The mention of using pig whipworms was based on German research. Pig whipworms can only live in the human body for a couple of weeks, and so were ideal to work with, and had good results.
Importantly, using hookworm for severe asthma only seems to work as long as the infection is maintained, so in past, those individuals that used the treatment had to breed their own hookworms and reinfect themselves at intervals. Hookworms are more dangerous than whipworms, especially to children, because they may cause anemia.
Hookworm used to be endemic the the southern US, but was generally wiped out by the wearing of shoes, and the use of latrines. Several species are still around in the world, and are carried by dogs and cats. In the developing world, there has been considerable effort to wipe it out.
There is considerable potential for research, as over 100 different types of worms are known to infest humans.
Dr. Joel Weinstock of the University of Iowa did some amazing research in this area.