Shawn Afvari, BS

With the growing popularity of water sports and a proliferation of invasive species, human injuries from marine animal envenomation continue to rise.^1-3 Members of the scorpionfish family Scorpaenidae are second only to stingrays as the leading cause of the 40,000 to 50,000 injuries annually from marine life worldwide.⁴ Because scorpionfish represent a growing threat and competition with native species, it has been suggested that they could replace endangered species on restaurant menus.^5-8 Scorpionfish have been introduced by humans from tropical to temperate seas and are now common off the coast of California and the eastern coast from New York to Florida, as well as in the Caribbean, the Bahamas, and off the southern coast of Brazil. Victims of scorpionfish stings experience considerable pain and may require days to weeks to fully recover, highlighting the socioeconomic costs and burden of scorpionfish envenomation.^9,10 Fishers, divers, swimmers, and aquarium owners are most often affected.

Family

The common term scorpionfish refers to both the family Scorpaenidae and the genus Scorpaena. Members of this family possess similar dorsal, anal, and pelvic fins, though they vary between genera in their size and the potency of the venom they insulate. Other familiar members include the genus Pterois (lionfish) and Synanceja (stonefish). Synanceja are the most venomous within the group, but scorpionfish stings more commonly arise from Pterois and Scorpaena.⁸ Because of the rare shapes and vibrant colors of scorpionfish, some traders and aquarium owners will seek and pay high prices for these fish, providing further opportunity for envenomation.^11,12

Characteristics

Scorpionfish have with a high variation in color, ranging from lighter grays to intense reds depending on their geographic location and habitat. Synanceja are bland in coloration, blending in with rocks and gravel, but the more dramatic-appearing Scorpaena exhibit a large cranium and wide range of multicolored patterns (Figure 1).¹³Pterois serve as the most conspicuous member of the group with brightly colored red and white stripes (Figure 2). Scorpionfish commonly grow up to 19 inches long and boast 12 dorsal, 2 pelvic, and 3 anal spines housing 5 to 10 mg of venom.¹⁴ An integumentary sheath encapsulates each spine housing the glandular tissue that produces the potent venom.

Photograph courtesy of Sylvain Le Bris (https://www.inaturalist.org/). Republished under the Creative Commons Attribution (CC BY-NC 4.0).

Toxin Properties

Unlike Pterois and Synanceja, Scorpaena do not have venom ducts around their glands, complicating the work of marine biologists aiming to extract and study the venomous toxins. Several studies have managed to isolate scorpionfish venom and overcome its unstable heat-labile nature to investigate its biologic properties.^15-20 Several high-molecular-weight proteins (50–800 kDa) comprise the venom, including hyaluronidase, integrin-inhibiting factors, capillary permeability factor, proteases, and some less-understood cytolytic toxins. These factors provoke the inflammatory, proteolytic, hemorrhagic, cardiovascular, and hemolytic biologic activities at both the local and systemic levels, directing damage to wounded tissues and inducing vascular and tissue permeability to reach cellular processes far and wide. Mediators of inflammation include tumor necrosis factor, IL-6, and monocyte chemoattractant protein 1, followed by neutrophils and other mononuclear cells, initiating the immune response at the wound site. Toxin potency remains for up to 2 days after fish death.¹

Photograph courtesy of 808_Diver (https://www.inaturalist.org/). Republished under the Creative Commons Attribution (CC BY-NC 4.0).

Clinical Manifestation

Physicians may be guided by clinical symptoms in identifying scorpionfish stings, as the patient may not know the identity of their marine assailant. Initially, individuals punctured by scorpionfish spikes will experience an acute pain and burning sensation at the puncture site that may be accompanied by systemic symptoms such as nausea, vomiting, diarrhea, tachycardia, hypotension, loss of consciousness, difficulty breathing, and delirium.^9,21-23 The pain will intensify and radiate distal to the site of envenomation, and the wound may exhibit vesiculation, erythema, bruising, pallor, and notable edema.^4,24 Pain intensity peaks at 30 to 90 minutes after envenomation, and other systemic symptoms generally last for 24 to 48 hours.²⁵ If patients do not seek prompt treatment, secondary infection may ensue, and the lingering venom in the blister may cause dermal necrosis, paresthesia, and anesthesia. Chronic sequelae may include joint contractures, compartment syndrome, necrotic ulcers, and chronic neuropathy.¹

Management

Treatment of scorpionfish stings primarily is palliative and aimed at symptom reduction. Patients should immediately treat wounds with hot but not scalding water immersion.^26,27 Given the thermolabile components of scorpionfish venom, the most effective treatment is to soak the affected limb in water 42 °C to 45 °C for 30 to 90 minutes. Any higher temperature may pose risk for scalding burns. Children should be monitored throughout treatment.²⁸ If hot water immersion does not provide relief, oral analgesics may be considered. Stonefish antivenom is available and may be used for any scorpionfish sting given the shared biologic properties between genera. Providers evaluating stings could use sterile irrigation to clean wounds and search for foreign bodies including spine fragments; probing should be accomplished by instruments rather than a gloved finger. Providers should consider culturing wounds and prescribing antibiotics for suspected secondary infections. A tetanus toxoid history also should be elicited, and patients may have a booster administered, as indicated.²⁹

With the growing popularity of water sports and a proliferation of invasive species, human injuries from marine animal envenomation continue to rise.^1-3 Members of the scorpionfish family Scorpaenidae are second only to stingrays as the leading cause of the 40,000 to 50,000 injuries annually from marine life worldwide.⁴ Because scorpionfish represent a growing threat and competition with native species, it has been suggested that they could replace endangered species on restaurant menus.^5-8 Scorpionfish have been introduced by humans from tropical to temperate seas and are now common off the coast of California and the eastern coast from New York to Florida, as well as in the Caribbean, the Bahamas, and off the southern coast of Brazil. Victims of scorpionfish stings experience considerable pain and may require days to weeks to fully recover, highlighting the socioeconomic costs and burden of scorpionfish envenomation.^9,10 Fishers, divers, swimmers, and aquarium owners are most often affected.

Family

The common term scorpionfish refers to both the family Scorpaenidae and the genus Scorpaena. Members of this family possess similar dorsal, anal, and pelvic fins, though they vary between genera in their size and the potency of the venom they insulate. Other familiar members include the genus Pterois (lionfish) and Synanceja (stonefish). Synanceja are the most venomous within the group, but scorpionfish stings more commonly arise from Pterois and Scorpaena.⁸ Because of the rare shapes and vibrant colors of scorpionfish, some traders and aquarium owners will seek and pay high prices for these fish, providing further opportunity for envenomation.^11,12

Characteristics

Scorpionfish have with a high variation in color, ranging from lighter grays to intense reds depending on their geographic location and habitat. Synanceja are bland in coloration, blending in with rocks and gravel, but the more dramatic-appearing Scorpaena exhibit a large cranium and wide range of multicolored patterns (Figure 1).¹³Pterois serve as the most conspicuous member of the group with brightly colored red and white stripes (Figure 2). Scorpionfish commonly grow up to 19 inches long and boast 12 dorsal, 2 pelvic, and 3 anal spines housing 5 to 10 mg of venom.¹⁴ An integumentary sheath encapsulates each spine housing the glandular tissue that produces the potent venom.

Photograph courtesy of Sylvain Le Bris (https://www.inaturalist.org/). Republished under the Creative Commons Attribution (CC BY-NC 4.0).

Toxin Properties

Unlike Pterois and Synanceja, Scorpaena do not have venom ducts around their glands, complicating the work of marine biologists aiming to extract and study the venomous toxins. Several studies have managed to isolate scorpionfish venom and overcome its unstable heat-labile nature to investigate its biologic properties.^15-20 Several high-molecular-weight proteins (50–800 kDa) comprise the venom, including hyaluronidase, integrin-inhibiting factors, capillary permeability factor, proteases, and some less-understood cytolytic toxins. These factors provoke the inflammatory, proteolytic, hemorrhagic, cardiovascular, and hemolytic biologic activities at both the local and systemic levels, directing damage to wounded tissues and inducing vascular and tissue permeability to reach cellular processes far and wide. Mediators of inflammation include tumor necrosis factor, IL-6, and monocyte chemoattractant protein 1, followed by neutrophils and other mononuclear cells, initiating the immune response at the wound site. Toxin potency remains for up to 2 days after fish death.¹

Photograph courtesy of 808_Diver (https://www.inaturalist.org/). Republished under the Creative Commons Attribution (CC BY-NC 4.0).

Clinical Manifestation

Physicians may be guided by clinical symptoms in identifying scorpionfish stings, as the patient may not know the identity of their marine assailant. Initially, individuals punctured by scorpionfish spikes will experience an acute pain and burning sensation at the puncture site that may be accompanied by systemic symptoms such as nausea, vomiting, diarrhea, tachycardia, hypotension, loss of consciousness, difficulty breathing, and delirium.^9,21-23 The pain will intensify and radiate distal to the site of envenomation, and the wound may exhibit vesiculation, erythema, bruising, pallor, and notable edema.^4,24 Pain intensity peaks at 30 to 90 minutes after envenomation, and other systemic symptoms generally last for 24 to 48 hours.²⁵ If patients do not seek prompt treatment, secondary infection may ensue, and the lingering venom in the blister may cause dermal necrosis, paresthesia, and anesthesia. Chronic sequelae may include joint contractures, compartment syndrome, necrotic ulcers, and chronic neuropathy.¹

Management

Treatment of scorpionfish stings primarily is palliative and aimed at symptom reduction. Patients should immediately treat wounds with hot but not scalding water immersion.^26,27 Given the thermolabile components of scorpionfish venom, the most effective treatment is to soak the affected limb in water 42 °C to 45 °C for 30 to 90 minutes. Any higher temperature may pose risk for scalding burns. Children should be monitored throughout treatment.²⁸ If hot water immersion does not provide relief, oral analgesics may be considered. Stonefish antivenom is available and may be used for any scorpionfish sting given the shared biologic properties between genera. Providers evaluating stings could use sterile irrigation to clean wounds and search for foreign bodies including spine fragments; probing should be accomplished by instruments rather than a gloved finger. Providers should consider culturing wounds and prescribing antibiotics for suspected secondary infections. A tetanus toxoid history also should be elicited, and patients may have a booster administered, as indicated.²⁹

With the growing popularity of water sports and a proliferation of invasive species, human injuries from marine animal envenomation continue to rise.^1-3 Members of the scorpionfish family Scorpaenidae are second only to stingrays as the leading cause of the 40,000 to 50,000 injuries annually from marine life worldwide.⁴ Because scorpionfish represent a growing threat and competition with native species, it has been suggested that they could replace endangered species on restaurant menus.^5-8 Scorpionfish have been introduced by humans from tropical to temperate seas and are now common off the coast of California and the eastern coast from New York to Florida, as well as in the Caribbean, the Bahamas, and off the southern coast of Brazil. Victims of scorpionfish stings experience considerable pain and may require days to weeks to fully recover, highlighting the socioeconomic costs and burden of scorpionfish envenomation.^9,10 Fishers, divers, swimmers, and aquarium owners are most often affected.

Family

The common term scorpionfish refers to both the family Scorpaenidae and the genus Scorpaena. Members of this family possess similar dorsal, anal, and pelvic fins, though they vary between genera in their size and the potency of the venom they insulate. Other familiar members include the genus Pterois (lionfish) and Synanceja (stonefish). Synanceja are the most venomous within the group, but scorpionfish stings more commonly arise from Pterois and Scorpaena.⁸ Because of the rare shapes and vibrant colors of scorpionfish, some traders and aquarium owners will seek and pay high prices for these fish, providing further opportunity for envenomation.^11,12

Characteristics

Scorpionfish have with a high variation in color, ranging from lighter grays to intense reds depending on their geographic location and habitat. Synanceja are bland in coloration, blending in with rocks and gravel, but the more dramatic-appearing Scorpaena exhibit a large cranium and wide range of multicolored patterns (Figure 1).¹³Pterois serve as the most conspicuous member of the group with brightly colored red and white stripes (Figure 2). Scorpionfish commonly grow up to 19 inches long and boast 12 dorsal, 2 pelvic, and 3 anal spines housing 5 to 10 mg of venom.¹⁴ An integumentary sheath encapsulates each spine housing the glandular tissue that produces the potent venom.

Photograph courtesy of Sylvain Le Bris (https://www.inaturalist.org/). Republished under the Creative Commons Attribution (CC BY-NC 4.0).

Toxin Properties

Unlike Pterois and Synanceja, Scorpaena do not have venom ducts around their glands, complicating the work of marine biologists aiming to extract and study the venomous toxins. Several studies have managed to isolate scorpionfish venom and overcome its unstable heat-labile nature to investigate its biologic properties.^15-20 Several high-molecular-weight proteins (50–800 kDa) comprise the venom, including hyaluronidase, integrin-inhibiting factors, capillary permeability factor, proteases, and some less-understood cytolytic toxins. These factors provoke the inflammatory, proteolytic, hemorrhagic, cardiovascular, and hemolytic biologic activities at both the local and systemic levels, directing damage to wounded tissues and inducing vascular and tissue permeability to reach cellular processes far and wide. Mediators of inflammation include tumor necrosis factor, IL-6, and monocyte chemoattractant protein 1, followed by neutrophils and other mononuclear cells, initiating the immune response at the wound site. Toxin potency remains for up to 2 days after fish death.¹

Photograph courtesy of 808_Diver (https://www.inaturalist.org/). Republished under the Creative Commons Attribution (CC BY-NC 4.0).

Clinical Manifestation

Physicians may be guided by clinical symptoms in identifying scorpionfish stings, as the patient may not know the identity of their marine assailant. Initially, individuals punctured by scorpionfish spikes will experience an acute pain and burning sensation at the puncture site that may be accompanied by systemic symptoms such as nausea, vomiting, diarrhea, tachycardia, hypotension, loss of consciousness, difficulty breathing, and delirium.^9,21-23 The pain will intensify and radiate distal to the site of envenomation, and the wound may exhibit vesiculation, erythema, bruising, pallor, and notable edema.^4,24 Pain intensity peaks at 30 to 90 minutes after envenomation, and other systemic symptoms generally last for 24 to 48 hours.²⁵ If patients do not seek prompt treatment, secondary infection may ensue, and the lingering venom in the blister may cause dermal necrosis, paresthesia, and anesthesia. Chronic sequelae may include joint contractures, compartment syndrome, necrotic ulcers, and chronic neuropathy.¹

Management

Treatment of scorpionfish stings primarily is palliative and aimed at symptom reduction. Patients should immediately treat wounds with hot but not scalding water immersion.^26,27 Given the thermolabile components of scorpionfish venom, the most effective treatment is to soak the affected limb in water 42 °C to 45 °C for 30 to 90 minutes. Any higher temperature may pose risk for scalding burns. Children should be monitored throughout treatment.²⁸ If hot water immersion does not provide relief, oral analgesics may be considered. Stonefish antivenom is available and may be used for any scorpionfish sting given the shared biologic properties between genera. Providers evaluating stings could use sterile irrigation to clean wounds and search for foreign bodies including spine fragments; probing should be accomplished by instruments rather than a gloved finger. Providers should consider culturing wounds and prescribing antibiotics for suspected secondary infections. A tetanus toxoid history also should be elicited, and patients may have a booster administered, as indicated.²⁹

Clinical Importance

Western poison ivy (Toxicodendron rydbergii) is responsible for many of the cases of Toxicodendron contact dermatitis (TCD) reported in the western and northern United States. Toxicodendron plants cause more cases of allergic contact dermatitis (ACD) in North America than any other allergen¹; 9 million Americans present to physician offices and 1.6 million present to emergency departments annually for ACD, emphasizing the notable medical burden of this condition.^2,3 Exposure to urushiol, a plant resin containing potent allergens, precipitates this form of ACD.

An estimated 50% to 75% of adults in the United States demonstrate clinical sensitivity and exhibit ACD following contact with T rydbergii.⁴ Campers, hikers, firefighters, and forest workers often risk increased exposure through physical contact or aerosolized allergens in smoke. According to the Centers for Disease Control and Prevention, the incidence of visits to US emergency departments for TCD nearly doubled from 2002 to 2012,⁵ which may be explained by atmospheric CO₂ levels that both promote increased growth of Toxicodendron species and augment their toxicity.⁶

Cutaneous Manifestations

The clinical presentation of T rydbergii contact dermatitis is similar to other allergenic members of the Toxicodendron genus. Patients sensitive to urushiol typically develop a pruritic erythematous rash within 1 to 2 days of exposure (range, 5 hours to 15 days).⁷ Erythematous and edematous streaks initially manifest on the extremities and often progress to bullae and oozing papulovesicles. In early disease, patients also may display black lesions on or near the rash⁸ (so-called black-dot dermatitis) caused by oxidized urushiol deposited on the skin—an uncommon yet classic presentation of TCD. Generally, symptoms resolve without complications and with few sequalae, though hyperpigmentation or a secondary infection can develop on or near affected areas.^9,10

Taxonomy

The Toxicodendron genus belongs to the Anacardiaceae family, which includes pistachios, mangos, and cashews, and causes more cases of ACD than every other plant combined.⁴ (Shelled pistachios and cashews do not possess cross-reacting allergens and should not worry consumers; mango skin does contain urushiol.)

Toxicodendron (formerly part of the Rhus genus) includes several species of poison oak, poison ivy, and poison sumac and can be found in shrubs (T rydbergii and Toxicodendron diversilobum), vines (Toxicodendron radicans and Toxicodendron pubescens), and trees (Toxicodendron vernix). In addition, Toxicodendron taxa can hybridize with other taxa in close geographic proximity to form morphologic intermediates. Some individual plants have features of multiple species.¹¹

Etymology

The common name of T rydbergii—western poison ivy—misleads the public; the plant contains no poison that can cause death and does not grow as ivy by wrapping around trees, as T radicans and English ivy (Hedera helix) do. Its formal genus, Toxicodendron, means “poison tree” in Greek and was given its generic name by the English botanist Phillip Miller in 1768,¹² which caused the renaming of Rhus rydbergii as T rydbergii. The species name honors Per Axel Rydberg, a 19th and 20th century Swedish-American botanist.

Distribution

Toxicodendron rydbergii grows in California and other states in the western half of the United States as well as the states bordering Canada and Mexico. In Canada, it reigns as the most dominant form of poison ivy.¹³ Hikers and campers find T rydbergii in a variety of areas, including roadsides, river bottoms, sandy shores, talus slopes, precipices, and floodplains.¹¹ This taxon grows under a variety of conditions and in distinct regions, and it thrives in both full sun or shade.

Toxicodendron rydbergii turns red earlier than most plants; early red summer leaves should serve as a warning sign to hikers from a distance (Figure 1). It displays trifoliate ovate leaves (ie, each leaf contains 3 leaflets) on a dwarf nonclimbing shrub (Figure 2). Although the plant shares common features with its cousin T radicans (eastern poison ivy), T rydbergii is easily distinguished by its thicker stems, absence of aerial rootlets (abundant in T radicans), and short (approximately 1 meter) height.⁴

Photograph courtesy of Thomas W. McGovern, MD.

Curly hairs occupy the underside of T rydbergii leaflets and along the midrib; leaflet margins appear lobed or rounded. Lenticels appear as small holes in the bark that turn gray in the cold and become brighter come spring.¹³

Photograph courtesy of Thomas W. McGovern, MD.

The plant bears glabrous long petioles (leaf stems) and densely grouped clusters of yellow flowers. In autumn, the globose fruit—formed in clusters between each twig and leaf petiole (known as an axillary position)—change from yellow-green to tan (Figure 3). When urushiol exudes from damaged leaflets or other plant parts, it oxidizes on exposure to air and creates hardened black deposits on the plant. Even when grown in garden pots, T rydbergii maintains its distinguishing features.¹¹

“Western poison ivy” by Whitney Cranshaw is licensed under CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/us/).

Dermatitis-Inducing Plant Parts

All parts of T rydbergii including leaves, stems, roots, and fruit contain the allergenic sap throughout the year.¹⁴ A person must damage or bruise the plant for urushiol to be released and produce its allergenic effects; softly brushing against undamaged plants typically does not induce dermatitis.⁴

Pathophysiology of Urushiol

Urushiol, a pale yellow, oily mixture of organic compounds conserved throughout all Toxicodendron species, contains highly allergenic alkyl catechols. These catechols possess hydroxyl groups at positions 1 and 2 on a benzene ring; the hydrocarbon side chain of poison ivies (typically 15–carbon atoms long) attaches at position 3.¹⁵ The catechols and the aliphatic side chain contribute to the plant’s antigenic and dermatitis-inducing properties.¹⁶

The high lipophilicity of urushiol allows for rapid and unforgiving absorption into the skin, notwithstanding attempts to wash it off. Upon direct contact, catechols of urushiol penetrate the epidermis and become oxidized to quinone intermediates that bind to antigen-presenting cells in the epidermis and dermis. Epidermal Langerhans cells and dermal macrophages internalize and present the antigen to CD4⁺ T cells in nearby lymph nodes. This sequence results in production of inflammatory mediators, clonal expansion of T-effector and T-memory cells specific to the allergenic catechols, and an ensuing cytotoxic response against epidermal cells and the dermal vasculature. Keratinocytes and monocytes mediate the inflammatory response by releasing other cytokines.^4,17

Sensitization to urushiol generally occurs at 8 to 14 years of age; therefore, infants have lower susceptibility to dermatitis upon contact with T rydbergii.¹⁸ Most animals do not experience sensitization upon contact; in fact, birds and forest animals consume the urushiol-rich fruit of T rydbergii without harm.³

Prevention and Treatment

Toxicodendron dermatitis typically lasts 1 to 3 weeks but can remain for as long as 6 weeks without treatment.¹⁹ Recognition and physical avoidance of the plant provides the most promising preventive strategy. Immediate rinsing with soap and water can prevent TCD by breaking down urushiol and its allergenic components; however, this is an option for only a short time, as the skin absorbs 50% of urushiol within 10 minutes after contact.²⁰ Nevertheless, patients must seize the earliest opportunity to wash off the affected area and remove any residual urushiol. Patients must be cautious when removing and washing clothing to prevent further contact.

Most health care providers treat TCD with a corticosteroid to reduce inflammation and intense pruritus. A high-potency topical corticosteroid (eg, clobetasol) may prove effective in providing early therapeutic relief in mild disease.²¹ A short course of a systemic steroid quickly and effectively quenches intense itching and should not be limited to what the clinician considers severe disease. Do not underestimate the patient’s symptoms with this eruption.

Prednisone dosing begins at 1 mg/kg daily and is then tapered slowly over 2 weeks (no shorter a time) for an optimal treatment course of 15 days.²² Prescribing an inadequate dosage and course of a corticosteroid leaves the patient susceptible to rebound dermatitis—and loss of trust in their provider.

Intramuscular injection of the long-acting corticosteroid triamcinolone acetonide with rapid-onset betamethasone provides rapid relief and fewer adverse effects than an oral corticosteroid.²² Despite the long-standing use of sedating oral antihistamines by clinicians, these drugs provide no benefit for pruritus or sleep because the histamine does not cause the itching of TCD, and antihistamines disrupt normal sleep architecture.^23-25

Patients can consider several over-the-counter products that have varying degrees of efficacy.^4,26 The few products for which prospective studies support their use include Tecnu (Tec Laboraties Inc), Zanfel (RhusTox), and the well-known soaps Dial (Henkel Corporation) and Goop (Critzas Industries, Inc).^27,28

Aside from treating the direct effects of TCD, clinicians also must take note of any look for signs of secondary infection and occasionally should consider supplementing treatment with an antibiotic.

Clinical Importance

Western poison ivy (Toxicodendron rydbergii) is responsible for many of the cases of Toxicodendron contact dermatitis (TCD) reported in the western and northern United States. Toxicodendron plants cause more cases of allergic contact dermatitis (ACD) in North America than any other allergen¹; 9 million Americans present to physician offices and 1.6 million present to emergency departments annually for ACD, emphasizing the notable medical burden of this condition.^2,3 Exposure to urushiol, a plant resin containing potent allergens, precipitates this form of ACD.

An estimated 50% to 75% of adults in the United States demonstrate clinical sensitivity and exhibit ACD following contact with T rydbergii.⁴ Campers, hikers, firefighters, and forest workers often risk increased exposure through physical contact or aerosolized allergens in smoke. According to the Centers for Disease Control and Prevention, the incidence of visits to US emergency departments for TCD nearly doubled from 2002 to 2012,⁵ which may be explained by atmospheric CO₂ levels that both promote increased growth of Toxicodendron species and augment their toxicity.⁶

Cutaneous Manifestations

The clinical presentation of T rydbergii contact dermatitis is similar to other allergenic members of the Toxicodendron genus. Patients sensitive to urushiol typically develop a pruritic erythematous rash within 1 to 2 days of exposure (range, 5 hours to 15 days).⁷ Erythematous and edematous streaks initially manifest on the extremities and often progress to bullae and oozing papulovesicles. In early disease, patients also may display black lesions on or near the rash⁸ (so-called black-dot dermatitis) caused by oxidized urushiol deposited on the skin—an uncommon yet classic presentation of TCD. Generally, symptoms resolve without complications and with few sequalae, though hyperpigmentation or a secondary infection can develop on or near affected areas.^9,10

Taxonomy

The Toxicodendron genus belongs to the Anacardiaceae family, which includes pistachios, mangos, and cashews, and causes more cases of ACD than every other plant combined.⁴ (Shelled pistachios and cashews do not possess cross-reacting allergens and should not worry consumers; mango skin does contain urushiol.)

Toxicodendron (formerly part of the Rhus genus) includes several species of poison oak, poison ivy, and poison sumac and can be found in shrubs (T rydbergii and Toxicodendron diversilobum), vines (Toxicodendron radicans and Toxicodendron pubescens), and trees (Toxicodendron vernix). In addition, Toxicodendron taxa can hybridize with other taxa in close geographic proximity to form morphologic intermediates. Some individual plants have features of multiple species.¹¹

Etymology

The common name of T rydbergii—western poison ivy—misleads the public; the plant contains no poison that can cause death and does not grow as ivy by wrapping around trees, as T radicans and English ivy (Hedera helix) do. Its formal genus, Toxicodendron, means “poison tree” in Greek and was given its generic name by the English botanist Phillip Miller in 1768,¹² which caused the renaming of Rhus rydbergii as T rydbergii. The species name honors Per Axel Rydberg, a 19th and 20th century Swedish-American botanist.

Distribution

Toxicodendron rydbergii grows in California and other states in the western half of the United States as well as the states bordering Canada and Mexico. In Canada, it reigns as the most dominant form of poison ivy.¹³ Hikers and campers find T rydbergii in a variety of areas, including roadsides, river bottoms, sandy shores, talus slopes, precipices, and floodplains.¹¹ This taxon grows under a variety of conditions and in distinct regions, and it thrives in both full sun or shade.

Toxicodendron rydbergii turns red earlier than most plants; early red summer leaves should serve as a warning sign to hikers from a distance (Figure 1). It displays trifoliate ovate leaves (ie, each leaf contains 3 leaflets) on a dwarf nonclimbing shrub (Figure 2). Although the plant shares common features with its cousin T radicans (eastern poison ivy), T rydbergii is easily distinguished by its thicker stems, absence of aerial rootlets (abundant in T radicans), and short (approximately 1 meter) height.⁴

Photograph courtesy of Thomas W. McGovern, MD.

Curly hairs occupy the underside of T rydbergii leaflets and along the midrib; leaflet margins appear lobed or rounded. Lenticels appear as small holes in the bark that turn gray in the cold and become brighter come spring.¹³

Photograph courtesy of Thomas W. McGovern, MD.

The plant bears glabrous long petioles (leaf stems) and densely grouped clusters of yellow flowers. In autumn, the globose fruit—formed in clusters between each twig and leaf petiole (known as an axillary position)—change from yellow-green to tan (Figure 3). When urushiol exudes from damaged leaflets or other plant parts, it oxidizes on exposure to air and creates hardened black deposits on the plant. Even when grown in garden pots, T rydbergii maintains its distinguishing features.¹¹

“Western poison ivy” by Whitney Cranshaw is licensed under CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/us/).

Dermatitis-Inducing Plant Parts

All parts of T rydbergii including leaves, stems, roots, and fruit contain the allergenic sap throughout the year.¹⁴ A person must damage or bruise the plant for urushiol to be released and produce its allergenic effects; softly brushing against undamaged plants typically does not induce dermatitis.⁴

Pathophysiology of Urushiol

Urushiol, a pale yellow, oily mixture of organic compounds conserved throughout all Toxicodendron species, contains highly allergenic alkyl catechols. These catechols possess hydroxyl groups at positions 1 and 2 on a benzene ring; the hydrocarbon side chain of poison ivies (typically 15–carbon atoms long) attaches at position 3.¹⁵ The catechols and the aliphatic side chain contribute to the plant’s antigenic and dermatitis-inducing properties.¹⁶

The high lipophilicity of urushiol allows for rapid and unforgiving absorption into the skin, notwithstanding attempts to wash it off. Upon direct contact, catechols of urushiol penetrate the epidermis and become oxidized to quinone intermediates that bind to antigen-presenting cells in the epidermis and dermis. Epidermal Langerhans cells and dermal macrophages internalize and present the antigen to CD4⁺ T cells in nearby lymph nodes. This sequence results in production of inflammatory mediators, clonal expansion of T-effector and T-memory cells specific to the allergenic catechols, and an ensuing cytotoxic response against epidermal cells and the dermal vasculature. Keratinocytes and monocytes mediate the inflammatory response by releasing other cytokines.^4,17

Sensitization to urushiol generally occurs at 8 to 14 years of age; therefore, infants have lower susceptibility to dermatitis upon contact with T rydbergii.¹⁸ Most animals do not experience sensitization upon contact; in fact, birds and forest animals consume the urushiol-rich fruit of T rydbergii without harm.³

Prevention and Treatment

Toxicodendron dermatitis typically lasts 1 to 3 weeks but can remain for as long as 6 weeks without treatment.¹⁹ Recognition and physical avoidance of the plant provides the most promising preventive strategy. Immediate rinsing with soap and water can prevent TCD by breaking down urushiol and its allergenic components; however, this is an option for only a short time, as the skin absorbs 50% of urushiol within 10 minutes after contact.²⁰ Nevertheless, patients must seize the earliest opportunity to wash off the affected area and remove any residual urushiol. Patients must be cautious when removing and washing clothing to prevent further contact.

Most health care providers treat TCD with a corticosteroid to reduce inflammation and intense pruritus. A high-potency topical corticosteroid (eg, clobetasol) may prove effective in providing early therapeutic relief in mild disease.²¹ A short course of a systemic steroid quickly and effectively quenches intense itching and should not be limited to what the clinician considers severe disease. Do not underestimate the patient’s symptoms with this eruption.

Prednisone dosing begins at 1 mg/kg daily and is then tapered slowly over 2 weeks (no shorter a time) for an optimal treatment course of 15 days.²² Prescribing an inadequate dosage and course of a corticosteroid leaves the patient susceptible to rebound dermatitis—and loss of trust in their provider.

Intramuscular injection of the long-acting corticosteroid triamcinolone acetonide with rapid-onset betamethasone provides rapid relief and fewer adverse effects than an oral corticosteroid.²² Despite the long-standing use of sedating oral antihistamines by clinicians, these drugs provide no benefit for pruritus or sleep because the histamine does not cause the itching of TCD, and antihistamines disrupt normal sleep architecture.^23-25

Patients can consider several over-the-counter products that have varying degrees of efficacy.^4,26 The few products for which prospective studies support their use include Tecnu (Tec Laboraties Inc), Zanfel (RhusTox), and the well-known soaps Dial (Henkel Corporation) and Goop (Critzas Industries, Inc).^27,28

Aside from treating the direct effects of TCD, clinicians also must take note of any look for signs of secondary infection and occasionally should consider supplementing treatment with an antibiotic.

Clinical Importance

Western poison ivy (Toxicodendron rydbergii) is responsible for many of the cases of Toxicodendron contact dermatitis (TCD) reported in the western and northern United States. Toxicodendron plants cause more cases of allergic contact dermatitis (ACD) in North America than any other allergen¹; 9 million Americans present to physician offices and 1.6 million present to emergency departments annually for ACD, emphasizing the notable medical burden of this condition.^2,3 Exposure to urushiol, a plant resin containing potent allergens, precipitates this form of ACD.

An estimated 50% to 75% of adults in the United States demonstrate clinical sensitivity and exhibit ACD following contact with T rydbergii.⁴ Campers, hikers, firefighters, and forest workers often risk increased exposure through physical contact or aerosolized allergens in smoke. According to the Centers for Disease Control and Prevention, the incidence of visits to US emergency departments for TCD nearly doubled from 2002 to 2012,⁵ which may be explained by atmospheric CO₂ levels that both promote increased growth of Toxicodendron species and augment their toxicity.⁶

Cutaneous Manifestations

The clinical presentation of T rydbergii contact dermatitis is similar to other allergenic members of the Toxicodendron genus. Patients sensitive to urushiol typically develop a pruritic erythematous rash within 1 to 2 days of exposure (range, 5 hours to 15 days).⁷ Erythematous and edematous streaks initially manifest on the extremities and often progress to bullae and oozing papulovesicles. In early disease, patients also may display black lesions on or near the rash⁸ (so-called black-dot dermatitis) caused by oxidized urushiol deposited on the skin—an uncommon yet classic presentation of TCD. Generally, symptoms resolve without complications and with few sequalae, though hyperpigmentation or a secondary infection can develop on or near affected areas.^9,10

Taxonomy

The Toxicodendron genus belongs to the Anacardiaceae family, which includes pistachios, mangos, and cashews, and causes more cases of ACD than every other plant combined.⁴ (Shelled pistachios and cashews do not possess cross-reacting allergens and should not worry consumers; mango skin does contain urushiol.)

Toxicodendron (formerly part of the Rhus genus) includes several species of poison oak, poison ivy, and poison sumac and can be found in shrubs (T rydbergii and Toxicodendron diversilobum), vines (Toxicodendron radicans and Toxicodendron pubescens), and trees (Toxicodendron vernix). In addition, Toxicodendron taxa can hybridize with other taxa in close geographic proximity to form morphologic intermediates. Some individual plants have features of multiple species.¹¹

Etymology

The common name of T rydbergii—western poison ivy—misleads the public; the plant contains no poison that can cause death and does not grow as ivy by wrapping around trees, as T radicans and English ivy (Hedera helix) do. Its formal genus, Toxicodendron, means “poison tree” in Greek and was given its generic name by the English botanist Phillip Miller in 1768,¹² which caused the renaming of Rhus rydbergii as T rydbergii. The species name honors Per Axel Rydberg, a 19th and 20th century Swedish-American botanist.

Distribution

Toxicodendron rydbergii grows in California and other states in the western half of the United States as well as the states bordering Canada and Mexico. In Canada, it reigns as the most dominant form of poison ivy.¹³ Hikers and campers find T rydbergii in a variety of areas, including roadsides, river bottoms, sandy shores, talus slopes, precipices, and floodplains.¹¹ This taxon grows under a variety of conditions and in distinct regions, and it thrives in both full sun or shade.

Toxicodendron rydbergii turns red earlier than most plants; early red summer leaves should serve as a warning sign to hikers from a distance (Figure 1). It displays trifoliate ovate leaves (ie, each leaf contains 3 leaflets) on a dwarf nonclimbing shrub (Figure 2). Although the plant shares common features with its cousin T radicans (eastern poison ivy), T rydbergii is easily distinguished by its thicker stems, absence of aerial rootlets (abundant in T radicans), and short (approximately 1 meter) height.⁴

Photograph courtesy of Thomas W. McGovern, MD.

Curly hairs occupy the underside of T rydbergii leaflets and along the midrib; leaflet margins appear lobed or rounded. Lenticels appear as small holes in the bark that turn gray in the cold and become brighter come spring.¹³

Photograph courtesy of Thomas W. McGovern, MD.

The plant bears glabrous long petioles (leaf stems) and densely grouped clusters of yellow flowers. In autumn, the globose fruit—formed in clusters between each twig and leaf petiole (known as an axillary position)—change from yellow-green to tan (Figure 3). When urushiol exudes from damaged leaflets or other plant parts, it oxidizes on exposure to air and creates hardened black deposits on the plant. Even when grown in garden pots, T rydbergii maintains its distinguishing features.¹¹

“Western poison ivy” by Whitney Cranshaw is licensed under CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/us/).

Dermatitis-Inducing Plant Parts

All parts of T rydbergii including leaves, stems, roots, and fruit contain the allergenic sap throughout the year.¹⁴ A person must damage or bruise the plant for urushiol to be released and produce its allergenic effects; softly brushing against undamaged plants typically does not induce dermatitis.⁴

Pathophysiology of Urushiol

Urushiol, a pale yellow, oily mixture of organic compounds conserved throughout all Toxicodendron species, contains highly allergenic alkyl catechols. These catechols possess hydroxyl groups at positions 1 and 2 on a benzene ring; the hydrocarbon side chain of poison ivies (typically 15–carbon atoms long) attaches at position 3.¹⁵ The catechols and the aliphatic side chain contribute to the plant’s antigenic and dermatitis-inducing properties.¹⁶

The high lipophilicity of urushiol allows for rapid and unforgiving absorption into the skin, notwithstanding attempts to wash it off. Upon direct contact, catechols of urushiol penetrate the epidermis and become oxidized to quinone intermediates that bind to antigen-presenting cells in the epidermis and dermis. Epidermal Langerhans cells and dermal macrophages internalize and present the antigen to CD4⁺ T cells in nearby lymph nodes. This sequence results in production of inflammatory mediators, clonal expansion of T-effector and T-memory cells specific to the allergenic catechols, and an ensuing cytotoxic response against epidermal cells and the dermal vasculature. Keratinocytes and monocytes mediate the inflammatory response by releasing other cytokines.^4,17

Sensitization to urushiol generally occurs at 8 to 14 years of age; therefore, infants have lower susceptibility to dermatitis upon contact with T rydbergii.¹⁸ Most animals do not experience sensitization upon contact; in fact, birds and forest animals consume the urushiol-rich fruit of T rydbergii without harm.³

Prevention and Treatment

Toxicodendron dermatitis typically lasts 1 to 3 weeks but can remain for as long as 6 weeks without treatment.¹⁹ Recognition and physical avoidance of the plant provides the most promising preventive strategy. Immediate rinsing with soap and water can prevent TCD by breaking down urushiol and its allergenic components; however, this is an option for only a short time, as the skin absorbs 50% of urushiol within 10 minutes after contact.²⁰ Nevertheless, patients must seize the earliest opportunity to wash off the affected area and remove any residual urushiol. Patients must be cautious when removing and washing clothing to prevent further contact.

Most health care providers treat TCD with a corticosteroid to reduce inflammation and intense pruritus. A high-potency topical corticosteroid (eg, clobetasol) may prove effective in providing early therapeutic relief in mild disease.²¹ A short course of a systemic steroid quickly and effectively quenches intense itching and should not be limited to what the clinician considers severe disease. Do not underestimate the patient’s symptoms with this eruption.

Prednisone dosing begins at 1 mg/kg daily and is then tapered slowly over 2 weeks (no shorter a time) for an optimal treatment course of 15 days.²² Prescribing an inadequate dosage and course of a corticosteroid leaves the patient susceptible to rebound dermatitis—and loss of trust in their provider.

Intramuscular injection of the long-acting corticosteroid triamcinolone acetonide with rapid-onset betamethasone provides rapid relief and fewer adverse effects than an oral corticosteroid.²² Despite the long-standing use of sedating oral antihistamines by clinicians, these drugs provide no benefit for pruritus or sleep because the histamine does not cause the itching of TCD, and antihistamines disrupt normal sleep architecture.^23-25

Patients can consider several over-the-counter products that have varying degrees of efficacy.^4,26 The few products for which prospective studies support their use include Tecnu (Tec Laboraties Inc), Zanfel (RhusTox), and the well-known soaps Dial (Henkel Corporation) and Goop (Critzas Industries, Inc).^27,28

Aside from treating the direct effects of TCD, clinicians also must take note of any look for signs of secondary infection and occasionally should consider supplementing treatment with an antibiotic.