Humans come in frequent contact with varying numbers of more than one million varieties of insects that inhabit the planet. A limited number of these cause more than a fleeting annoyance or discomfort. Some insects inject venom into humans as a means of self-protection. Others use the human for a blood meal. Although almost everyone has experienced a bite or sting, only about 1% of pediatric and 3% of adult population experiences an allergic reaction. Insects also transmit some diseases (Table 12-1
A. Arachnida: a class of eight-legged insects that includes scorpions, spiders, ticks, and mites.
a. Found in all continents except Antarctica. In North America, the scorpion is found in Mexico and southwestern states of the United States.
b. Obtaining a length of 9 mm to 20 cm, it has a forebody, six projecting legs, large anterior claws (pedipolyps), and a hind body with a hooked caudal stinger which it uses to inject venom into its prey.
c. Major function is control of insect population.
d. Stings occur most frequently on the legs, thighs, and buttocks of its human victims, often the result of sitting or walking in the scorpion’s territory.
e. Sting produces an immediate sharp burning sensation. Local edema soon surrounds the small puncture wound often followed by skin discoloration. Numbness may extend beyond the puncture site.
f. The only scorpion in the United States with enough venom to cause serious side effects is the bark scorpion found in the southwest desert. Its bite may produce hypersalivation, tachycardia, decreased blood pressure, peripheral motor symptoms, and severe agitation.
g. Children due to their size may have a more severe reaction. Debilitated or immunocompromised adults have increased risks of complications of bites. Scorpion bites in the United States rarely are associated with death.
Black widow spider: Latrodectus mactans
a. Found in the entire continental United States except Alaska.
b. The female is 1.5 cm, glossy black with the characteristic red hourglass on the underside of the abdomen. The male of the species is smaller with four pairs of red markings on the abdomen.
c. Diet consists of insects, centipedes, and other spiders. After mating, the female often ensnares and devours her mate from which she derives the name, black widow.
d. Often resides in yard debris and wood piles. The spider can also be found inside of homes. It is generally not aggressive. It will bite when threatened or when guarding an egg sac.
e. The bite feels like a pinprick and leaves a mildly erythematous 3- to 4-mm papule with a central punctum.
TABLE 12-1 Potential Vector Diseases from Blood-Sucking Insects
Plaque, typhus, tapeworms, filariasis, brucellosis, and melioidosis
Malaria, encephalitis, dengue, and yellow fever
Rickettsial typhus, viral encephalitis, and pasteurella plaque
Oriental sore, Chagas disease, Kala-azar, relapsing fever, and perhaps hepatitis B. Note all of which are rare occurrences
Tick—specifically deer tick
Rocky Mountain spotted fever, Lyme disease, and relapsing fever
(1) The injected venom contains α-latrotoxin, which is a neurotoxin that produces massive presynaptic release of acetylcholine.
(2) Diaphoresis and hypertension may occur.
(3) Severe muscle spasms occur within 1 hour of the bite and may mimic myocardial infarction or an acute abdomen.
Brown recluse spider: Loxosceles reclusa
a. The highest concentration of spider population is found in Midwest and southern states of the United States.
b. The spider has a cephalothorax with a characteristic violin-shaped marking, joined to abdomen, and is light to medium brown in color. This violinshaped marking is not unique to the brown recluse spider. The spider can be identified due to its six eyes as compared to other spiders having eight eyes (two eyes in front and two sets of eyes on the cephalothorax).
c. Spider prefers an insect diet.
Bites occur most frequently on the extremities of its victim (Figure 12-4
(1) Bite may feel like a pinprick or not even noticed by the victim.
FIGURE 12-1. Scorpion: Centruroides sculpturatus. (Courtesy of John K. Randall, MD.)
FIGURE 12-2. Black widow spiders: Latrodectus mactans. (Courtesy of John K. Randall, MD.)
FIGURE 12-3. Brown recluse spider: Loxosceles reclusa. (Courtesy of John K. Randall, MD.)
FIGURE 12-4. Brown recluse spider bite on the left lateral thigh. (From Lugo-Somolinos, A., et al. (2011). VisualDx: Essential dermatology in pigmented skin. Philadelphia, PA: Wolters Kluwer.)
FIGURE 12-5. Chigger bites on the lower legs and ankles. (From Goodheart, H. P. (2003). Goodheart’s photoguide of common skin disorders (2nd ed.). Philadelphia, PA: Lippincott Williams & Wilkins.)
(2) A coagulotoxin is injected at the time of the bite.
(3) Pain develops 2 to 8 hours after the initial bite.
(4) Presentation is very unpredictable often with the presence of a gray-blue halo surrounding the puncture site within 24 hours.
(5) Early signs of necrosis include hyperesthesia, bullae, cyanosis, red-blue ulcer, and/or painful eschar.
(6) Systemic reactions (loxoscelism) can occur in some victims. This may vary from a mild flu-like presentation to anaphylactic shock. Symptoms caused by coagulotoxin may include deep vein thrombosis, pyoderma gangrenosum, intravascular hemolysis, renal failure, pulmonary edema, and cardiac arrhythmias.
FIGURE 12-6. Tick: Ixodidae on the plantar surface toe. (Courtesy of John K. Randall, MD.)
a. Present in most grain-growing regions of the United States. They are most prevalent in the south during the summer and fall. They may be known by many other names such as red bugs, jiggers, harvest mites, harvest lice, and harvest bugs.
b. A barely visible reddish colored mite, the larval stage of the chigger attaches to the skin of the host by means of a hooked mouthpart.
c. It ingests a blood meal while attached to the human host. The mite does not burrow and releases after engorgement.
d. Lesions occur most frequently after contact with infested hay or grains. The mite prefers an area where it can feed undisturbed, especially under constrictive clothing.
e. Pruritic macules or papules up to 5 mm in size are very common after the mite bite. If the individual is sensitized, the eruption may vary from urticarial to a more severe granulomatous reaction. Lesions slowly regress over 1 to 2 months.
a. Classification: Ixodidae—“hard tick” with hard-like shield with the head visible. Argasidae—“soft tick” with leathery skin and the head hidden by the body of the tick.
b. This parasite is widespread, preferring thick vegetation or grasses on which to cling when not in contact with an animal or human host.
c. Most species have three stages in their 2-year life cycle. Larva stage usually hatches in the early spring. The nymph stage in which the tick is about the size of a poppy seed. The adult tick is about the size of an apple seed. Ticks only require one blood meal for each stage of its life cycle. Diseases can be transmitted from tick bites during any of their three life cycle stages.
The tick has four pairs of clawed legs and a specialized mouthpart used for grasping and slicing a hole in its victim to enable the tick to suck blood.
Blood meals are generally provided by small rodents (mice) in the larva stage. Larger mammals (deer and humans) provide the blood meals for the nymph and adult ticks.
FIGURE 12-7. Tick bite with inflammation. (Courtesy of Charles E. Lewis, MD.)
FIGURE 12-8. Rocky Mountain spotted fever rash on the hand and arm. (From Harvey, R. A., & Cornelissen, C. N. (2012). Microbiology. Philadelphia, PA: Wolters Kluwer.)
e. Ticks will attach to any exposed skin and often travels under clothing or around body parts until they reach a constricted area. When the tick attaches to the victim, it emits a cement-like substance to help keep the tick adhered to the host.
Rocky Mountain spotted fever (RMSF; Figure 12-8
) is a rickettsia disease transmitted to humans after the bite of a deer or dog tick (Box 12-1
Lyme disease (Figure 12-9
) is a spirochete Borrelia burgdorferi
infection transmitted to humans generally by the bite of a deer tick (Box 12-2
Insecta: class of six-legged insects, many with wings, which includes bees, wasps, and ants. Hymenoptera is a superorder of the class of insects containing wings. Hymenoptera can cause severe hypersensitivity reactions (Box 12-3
a. Largest of the Hymenoptera, the bumblebee is found throughout the United States and is better adapted to cold regions than other bees.
b. The bumblebee is hairy and usually a black and yellow coloration.
c. The bee is responsible for pollination of a wide variety of plants.
d. Generally not aggressive, often stings when its victim disturbs it while walking in patches of clover.
e. The bee produces one painful, stabbing defensive sting into its victim that may leave behind a barbed stinger. The sting injects formic acid and proteins into its victim. The venom sac is attached to the stinger and will continue to contract and inject additional venom if it is not removed.
f. The skin lesion is an erythematous papule, which becomes urticarial and edematous. It may remain painful for hours.
a. Found in all of the United States, the honeybee is found in any area with flowers and fruit.
b. Smaller and somewhat similar in appearance to the bumblebee. It is generally yellow, brown, or gray in color. It measures 1 to 1.5 cm in length.
FIGURE 12-9. Lyme disease; erythema migrans. (From Goodheart, H. P. (2003). Goodheart’s photoguide of common skin disorders (2nd ed.). Philadelphia, PA: Lippincott Williams & Wilkins.)
c. Very important to pollination, the honeybee is generally not aggressive, unless it perceives a threat.
(1) Since experiments in Brazil sought to cross the native honeybee with an African cousin, a much more aggressive bee has developed.
(2) The Africanized honeybee migrated into the United States in the early 1990s.
(3) This aggressive bee is capable of chasing a fleeing victim for up to one quarter of a mile.
(4) The sting of this bee is no more venomous than the native honeybee.
d. If the entire hive is disturbed, a swarm of insects may attack, stinging any body surface. The honeybee may leave a barbed stinger with a venom sac that may continue to inject venom.
e. The sting produces an edematous erythematous papule.
a. The paper wasp lives in communities, while the mud dauber, potter wasp, and digger wasp are solitary.
b. The wasp has a smooth body that ranges in color from mahogany to black. It ranges from 2 to 5 cm in length. The wasp legs hang down from the body when it flies.
c. The diet consists of other insects and vegetable matter. Some species are pollinators of crops.
d. Like other members of Hymenoptera, wasps sting when disturbed or threatened.
e. Produces a similar lesion to other stings, the venom of all wasps contains histamines and a factor that dissolves red blood cells.
4. Yellow jacket
a. A member of the wasp family, closely related to the hornet, and widely distributed throughout the United States.
b. Named for its characteristic yellow markings, it is hairless and has three sets of legs and two sets of wings. It measures 1 to 1.5 cm in length.
c. The diet consists chiefly of insects and rotting fruit.
d. Stings may be inflicted on any body part when the nest or individual yellow jacket is disturbed.
e. Sting is similar to that of the honeybee.
f. Nests are generally underground or close to the ground and may contain thousands of yellow jackets.
A member of the wasp family, the hornet, may be found in unique football-shaped nests made of
papery material comprising of masticated plant foliage and wood suspended from a tree limb. The hornet’s habitat is more evident in the northern United States.
FIGURE 12-10. Hornet. (Courtesy of Charles E. Lewis, MD.)
b. The hornet is about 3 cm in length and is generally white faced with black and white markings on its segments.
c. The diet consists largely of other insects and ripe fruit.
d. The hornet stings to protect its environment and is likely to swarm if the nest is disturbed, whether by accident or intentionally.
e. The sting produces a painful, edematous papule.
a. The wingless member of the order Hymenoptera; the imported fire ant (IFA) is the species most frequently involved in painful stings and allergic reactions. Introduced into the United States from South America; it is prevalent in most of the south.
FIGURE 12-11. Fire ant bite on the thumb. (Courtesy of Charles E. Lewis, MD.)
b. Ants are generally 2 to 5 mm in length, colors varying from tan, red to black. The IFAs live in large colonies, often producing large mounds of dirt or sand especially noticeable after rains.
c. The ant serves a useful purpose as an aerator and mixer of soil. Most species are omnivorous. The IFA can be dangerous to young or injured livestock as well as humans.
d. Walking, gardening, and sitting in the habitat of the ant may result in one or more stings. Multiple stings, with the increased venom load, increase the risk of systemic reaction for small children and debilitated adults.
e. IFA stings create erythematous sterile pustules with high levels of neutrophils. The sterile pustules result from piperidine venom alkaloids.
a. A member of the order of Siphonaptera, fleas can be found all over the world with cat and dog fleas causing the highest proportion of infestation in households. Human fleas are rare in the United States.
b. Wingless, and with three pairs of legs, fleas can jump distances out of proportion to their size.
c. Fleas require a blood meal for survival. However, they can live up to a year without contact with humans or pets.
d. Bites occur most frequently about the ankle of the human but may be scattered in any area as the flea attempts to hide from lights.
e. The bite produces an erythematous papule that may progress to a wheal or blister. Bites may be intensely itchy.
FIGURE 12-12. Flea bites on freckled skin. (Courtesy of Charles E. Lewis, MD.)
a. Sandfly is a colloquial name for the genus of flying biting dipteran. There are many types of dipterans that may be extremely small to as large as a horsefly. The female bites to obtain a blood meal of which the protein is necessary to form her eggs. The sandfly discussed in this section is the type responsible for development of leishmaniasis.
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