Infant Listeriosis Testing

An interesting question with practical application occurred to me while learning about the ontogeny of antibody responses. The question is this: If an infant tests positive for IgG to Listeriosis at 6 months of age, does this conclusively rule out the possibility that listeriosis was the cause of seizures and neurological damage apparent within 1 week after birth?

Listeriosis is a serious bacterial infection caused by Listeria monocytogenes. The bacteria can be found in soil, and can cause disease in some animals commonly used for meat and dairy for humans. Listeriosis is most commonly transmitted to humans via contaminated food, particularly uncooked meats and unpasturised dairy.

Listeriosis rarely affects people with a healthy immune response. However it may affect people of advanced age, pregnant women, newborns and people with weakened immune systems. Pregnant women are about 20x more likely to contract the disease than other healthy adults, and it is the risks associated with infection during pregancy, and during the post-natal period, that interest me most. An infected pregnant woman may experience only a mild, flu-like illness. Yet the infection can lead to miscarriage or stillbirth. Those infants that do survive have a reasonable chance of having seizures and neurological damage.

This question has direct relevance to my family because while pregnant with my youngest brother, my mother handled some goats on our property that were seriously ill with what looked very much like listeriosis. The goats had obvious neurological problems and major convulsions, and died. My mother's pregancy seemed healthy and went to full term. However a week after birth my brother began having convulsions and went on to display significant intellectual impairment. In retrospect he had a slight fever at birth, which went untreated. When my brother was six months old he and my mother were both tested for listeriosis. They both tested positive (though this was in 1989 and the test then had a higher rate of false positives than you might hope for). My brother had IgG against listeriosis. IgM was also tested, though we can't recall the result I would assume it was also positive.

When studying the ontogeny of antibody responses in IMMU7630 we learned that a foetus makes only IgM in utero. Maternal IgG crosses the placenta and so is present after birth. It has a half life of 3 weeks, so by 18 weeks after birth there should be only about 1% of maternal IgG remaining in the infant. The infant begins making its own IgG at around 12 weeks.

Without having studied the matter further, this suggests to me that an infant with IgG antibodies to listeriosis at 6 mo must have been exposed to lysteriosis after 12 weeks of age. This would rule out listeriosis as the cause for neurological damage. Yet is it possible that other mechanisms are in play? Could, for example, a B cell that began producing IgM to listeriosis in utero switch to IgG production as the infant's immune system matures? Are there other mechanisms that I have not yet considered?

I would be interested to hear your responses to these questions. It has piqued my curiosity, and I will certainly do some more research into the matter.