLV-1 infection rate in breastfed children compared with bottle-fed children and 4) long-term prospective data showing that MTCT rates were 20.5% in infants breastfed for 6 months or more, 8.3% in those breastfed for< 6 months and 2.4% in infants exclusively formula-fed (Fig. 1). 10 These data indicate that breastfeeding is the most prevalent, but not the sole route of

MTCT of HTLV-1, and that a longer duration of breastfeeding increases the risk of MTCT. The source of the virus is thought to be infected lymphocytes in breast milk, and proviral load in breast milk11 or maternal blood12 appears to contribute to milk-borne transmission. A Jamaican study showed that MTCT occurred at 4.7 and 28.7 per 1000 person months at low and high proviral loads in breast milk, respectively. 11 Transmission of HTLV-1 infection to exclusively formula-fed infants indicates that other largely unknown routes of MTCT. Transplacental infection or placental microtransfusion are less likely, as proviral HTLV-1 DNA in cord blood of infected mothers is not indicative of intrauterine infection, and none of the cord blood samples derived from exclusive formula-fed HTLV-1 infected Japanese