A male alpaca acutely developed signs of anorexia and fever. Within 2 days, neurologic signs (head tremors and asymmetric ataxia) developed. West Nile virus (WNV) infection was considered a primary differential diagnosis on the basis of 6 previous cases on nearby alpaca farms on which animals had similar clinical signs. Four days after the male alpaca became ill, a female alpaca from the same farm developed similar neurologic signs. In addition to anti-inflammatory and supportive treatments, both alpacas received a transfusion of llama plasma with antibodies against WNV Seven days after the onset of clinical signs, the female alpaca had made a full recovery; however, the more severely affected male died. West Nile virus infection was confirmed post mortem by use of reverse transcriptase-polymerase chain reaction assay and immunohistochemical staining.