Transmission of plague

Plague is due to a Gram negative enterobacterium, Yersinia pestis, and it is a disease of rodents transmitted by fleas in the form of bubonic plague. This disease is still endemic in several regions of the world due to the fact that it is a zoonosis primarily infecting rodents (e.g. rats and other wild rodents). Its emergence in humans depends both on the frequency of infection among rodents and the promiscuity of human with these animals. From 1989 to 2003, 38 310 human cases and 2845 deaths were reported from 25 countries in the world, but 82% of all cases occur in Africa.37 Twelve African countries had notified cases of plague: Algeria, Botswana, the Democratic Republic of Congo, Kenya, Madagascar, Malawi, Mozambique, Namibia, Tanzania, Uganda, Zambia and Zimbabwe, but there may also have been occurrences in other countries. Madagascar is particularly affected by plague, especially on the central and Northern Highlands, typically above 800 m altitude. There are three foci: (1) rural foci in the Highlands, the reservoir of pathogens is Rattus rattus; (2) urban foci in the Highlands and the capital, Antananarivo, Rattus norvegicus; and (3) a coastal urban focus at Mahajunga, Rattus rattus and R. norvegicus. The vectors are Xenopsylla cheopis (indoors: 95%) and Synopsyllus fonquerniei (outdoors: 86% to 95%). 14% of the world's cases occur in eight countries of Asia: China, India, Indonesia, Kazakhstan, Laos, Mongolia, Republic of the Union of Myanmar and Vietnam. 4% of the world's cases have been reported in five

American countries: The United States of America, Peru, Brazil, Bolivia and Ecuador. No cases of plague have been reported recently in Oceania or Europe. In France, the last cases occurred in 1945 in Corsica.

One of the characteristics of plague epidemics is their capacity to disappear during several years, before reappearing brutally in an epidemic form. In 1994, an epidemic of bubonic plague (128 cases) was recorded in Mozambique after more than 15 years of silence, which was then propagated to Zimbabwe and Malawi. At about the same time, an epidemic occurred in Peru (1031 cases in 1993-1994). In spite of their close temporal appearance, there is probably no epidemiologic link between these epidemics. More recently (June 2003), the plague reappeared in Algeria (Oran) after a period of 50 years of inter-epidemic silence. A pulmonary epidemic of plague was recently declared in the Democratic Republic of Congo (December 2004) and an epidemic of bubonic plague has been reported (June 2009) in Libya near the Egyptian border.

In man, the disease takes two principal forms: (1) bubonic (contracted by the flea bite) and (2) pulmonary (airborne transmission). The bubonic plague (most frequent) is characterized by a very severe infectious syndrome with strong fever, accompanied by a hypertrophy of the lymphatic ganglion (bubo). In 20% to 40% of cases, the patient recovers after a rather long time of convalescence. Otherwise, the disease develops towards an acute and fatal septicaemia. In certain cases, the bacteria reach the lungs and the disease develops towards a pulmonary plague. The inter-human contagion occurs via the infected expectorations. In the absence of an early and suitable treatment, pulmonary plague is systematically fatal within three days.

The plague has played an important role in human history in some spectacular deadly epidemics. Three great pandemics can be noted: (1) during the 6th century ''the plague of Justinien'' occurred in the countries of the Mediterranean basin and Northern Europe. There were one hundred million deaths with a 60-year epidemic, or over two centuries with regular reoccurences every 10-12 years; (2) during the 14th century (1348-1350), the Black Death came from foci of Central Asia and killed a quarter of the European population, with a long series of epidemic reocurrences through Europe, India and China; and (3) in 1855, a third pandemic started from the ancestral Chinese foci of Yunnan and spread across the whole world (Bombay in 1896; Calcutta in 1898, Alexandria 1899, the Mediterranean basin in 1901, northern Europe in 1908, and eventually to reach the New World and South Africa). There were 12 million deaths.

Plague involves flea-rodent-host transmission with three cycles: (1) wild plague prevails in wild rodents (squirrels, marmots, gerbils, prairie dogs, etc.) where the attack of humans is exceptional; (2) rural plague passes from wild rodents to the rats of villages; (3) urban plague is imported downtown from a rural focus, and is quickly propagated from rat to rat. The black rat (Rattus rattus) has the greatest sensitivity to the bacillus of plague with a phase of septicaemia favourable to the infection of fleas, and because of its contact with man, it can ''transport'' the infection from one continent to another, like from one house to another. The grey rat (Rattus norvegicus), a rodent of the cellars and sewage systems, has a stronger resistance to the infection.

There are more than one hundred species of fleas associated with the plague. The main vectors are: Pulex irritans, best adapted to man in temperate climates and cosmopolitan, involved in epidemics in Morocco, England, Algeria, Nepal, Brazil, etc.; Xenopsylla cheopis, cosmopolitan, related to black rat and bites in humans; X. brasiliensis, vector of plague in rural areas of South America, Africa and India; X. astia, vector in Asia; and Nosopsyllus fasciatus, a cosmopolitan species.

Transmission of other pathogens

Rickettsia: Fleas transmit Rickettsia mooseri (or R. typhii) responsible for the murine typhus which affects rats, mice and sometimes humans. This typhus is especially present in tropical and sub-tropical areas, especially in ports. The natural reservoir is the rat; the vectors are X. cheopis, N. fasciatus and Lep-topsylla segnis. Contamination occurs through the faeces of fleas, where the concentration and pathogenicity of Rickettsiae is high.

Other bacillus and viruses: Fleas can accidentally transmit the bacillus of tularaemia (Francisella tularensis), contamination occurring as a result of contact with the infectious faeces of fleas present in the fur of contaminated animals handled by man. The flea, Xenopsylla cuniculi, transmits the virus of the myxomatosis (Leporipox virus) to rabbits and hares in Australia, Europe and the United States.

Animal pathogens: Fleas can transmit Trypanosoma parasites to animals (T. lewisi of rats transmitted by N. fasciatus and X. cheopis; T. duttoni of mice transmitted by species of Ctenophtalmus or Nosopsyllus; T. nabiasi of rabbits by Spilopsyllus cuniculi). The fleas are also intermediate hosts of helminths in animals and occasionally in man, Dipylidium canidum, a cestode of cats and dogs, accidentally human; vectors: Ctenocephalides canis and Pulex irritans. The contamination is made when the larvae of fleas ingest the cestode eggs; the infection of man is done through ingestion of the fleas carrying cysticercoids. Jiggers and tungiasis

Jiggers are extremely small fleas, such as Tunga penetrans, which develop in the dry sandy soils of hot regions of America, Africa, India and China.38 The fertilized female buries herself into the skin of the feet of large mammals, humans and pigs. This causes a strong cutaneous irritation and ulcerations. The young female is small (1 mm), but the fertilized female can measure 1 cm, due to thousands eggs accumulated in the slack abdomen. Great care must be taken when extirpating the Tunga to avoid breaking their bodies and releasing several hundreds of eggs into the wound.

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