For the most part, bites from brown recluse spiders are not dangerous. In rare cases, a bite may cause tissue necrosis and a large ulceration which can take a long time to heal, and may expose the patient to the risk of infection. Even more rarely, brown recluse bites can lead to death, with deaths being isolated to very young children, older adults, and people who are in poor health.
One of the biggest problems with brown recluse bites is that they are overdiagnosed. These spiders are very shy, and typically will not bite unless antagonized or threatened. People often do not notice the spider when it bites, and a doctor may be inclined to diagnose a lesion which looks like a brown recluse bite as a spider bite, even if it later turns out to be something else. For example, many painful ulcerations of the skin which are thought to be brown recluse bites are actually examples of severe staphylococcus infection.
If a spider does bite, the patient can experience a sharp pain at the site of the bite. A white blister may appear, fading to a red dot which disappears in a few days or weeks. The patient may feel mildly ill with nausea and fever until the body can process the spider venom. Often, the only treatment required is ice on the wound and rest until the patient feels better.
If the spider injects a lot of venom and the patient is especially sensitive, brown recluse bites become necrotic, with tissue around the bite dying and sloughing off. This leads to an exposed ulcer which can be painful and very unpleasant to look at. As long as the ulcer is kept clean and dry, it should heal within six to eight weeks. Some ulcerations may require more extensive medical treatment, such as a skin graft or medications to control infections. In vulnerable patients, venom sensitivity can lead to death as a result of a brown recluse bite.
The important thing to be aware of is that brown recluse bites are incredibly rare. This spider is found mostly in the American South, and when people do encounter brown recluses, the spiders are usually quite reluctant to bite. People who develop skin ulcerations or lesions which they suspect may be the result of an insect or spider bite can seek medical treatment, and should definitely seek treatment if there are signs of infection or other complications, but they should encourage their doctors to rule out other causes for the lesion, rather than assuming that it is a bite. Failure to diagnose the correct cause can lead to a delay in getting the right kind of treatment.