Tickborne Relapsing Fever

Organism Borrelia dutoni, which is indistinguishable from B. recurrentis in stained blood films.

Clinical features Fever develops with a recurring or relapsing pattern as the name of the disease indicates. The period of fever lasts for a few days and then recurs after 2-5 days, with up to ten or more relapses occurring in the untreated case. The onset of fever is sudden with headache, myalgia and vertigo; a transient petechial rash can occur and a variety of other systems may be involved. There can be bronchitis, nerve palsies, hepato-splenomegaly and signs of renal damage.

Diagnosis Spirochaetes are found in the blood during febrile periods when blood slides should be taken and either stained or viewed by dark ground illumination. An improvement on this technique is the direct centrifugal method.

Transmission, occurrence and distribution Two different patterns of tick-borne relapsing fever occur; an endemic in Africa and epidemic in other parts of the world (Fig. 16.4).

In Africa, the vector Ornithodorus mou-bata is domestic in habit living in and around the home, transmitting the disease within the household. The reservoir is the tick, but humans act as a source of organisms. When a tick feeds on an infected person, spirochaetes are ingested with the blood, multiply in the gut and enter the haemocoel, where they increase to enormous proportions. The spirochaetes pierce all organs of the tick's body, including the salivary gland, the coxal organ and the reproductive system, leading to transovarial infection. Nymphal stages may already be infected when they take blood meals or can become so from their hosts. People are infected both by the bite of the tick and from the coxal fluid (see below), but not from the faeces. In O. moubata adults, the coxal fluid is the main source of spiro-chaetes, but in the nymphs and other species of Ornithodorus, it is the salivary glands.

Babies and young children are particularly susceptible to infection in endemic areas, with adults exhibiting immunity.

However, immunity is lost by pregnancy and congenital infection can occur. Relapsing fever is a cause of abortion, stillbirth and premature delivery. This is particularly the pattern in Central and East Africa.

In other parts of the world, the infection is a zoonosis with a transmission cycle maintained between rodents and their parasitic ticks. People enter this cycle as intruders or by accident in a similar way to sylvatic plague or scrub typhus. Rodent-inhabited caves or campsites near rodent burrows are areas where sporadic infection can occur. When temporary shelters or log cabins are erected near a zoonotic focus, rats invade these buildings and their ticks begin to feed regularly on humans. An endemic pattern, similar to that in Africa may then develop. Ticks responsible for transmitting relapsing fever in other parts of the world are O. tholozani in Asia, O. erraticus in North Africa, and O. rudis and O. talaje in Central and South America.

Soft ticks (Argasidae) have a retracted head and no scutum, which differentiates them from hard ticks (see below). As their name implies, soft ticks do not have a rigid structure, but a leathery body that looks like a collapsed bag. This hangs over the body structures, so viewed from above only the legs can be seen protruding from it (Fig. 16.4). When the tick takes a blood meal, its collapsed body fills and becomes greatly distended. The tick digests the blood meal utilizing a structure called a coxal gland, which is like a filter to remove excess fluid.

After hatching, there are several nymphs (four in O. moubata), each needing to take a blood meal before passing on to the next nymphal stage. Finally, the fourth instar changes into an adult and egg laying commences after the female has become engorged with blood. Ticks can live for several years so that many eggs can be laid in a lifetime. In contrast to hard ticks, the female does not die after egg laying, but is able to continue taking blood meals, laying eggs after each meal.

Ticks rest in cracks and crevices of poorly built houses, emerging at night to feed on sleeping occupants. They can

Tick taking blood meal on sleeping person J

Tick taking blood meal on sleeping person J

Ticks inhabiting cracks in wall and floor Endemic variety (Africa)

Ornithodorus moubata

Ornithodorus moubata

Picture Soft Tick

Soft tick

Rodent tick biting human

Rodent and its ticks living in burrow

Epidemic variety

Rodent tick biting human

Rodent and its ticks living in burrow

Epidemic variety

Soft tick

Fig. 16.4. The vector and epidemiology of tick-borne relapsing fever.

remain alive for up to 5 years after a single blood meal and are able to attack again after the owner re-occupies a house following a prolonged absence. The eggs are coated with a waxy protective layer allowing them to remain viable for several months - laid in walls, floors and furniture, they will hatch in 1-4 weeks if conditions are suitable. Soft ticks once established are very persistent occupants.

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