The pathogen Yersinia (syn. Pasteurella) pestis is a short, non-motile, gram-negative bacterium that stains at both poles. The facultatively anaerobic ellipsoid rods often encapsulate. They propagate rapidly and produce large amounts of toxin. They survive under dry conditions in the faeces of fleas for 4 weeks but under humid conditions and below 0 0C for up to 18 months. Sunshine and ordinary disinfectants kill them quickly.
The enzootic transmission between small wild mammals in steppes and highlands occurs without noticeable lethality and exclusively by their host-specific fleas. On rubbish dumps, these fleas also pass to rats. The epizootic killing of Rattus rattus (black rat) is triggered predominantly by Xenopsylla cheopis, the tropical rat flea; Rattus norwegicus (brown or grey rat), however, by Nosopsyllus fasciatus, the European rat flea (fig. 4.9, page 222).
For the transmission of Yersinia pestis on man, X. cheopis plays the most important part. It is widely distributed in warm countries. Plague bacteria become ready for transmission in the gut of X. cheopis at 15 20 0C and 85 95 % relative humidity within 5 days; at 20 0C, this requires 30 days. X. cheopis is carried, via ships, on R. rattus living off transported cereals. At Middle Europe, the bacteria persist in the pipes of heating systems of large cities. N. fasciatus is associated with R. norwegicus and is poorly anthropophilic. The flea species survives, when not infected, for 95 days at 7 10 0C without feeding, whereas infected fleas live for 47 days; at 27 0C, the fleas live only for 28 days. The flea species therefore establish a reservoir for plague bacteria.
Plague manifests itself primarily by attacking regional lymph nodes in bubonic plague. It is transmitted solely by fleas, not by contact, and after haematogenous dissemination as septaemic plague. In both cases, the plague bacteria propagate in the gut of the flea. Only pneumonic plague is transmitted by aerosol infection without participation of fleas. Rapid diagnosis is possible only by bacteriological and molecular-biological techniques. The early treatment of suspected cases by antibiotics is extremely important and should not await laboratory confirmation.
Biology of Siphonaptera: Fleas are holometabolic and temporary house parasites of warm-blooded animals with high host specifity. Their eggs are dropped into the nests, sleeping places and living structures of their hosts. The embryogenesis of N. fasciatus lasts 2 14 days at a temperature of at least 5 0C, whereas X. cheopis requires at least 13 0C and moderate humidity. The legless larvae moult three times at intervals of 1 24 weeks, depending upon the temperature. They feed on detritus and require dried blood as a source of iron. For this purpose, adult fleas drop fresh anal blood during feeding. The pupae develop in a cocon covered with detritus. After one to several weeks, the imago hatches and lives for 250 (males) or 300 (females) days. It can withstand starvation for up to 17 months (Xenopsylla spec.).
Transmission of plague bacteria: fleas lack a peritrophic membrane. The bacteria propagate in the midgut and colonize the proventriculus retrogradely. Its basket of backward-directed chitinous lamellae becomes blocked because of a coagulase of the bacteria, which lyses fibrin above 30 0C. Feeding attempts of the flea releases bacteria into the stab wound by regurgitation. Before this, the flea disposes of the bacteria via its faeces.
Morphology of the fleas (fig. 4.10, page 223). The body shape is compressed and the flea can push its way quickly even through dense hairs. The (variable) jumping ability is based on the joint of the third coxa, which is provided with a layer of resilin. The club-shaped antennae are hidden in a pouch of the head; the complex eyes, if present, posess a common lens. The biting apparatus consists of the immobile epipharynx forming the feeding channel, the paired lacinia as piercing stylettos (each with a salivary channel) and the maxillary lobes carrying a long four-segmented palpus. The penetration power is stored by the compression of the resilin and is released by a minute change of the position of the hypopharynx, which is constructed according to the principle of a pneumatic hammer (fig. 4.11, page 224). The thorax is secondarily wingless; the abdomen consists of 9 segments, containing the dorsally bowed copulation organs in males, and carries a sensory plate on the 8th segment, the pygidium. In the female, the shape of the receptaculum seminis is important for the identification of the genus, which can be recognized further by the structures of the head and the pronotum: ctenidia, bristles, eyes (fig. 4.9, page 222).
Fleas propagate only in presence of their blood hosts. Echidnophaga gallinacea, the chicken flea, lives a sedentary life on the cocks comb; the male copulates stimulated by warmth. The female of Tunga penetrans, the burrowing flea, burrows deeply into the skin, feet or other parts of the body of the host. The male does not feed on blood and copulates just before the skin around the female has closed completely. The tracheae of T. penetrans are smooth tubules without the coiling structures common to other fleas and insects (important for histological identification). The rabbit flea (Spilopsyllus cuniculi) sychronizes its propagation exactly with that of its host: The wild rabbit (Oryctolagus cuniculus) constructs separate burrows for living and breeding. Its fleas, having settled in the ears of the rabbit, start the ripening of their eggs as soon as the rabbit has experienced multiple ovulation at copulation. The increasing corticosteroids in the blood of the rabbit are resorbed by the fleas through their cuticle. The fleas migrate via the mouth region of the rabbit into the nest, feed on the first litter and oviposit. The eggs develop rapidly in the warm nest and the new generation of fleas is ready to feed when the second litter appears. These fleas remain on the individuals of the second litter and leave the nest with them. The original parental generation of fleas, after a second oviposition, returns into the ears of the mother rabbit (details see fig. 4.12, page 227).