Brown Recluse Spider Bite Poisoning (Loxoscelism) in Dogs

A brown recluse spider can be identified by the violin shape on its back, as well as its six eyes, as opposed to most spiders’ eight eyes.

Many cases of loxoscelism do not cause severe tissue death and wounds, but those that do involve painful, slow-healing wounds that sometimes take several months to resolve. Rare cases result in damage to the red blood cells leading to anemia and acute kidney injury which is sometimes fatal. Dogs that have a confirmed or suspected spider bite require prompt veterinary attention. A dog with a known BRS bite requires emergency medical attention. In most cases, the prognosis for BRS bites is good.

Symptoms of brown recluse spider bites vary significantly, based on the amount of venom transmitted via the bite. Some cases are mild and result in no symptoms or minor skin wounds.

More severe cases result in severe, ulcerated skin wounds. In rare cases the spider bite triggers destruction of red blood cells, kidney injury, and rarely, death.

Brown recluse spiders are not endemic to most areas of the US and Canada, so dogs living outside of the south central and southwestern United States are not at risk of brown recluse spider bites. Even among dogs living in endemic areas, brown recluse spider bites in dogs are rare. Brown recluse spiders prefer dark, secluded places to hide indoors such as basements or closets, and outside under logs or rocks.

Loxoscelism is caused by a bite from the Loxosceles species of spider. The most common example in North America is the brown recluse spider, but other Loxosceles spider species have worldwide distribution

Brown recluse spiders are non-aggressive and require close contact to bite. Most bites result from dogs lying or walking on a spider accidentally.

Symptoms of loxoscelism can be directly related to the skin, or in severe cases systemic illness can develop.

Symptoms on the skin at the site of the spider bite include:

• Pain
• Swelling
• ‘Target’ lesion
• Redness
• Blistering
• Bruising
• Necrosis (blackening of the tissue)
• Sloughing (falling off) of damaged skin resulting in an ulcerated wound

Systemic disease occurs if envenomation affects more than just the skin around the bite wound.

Investigation of loxoscelism is challenging as the symptoms are nonspecific, and can take hours to days to develop. Definitive diagnosis involves witnessing a spider bite and subsequent development of a skin lesion.

Diagnosis is usually presumptive and depends on a skin lesion consistent with a spider bite in a dog where BRS are known to be present in the environment.

Investigation involves:

• Physical examination
• Blood work
• Urine examination
• Spider identification (if available)

Treatment options include:

• Cold compresses of bite site
• Steroids
• Pain relief
• Antibiotics

Supportive care may be required in severe cases and can include

• Fluid therapy
• Blood transfusion
• Wound care

In cases where ulceration and/or tissue death is severe, surgical wound debridement (clearing of dead tissue) may be necessary.

Wounds from BRS bites may ulcerate, requiring intensive wound management, sometimes taking weeks, or months to heal.

Most cases do heal fully and long-term prognosis for recovery is good.

A minority of cases develop systemic signs requiring prolonged treatment of red blood cell destruction and kidney injury. A small number of these cases are fatal.

Prevention focuses on preventing brown recluse spider bites. Strategies include:

• Raising bedding off the floor
• Frequent cleaning of dark, warm areas of the home and areas where pet bedding is located
• Consulting a pest exterminator to reduce spider numbers in the house

The true incidence of BRS bites is unknown as mild bites may not develop symptoms and, in symptomatic cases, diagnosis is often presumptive as the spider is rarely seen.

Loxoscelism is most common in regions where the BRS is endemic and in dogs which spend time in dark, quiet areas such as basements or under porches.

• Cold compresses to reduce swelling where the bite is witnessed
• Steroids
• Pain relief
• Antibiotics
• Fluid therapy
• Blood transfusion
• Wound care
• Wound debridement surgery