Black Widow spider or Latrodectus are highly venomous and their bites could be fatal for both humans and most domestic pet animals. They are found in every state of US except Alaska and are often found residing around buildings and human habitations.
The female measures about 2–2.5 cm in length and is the larger of the genders. She is shiny black with a red or red-orange hourglass shaped mark on the underside of her abdomen, and in some females, there is also a patch of red on the top of the abdomen, above the spinnerets. The immature female is more of a brown color with red to orange or yellow stripes across the top abdomen that changes into the hourglass shape as she ages and darkens to black. The male is significantly smaller, about half the size, with light brown coloring, and lacking the red hourglass marking the female is recognized for. The male is not regarded as a threat, as it is typically the female that bites.
Bites may be dry, with no venom injected. The venom is a potent neurotoxin, opening channels at the presynaptic nerve terminal and causing massive release of acetylcholine and norepinephrine, both of which can cause sustained muscular spasms and paralysis.
Some common symptoms of that appear if the dog has been bitten by a black widow spider are paralysis, muscle tremors and cramping, abdominal rigidity, severe muscle pain in the back, chest and abdomen, manifested by howling, whining and loud vocalizations, trouble breathing, respiratory collapse due to abdominal muscle paralysis, excessive salivation and restlessness, increased blood pressure and heart rate, incoordination and inability to stand, vomiting, and diarrhea.