Children as young as that should never go to a malaria area in a risk period!! It is as simple as that, in my opinion.
At 5 months of age, the little one's immune system is not yet fully developed, so if they are unlucky enough to contract malaria, the progression of the disease is very rapid, and they can die within a day or two! Furthermore, it is not always easy to suspect malaria in such a young child - remember they cannot tell you they are ill, and they often pick up fevers at that stage of development, what with colds and flus and, of course, teething!
Moreover, remember that your baby is too young to take conventional antimalarial medications, so that adds more risk.
Personally, if I had a 5-month old, I would make alternative arrangements and not put the child at risk. Your suggestion, however, of day trips - thereby overnighting in a non-malarial area - is about your only proper option (if you cannot find someone to babysit, or don't trust anyone enough to do that), and is feasible. However, remember that you would have to drive quite a way to get to Kruger from the non-malarial areas (consult a malarial-risk areas map in SA), and then make sure you leave the malarial areas before dusk and enter them after the sun has risen. In that case, there is always the tiny chance that an infected mossie remains in your car, but this is often a very small risk.
Depending on when you are going to Kruger, I would also be concerned about the heat: a child's temperature can spike quickly around 5-months of age and the additional ambient heat could complicate matters. Also, the child may cry the entire time he/she is uncomfortable. Then there is the risk of sunburn.
There are other factors that can be taken into account, but, as I said at the beginning, unless you can guarantee being in the malarial area during the day only, and avoiding other complicating factors, it is best not to take your child with.
Disclaimer: My recommendations here - though based on some experience and some drug, and other, knowledge - are not absolute, and further consultation with suitable health-care professionals is suggested before a final decision is taken on whether to enter a malarial area, what prophylaxis to use, and any general factors and limitations that need to be taken into account. Furthermore, I only advise based on what information is given by the person(s) entering the malarial area, but I have no control on the information given to me, and so such information could possibly be incomplete or misleading. Moreover, people vary subjectively as to how they metabolise, and react to, drugs and other substances, which further accentuates that my suggestions here are only general suggestions, and therefore not to be taken as pertaining to every person alike.
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