Poison Plants

It starts with an itch. And then it gets itchier and itchier and then you know, you were exposed to poison ivy, oak, or sumac. 

There are many myths and misunderstandings that persist about these plants, so let's scratch the itch to learn more.

Both poison ivy and poison sumac are widely distributed here in New England. The old adage, "leaves of three, let them be," works great for poison ivy and poison oak (the latter is not typically found in New England), but poison sumac can have several different leave orientations. 

The plants are all in the toxicodendron family and include other species such as the Chinese Lacquer tree, and they all produce an oil called urushiol which is found in all parts of the plant. 

The chemical components of the urushiols are low in molecular weight (i.e., small) and easily penetrate the epidermis. They’re picked up by cells that live in the skin (dendritic cells and Langerhans cells), which then travel to the lymph nodes and activate the immune system via T-cells. The activated T-cells stimulate an inflammatory response which then results in the typical rash. That process takes several days on the first exposure—and the rash can take a week or more to show up, but once sensitized, the "exposure to rash" time can be as short as 12 to 48 hours. 

Here are some important facts about urushiol:

  • The oils are rapidly absorbed into the skin, within minutes in some cases.
  • Urushiol can be transferred to tools, clothes, or anything really, and can remain allergenic for up to 5 years.
  • Urushiol can penetrate rubber and latex gloves, so heavy-duty vinyl or leather gloves are best for protection if you plan on being in contact with the plants or oils.
  • Burned plants can make the urushiols airborne, and can land on the skin. This can also potentially lead to airway involvement.

Image 1 used with permission of the American Academy of Dermatology National Library of Dermatologic Teaching Slides.
The rash is often identified as linearly oriented lines of redness that may develop fluid filled blisters.  There will also typically be some swelling and additional redness, but the key is intense itching. The rash can appear to spread (see myths below), and may worsen over several days, but tends to peak in about 10-14 days and then gradually resolve on its own.

Those are some facts, here are some myths:

  • The rash “spreads.” After the initial contact, the only way the rash will spread is from subsequent exposure to the urushiol oils or if you have the oils on your hands and expose other parts of your body. Most often, the rash evolves over a few days as different parts of the skin can react slower or faster than others (based on the amount of oils and/or the thickness of the skin). This is why the rash may appear to spread, but it doesn’t extend beyond areas that the oils touched. 
  • Fluid in the blisters can cause the rash to spread. The blisters are filled with inflammatory fluid and, if draining, will not cause the rash to spread to other parts of the skin, nor will it cause the rash to be transmitted to others. 
  • Over the counter steroid creams, like hydrocortisone can help the rash clear up faster. Low-potency steroid creams don’t help much, and even high potency creams do little once blisters develop. 
  • Anti-itch creams and over-the-counter anti-histamines like Benadryl (diphenhydramine), can help with the intense itching. The reaction is not based on a histamine process, so anti-histamine medications do nothing other than perhaps make a person tired. 

So if hydrocortisone and Benadryl don't help, what can be done?

As with many illnesses and injuries, prevention is the best approach and avoiding contact with poison ivy and sumac dermatitis is no different. 

Being able to identify the plants is key to avoidance, but if you’re dumb like me, you don’t notice the plants until you’re knee deep in them.  That being said, it's good to scan the foliage before diving in. On a recent trail work day, one of the guys I was working with only realized the poison ivy after he had been digging in the area for almost an hour. Oops. 

Protective clothing is good, especially if you’re doing trail work or otherwise anticipating prolonged possible contact, but remember that the oils can get on to the clothing (and anyting else), risking possible repeat exposures. And again, the oils can cause a reaction even if they've been on clothes or tools for years. Barrier creams are sometimes advertised, and some people seem to love them, but I’ve not seen any products that are clinically proven to be beneficial.  

After avoidance, the best thing to do is to wash the oils away, as soon as possible.

UpToDate has the following recommendations regarding washing:

Washing — After a known exposure, patients should remove any contaminated clothing and wash the whole body with mild soap or dish soap on a damp washcloth under very warm or hot running water as soon as possible. One study found that after approximately 10 minutes on the skin, 50 percent of the urushiol can be removed. This number falls to 10 percent after 30 minutes and 0 percent after 1 hour. 

Despite this, washing even two hours after exposure significantly reduces the likelihood and severity of dermatitis. Some clinicians suggest washing the entire body three times while always wiping in one direction, not back and forth; this seems to reduce irritation and help remove the oils. If there is no rapid access to dishwashing liquid, plain water can be used to wipe the skin in the same fashion. This will at least remove some of the resin. 

Comparison of dishwashing liquid with more expensive products made for removing poison ivy oils did not show a difference in effectiveness. Clothing, tools, or other items that may be contaminated with the oleoresin should also be washed with warm, soapy water prior to reuse. 

In summary, you'll want to wash off as soon as possible, but even if you try to wash the oils off with 10 minutes, it's still possible to have a reaction, and, dish soap appears to be as effective as products marketed specifically for poison ivy oils. 

If you do get the rash, home remedies that can help include:

If the rash is really bad, and/or widespread, the best treatment is a course of an oral steroid such as prednisone. In some situations, people can be on a short course of steroids or a rapid taper, but for poison ivy, the course is about 21 days long. If the prescription is shorter, there's a good chance the rash will come back. 

For a health care provider, it's an easy diagnosis to make, and many people know what they've got because they've been in the woods (or their yard). 

In some cases, a bacterial infection occurs because bacteria find their way into small breaks in the skin, so it's important to keep the rash clean and limit the scratching. Don't use antibiotic ointments as they can irritate the skin and lead to more inflammation. 

And, because the world is just getting more dangerous. we have this to think about: Why poison ivy loves climate change (NPR)

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