Vertical transmission of a disease means that it can be passed through the placenta from mother to child during pregnancy. This mode of Lyme transmission makes sense to me. I see a lot of families where many of the children are infected, as well as the mother. Currently, there is family with 4 daughters, all of whom have Lyme disease. Their mother died at an early age of atypical multiple sclerosis. The family always suspected something else was going on. Unfortunately, many doctors diagnosis MS on the basis of MRI findings. Today, on my desk, a brain MRI report states that the findings are compatible with Lyme, but that MS is more likely. The patient has Lyme. Syphilis may be passed vertically from mother to child and cause a congenital syndrome seen at birth. Lyme does not cause obvious abnormalities seen at birth, to the best of our knowledge. It is a slow infection. Symptoms develop over time. Children become symptomatic at various ages from pre-school to early adulthood. Still based on what is known of the biological behavior of Borrelia and other spirochetes, it makes sense that vertical transmission occurs.
Horizontal transmission refers to person to person transmission of an infectious disease. Some believe that Lyme may be sexually transmitted. I believe this is false. Syphilis is transmitted via an open sore on the skin, a chancre with spread of the spirochete through the thin skin or mucous membranes of the genitalia. Lyme generally lives deep in tissues. The rashes which may have active spirochetes near the surface occur on the extremities and trunk. It is nearly impossible to find Lyme bacteria in any body fluids. Other sexually transmitted organisms are present in blood and body fluids. This is not the case for Lyme. Lyme DNA has been shown to be excreted through the urinary tract. After an antibiotic challenge urine PCR testing for Lyme DNA may be positive. This does not mean that intact viable organisms are present in genital secretions. In addition, there are specific factors in the tick bite and in tick saliva which have been shown to be important for infection to occur. Other insects like mosquitoes and flies may be infected with Borrelia, but there is no evidence that bites from these insects transmit the disease to humans. Lyme has a difficult time penetrating the skin barrier. But once it is in the body it has the ability to easily spread to tissues throughout the body. This is why vertical transmission makes sense and horizontal does not.