Understanding the Rationale of TCM Formula Design through Didang Tang in the Shanghan Lun

I grew up in the countryside. During the busy farming season I had to work the fields with my parents; rural children have followed the same path for thousands of years. The hardest work we did as children was probably transplanting rice seedlings. It required us to bend over and work in the paddy for hours on end. The leeches in the paddy were countless; we transplanted barefoot, and from time to time leeches would attach to our calves to suck blood—often several leeches feeding on our calves at once.

The leech is an especially tough opponent. It belongs to the family Hirudinidae, order Arhynchobdellida, and has another name: shui zhi (water leech). Its body is soft, and when attacked it can quickly curl into a ball and roll away to escape. But this creature is greedy when it feeds, and even when we want to drive it off it is extremely difficult. It is brazen because we cannot crush or pinch it to death; most of the time, after struggling to grab it and tear it off our calf, we can only fling it away. The leech cannot be crushed to death either; when thrown into another patch of water, it soon swims gracefully to and fro.

In my hometown, when one person in a relationship pesters the other relentlessly, we say he is as clinging as a leech. Of course, we must admit that love is indeed similar in essence to a leech sucking blood: we sweet-talk and cajole the object of our affection, lulling them—leeches also release toxins when they feed; leech venom contains more than a dozen components, some of which have a paralyzing effect on people. Once our courtship succeeds, we also make our spouse endure some pain, putting up with all the flaws we had hidden before. They must make great sacrifices for us, raise children with us, and bump into each other through the daily grind of oil, salt, soy sauce, vinegar, and tea.

If I had the talent to write songs, I might write a love song about leeches, and then, in a spring field, sit on a small hillside with a cigarette in my mouth and play the guitar and sing it: "Why do you cling to me like a leech, making my love for you so painful? Oh-oh-oh-oh—"

Mammalian blood is the leech's food. Over long evolution, the leech learned to live in symbiosis with mammals. It sucks blood from mammals, but does not make its food feel especially painful, nor does it kill the mammal it feeds on—I have never heard of any person or animal dying from a leech bite. Hirudin is basically harmless; the leech uses hirudin's powerful anticoagulant effect to feed, but hirudin does not poison anyone.

Hirudin is found in the leech's salivary glands and is the most potent thrombin-specific inhibitor yet identified. Bivalirudin (Angiomax), commonly used by clinicians as an anticoagulant in vascular angioplasty and other modern surgical procedures, is a small-molecule peptide based on a compound from the venom of the European medicinal leech (Hirudo medicinalis). It has passed FDA approval and has become one of the common drugs used by American clinicians.

From this it is not hard to understand why leeches can inhibit tumor angiogenesis; many modern TCM physicians use leech as one of their anti-cancer drugs. The mechanism by which leeches fight tumors is very similar to that of expensive bevacizumab; I believe that in the near future pharmaceutical companies will still refine anti-cancer drugs from hirudin to treat tumors.

But human pharmaceutical skill still pales beside the leech's. Many of the drugs we use clinically have major side effects, while the leech's impact on its food when feeding is very small. The leech is a born combination of surgeon and anesthetist: it bites a wound in our calf and feeds from there, while releasing toxins that numb our nerves so we feel no pain; by the time we discover it feeding, the leech has already had a fine meal.

This biochemical master grew up over long natural evolution. Whether animal or plant, every living thing adapted to nature through a long evolutionary process. That part of their ability to adapt to nature can be used to treat disease is no surprise. Our ancestors observed these phenomena and so used plants and animals to treat their illnesses.

Some people think that our ancestors' discovery of the healing effects of certain natural organic or inorganic substances was the result of accidental, random trial. I cannot quite agree; perhaps some drugs were indeed found by this blind-catches-a-random-dead-mouse method, but a considerable number were discovered through human wisdom—the leech being one.

At least a hundred thousand years ago, the brain capacity of our ancestor Homo sapiens was already on a par with modern humans; they observed and thought as we do—if they received our modern education, they could equally get into our universities today. Observing in life that leeches suck blood, then using leeches to treat diseases caused by static blood in the body, should have been a natural step, not a blind stumble as some claim.

Some toxinologists in Europe and America hold that humans have only six hundred years of history using leeches medicinally, which is obviously limited by their narrow outlook. In both the Shennong Bencao Jing (Divine Farmer's Materia Medica) and the Shanghan Lun, leech is already a medicinal, and what it treats is precisely the various diseases caused by static blood—including stroke, tumors, and traumatic injuries. So, from TCM's written record, the history of using leech medicinally is far more than six hundred years.

The Didang Tang in the Shanghan Lun is composed of four herbs: leech (dry-fried) 30 pieces, gadfly (wings and legs removed, dry-fried) 30 pieces, peach kernel (peeled, pointed end removed) 20 pieces, rhubarb (wine-washed) 3 liang. The gadfly here, like the leech, is a blood-sucking animal. In the eyes of toxinologists, the venoms of such blood-sucking animals are much the same, chiefly anticoagulant. Peach kernel also has an anticoagulant effect and a slight laxative effect. Rhubarb not only anticoagulates but also has a strong purgative effect; wine-washed rhubarb has a slightly weakened purgative action. Combined, this formula has both an anticoagulant action and a purgative action, able to expel static blood from the body more quickly. So its formula design is very exquisite, outdoing the modern pharmaceutical approach of taking only a single effect of a drug to treat disease.

Didang Tang is specifically used to treat various static-blood patterns. The Shanghan Lun and Jingui Yaolue record four indications for Didang Tang: