|MadSci Network: Virology|
Thank you for your question about hand foot and mouth disease. From the way you worded your question I can see you have spent some time learning about this disease from the WWW. And, I am not certain I can answer the specific question about why babies have the hand and foot rash while adults only have the oral and pharyngeal manifestations. I can offer some opinions, however.
As Hand Foot and Mouth Disease [HFMD] is not caused by a single virus. Viruses from the group called enteroviruses cause HFMD. The most common cause is coxsackievirus A16; sometimes, HFMD is caused by enterovirus 71 or other enteroviruses. The enterovirus group includes polioviruses, coxsackieviruses, echoviruses and other enteroviruses.
Infants, children, and adolescents are more likely to be susceptible to infection and illness from these viruses, because they are less likely than adults to have antibodies and be immune from previous exposures to them. Infection results in immunity to the specific virus, but a second episode may occur following infection with a different member of the enterovirus group.
It is possible for a baby to have no immunity to the virus even if the mother had had the disease herself when she was a baby because the antibodies she gave to the baby across the placenta would not neutralize it. She could have had one of the other enteroviruses which causes non- crossreactive immunity.
The fact that the baby and the mother both got the disease goes along with this theory.
The non-itching rash on the hands and soles of the feet that is part of the babies HFMD has always been interesting to me as only a few other conditions are known to cause this kind of rash and all of these are associated with strong immune reactions or allergy. Skin rash occurring during viral infections may not always be caused by viral invasion of the skin. Certainly, measles and chicken pox associated rashes are caused by viral infection in the skin but a the rash of Scarlet fever (related to strep infection) is not caused by the bacteria but rather the immune response to soluble bacterial antigen in the blood.
It is possible that the strong robust immune response to HFMD that happens on a baby, could result in formation of immune complexes that lodge in skin and cause the rash. I know of no studies where the skin rash had been biopsied to prove or disprove this idea. It would be simple to do and with the new polymerase chain reaction methods results could be had very soon. However, the cost both financial and ethical would not justify this kind of invasive test. HFMD is not so serious a disease as to warrant this kind of test and the emotional trauma the procedure would bring to a baby would far outweigh any benefit of this knowledge. So, it appears likely that we will never know the answer to your question, but my opinion is that the rash occurs because of the baby having too much immunity rather than not enough.
I hope this helps resolve your curiousity.
Here are some URLs from the web, which you probably already found, where you can find more information.
Try the links in the MadSci Library for more information on Virology.