Avian blood can contain various disease agents, including bacteria, protozoa, microfilariae, and, rarely, fungi. These organisms can be identified by microscopic examination of wet mounts, buffy coats, or blood smears or by appropriate culturing and molecular techniques.
Microscopically, some bloodborne organisms (eg, Plasmodium, Haemoproteus, Leucocytozoon, Isospora, Hepatozoon, Babesia, Aegyptianella) are within blood cells. Others (eg, Trypanosoma, microfilariae, various bacteria) are free in the plasma.
No bloodborne organisms live exclusively in the blood; most are found in tissues but are present in blood during part of their life cycle. Some, such as microfilariae and Plasmodium, have numbers or stages of parasites that vary with time. For these parasites, examining multiple blood smears at intervals increases the likelihood of obtaining a diagnosis.
Seasonal variations in rates of infection by bloodborne organisms relate to the activity of arthropod vectors.
Most bloodborne organisms either cause subclinical infections or are uncommonly associated with disease. Very young, weakened, or injured birds infected with hemoprotozoa may have higher mortality rates and slower recovery than uninfected birds.
Examination for bloodborne organisms should be included in the clinical and diagnostic procedures for ill birds.
Avian bloodborne organisms are best visualized in thin blood smears made immediately after blood is taken, if possible. Adding anticoagulants to, storing, and cooling blood can distort protozoal morphology and introduce artifacts. A Romanowsky-type stain that gives good polychromatic coloration (eg, Giemsa stain) should be used.
Pearls & Pitfalls
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Bloodborne organisms in plasma or in WBCs are concentrated in the buffy coat. Stained buffy coat smears are recommended to detect spirochetes and other bacteria, as well as Leucocytozoon, Trypanosoma, and Isospora.
Direct examination of the buffy coat by dark-field or phase-contrast microscopy is an excellent technique to identify low numbers of motile organisms such as spirochetes and microfilariae. The buffy coat and all of the plasma should be expressed onto a glass slide and covered with a coverslip, which is depressed slightly to spread the buffy coat. The buffy coat–plasma interface should be examined with dark-field or low-light microscopy to detect motile organisms.
Suspected intracellular blood protozoans observed on a thin blood film should first be excluded as either normal structures or artifacts before they can be confirmed as infectious agents. The following should then be determined: what the host cell is and whether it is normal or deformed beyond identification, whether pigment granules (hemozoin) are present or absent, and whether merogony (asexual reproduction) is occurring (see the table Characteristics of Protozoa in Avian Blood).
Characteristics of Protozoa in Avian Blood
Protozoan | Host Cell | Pigment Present | Merogony in Blood |
|---|---|---|---|
Plasmodium | RBC | Yes | Yes |
Haemoproteus | RBC | Yes | No |
Leucocytozoon | RBC or WBC; distorted and enlarged, often beyond recognition | No | No |
Isospora | WBC; in nuclear indentation | No | Noa |
Hepatozoonb | WBC; large, elongated oval shape | No | No |
Babesiab | RBC | No | Noc |
aMultiple intracellular parasites may be present in acute infections. | |||
bUncommon to rare. | |||
cBabesia is piriform and can be in a V, X, or fan pattern. | |||
Identification of an organism beyond genus is difficult and usually unnecessary for clinical purposes.
Cytological evaluation of organs (through impression smears) and of blood smears are useful adjuncts to histological examination for postmortem examination.
Serological and molecular diagnostic methods have been developed for avian hemoparasites. Molecular methods are very sensitive and can detect infection when parasite numbers are too low to be detected in blood or tissue smears or by histological examination.
Hemoparasites can also be studied by subinoculation of infectious blood in birds of the same or a known susceptible avian species. Bacteria can usually be identified by blood culture and molecular methods.
