Answer: Borrelia spp., relapsing fever group. Molecular studies are needed to definitively identify the causative agent. However, knowing the exposure history of the patient also helps narrow the differential. In this case, we later found out that the patient is from the northwestern part of North America, and therefore this is most likely Borrelia hermsii, one of the tick-borne relapsing fever Borrelia species. The other tick-borne Borrelia in North America are B. parkerii and B. turicatae. These species are transmitted through the bite of a soft-bodied tick in the genus Ornithodoros, and are usually associated with exposure to 'rustic' settings such as cabins and lean-tos. There is a nice article on the geographic distribution of B. hermsii that you can read HERE.
Borrelia recurrentis, the causative agent of louse-borne relapsing fever, is rarely seen in North America, given that body lice are relatively uncommon and usually associated with overcrowding and social disruption. However, B. recurrentis infection has a similar clinical presentation to the tick-borne relapsing fevers, and the spirochetes have an identical appearance on peripheral blood smear.
Borrelia spirochetes appear as long spiral-shaped bacilli (5-20 microns in length) with relatively regular undulations. As William mentions, there are other spiral-shaped or curved bacteria as well; some are curved or wavy rods (e.g. Vibrio, Campylobacter, Helicobacter), whereas others are true spirochetes (Borrelia, Treponema and Leptospira spp.). Of the true spirochetes, only the relapsing fever Borrelia are regularly seen in blood. Therefore, this thin blood film appearance in conjunction with the clinical history of recurrent (i.e. relapsing) fevers is consistent with tick-borne relapsing fever.
As Blaine and Idzi mention, Borrelia burgdorferi, the primary cause of Lyme disease in North America, does NOT reach sufficient levels in blood to be seen on peripheral blood smear (mean number of spirochetes is 0.1 per mL of blood). Having said that, some mentioned that the newly-described Borrelia mayonii HAS been noted to have high levels of bacteremia in peripheral blood, and therefore could potentially be seen on peripheral blood smear. Good catch! While spirochetes of Borrelia mayonii may be seen on peripheral blood smears, they are in relatively low numbers - ~2 spirochetes per 70 high power fields - so while visible, they would not be in such abundance as seen in this case.
Finally, some of you may be wondering why I've included a bacterial case on this blog devoted to parasites. The reason is that parasitologists can play an important role in identifying other microorganisms when seen on conventional parasitology preparations such as thin blood films. Borrelia spirochetes, Ehrlichia/Anaplasma morulae, and intracellular yeasts (e.g. Histoplasma capsulatum) are the more common non-parasites that can be seen on blood smears, and therefore it's helpful to be familiar with their appearance. We've seen all 3 in just the past year! Here are links to 2 past cases:
Case of the Week 165: Anaplasma phagocytophilum morulae within neutrophils
Case of the Week 492: Histoplasma capsulatum within phagocytic cells