Failure to prevent diseases will only compound the problem. Once again, get with the local veterinarian with the broadest background in local problems. Even if the ranch has been through drought cycles before you may not have seen or heard about all the possible problems. Parasites, both internal and external, need to be monitored and controlled as needed. When grass is short due to drought internal parasites may not be a problem.
Parasites require moisture to reproduce and move up leaf tissue before they can be consumed.
Many producers will deworm cows when they become thin just in case the cause is internal parasites. With the cost of deworming products today it would be best to have fecal exams performed on a random sample of cows before deciding to deworm the whole herd. Most often cows are thin due to poor quality and quantity of forage. When rainfall is received and pastures are short, start being concerned about parasites. Carefully, monitor the herd and watch for signs of heel flies.