The organism, Francisella tularensis , is extremely ubiquitous in nature. Hundreds of wild and domestic mammals and birds have been found infected, including common house pets. Incubation is generally 2-10 days. A positive test supports the clinical diagnosis of tularemia. A history of contact with rabbits, ticks, dogs, cats, or skunks is suggestive but a negative history does not rule out the diagnosis.
Titers decline over years.
Sera from normal patients may show positive agglutination with febrile antigens due to previous immunization, past infection, or cross-reacting antibodies to related antigens. These titers should be lower and constant. Active infection or recent immunization will show higher titers that rise over time. A progressive increase in titer is indicative of recent infection or immunization. Seroconversion indicated by a fourfold increase in titers during illness is considered diagnostic.