Page 823 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

782 SECTION | XII Poisonous and Venomous Organisms

VetBooks.ir fangs of the male are considered too short to inflict an and inability to stand may precede an atonic paralysis.
Cheyne Stokes respiratory pattern may develop prior to
envenomating bite to humans or other mammals (Lucas
death.
and Meier, 1995b). In the United States, there are five
major species of widow spiders (Goddard, 2003):
Latrodectus mactans (black widow spider), Latrodectus
Treatment
hesperus (western black widow spider), Latrodectus var-
Treatment is largely symptomatic, with control of pain by
iolus (northern black widow spider), Latrodectus bishopi
opioids and use of muscle relaxants such as diazepam and
(red widow spider and red-legged widow spider), and
methocarbamol to control muscle rigidity. Calcium gluco-
Latrodectus geometricus (brown widow spider).
nate, which was once used for treatment of widow enven-
Widow spiders are found mostly outside of living
omation, is no longer recommended because it was found
spaces, preferring dark secluded areas such as piles of
to be less effective at controlling pain than opioids and
debris and densely growing plants. Inside houses, they
muscle relaxants (Clarke et al., 1992; Peterson and
prefer dark, rarely disturbed spaces and will often settle
McNalley, 2006a). An equine-origin antivenin (Lycovac,
under appliances and cabinets. Adult spiders are most
available in the United States from Merck and Co.) has
active in the warm months and will die during the colder
been used in humans. It is generally administered to high-
months if not in heated spaces. They build webs to
risk patients (the very young or old) or in those whose
ensnare prey. In general, widow spiders are shy and do
signs are progressing despite other medical care. The anti-
not bite larger animals unless provoked (Goddard, 2003).
venin is extremely effective; in one study, all 58 patients
receiving the agent had resolution of signs within 30 min
Mechanism of Action
(Clarke et al., 1992). In one case report (Twedt et al.,
Widow spider venom is a complex mixture of approxi- 1999), a cat with suspected latrodectism was treated with
mately six neuroactive proteins; there are also some pro- antivenom approximately 26 h after signs began. The cat
teolytic enzymes. The principal toxin for mammals is rapidly recovered neurologic function.
α-latrotoxin, a polypeptide that causes a large release and
then depletion of acetylcholine and norepinephrine at Recluse or Violin Spiders (Loxesceles spp.)
postganglionic sympathetic synapses (Lucas and Meier,
Background
1995b).
Although there are at least 50 species of Loxosceles, few
Toxicity can inflict the medically significant envenomation known
as loxoscelism. In the United States, where several
The venom of widow spiders is extremely potent; the
Loxosceles species occur, the brown recluse spider
LD 50 for whole venom of L. mactans tredecimguttatus
(L. reclusa) is considered the only one of medical impor-
(European black widow) in guinea pigs is 0.0075 mg/kg
tance. L. reclusa is found principally in Missouri,
and for mice is 0.9 mg/kg (White et al., 1995). The syn-
Arkansas, Oklahoma, Kansas, and Tennessee, but its
drome caused by widow venom is called latrodectism.
range extends from the southern Gulf states and southern
Following a widow spider bite, venom is taken up by
Atlantic states through Indiana and Illinois. Other species
lymphatics and then enters the bloodstream (White et al.,
of Loxosceles are found principally in the southwest
1995). Clinically, there may be short-lived, localized pain
United States and California, and the Brazilian native
at the bite site followed in approximately 30 120 min by
Loxesceles laeta appears to have been introduced into the
myalgia and muscle cramps near the site of envenom-
Los Angeles area (Goddard, 2003). Brown recluse spiders
ation. Pain begins to spread to the large muscles groups
(L. reclusa) are nocturnal, nonaggressive spiders. They
of the legs, thorax, back, and abdomen, peaking in
are recognizable by the “fiddle”-shaped marking on the
approximately 2 3 h. Mild to moderate hypertension and
dorsal surface of the cephalothorax.
tachycardia are common. In most cases, signs resolve in
48 72 h, but signs such as weakness, fatigue, and insom-
nia may persist for weeks to months (White et al., 1995; Mechanism of Action
Goddard, 2003). The venom of recluse spiders contains several necrotizing
Cats are very sensitive to the effects of widow venom. enzymes, including hyaluronidase, esterases, and alkaline
In one study, 20 of 22 cats died after widow spider bites, phosphatases. Sphingomyelinase D, a phospholipase,
with an average survival time of 115 h (Peterson and appears to be the most important component; it binds to
McNalley, 2006a). Paralysis occurs early in the course; cell membranes and causes migration and activation of
severe pain is evidenced by howling and other vocaliza- neutrophils in the area of the envenomation. In addition,
tions. Hypersalivation, restlessness, vomiting, and diar- the venom inactivates serum hemolytic complement, lead-
rhea were common. Muscle tremors, cramping, ataxia, ing to intravascular coagulation, occlusion of small