Description and Distribution
Leucocytozoonosis is a disease caused by a protozoan blood parasite transmitted by the bite of a blackfly (Simuliidae). Both wild and domestic avian species are susceptible to infection with Leucocytozoon species: L. simondi (ducks and geese), L. smithi (turkeys), L. bonasae (grouse and ptarmigan) and L. marchouxi (pigeons and doves). An unspecified species has been found in an osprey in Michigan. L. simondi has been implicated in major die-offs of Canada geese at the Seney National Wildlife Refuge in the past, occurring approximately every 4 to 5 years. It was believed to be one of the major factors limiting the growth of the Seney Canada goose population. Die-offs generally occur in the spring (mortality may reach 100% in the goslings) but deaths due to leucocytozoonosis can be seen anytime during the blackfly season (May-August). In North America, the distribution of L. simondi is the northeastern and northern midwestern U.S. and Canada. L. smithi infects both wild and domestic turkeys and has been responsible for economic losses to the southeastern U.S. turkey growers. Leucocytozoonosis has not been demonstrated to be a disease problem in wild turkeys.
Transmission and Development
Leucocytozoon transmission begins each year with a spring relapse in previously infected birds. Birds with circulating elongate gametocytes are infective while those with circulating round gametocytes do not experience this relapse and are not infective. The circulating elongate gametocytes from the bird enter a blackfly during a blood meal. Sexual development takes place within the blackfly digestive system and sporozoites enter the salivary glands of the blackfly where they are injected into a susceptible avian host during a subsequent blood meal.
In ducks and geese, the development of L. simondi progresses through 2 asexual tissue stages, with initial hepatic schizogony developing into round gametocytes in red blood cells and later megaloschizonts found primarily in the spleen, developing into elongate gametocytes in white blood cells. The rupture of hepatic schizonts has been observed to occur around post exposure day 5 and rupture of splenic megaloschizonts occurs around day 10.
The appearance of both round and elongate gametocytes is related to the pathogenicity of the Leucocytozoon strain, with the more pathogenic forms progressing to the elongate stage. Circulating L. simondi gametocytes can be found in the bloodstream of infected birds from the time of spring relapse until early fall (April-September). In turkeys, L. smithi gametocytes persist in the bloodstream and may be found throughout the year, although in greatly reduced numbers during the winter months.
Clinical Signs and Pathology
The majority of birds affected with leucocytozoonosis exhibit no clinical signs. Those that are visibly affected show mild to severe signs of anorexia, ataxia, weakness, anemia, emaciation, and have difficulty breathing. Birds may die acutely at the time of hepatic schizont or splenic megaloschizont rupture, or chronically as a result of rupture of slower developing brain megaloschizonts. It is believed that the mortality in adult birds occurs as a result of debilitation and increased susceptibility to secondary infection.
A diagnosis can be made by the demonstration of gametocytes in blood smears. Histopathological examination of the liver, spleen and brain can show developing Leucocytozoon megaloschizonts. Necropsy may reveal an enlarged spleen and liver. Since the majority of birds are subclinically infected with Leucocytozoon, other causes of death must be ruled out even with the presence of Leucocytozoon gametocytes in peripheral blood smears.
Treatment and Control
Control of the Leucocytozoon organism in domestic avian species has been limited to control of the blackfly vector. Findings indicate that Clopidol (active ingredient in Coyden 25 coccidiostat - Dow Chemical Co., Midland, MI) may be useful in the control of leucocytozoonosis in domestic and wild waterfowl. Clopidol has been used successfully to control leucocytozoonosis in domestic turkey rearing operations in the southeastern U.S.
There is a potential for leucocytozoonosis outbreaks in waterfowl breeding grounds in the spring at Seney National Wildlife Refuge where severe losses may greatly depress population growth. Domestic duck rearing operations have been removed from parts of the northern Lower and Upper Peninsulas of Michigan due to leucocytozoonosis. Leucocytozoonosis is of no public health significance since it is not infective to humans, and infected waterfowl are safe for human consumption.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.