What are the economic importance of diseases in livestock management?

Diseases are not an economic importance. They are rather more of a nuisance to livestock management than an importance, since any producer would dream of having disease-free animals all the time without any sort of input to keep them disease free.

But I believe you are referring to the economic importance in disease prevention and herd health practices in livestock management. THAT is very important in raising livestock. Having good prevention practices and a good herd/flock health program means more money in the bank when selling excess stock that are healthy. It also means less death losses and production issues when you have good disease prevention practices on your farm. In order to have a good herd/flock health program, you must have a good annual vaccination program for young stock and breeding stock, having and maintaining a quarantine station and program for any new animals that are to be introduced into the herd. The quarantine program ensures that these new animals are tested for diseases that could be detrimental to the rest of the herd, and to keep these animals quarantined until they are deemed safe or not. Finally, culling any and all animals that are sick or tested carriers of a disease is the third and the most important part of a good herd/flock health program. Often it is much harder to maintain 100% prevention rate in your herd or flock if you keep any animals that are sick or are carriers for a certain disease, because they could still pose a risk to the rest of those that are susceptible to that particular illness or disease.