Spider experts say that bites do not cause bacterial infection

Tim Baca/HIGHLANDER
Tim Baca/HIGHLANDER

A study led by a former UCR entomologist suggests that, contrary to popular belief, spider bites do not lead to bacterial infections in humans. Headed by spider expert and retired UCR staff research associate Richard S. Vetter, the study could contradict thousands of spider bite diagnoses doled out by the medical community in cases where skin infections are assumed to be caused by spiders.

Vetter began the research in the summer of 2013, and the findings were published in the January 2015 edition of Toxicon, a journal dedicated to the study of toxins in plants, animals and microorganisms. The study combed spider bite data spanning from 1935 to 2014 to show that it is likely not possible for spiders to vector human pathogens.

The problem is that medical providers often treat patients with skin lesions or other infections that have “no obvious cause,” and speculate that they are the result of a spider bite, according to the study.

Vetter attributes this common misdiagnosis — the motivation behind the study — to several factors, including the medical community’s lack of knowledge about spiders. “They’re shooting from an area where they have no knowledge … so they just try to fill the void with some statement that sounds tenable,” he said.

The study indicates a very low likelihood that spiders are “capable of vectoring human pathogens,” referring to a process by which one organism transmits infection to another.

“The mere presence of bacteria on spider fangs or mouthparts does not predicate spiders as vectors,” the report says. It lists several cases in which bacterial infections were thought to have been caused by spider bites, including an instance in Australia where a type of bacteria was thought to have contributed to necrotic arachnidism, the tissue death caused by a bite. This was shown to be highly improbable due to the bacteria’s inability to stay alive on spider fangs, as well as the bacteria not being easily transmitted through simulated bites.

The study also cites the lack of skin infections that occur with verified bites, indicating that infections associated with hastily diagnosed bites are unlikely to have come from spiders. If spider bites did cause infection, “it should be obvious, common, and a routinely reported manifestation of envenomation,” the study says.

Vetter added that many types of spider bites “have never been associated with skin lesions in the centuries of dealing with them throughout the world.” A 2002 study was cited which reported an infection rate of 0.9 percent in 750 verified bites, “although the infections were non-confirmed and based on nonspecific findings of redness, swelling and pain.”

Vetter further stated that most diagnoses of brown recluse bites, for example, occur in places where there is no brown recluse population, further decreasing the likelihood that the diagnoses are accurate.

However, Vetter was careful to note that this study does not actually prove that spider bites do not cause infection, but rather presents a strong argument for why it is likely not possible. “I’m not showing definitive proof,” he said. “I’m hanging a question mark on their comments.”

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