Vetch is not vetch is not vetch. Poison ivy is not an ivy, and poison oak is not an oak. Yes, names for common plants can be delightfully whimsical and inconsistent, but there’s nothing delightful about the toxins in these plants. Especially because these toxins do not play well with our immune systems. Let’s review two situations that can be quite serious, for our livestock and for us.
First, a potentially toxic vetch. In the biological world, lots of plant species bear the name “vetch,” and quite a few of them are used as forages. One popular forage is hairy vetch (Vicia villosa) – the primary forage vetch in the U.S. It’s planted extensively as a cover crop and for hay and grazing. But hairy vetch also has a toxic dark side.
As a forage plant, hairy vetch is a palatable annual legume with a shallow taproot, tiny compound leaves, clusters of purple flowers and, of course, hairy stems and leaves. It is very tolerant of cold weather and a wide range of rainfall and soil pH conditions, although it does best in well-drained soils. Hairy vetch has no rhizomes or stolons, but it is a viny plant that likes to climb. It’s very good at using grasses to support it like a scaffold. Farmers often plant hairy vetch with high-yielding grasses like small grains or annual ryegrass. If properly managed, this combination can result in a huge mass of forage – which is both good and bad. Good because it’s a huge mass of forage. Bad because there’s so much forage and it’s so thick it can plug up machinery. But either way, hairy vetch boasts one other attractive feature: It does not cause legume bloat.
Toxicity
Now the toxicity. This has been well documented in cattle and horses, but interestingly, not in sheep or goats. After grazing hairy vetch for at least two weeks, cattle can become unthrifty, showing symptoms of hair loss, coughing, ulcers in the membranes around the mouth, reduced feed intake and occasionally red-tinged urine and sporadic abortions. Older cattle tend to be more susceptible than heifers or weaned calves. The prognosis for cattle with severe symptoms is grim; their mortality rate can approach 100%.
Toxicity problems in horses are less common, and their symptoms are somewhat different than cattle. While horses also become unthrifty, they can suffer a general dermatitis, hair loss, edema (swelling) around the lips and eyes, cornea ulcerations and scours. There are no effective treatments for the toxicosis in either livestock species. Veterinarians simply try to help the animal get through the episode and feel better.
An unnerving point about this toxicity is that we don’t know the cause. Researchers have not identified any specific toxic substance or a clear mechanism of action. But the observation that symptoms are so disparate and occur across so many physiological systems may give us a clue – that hairy vetch toxicity may be a hyperactive response of the immune system, either to a specific compound in the plant or to a small plant molecule that attaches itself to an internal protein, causing the immune system to launch into overdrive.
One comforting thing about hairy vetch, however, is that its toxicity is not that common. In practice, most cattle graze hairy vetch or consume the hay and show no symptoms, especially if the vetch does not constitute a high proportion of the diet. And that gives us a practical strategy: We should be aware of the potential risks and then watch our animals. At the first signs of symptoms, move the livestock off that field or stop feeding that hay. The bottom line is that including hairy vetch in a seed mixture can be a good agronomic choice, but we should do it with knowledge.
Now let’s switch gears to another plant, or rather, a notorious family of plants: poison ivy, poison oak and their cousin, poison sumac. Here’s where we can get up close and personal. How many of you have firsthand experience with this toxicity? You’re not alone. This is perhaps the most common toxic plant problem in the U.S. – millions of cases occur every year – from young children and gardeners to outdoor folks like hikers and Forest Service employees.
Urushiol
So, what is it? Poison ivy (Toxicodendron radicans), poison oak (T. diversilobum) and poison sumac (T. vernix) all belong to the same genus, and all produce the same toxin: urushiol. All three species are native to North America. Captain John Smith encountered poison ivy in the Jamestown colony in the early 1600s and misnamed it after the look-alike English ivy of his home country. Early Mexican settlers in California knew all about the ivy-like poison oak and described it in their writings. And these three plants are not alone in producing urushiol. They are accompanied by a couple of species in Malaysia and China, and also the Japanese lacquer tree (T. vernicifluum). In fact, the word “urushiol” is derived from the Japanese word “urushi,” meaning a beautiful lacquer made from the sap of this tree.
"Urushiol.” It’s a graceful, delicate word for such a malevolent molecule. Urushiol occurs in the resinous sap on all the surfaces of these plants: leaves, stems, roots and flowers. Although urushiol is not volatile, it can be carried in dust particles and in the ash and smoke from campfires and burn piles – a particularly dangerous situation because the toxin can then enter the lungs. Also, urushiol does not dissipate when the plant dies; it can remain toxic on dead plant surfaces for years. I can testify to this from personal experience.
But urushiol’s mode of action is truly fascinating. When a person brushes against a leaf or stem, a tiny bit of sap clings to the skin (or clothing, which can ultimately touch the skin). This sap contains urushiol. Within a few hours, this molecule penetrates the epithelial cells and binds to specialized white blood cells in the epidermis called “Langerhans cells.” These cells migrate to a nearby lymph node and activate some helper T cells that release cytokines, which are communication molecules. These cytokines effectively shout, “Help, help! All points alert!” which attracts a battalion of macrophages and killer T cells into the area. These cells, especially the killer T cells, then release destructive enzymes and other cell-toxic proteins throughout that region of the skin. Fluid oozes from tiny blood vessels, and skin tissues die. Within a few days, the area shows the well-known symptoms of poison ivy toxicity: rash, itching, redness and small blisters.
Essentially, the urushiol toxin causes a delayed allergic reaction; it unleashes an immunological process that destroys skin tissue. Like many allergies, some people are sensitive and some are not, but this status is not written in stone. Over time, many people will develop sensitivity when exposed multiple times. And only a tiny amount – a mere 2 micrograms – of urushiol can cause a reaction. That’s not very much. (1 microgram = 1 millionth of a gram.) In comparison, a grain of salt weighs 60 micrograms.
Fighting back
On a practical level, what should you do when exposed? The most effective strategy is to remove the urushiol immediately while it’s still on the skin surface, before it penetrates the epidermis. Once this molecule reaches the Langerhans cells, the game is over; the immune system kicks into action, and skin damage occurs. This only takes a couple of hours. You can look online and find dozens of recommendations about how to deal with the poison. Some folks, for example, use lots of soap and water. But in reality, soap is usually not enough. Sometimes it can actually spread the sap to other areas. (Again, I can testify to this from personal experience.) Personally, I prefer to apply the commercial product Tecnu within an hour and follow the directions religiously.
Livestock, however, seem immune to its effects. Sheep, cattle, horses and deer can happily graze these plants and show no symptoms. Their immune systems apparently do not react to the urushiol. I’ve never seen cows or horses lining up in drugstores to buy Tecnu. But here’s a practical warning: Sheep can carry plant particles in their wool; ask any shearer. Again, I can testify to this from personal experience.
So, there we have it: two toxins, two plant families and two very different syndromes (one systemic, one localized). But these toxins have one common strategy: They manipulate the immune system. Subtle devils.
We can learn a thing or two from these plants. With hairy vetch, we should plant it with knowledge. With the poison ivy trio, we should follow the sing-song warning that is still very true: “Leaves of three, let it be; berries white, poisonous sight.”
But in a larger sense, we should recognize that these plants have developed toxins that manipulate the immune system. How did that scary mechanism evolve? And urushiol is a toxin that is almost ironic. Its name comes from a Japanese word, and the urushiol molecule invokes a biochemical mechanism that resembles the Japanese martial arts style of aikido, which strives to redirect the energy of an enemy against itself.
