We present a series of patients with transverse myelitis (TM) following insect bite in order to describe an under-recognized etiology of TM.

Transverse myelitis (TM) is often associated with an infectious trigger or a demyelinating disorder, such as multiple sclerosis. However, between one-fifth and two-thirds of cases remain idiopathic. Idiopathic TM may be secondary to previously unidentified triggers with molecular mimicry to self-antigens. To our knowledge, there have been few descriptions of insect bites as an inciting trigger for TM.

We identified patients seen at Stanford Health Care between 2016 and 2021 who reported an insect bite shortly before symptom onset and were diagnosed with TM without a clear alternative etiology. We reviewed the medical record for details regarding their case, including medical history, serum and CSF testing, and imaging.

Two middle-aged patients were found to have insect bites (from a mosquito and a wasp, respectively) as a possible trigger for TM. Both developed dysesthesias of the bitten limb the day after the bite and had progressive dysesthesias followed by weakness. Serum vitamin D level was either borderline-low or markedly low at 29ng/mL and 12ng/mL, respectively. Both patients were found to have enhancing cervical cord lesions and had gradual recovery after receiving high-dose steroids, with mild residual sensory loss in both cases and residual weakness in one case.

Insect bites may trigger a monophasic TM in predisposed patients. Envenomation by various organisms, including mosquitos and wasps, upregulates antigen presentation and T-cell recruitment. Low vitamin D levels are associated with a further shift away from a homeostatic regulatory T-cell environment. We hypothesize that insect bites promote immune dysregulation, causing a monophasic TM in predisposed individuals.