Leukocytozoon

Leukocytozoon is a genus of parasitic protozoa belonging to the phylum Apicomplexia. The parasites were first seen by Danilewsky in 1884. The genus was created by Ziemann in 1898. This taxon was revised by Berestneff in 1904 and then by Sambon in 1908.

The species of this genus use blackflies (Simulium species) as their definitive host and birds as their intermediate host. There are over 100 species in this genus. Over 100 species of birds have been recorded as hosts to these parasites.

Life cycle
Sporozoites are found in the salivary glands of an infected Simulium fly and are transmitted to the bird when the fly bites the bird to feed on its blood. The injected sporozoites find their way to the liver where they infect hepatocytes and develop initially into trophozoites and subsequently into schizonts. These schizonts divide into merozoites after 4-6 days. The duration of this stage depends in part upon the species.

The newly released merozoites infect either erythrocytes, leukocytes, macrophages or endothelial cells. Those infecting the macrophages or endothelial cells develop into megaloschizonts. The megaloschizonts divide into primary cytomeres, which in turn multiply into smaller cytomeres, which mature into schizonts, which in turn divide into merozoites.

In erythrocytes or leukocytes, merozoites develop into gametocytes.

If a non-infected fly feeds on an infected bird at this point they will ingest the gametocytes.

The gametocytes mature in the insect midgut into macrogametocytes (female) and microgametocytes (male) which fuse to form an ookinete.

The ookinete penetrates an intestinal cell of the fly and matures into an oocyst. After several days the oocyst produces sporozoites that leave and migrate to the salivary glands of the fly.

Diagnostic criteria
Form gamonts in white blood cells and/or erythrocytes. No merogony occurs in either leucocytes or erythrocytes.

Merogony occurs in the parenchyma of liver, heart, kidney, or other organs. Meronts may form large bodies divided into cytomeres.

Hemozoin deposits are not formed.

Oocysts develop rapidly in 3 - 5 days. The oocysts are small and nonexpanding, reaching 13 micrometres in diameter and typically have less than 100 short, thick sporozoites.

The vectors are Simulium or Culicoides species.

The vertebrate hosts are birds.

Type species: Leucocytozoon ziemanni

Pathology
The typical pathology of infection with these parasites includes anaemia and enlargement of the liver and spleen. Gross lesions also include pulmonary congestion and pericardial effusion.

Megaloschizonts appear as grey-white nodules found in the heart, liver, lung or spleen. Microscopically there is ischemic necrosis and associated inflammation in the heart, brain, spleen and liver due to occulsion of blood vessels by megaloschizonts in endothelial cells. Ruptured schizonts may induce granulomatous reactions in the surrounding tissues.

Clinically the majority of birds affected with leucocytozoonosis exhibit no signs. Among those that do the signs include mild to severe signs of anorexia, ataxia, weakness, anemia, emaciation and difficulty breathing.

The excess mortality due to Leukocytozoon in adult birds seems to occur as a result of debilitation and increased susceptibility to secondary infection.

Host range
Bird hosts


 * L. anatis - ducks
 * L. andrewsi - chickens (Gallus gallus domesticus)
 * L. anseris - geese
 * L. artamidis - wood-swallows (Artamidae species)
 * L. atkinsoni - Old World babblers (Timaliidae species)
 * L. balmorali - bushshrikes (Malaconotidae species)
 * L. bennetti - hook-billed vanga (Vanga curvirostris)
 * L. berestneffi - yellow-billed magpie (Pica nuttalia)
 * L. bishopi - parrotbills (Paradoxornithidae species)
 * L. bonasae - ruffed grouse (Bonasa umbellus), spruce grouse (Canachites canadensis),  ptarmigan
 * L. bouffardi - weaver birds - (Ploceidae species)
 * L. brimonti - bulbuls (Pycnonotus species)
 * L. caulleryi - chickens (Gallus gallus domesticus)
 * L. gentili - house sparrows (Passer domesticus)
 * L. hamiltoni - Bukharan great tit (Parus bokharensis)
 * L. lovati - grouse, hawks, wild rock ptarmigan (Lagopus mutus)
 * L. macleani - chickens (Gallus gallus domesticus)
 * L. marchouxi - pigeons and doves
 * L. nectariniae - sunbird (Nectarinia olivacea)
 * L. neavei - guinea fowl
 * L. pogoniuli - Pogoniulus subsulphureus, Pogoniulus atroflavus
 * L. sabrazesi - chickens (Gallus gallus domesticus)
 * L. schoutedeni - chickens (Gallus gallus domesticus)
 * L. simondii - ducks, geese, Emperor Goose (Chen canagica)
 * L. smithi - turkeys (Meleagris species)
 * L. roubaudi - estrildid finches (Estrildidae species)
 * L. toddi - grouse and hawks including the sharp-shinned hawks (Accipiter striatus), Chimango Caracara (Milvago chimango)
 * L. trachyphoni - Trachyphonus purpureus

Simulium hosts


 * L. bonasae - Simulium aureum and Simulium latipes
 * L. simondi - Simulium rugglesi