MadSci Network: Anatomy

Re: How do babies get aids?

Area: Anatomy
Posted By: Tom Wilson, M.D./PhD, Pathology, Div. of Molecular Oncology, Washington University School of Medicine
Date: Thu Jul 3 23:56:55 1997
Area of science: Anatomy
ID: 865637056.An
This is a very good question.  Most people now know (or should learn) that 
AIDS is spread to adults in three main ways - having sex, high-risk drug 
use (like needle-sharing), and blood transfusions.  All of these are ways 
that the HIV virus can get from one personís blood to another personís 
blood.  But obviously babies donít do any of these things, except maybe for 
a small number of blood transfusions.  And yet AIDS is definitely a problem 
in pediatrics as well as adult medicine.  So how do babies and little kids 
get it?

Almost all babies with AIDS got it from their HIV-infected mother while 
they were inside the uterus (also known as the womb).  In other words, the 
HIV virus was spread from the motherís blood to the babyís blood before the 
baby was born.  Even if they donít get AIDS until years later, 
little kids were still almost always infected with HIV when they 
were inside their mother.

The thing is, only about one-third of babies born from mothers who have 
AIDS will actually get infected with HIV themselves (and treatment of the 
mother with AZT can drop this down even farther).  This is because there is 
actually an inherent resistance to transmitting viruses from mother to 
baby, but this resistance is just not perfect.

We can understand how all of this works by considering the special anatomy 
of the pregnant woman.  The mother "feeds" the baby, i.e. nutrients pass 
from the motherís blood to the babyís blood, through a special organ called 
the placenta (also known as the afterbirth).  In the placenta, blood 
from the mother washes over small blood vessels from the baby, allowing 
sugar, oxygen and other good things to diffuse across.  But the 
blood never actually mixes, it is separated by a blood vessel wall, and so 
big things like viruses canít get into the babyís blood.  As an aside, the 
babyís blood vessels get to the placenta through the umbilical cord, which 
was attached to your body where your belly button is now.

The problem is that the placenta has SO MANY of these little blood vessels 
that sometimes they get damaged and torn.  This rarely causes a major 
bleeding problem, but it does allow the motherís blood and the babyís blood 
to mix just a little bit.  We call this a "maternal-fetal transfusion", 
which literally means passing of blood between mother and baby.  The more 
virus there is in the motherís blood, the more likely it is that these 
small blood exchanges will infect the baby.

Hope this helps you understand.  And remember, please, talk to your parents 
or teachers about how to PROTECT YOURSELF FROM AIDS!!!

Tom Wilson, MD PhD

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