Spider Bite

Chapter 96 Spider Bite Envenomation



Michael E. Peterson, DVM, MS



KEY POINTS



• There are two genera of venomous spiders of clinical importance in North America, Latrodectus (black widow) and Loxosceles (brown recluse).

• Black widow spider bites induce no significant local symptoms but do cause acutely painful bites with muscle cramping, systemic hypertension, and possibly death.

• Cats are particularly sensitive to black widow spider venom, exhibiting extreme pain, vocalization, and muscle cramping, leading to flaccid paralysis and a high fatality rate.

• Black widow envenomations can be managed initially with pain management; however, antivenin is available and very efficacious.

• Brown recluse spider bites can cause one of two manifestations: an indolent dermatonecrotic ulcerative lesion or systemic hemolysis that can be life threatening.

• Brown recluse spiders are endemic only in certain parts of the United States, the diagnosis of brown recluse spider bite should be considered only in patients living in these regions.


BLACK WIDOW SPIDER ENVENOMATION


Five primary species that belong to the genus Latrodectus reside in the United States. These are L. mactans, L. variolus, L. bishopi, L. hesperus, and L. geometricus. L. mactans (the black widow) is the predominant species found throughout North America. These spiders are found throughout the continental United States and north into the southern Canadian provinces. They inhabit funnel-shaped webs in dry, dimly lit, secluded places. The web is irregularly shaped, has a tattered “cobwebbed” appearance, and usually is found in corners. The spiders commonly are found around houses where outside lights help to attract prey insects.


Male black widow spiders are of little medical importance, because they are unable to penetrate mammalian skin. The female can be identified by the hourglass pattern, red or orange in color, on the ventral aspect of her shiny, globose black abdomen. The hourglass becomes more prominent as the spider ages. It is important to recognize the immature female, which has a colorful pattern of red, brown, and beige on the dorsal surface of her abdomen, because she is fully capable of delivering a severe envenomation.


Black widow spiders control the amount of venom they inject using striated muscle. Therefore Latrodectus bites do not necessarily indicate envenomation. It is estimated that 15% of bites in humans are not envenomating. Cats are very sensitive to the venom, and deaths are common in envenomated victims. In the dog the toxin provokes severe symptoms, although dogs are considered more resistant than cats.1 A single bite is fully capable of delivering a lethal dose of venom to companion animals. Evidence suggests that the venom has increased toxicity in spiders living in areas with higher environmental temperatures. Although some controversy exists, it appears that venom toxicity is highest in autumn and lowest in spring. The incidence of black widow spider bites in veterinary medicine is unknown. Diagnosis of Latrodectus envenomation in domestic animals rarely is made, primarily because of veterinary unfamiliarity with the clinical manifestations.


Black widow spider venom contains no locally acting toxins that would provoke a significant inflammatory reaction at the bite site. The venom contains a potent mammalian neurotoxin called α-latrotoxin, which induces neurotransmitter release from nerve terminals. This depolarization promotes calcium-independent release of the neurotransmitters acetylcholine and norepinephrine (and others) down concentration gradients and then inhibits their subsequent reuptake. Acetylcholine, noradrenaline, dopamine, glutamate, and enkephalin systems are all susceptible to the toxin.2



Clinical Signs


The onset of clinical signs usually occurs during the first 8 hours after envenomation. Local tissue changes are generally absent, and swelling at the bite site is uncommon. Systemic manifestations depend on two sets of variables. Spider-dependent variables include the size of the spider, motivation of the spider (e.g., quantity of venom it decides to inject), and time of year (e.g., altered venom toxicity). Victim-dependent variables would include species and size of victim, location of bite, underlying health problems, and age of the victim (e.g., pediatric and geriatric victims are more severely afflicted).


Initial regional numbness often is observed in dogs. Tenderness in adjacent lymph nodes may precede hyperesthesia, progressive muscle pain, and fasciculations in the affected region. Cramping of the muscles of the chest, abdomen, and lumbar and other large muscle masses is common. Abdominal rigidity without tenderness is a hallmark symptom of Latrodectus envenomation. The condition is extremely painful in moderate to severe envenomations. Significant respiratory distress may become evident if muscle cramping is strong. Marked restlessness, writhing, and muscular contortions may occur. Hypertension and tachycardia should be anticipated. In high-risk patients (i.e., those with underlying health problems or at either end of the age spectrum) these cardiovascular manifestations may lead to stroke, exacerbation of heart failure, and possibly myocardial ischemia. Signs of motor restlessness may abate over 10 to 20 hours with the possible onset of paralysis. Death is usually due to respiratory or cardiovascular collapse.


Cats are extremely susceptible to Latrodectus venom. In one study, 20 of 22 feline victims died subsequent to black widow envenomation; the average survival time after the bite was 115 hours.1 Paralytic signs may appear early and are particularly marked. Severe pain is manifested by howling and loud vocalizations. Excessive salivation and restlessness are common, and vomiting and diarrhea may occur. Muscular tremors, cramping, ataxia, and inability to stand precede complete paralysis. The body becomes adynamic and atonic. A Cheyne-Stokes respiratory pattern may develop, and death ensues.

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  4. Allergic Airway Disease in Dogs and Cats and Feline Bronchopulmonary Disease

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Spider Bite

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