RELAPSING FEVER SPIROCHAETES
Relapsing fever (synonym: typhinia) is an acute infectious disease caused by different Borrelia species, and is characterized by febrile episodes separated by afebrile intervals.
The term relapsing fever (RF) has traditionally been used in connection with human disease (human relapsing fever (HRF). Disease caused by RF spirochaetes in animals is often designated spirochaetosis. Details on RF and the causative spirochaetes can be found in the reviews by Felsenfeld (1965)(2), Barbour & Hayes (1986)(3), Larsson (2007)(4) and Barbour & Guo, (2010)(26).The severity of HRF ranges from subclinical to fatal. The main clinical sign is recurrent fever, which coincides with large numbers of spirochaetes in the blood (spirocha- etaemia). The characteristic relapsing phenomenon is associated with the presence of multiphasic antigenic variation. As an immune response develops to the predominant antigenic strain, shifting of the outer surface proteins of the spirochaete produces variant strains that multiply and cause a recrudescing infection(26).
I nfections caused by the RF spirochaetes are vector- borne, primarily by the soft-body ticks in the genus Orni- thodoros (Family Argasidae), and are referred to as tick- borne relapsing fever (TBRF). An exception is Bor- relia recurrentis, which is transmitted by the human body louse (Pediculus humanus humanus) and causes louse-borne relapsing fever (LBRF). The louse is strictly specific to humans with no animal reservoir for B. recurrentis. Large outbreaks of LBRF have occurred in Africa and Eurasia throughout the past century, but at present, the disease is primarily found in Ethiopia and adjacent areas.
Most TBRF are zoonoses. The TBRF persists in endemic foci around the world, but it is particularly a burden for humans in several African countries.
In Table 27.1, the different TBRF Borrelia species with their vector, typical host(s) and geographical distribution are presented. The responsible B orrelia species is closely associated with its tick vector and many of them share parallel nomenclature. Although most TBRF spirochaetes are transmitted by Ornithodoros ticks, B. anserina, causing avian spirochaeto- sis in domestic fowl, is transmitted by several Argas ticks. Ornithodoros ticks are usually nocturnal feeders and feed for short periods of time (around 30 minutes), and inoculation of the spirochaetes in the host happens within minutes. The ticks are sedentary and inhabit sheltered environments close to their hosts, such as rodent burrows, bird nests, caves and human-made shelters(2).Typical reservoir hosts for the TBRF spirochaetes are various wild rodents, but they also include insectivores, birds and other mammals (Table 27.1) (2). Mice are reservoirs for several B orrelia spp. in nature, and the house mouse (Mus musculus) is the most commonly used species in animal experiments on RF. Mice develop a disease that is similar to HRF(4). Details on the susceptibility of and pathology in various experimental animals used in RF studies can be found in the review by Felsenfeld (1965)(2) and Barthold et al. (2010)(26). The pathogenesis and interactions between the TBRF spirochaetes and their host (both ticks and vertebrates) have been reviewed in depth by Barbour & Hayes(3) (1986) and Barbour & Guo (2010)(26).