I

FIGURE 10.17 Haemoproteas sp., avian protozoan developing in blood cells of mourning dove, transmitted by biting midges. (Photo by Mary E. Hayes/Rogers.)

gametocytes, the sexual stage of the parasite. In the midgut of the midge, the gametocytes are released, where they unite to form a motile zygote, the ookinete. The ookinete typically penetrates the peritrophic membrane and midgut tissue to form a cystlike structure, or oocyst, on the outer midgut wall. Within the oocyst, sporozoites are produced asexually, eventually rupturing from the mature oocysts into the hemocoel, where they make their way to the salivary glands and accumulate there. The sporozoite is the infective stage which is transmitted via the saliva to suitable hosts when the biting midge subsequently blood feeds. Development of the parasite in Culicoides species usually takes about 6— 10 days.

Upon entering the vertebrate host, sporozoites invade cells of fixed tissues, notably the endothelium of various organs, and myofibroblasts, precursor cells that form muscle fibers. There they undergo one or more cycles of asexual reproduction, called schizogony, to produce mero-zoites. The merozoites then invade the blood and penetrate circulating erythrocytes. There they develop into gametocytes, thereby completing the life cycle.

The species of blood protozoans which are known to be transmitted by biting midges are summarized in Table III. With the exceptions of Haemoproteus kochi, which parasitizes Old World monkeys, and H. brayi, which parasitizes Malaysian squirrels, these haemospori-dians are parasites of birds. The species are primarily members of the genus Haemoproteus, all of which are transmitted by Culicoides spp. Relatively few details are known about most of these arthropod-borne haemo-sporidians and their associated Culicoides vectors, What is known is based primarily on studies of the following two species, which are parasites in poultry.

Haemoproteus meleagridis

H. meleagridis is primarily a parasite of wild and domestic turkeys, It also can cause at least transient infections

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