Leucocytozoon spp are common blood parasites in birds, especially nondomestic birds (see red-tailed hawk blood image). More than 30 species have been reported. Leucocytozoon spp are host specific at the avian order level and sometimes at the family level.
Courtesy of Dr. H. J. Barnes.
Infections with Leucocytozoon spp are usually subclinical; occasionally, however, they cause clinical signs and even death. Mortality rates vary greatly with the parasite strain, species, degree of exposure, age, immune status, and other factors.
Outbreaks of leucocytozoonosis have been reported in chickens (Asia, Africa), turkeys (North America), waterfowl (North America, Europe, Asia), and a number of free-living and captive avian species throughout the world. Species found in domestic birds include L simondi in waterfowl; L smithi in turkeys; and L caulleryi, L sabrazesi, L andrewsi, and L schoutedeni in chickens.
L caulleryi can be highly pathogenic, causing a lethal hemorrhagic disease of chickens in Southeast Asia. L simondi can cause death in ducks and geese, and L smithi can cause death in turkeys. Numerous Leucocytozoon spp infect nondomestic birds.
Clinical signs of leucocytozoonosis and death result from anemia caused by antierythrocytic factors produced by the parasite, high numbers of the large gametocytes blocking pulmonary capillaries, or parasites invading the endothelium of vessels in tissues (brain, heart, etc), where they form megaloschizonts that occlude vessels and result in multifocal necrosis.
In the Northern Hemisphere, parasitemia often increases dramatically in late April and early May (called spring rise), just before populations of arthropod vectors, black flies (Simulium spp), and biting midges (Culicoides spp) increase.
Ducks that have recovered from infection with L simondi might relapse when light cycles are manipulated to increase egg production. Increased levels of prolactin have been suggested as a possible cause.
Acute leucocytozoonosis occurs more often in young birds with high parasitemia and when black flies or biting midges are most abundant. Subacute or chronic disease occurs in the young outside of fly season and in older birds in any season; parasitemia is usually low.
Recovered birds remain carriers of leucocytozoonosis and serve as a reservoir for young, susceptible birds.
Clinical Findings of Leucocytozoonosis in Poultry
Anemia
Respiratory signs
Neurological signs
Ducklings or turkey poults with leucocytozoonosis are listless and show various combinations of the following clinical signs: anemia, leukocytosis, tachypnea, anorexia, diarrhea with green droppings, and CNS signs.
Mortality rates in ducklings can reach 70%. Mortality rates are low in adult ducks or turkeys.
Egg production and hatchability are decreased.
Clinical signs of leucocytozoonosis are evident approximately 1 week after infection and coincide with the onset of parasitemia. Visibly affected birds die after 7–20 days or might recover with sequelae of poor growth and poor egg production.
Lesions of Leucocytozoonosis
Hemorrhages, splenomegaly, and hepatomegaly occur with leucocytozoonosis. Grossly visible white dots in affected organs are Leucocytozoon megaloschizonts. Histological lesions are associated with megaloschizont development in the spleen, liver, heart, and other organs.
Infection with L caulleryi in chickens has a tropism for the reproductive tract and is associated with oviduct inflammation and edema, as well as decreased egg production. Peritoneal, perirenal, and subdural hemorrhages are reported with severe disease.
Diagnosis of Leucocytozoonosis in Poultry
Blood smears
Tissue cytological or histological examination
Serological testing
PCR assay
Diagnosis of leucocytozoonosis is typically based on blood smears. Leucocytozoon gametocytes can be observed in blood smears (see Muscovy duck lung image), especially along the edges and tail of the smear. Leucocytozoon is identified by large gametocytes that lack pigment and distort the host cell (RBC or WBC), making it no longer identifiable. The gametocyte shape varies: some are elongated with long tapering extremities; others are round.
Courtesy of Dr. A. J. Van Wettere.
Serological testing can detect prior infection with Leucocytozoon.
PCR tests have been developed for diagnosis of leucocytozoonosis.
Treatment and Control of Leucocytozoonosis in Poultry
Treatment not usually effective
Preventive medication for control
Treatment of leucocytozoonosis usually is not effective. Preventive medication using pyrimethamine and sulfadimethoxine combined in the feed is used to control L caulleryi. Clopidol is used to control L smithi. Measures to control invertebrate vectors are helpful.
Humoral immunity resulting from vaccination can protect against L caulleryi infection.
In Leucocytozoon-infected raptors treated with quinacrine hydrochloride or trimethoprim/sulfamethoxazole solution, parasitemia is decreased, but the infection is not cleared. Local guidance must be followed for withdrawal and residue avoidance times for all medications in food-producing species.
Key Points
Leucocytozoon spp infection is usually subclinical; however, anemia and respiratory or neurological signs can occur in chicken, turkeys, and waterfowl.
Diagnosis is based on examination of blood smears, tissue cytological evaluation, histological examination, and/or PCR assay.
Drug treatment can decrease parasitemia but does not eliminate parasites.
Administering preventive medication and limiting insect vectors help to control the disease in susceptible species.
For More Information
McDougald LR, Cervantes HM, Jenkins MC, Hess M, Beckstead R. Protozoal infections. In: Swayne DE, ed. Boulianne M, Logue CM, McDougald LR, Nair V, Suarez DL, associate eds. Diseases of Poultry. 14th ed. Wiley Blackwell; 2020:1192-1254.
Forrester DJ, Grainer EC. Leucocytozoonosis. In: Atkinson CT, Thomas NJ, Hunter BD, eds. Parasitic Diseases of Wild Birds. Wiley-Blackwell; 2008:54-107.
