4.3 Relapsing fever and soft ticks

African endemic tick-borne relapsing fever. The pathogen is Borrelia duttoni (Spirochaetales), a mobile, Gram-negative, screw-shaped bacterium. The vector is the soft tick Ornithodorus moubata. After incubation of 3 – 10 days, attacks of fever last several days but are interrupted by intervals of 7 – 10 days, connected with a change of the antigenic specifity of B. duttoni. Transovarial transmission is accompanied by a change of shape in the borrelliae. Strain-specific acquired protective immunity has been noted. A natural reservoir is unknown; the disease is a typical endemic anthroponosis.

Biology of Ornithodorus moubata: The imago takes a blood meal several times, after each of which oviposition occurs into the floor of human dwellings or the holes of warthogs and porcupines. The egg hatches into a six-legged first larva that immediately moults to the second larva with eight legs. The second to the sixth larvae, the nymph and the imago feed on blood and moult in the middle of each digestion phase of 3 – 6 weeks. During the blood meal, coxal liquid is secreted, which may contain borreliae, from the coxal glands. The midgut ends blindly. The Malpighian tubules harbour excretion symbionts and discharge uric acid via the rectal ampulla. At copulation, a retort-shaped spermatophore is transferred that contains the spermiophores, each of which holds a single spermium. One of the spermium penetrates into a cell of the proximal part of the oviduct and triggers the oviposition of 30 – 80 eggs, which are inseminated when passing the ampulla (fig. 6.19, page 263). Embryogenesis lasts 10 days (25 0C) and one generation requires at least 8 months. All stages can survive starvation for weeks and transmit spirochaetes. The latter propagate in the haemolymph, salivary glands, coxal glands, Malpighian tubules, gonads and cerebral ganglion, but not in muscles or the rectal ampulla (fig. 4.8, page 219). In cases of transovarial transmission, 40 – 60 % of the eggs may contain borreliae.

Louse-borne relapsing fever. The pathogen is Borrelia recurrentis and the vector is Pediculus humanus corporis or P.h.capitis. The lethality of the typical epidemic anthroponosis reaches 60 %. In Europe during the 19th and 20th Centuries, extended epidemics struck the impoverished Irish population and during the Russian revolution.

Lyme’s disease/borreliosis. The pathogen is Borrelia burgdorferi and the vectors are hard ticks, such as Ixodes ricinus, the larvae of which are infected up to 1 % transovarially. Up to 20 – 30% of nymphs and adults are infected in the field. The natural reservoir is located in small wild-living mammals; the disease is a zoonosis.

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