Incubation period days

Period of communicability An unblocked infected flea can remain alive for several months able to transmit infection. Pneumonic plague is highly infectious and can spread rapidly within a concentration of people although infected individuals will remain alive for only a few days.

Occurrence and distribution Plague is a classic example of an ectoparasite zoonosis, the greatest of all epidemic diseases. It has ravaged the Orient, Asia and Europe, altering the course of history. Today it is confined to established foci (Fig. 16.2) from which it erupts from time to time, but fortunately effective control now prevents the uncontrollable pandemics of the past.

Plague is a disease of civil disturbance and war. In recent history, the largest human focus has been in Vietnam. Persistent endemic foci in Madagascar and East Africa continue to produce cases, while a worrying exacerbation was an outbreak of pneumonic plague in Ecuador in 1998.

Control and prevention The methods of control depend upon the transmission cycle involved (Fig. 16.1).

Wild rodent plague foci are often extensive and harmless, and to try and destroy them a considerable task. If they are localized and close to habitation, then it might be feasible to alter the environment by cultivation or in a way that discourages rodents. Precautions need to be taken that a plague epidemic is not generated by such activity. Where hunters or soldiers have to pass through a plague focus, then personal protection can be obtained from long trousers tucked into socks, treated with repellents or insecticides. Warnings should be given about the danger of touching or eating any animals killed.

Domestic rodent plague depends upon the two components of the rat and the flea, but the order in which they are attacked is crucial according to the stage of the disease. To kill rats during a plague epidemic only makes the infected fleas search for a human host and increase spread. In the presence of plague, the fleas must be controlled first. Using insecticide powder (permethrin, bendiocarb, carbaryl or fenitrothione), burrows can be insufflated and rat runs liberally dusted. Rats pick up insecticide on their fur and take it into their nests with them. Fleas do not like cleanliness and

Fig. 16.2. Known and probable foci of plague, 1959-1979. (Reproduced by permission, from WHO (1980) Weekly Epidemiological Record 32, p. 234. World Organization, Geneva.)

Fig. 16.2. Known and probable foci of plague, 1959-1979. (Reproduced by permission, from WHO (1980) Weekly Epidemiological Record 32, p. 234. World Organization, Geneva.)

Health

NJ lu people should be encouraged to wash with soap and warm water. Clothes can be searched and fleas picked off, but it is preferable to boil clothing (see Box 16.1 for the control of rats).

Plague vaccine will protect persons at risk for several months, but should not be relied upon. Chemoprophylaxis with tetra-cycline 250 mg four times a day or doxycy-cline 100 mg a day for a week should be given to close contacts of cases and medical workers at risk.

Quarantine of all cases is required by international health regulations for a period of 6 days. All persons should be dusted with insecticides to remove fleas and precautions taken to prevent aerosol spread from pneumonic cases. Any cases dying of plague should be buried or burnt with aseptic precautions.

Treatment Effective treatment depends upon the speed of making a diagnosis and treating early. If plague has already been diagnosed in the area, then a confirmatory test should not be awaited. The clinical presentation of fever and bubo in a severely ill patient is sufficient and treatment needs to be started immediately. This is either by:

• Streptomycin 1 g followed by 0.5 g every 4 h, up to a total of 20 g;

• Tetracycline 3 g immediately, followed by 1 g three times a day for 12 days;

• Doxycycline 100 mg every 12 h for 7 days;

• Chloramphenicol 500 mg every 6 h for 7-10 days.

Gentamicin and co-trimoxazole have also been used.

Streptomycin is the treatment of choice, but can cause a Herxheimer reaction, so tetracycline is preferable in the critically ill. Resistant strains have occurred and the sensitivity of the organism should be monitored.

Isolation of cases is mandatory and the terminal bubonic case with pneumonia or pneumonic plague is highly infectious and, therefore, extreme precautions should be taken. Gowns and full-face masks should be worn, while goggles are required to protect the eyes as Y. pestis can be absorbed through the conjunctiva.

Surveillance of foci should be maintained with regular trapping of rodents to examine them for infection and their flea populations. Notification of any confirmed or suspected case of plague must be made to WHO and neighbouring countries. Any person travelling from an area where there have been cases of plague should be placed under surveillance for 6 days.

16.2 Typhus

There are many similarities between the epidemiology of typhus and plague, and it is convenient to approach the disease in the reverse order to which it is normally described in order to assist in its description. While plague is a composite disease of three different cycles utilizing the same organism and vector, there are three different forms of typhus (scrub, murine and epidemic), each with its own organism and vector (Fig. 16.3).

Organism The causative organism of typhus is a Rickettsia or Orientia, an intracellular bacteria which requires cellular tissue of the host or ectoparasite to develop and reproduce. It can survive in the environment if suitable conditions prevail (e.g. in louse faeces); otherwise, it is sensitive to heat (being killed by a temperature of 60°C for 30 min) and easily by antiseptics.

The typhus-producing organisms and their ectoparasites are as follows:

Scrub

Orientia

Trombiculid

typhus

tsutsugamushi

mites

Murine

Rickettsia typhi

Flea, X.

typhus

(R. mooseri)

cheopis

Epidemic

typhus

R. prowazekii

Human louse

Clinical features Typhus was confused with typhoid for a considerable period of time because they both produced fever, prostration and a rash. Indeed, typhoid obtained its name only when it was finally separated from typhus as being a less infectious disease, with markedly abdominal symptoms and a milder rash.

In the most severe form, epidemic typhus, there is a sudden onset with headache, pains, rigours and malaise as the temperature rapidly rises to 40°C or more; where it remains for the duration of the illness. The characteristic rash appears between the fourth and seventh day and consists of petechial haemorrhages on the trunk and limbs, but sparing the face, palms and soles. As the disease progresses, the patient becomes semi-stuporous with confusion, anxiety and considerable dullness. The patient appears unable to hear, talks nonsense and has to be fed. By the third week, if treatment has not been given, the patient will progressively recover or else sink further into heart failure, bronchopneumonia and death.

In scrub typhus, the illness similarly commences with fever, progressive prostration and a macular rash, but after a few days, the infective mite bite develops into an eschar. This is a red indurated area with central vesicle that subsequently breaks down to leave a black scab. The severity of the disease varies markedly from area to area, being a severe and fatal illness, similar to epidemic typhus in some places, while in others, so mild and innocuous that it passes as 'flu'. I remember visiting a school near a well-known mite island, which expected all new students to have a minor illness for a day or two and then be immune for the rest of their academic stay.

In murine typhus, there is fever, followed by a rash, but the illness is milder than epidemic typhus, mortality is low and complications rare so that most people fully recover in 7-10 days.

Diagnosis will probably be on clinical grounds, the escar of scrub typhus being characteristic, but where available, the indirect fluorescence antibody test, labelled enzyme with ELISA, latex agglutination or PCR can be used.

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