Family Trypanosomatidae

The hemoflagellates all belong to the family Trypanosomatidae. Members of this family have a leaf-like or sometimes a rounded body containing one nucleus. They have a single flagellum which arises from a basal granule or blepharoplast posterior to the end of an elongate blind pouch or reservoir and passes anteriorly, usually extending beyond the body. A contractile vacuole opens into the reservoir, but both of these structures can be seen only with the phase microscope and not with the ordinary light microscope (Ketterer, 1952; Cosgrove and Kessel, 1958; Clark, 1959). The flagellar axoneme is composed of 9 peripheral and 2 central fibrils (Anderson, Saxe and Beams, 1956). An undulating membrane is present in some genera; the flagellum lies in its outer border. Posterior to the basal granule is a rod-shaped or spherical kinetoplast containing deoxyribonucleic acid. The structure of the kinetoplast as seen in electron micrographs has been interpreted in different ways. According to Anderson, Saxe and Beams (1956), it consists of lamellae oriented at right angles to its long axis; Meyer and Queiroga (1960) called it an apparently lamellar mass located in a vacuole-like space; Hans Ris (unpublished) said that the lamellae represent sections of a continuous spiral; Clark and Wallace (1958) considered the kinetoplast to be a mitochondrion containing antero-posteriorly oriented anastomosing fibers. Under the ordinary light microscope, the kinetoplast and blepharoplast may appear to be fused. Mitochondria and volutin granules have also been seen in electron micrographs.

Members of this family were originally parasites of the intestinal tract of insects, and many are still found only in insects. Others are heteroxenous, spending part of their life cycle in a vertebrate and part in an invertebrate host.

In the course of their life cycles, members of one genus may pass thru forms morphologically similar to those of other genera. These stages are named for the genera which they resemble. In the Meyer and Queiroga (1960) called it an apparently lamellar mass located in a vacuole-like space; Hans Ris (unpublished) said that the lamellae represent sections of a continuous spiral; Clark and Wallace (1958) considered the kinetoplast to be a mitochondrion containing antero-posteriorly oriented anastomosing fibers. Under the ordinary light microscope, the kinetoplast and blepharoplast may appear to be fused. Mitochondria and volutin granules have also been seen in electron micrographs.

Members of this family were originally parasites of the intestinal tract of insects, and many are still found only in insects. Others are heteroxenous, spending part of their life cycle in a vertebrate and part in an invertebrate host.

In the course of their life cycles, members of one genus may pass thru forms morphologically similar to those of other genera. These stages are named for the genera which they resemble. In the rounded and the flagellum has degenerated into a tiny fibril which remains inside the body (Fig. 1). Further information on life cycles and morphology is given by Noble (1955).

There are several genera in the family Trypanosomatidae. Members of the genus Trypanosoma are heteroxenous and pass thru leishmanial, leptomonad, crithidial and trypanosome stages in their life cycle. In some species, only trypanosome forms are found in the vertebrate host, while in other, more primitive ones, both leishmanial and trypanosome forms are present.

Forms of the Trypanosomatidae

Members of the genus Blastocrithidia are monoxenous in arthropods and other invertebrates. They pass thru crithidial, leptomonad and leishmanial stages in their life cycle. This generic name was introduced by Laird (1959) for the crithidial species commonly and erroneously assigned to the genus Crithidia. As both Laird (1959), Wallace (1943) and Clark (1959) have shown, the type species of Crithidia, from mosquitoes, C. fasciculata, is a short, truncate form with a stiff flagellum emerging from a funnelled anterior depression, and never has an undulating membrane. However, the term crithidial is so deeply Embedded in our terminology as referring to forms with an undulating membrane that it is best to retain it.

Members of the genus Crithidia are monoxenous in arthropods. Despite their name, they have only a leptomonad stage.

Members of the genus Leptomonas are monoxenous in invertebrates. They pass thru leptomonad and leishmanial stages in their life cycle.

Members of the genus Leishmania are heteroxenous, passing thru the leishmanial stage in their vertebrate host and the leptomonad stage in their invertebrate host or in culture.

Members of the genus Herpetomonas are monoxenous in invertebrates. They pass thru trypanosome, crithidial, leptomonad and leishmanial stages in their life cycle. The trypanosome form in this genus differs from that of Trypanosoma in that the undulating membrane lies in a long reservoir which runs the whole length of the body and opens at the anterior end, whereas in Trypanosoma the reservoir is very short and opens laterally near the posterior end so that the undulating membrane runs along the side of the body (Clark, 1959).

Members of the genus Phytomonas are heteroxenous in the latex of plants and hemipterous insects, passing thru leptomonad and leishmanial stages in their life cycle. They are found in milkweeds and related plants, and cause the normally milky sap to become colorless.

The only genera parasitic in domestic animals and man are Trypanosoma and Leishmania. Since, however, their stages in the invertebrate vector are morphologically similar to those of the genera confined to invertebrates, one cannot be positive, when he finds an infected invertebrate, whether it is infected with a parasite of vertebrates or with one of its own. It is possible, too, that some of the forms which we now think are confined to invertebrates may actually be normal parasites of some wild vertebrates.