Typically, phase 1 presents with a fever, headache, nausea and vomiting, while phase 2 shows toxicity, jaundice, shock and death. The virus producing these diseases initially replicates in the skin at the site of the mosquito bite. Next, lymph nodes and the RE system become involved and viremia results. Fever and rash lasting 3-9 days defines the symptomology. Togaviruses induce interferon and are susceptible to its effects.
Infections usually resolve once antiviral antibody is produced.
The role of the cell-mediated response is less well known but is probably important. Alphaviruses are generally associated with birds and horses and their mosquitoes. Yellow fever and Dengue fever are human in origin. The epidemiology of the infection along with clinical suspicion and laboratory results are diagnostic. One can isolate the virus from the blood during the viremic phase.