While millipedes suffer from less observable health problems than might befall a mammal or avian, there are still a few which are cause for concern on captive animals. One of the most common are damaged segments, dented segments, and scratched exoskeletons. These are usually the result of mismolts or damage received during a molt, and are especially common on wild caught specimens. Generally speaking they are not life-threatening. Over time captive populations can also develop small deformities not seen on wild specimens.
Other common deformities seen on mismolted or wild caught specimens are missing antenna or legs, smaller than normal antenna or legs, and cracked segments. The first two are usually not a major cause of concern as both legs and antenna will be grown back within a few molts. Regrown antenna or legs are often shorter than non-regrown legs and take time to regain full size (though it is more likely that they will never grow quite as big as the originals). The last can be dangerous however and cause problems with molting, while also opening up the animal to infection; a specimen with segments that are already cracked will usually die after being dropped due to hemolymph loss.
A very common malady afflicting usually wild caught specimens is foot rot, which is caused by a bacteria that attacks the legs, antenna, and sometimes the body of millipedes, causing them to at first appear blackened. Over time legs will eventually start to take on a clipped appearance as the bacteria eats away the legs, and if not treated many of the legs can be completely eaten away. It is usually fatal over the long-term if the specimen does not molt out of it.
Foot rot bacterial attacks manifest themselves by afflicting the legs of millipedes, eventually reducing them to blackened stumps. The bacteria will gradually consume the entire leg and it can also affect other areas as well such as antennae or damaged areas of the exoskeleton. This affliction generally occurs amongst wild caught specimens, and as such it is extremely important to inspect each and every new wild-caught specimen you acquire, or any that you suspect were housed with wild-caught individuals, as the bacteria is very opportunistic and aggressive. It is also very important to separate any millipedes that do not have foot rot from the ones that do as the bacteria is quite contagious. It is recommended that you clean the non-afflicted millipedes with a clean toothbrush under room temperature water under the sink and gently scrub every segment, making note to be extra careful when scrubbing the legs. Be mindful to not submerge all of the millipede under running water as the millipede breathes through small holes called spiracles present on each side of it’s segments, so you only want to have a small stream of water that runs over a small part of the millipede at a time. Cleaning the millipede may help to remove any of the bacteria before it can become a problem. It is very important that you keep the non-afflicted millipedes in a separate enclosure from the ones that are afflicted.
For the millipedes that are heavily afflicted with foot rot, you must use a different clean toothbrush for them to clean their segments and their feet in the sink with a thin stream of water. Do not share the toothbrush between the afflicted and non-afflicted as you will spread the infection to the healthy millipedes. Cleaning the millipede may help remove some of the bacteria and perhaps slow down the progress of the infection until the millipede goes into premolt, which is currently the only way known for specimens to become completely free of foot rot. You must never disturb your millipede if they have burrowed deep down into the substrate to molt as they are extremely delicate during this stage.
Care for the millipedes with foot rot involves keeping the top layer of the enclosure dryer then usual, as warm and excessively wet conditions will exacerbate the spread of the bacteria and it will consume the legs at an accelerated rate. The goal is to slow down the spread of the foot rot and manage the condition until the millipede can molt and rid itself of the infected old exoskeleton. Once you have spotted a millipede that has molted free of its foot rot and is fully hardened, you must immediately remove the millipede from the enclosure and take it to the sink to gently clean it with a toothbrush following the same steps as before. Once the millipede is cleaned, dry the millipede off, making sure that the spiracles are not wet, and do NOT place them back in their old enclosure unless it has been thoroughly cleaned and disinfected. It is extremely important that the specimens are placed into a clean enclosure with brand new substrate that is likewise somewhat dryer in the upper layers. However, it is also necessary to make sure that the bottom layers of the substrate are kept adequately damp, so that if the millipede needs to rehydrate itself all it needs to do is burrow down. If it is kept too dry all the way through to the bottom, the millipede will die of dehydration. If you are concerned with keeping the top layers of the enclosure drier, you can also mist a small area in one of the corners of the enclosure and provide a small drinking bowl (ensure that it is shallow enough that the millipede cannot drown in it), though keep in mind if the millipede needs more moisture it will instinctively burrow to acquire it.
Foot rot is not always fatal, and there is a very good prognosis when specimens with it are kept under the right circumstances. It is entirely possible to keep the millipede alive long enough to molt out of it as long as special care is taken to not keep the millipede on overly damp conditions that exacerbates the growth of the bacteria. Following the care listed above, there is a very high chance of your millipede surviving and recovering from the infection, albeit this is not guaranteed.