When plague is prevalent in a locality, the diag nosis is easy in fairly well-marked cases of the bubonic type, but less so in the other varieties. When it is not prevalent the diagnosis is never easy, and in pneumonic and septicaemic cases it is impossible without bacteriological assistance. The earliest cases have hardly ever been even suspected at the time in any out break in a fresh locality. It may be taken at first for almost any fever, particularly typhoid, or for venereal disease or lymphangitis. In plague countries the diseases with which it is most liable to be confounded are malaria, relapsing fever and typhus, or broncho pneumonia in pneumonic cases.
The investigations of the Indian plague corn mission working in conjunction with the advisory committee showed that bubonic and septicaemic plague are transmitted by the agency of fleas. Highly susceptible animals are not infected by their plague-stricken fellows so long as fleas are excluded from the cages, but with the introduction of these ectoparasites the disease forthwith spreads from the sick to the healthy. Sub sequent experience has confirmed the main findings of the com mission, and outbreaks of human plague are unquestionably mere extensions of preceding or concurrent plague affecting animals. Rats (Rattus rattus) are of outstanding importance as reservoirs of infection through their close association with man, but mar mots (Arctomys bobac), gerbilles (Tatera lovengulae) and ground squirrels (Citellus beecheyi) have been responsible for epidemic outbreaks in China, South Africa and California respectively.
The plague commission concluded that the disease can be trans mitted by the faeces of infected fleas, but did not state that this is the usual method of spread. Bacot and Martin later demon strated that the entrance to the flea's stomach becomes obstructed by a mass of plague bacilli. Such fleas in their persistent efforts to feed merely succeed in distending the gullet with blood, which, now contaminated, regurgitates on to the skin of the host. The organisms then enter the body either through the flea's bite or through some abrasion of the skin.
The flea most commonly incriminated as a plague-vector is Xenopsylla cheopis, but several other species of rat fleas (Cerato phyllus fasciatus, Stivalius ahalae), the common mouse flea (Lep topsylla musculi), the dog flea (Ctenocephalus canis), a marmot flea (Ceratophyllus silantievi) and the human flea (Pulex irritans), amongst others, are proven carriers of plague.
Laborious studies on the biology of fleas have added to our knowledge of the epidemiology of plague. These insects cannot withstand desiccation, and their survival apart from their host is proportional to the rate at which they lose moisture. Bacot and Martin (1924) found the water content of fleas to be 8o% of their weight, and death occurred before the loss of water had reached 6o% of their weight. This goes far to explain the observation that plague does not maintain itself in epidemic form when the temperature rises above 8o° F, accom panied by a saturation deficiency of over .30 of an inch.
The seasonal variations in the spread of plague have been recognised for many centuries. In temperate countries epidemics attain their maximum in the summer months, whereas in tropical climates the onset of hot, dry weather brings an existing epidemic to an end. In each case, the peak of the plague curve coincides with meteorological conditions most favourable to the activity and longevity of fleas. In contradistinction to the foregoing, pneumonic plague, once established, spreads independently of animals and fleas. In outbreaks of ordinary plague, a secondary involvement of the lungs supervenes in a small proportion of cases, and if atmospheric and environmental conditions favour the survival and transmission of the bacilli an epidemic of pneumonic plague may result, the virus being transmitted from the cougher to the victim up to a range of about three feet.
Unfortunately, there is no plague specific. Serum, to be really effective, must be given before the symptoms of the disease are manifest, an impossible procedure in most cases. Still, when available, serum should be administered intravenously in doses of 8o–ioo c.c.'s, and repeated. Some ad vocate injection of tincture of iodine (7-10 minims), or other antiseptics, intravenously, and also into the buboes. These measures are of doubtful benefit. Buboes showing signs of sup puration should be incised. Otherwise, treatment is on general lines, and cardiac stimulants should be administered early.