Xenodiagnosis

XENODIAGNOSIS (from Greek xenos—stranger, guest), 'diagnosis with the help of a host' (Brumpt; 1914), is a method of discovering a parasite, establishing its species or properties by using an intermediate host or vector for the possible cultivation of the microorganism being studied within it. Sometimes parasites are very rare or completely invisible in the blood. By feeding their suspected intermediate host or vector on an animal with such parasites, one can discover the sought-after parasites in the intestine of the former in various stages of development. For such a 'hemoculture' of a parasite in the intestine of a vector or host, not only natural hosts but also third-party bloodsuckers can be used. For example, trypanosomes of vertebrates can be detected during a latent infection in leeches or in kissing bugs if they are fed on the corresponding vertebrate. The xenodiagnosis method is used as an auxiliary means for determining biological (species) differences of protozoa, as well as spirochetes, which are morphologically indistinguishable from one another: for example, the tick Rhipicephalus appendiculatus, fed on a cow infected with Theileria dispar, does not transmit theileriosis to another cow; at the same time, larvae of the same tick from the same brood transmit Theileria parva. Hence the conclusion that, by biological properties, Theileria dispar is not identical to Th. parva. Similar methods are also used for the differentiation of relapsing fever spirochetes. For example, the tick Ornithodorus moubata transmits Spirochaeta Duttoni, Sp. crocidurae (Dakar), Sp. hispanicum, Sp. normandii (Tunisia), Sp. sogdianum (Central Asia) and does not transmit Sp. venezuelense (South America) and Sp. gondii (Tunisia); Ornithodorus marocanus transmits Sp. hispanicum and Sp. sogdianum and does not transmit Sp. Duttoni. Xenodiagnosis as a method undoubtedly has important significance and should be used among other auxiliary means for establishing species differences of microorganisms and metazoan parasites. However, negative results must be studied particularly carefully. The fact is that even a genuine vector does not always actually transmit the microorganism characteristic of it. Various environmental factors can hinder transmission. Furthermore, the age (resp. condition) of the vector can also play a similar role. Theiler and du Toit recently (1928) provided an interesting example of such relationships. In their experiments, adult Rhipicephalus appendiculatus ticks aged 144 days or more did not transmit theileria when emulsions from infected ticks were injected into the veins of a cow; however, experiments with the same ticks, but 16 days of age, caused infection of cows with theileriosis. At the same time, the age of the nymphs in similar experiments was irrelevant.