San Antonio stroll identifies characteristics of common allergenic trees

Key takeaways:

• Oak and maple are the most common trees in North America.
• Dutch elm disease caused recent backlogs in extract orders.
• Patients who test positive for queen palm likely are allergic to profilin.

SAN ANTONIO — Tricia Sowers, PhD, introduced attendees to allergenic trees along the River Walk and discussed how immunotherapy can address them at the 16th Annual Allergy, Asthma & Immunology CME Conference.

“Cross reactivity is a huge area,” Sowers, chief scientist at Strategy Lift, told attendees as she led a stroll exploring the flora of the River Walk, which follows the San Antonio River through the city.

Sowers emphasized the importance of allergenicity — which she said refers to how well an allergen binds IgE — how much pollen is produced and the size of the pollen. Also, she said that mild allergens are not going to be very clinically relevant in most cases.

“It’s the moderate use of your allergens that you really want to think about from a patient treatment standpoint,” she said.

Oak trees

The walk began with a red oak tree, which is part of the Fagaceae or beech family. Sowers cited a 2019 paper that said oak and birch have a very high degree of cross reactivity.

“If you look at patient reactions as well as IgE inhibition studies, you can see that if you actually treat a patient with birch allergen and they’re sensitized to oak, that you get significant improvement in clinical symptoms,” she said.

“This is really key,” she continued. “This is a new advancement that can help simplify patients’ treatment vials quite substantially.”

Oak trees constitute the largest biomass and are the most prevalent trees in North America, Sowers explained, with approximately 400 species. Oak also is most clinically relevant for most patients with tree allergies, she said.

“It doesn’t matter what the species is. You can use one oak species to treat any and all of them,” Sowers said. “You don’t need to use a mix.”

Oak also has a protein nitrogen count (PNU) of 40,000 per milliliter, which is approximately standard for most tree species that allergists regularly treat.

Additionally, oak is known for its catkins, which are the long structures that hang down from these trees and store pollen.

“You know that a tree is allergenic if you see those catkins that develop because the wind is able to come through, blow the pollen off that catkin and disperse it everywhere,” Sowers said.

“And if you are in an area where oak trees are, in the early spring, you see everything covered in yellow,” she continued. “It’s because there’s so much pollen that’s produced by these oak trees.”

Depending on the region, oak trees disperse pollen for approximately 3 to 4 weeks, with different species releasing it at different times.

“This is why you get seasonal extension in and among the oak trees,” Sowers said.

Maple trees

Maple trees are the second most common tree in North America, Sowers said, adding that red maples have a PNU count of 38,000 per milliliter. Recent literature indicates that maple and box elder trees are highly cross reactive, she continued.

“But some of the original studies actually show that there are some unique allergens in box elder that you don’t find in maple,” she said.

Practice parameters recommend using a mix of box elder and maple in diagnostics, Sowers said, adding that she is curious to see if there will be updates with recommendations to use just one representative species.

“Box elder is actually going to be a little more potent. So, if you want to select one, I would actually recommend using box elder vs. maple from both the diagnostic standpoint as well as the treatment standpoint,” Sowers said. “You get more bang for your buck.”

Maple trees can be male, female or both, or they can change sexes when needed, depending on the sexes of nearby maple trees.

“In one year, they can be both male and female so that they can reproduce wherever they are and increase their population,” Sowers said. “If they have a lot of trees, then perhaps they’ll just be male, so that they can put out the pollen and it can spread.”

Maple trees are pollinated by insects and by the wind.

“Normally with a tree, you can tell if it’s going to be an allergen or not based off of if you see bees landing on them or not. So, if you see a bunch of bees that are in and around a plant species, you can assume it’s probably not an allergen,” Sowers said.

That is not the case with maples, she said. Maples produce hanging flower clusters with an extended stamen that allows the wind to pick up and disperse pollen.

“Insects are important as well, but not as important as the wind dispersal,” Sowers said.

Cedar elm

American elm is the most prevalent elm tree across the United States, particularly in the suburbs of the Northeast, where it was planted due to its rapid and large growth. But it also is susceptible to Dutch elm disease.

“It has devastated the elm population,” Sowers said, adding that these losses caused the back orders for elm extracts over the past couple of years. “They couldn’t make the extracts that you needed to have within your clinic.”

Cedar elm resists Dutch elm disease, along with Siberian and Chinese elm, which are part of the same genus.

“You can actually substitute one for the other without having any issues from a treatment standpoint, safety profile standpoint, or from a symptom improvement perspective,” Sowers said.

Most elm species pollinate in the spring, but cedar elm also pollinates in the fall.

“This is important to know,” Sowers said.

Cedar elms can be male, which produces pollen, or they can be female or both.

“If you’re looking at a female tree, you may not see any pollen structures,” Sowers said.

Also, cedar elm has a PNU count of 36,000 per milliliter. Patients with cedar elm and oak allergies may have similar skin prick test wheal sizes and sensitivity.

“If an extract is actually stronger, it has the opportunity to help elicit a stronger skin prick test response that may not actually be correlated with the degree of sensitization of the patient,” Sowers said.

Bald cypress

There are three different species of bald cypress in the United States, with a PNU count of 8,000 per milliliter.

“It has very low protein concentration,” Sowers said, making it difficult to compare its SPTs with those of other batch tree species due to the reduced sensitivity.

Cone structures shaped like little balls that hang from these trees carry their pollen. Bald cypress also is plentiful in San Antonio.

“You have a patient that comes in, is skin tested, and they don’t have a large wheal response,” Sowers said. “But everything tells you they spend all of their days down on the Riverwalk in San Antonio. You know that bald cypress is actually going to be driving some of your symptoms.”

Practice parameters for treatment indicate a dose of 0.5 mL of extract per 5 mL dose, Sowers said.

“But if you know that an extract has a lower potency, you may need to use a little bit more to actually get the same symptom improvement than you would with other species,” she said.

Juniper and mountain cedar have similarly low PNU counts, she said.

“If you just stick with utilizing 0.5 mL, in many cases, you won’t get symptom improvement,” she said.

Palm trees

Sowers classified palm trees as a mild to moderate allergen with a PNU count of 26,000 per milliliter. She also said that clinicians can use queen palm as a diagnostic tool.

“If you live in an area where you don’t have palm trees everywhere, you can actually test your patients for queen palm,” she said.

Sowers noted that 20% of the patient population is sensitized to pan-allergens instead of individual species.

“You don’t need to be putting everything into a patient vial,” she said. “If it’s profilin, which is one of the primary proteins in the palm tree, then perhaps you could be only utilizing an extract that is high in profilin concentration and actually be able to treat a patient that way.”

Profilin is found in every single pollen species, she continued, regardless of what it is, where it is expressed or where it lives. Also, she said, queen palm extract is available from all the extract manufacturers.

“If they test positive for all trees, weeds and grasses, and they test positive for queen palm, this signifies that the patient is likely profilin allergic,” she said. “It gives you a better and deeper understanding of how to go about approaching it clinically.”

Additionally, Sowers noted that queen palm extract includes 10 µg/mL of profilin, and timothy grass has 15 µg/mL.

“If you had a patient that you see is profilin allergic, you could consider using timothy grass,” she said.

Tree of heaven

Sowers called tree of heaven a “highly invasive species” that grows “like a weed.” With 6 feet of growth each year, it is the fastest growing tree in North America.

Also, Sowers classified tree of heaven as a mild allergen that produces less pollen than other trees. But with a PNU count of 76,000 per milliliter, it still can provoke symptoms in highly sensitized individuals.

“You could expect to see a potentially bigger skin prick test response utilizing the tree of heaven vs. some of the other skin prick test diagnostics,” she said. “You may have patients that come in thinking they’re sensitized.”