The species of flea commonly found on rats are five in number, and the readiness with which they bite human beings has been carefully studied.
1. Xenopsylla cheopis. This is the oriental rat-flea first described by Mr Charles Rothschild from specimens collected in Egypt. The true home of this flea appears to be the Nile valley, where it may be found in plenty on various hosts. Many of these are desert animals and the flea shows a preference for rodents. Having been distributed all over the world by rats, it now occurs, occasionally, in all warm climates. It is the common rat-flea of the tropical and sub-tropical world. In India it often happens that the whole of the fleas collected from rats prove to be of this species. But it cannot, apparently, flourish in cold countries. In the warmer temperate zones, such as the Mediterranean and Australian seaports, it occurs in varying proportions according to the time of year. The numbers decline with cold weather. It readily bites man and is more active than any other flea in the transmission of plague. For this reason it is sometimes spoken of as “the Plague-flea.” It is a smaller and a lighter coloured insect than the human flea.
2. Ceratophyllus fasciatus. This is the common European rat-flea. It is the rat-flea of the temperate as opposed to the hot countries of the world. It is commonly found on black and brown rats in the British Islands and the other countries of Northern and Central Europe. It readily bites man, and there is no reason to suppose that, other conditions being equal, it would not be as efficient an agent in spreading plague as the last species has been shown to be in India.
3. Ceratophyllus anisus. This is a closely allied species of rat-flea which replaces the last in China and Japan.
4. Leptopsylla musculi. This is the mouse-flea and it is as widely distributed over the globe as its host. From mice it frequently moves to rats, and it has been found on them in various parts of Europe, America, Australia, and Japan. It occasionally bites man, but evinces little inclination to do so.
5. Ctenophthalmus agyrtes. This flea is commonly found as a parasite of voles and field-mice. When farm-rats take to an open life in the fields they pick up this species from the rustic rodents. In Hertfordshire, Hampshire and Suffolk one half the fleas from rats, collected in farmyards and hedgerows, were found to belong to this species; but whether it is as common on rats all over England is unknown. It appears not to bite man. A closely allied flea (Ct. assimilis) is found in central Europe on field-mice and equally on rats which live under the same conditions. It has not been found in England.
The principal occasional parasites of rats are dog-fleas, cat-fleas, fowl-fleas, and human fleas. The proportions in which they and rat-fleas are found vary greatly in different parts of the world. For instance, in San Francisco nine per cent. of the fleas collected from rats have sometimes been found to be human fleas; whilst in Italy as many as twenty-five per cent. have been identified as cat-and dog-fleas.
It must be borne in mind that when new countries are opened up by man the rats, which follow in his rear, exterminate numbers of the weakly native small mammals and take on their fleas. A change of habitat may be followed by an exchange of fleas.
Some interesting work has been done in testing the appetite of different kinds of flea for human blood. The oriental rat-flea (X. cheopis) has been kept alive for three weeks on that diet. Other species show repulsion for mankind and refuse to suck. The experiments confirm the popular belief that fleas have a marked preference for certain individuals. When the flea has refused to bite the human arm, it becomes necessary to check the experiment by trying whether the refusal is merely due to want of hunger. For this purpose a rat must be at hand. It can be secured on a board by two bandages fixed at each end by drawing pins. The rat lies, of course, on its back with its head comfortably supported by a little pillow of cotton wool. A portion of the rat’s abdominal wall is left exposed and shaved. The flea, in an inverted test-tube, can then be put on the hairless patch of the abdomen and given an opportunity of biting, which it may or may not accept.