Emily Martin, Andrew Brooks, Felipe Reggeti, John R Barta, Natasha Slawnych, Perryn Kruth 

 Animal Health Laboratory, University of Guelph, Guelph, ON (Martin, Brooks, Reggeti); Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, ON (Barta, Slawnych, Kruth)

 AHL Newsletter 2023;27(3):20. 

 Since the spring of 2023, the AHL has received 3 submissions of different avian species with a history of sudden death (cygnets, hawk, duckling) that are either confirmed or suspicious for leucocytozoonosis, a blood parasite of birds in the phylum Apicomplexa.  The life cycle requires 2 hosts; black flies (Simuliidae) are the definitive host and transmit the parasite to birds who are the intermediate host.  It is rare that we determine this parasite as the primary cause of death, as most cases have an underlying disease; however, in many areas of southern Ontario, the rainfall is normal to above average for this time of year.  This provides an environment for black fly development since they lay their eggs in moving water.  Once flies emerge, they have an average life span of 3 weeks.  

Figure 1.  Life cycle of Leucocytozoon spp.

Clinical signs of leucocytozoonosis in birds can be related to the location of the developing parasite (Fig. 1):

• listlessness, pallor (anemia - parasite produces antierythrocytic factors);
• dyspnea (large gametocytes occlude pulmonary capillaries);
• CNS signs (megaloschizonts form in vessel endothelium that could occlude vessels causing multifocal necrosis – multiple organs).

Clinical signs are observed approximately 1 week after infection during the onset of parasitemia.  Mortality can vary with strain, species, degree of exposure, age, and immune status.  Death can occur within 24 hours of onset of clinical signs.  Mortality can be up to 70% in ducklings, but is low in adult birds that may have decreased egg production and hatchability.  Approximately 60% of deaths occur 11-19 days post exposure.  Death can occur in commercial species as well, including turkeys, ducks, geese and chickens.

Acute disease is more common in areas with high black fly populations and in young birds with high parasitemia.  Subacute or chronic disease develops in young birds during low insect exposure or in older birds (low parasitemia).  NOTE:  Recovered birds remain carriers and a reservoir of infection for young susceptible birds.  On postmortem examination the blood can appear thin and watery. There may be splenomegaly, hepatomegaly, and hemorrhages as well as visible white foci in multiple organs (megaloschizonts).

Leucocytozoon spp. cannot parasitize other vertebrates, and have wide genetic diversity making diagnosis challenging.  Diagnosis is by identification of gametocytes on blood smears, schizonts in tissue sections, or PCR.  There is no treatment, and control aims to reduce or eliminate the insect vector from the environment of the birds.

In areas with high black fly populations and death primarily in young birds on a property, remember to include this disease on your list of rule outs.  AHL

References

1. Van Wettere, AJ.  Merck Veterinary Manual.  https://www.merckvetmanual.com/poultry/bloodborne-organisms/leucocytozoonosis-in-poultry [1]

2. Valkiūnas G, Iezhova TA.  Insights into the biology of Leucocytozoon species (Haemosporida, Leucocytozoidae): Why is there slow research progress on agents of leucocytozoonosis? Microorganisms. 2023;11(5):1251. doi: 10.3390/microorganisms11051251. PMID: 37317225; PMCID: PMC10221462.

3. Government of Canada EEEC web site.  Wild species 2010: Insects: Black flies.  https://www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/publications/wild-species-2010/chapter-16.html.