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An Overview of Atoxoplasmosis in Birds
K. Leigh Sheridan, DVM and Kenneth S. Latimer, DVM,
PhD
Class of 2002 (Sheridan) and Department of Pathology
(Latimer), College of Veterinary Medicine, The University of Georgia, Athens,
GA 30602-7388
Introduction
Atoxoplasmosis is a parasitic disease primarily of passerine
birds, especially canaries, finches, sparrows, grosbeaks, starlings, and mynahs.
It is caused by species of the coccidian protozoan Atoxoplasma, a host-specific
parasite. Atoxoplasma serini, the species found in canaries, is not
infectious to sparrows; likewise, the species infecting sparrows is non-infectious
to canaries.1,2 The parasite is transmitted via the fecal-oral
route and not by mites as was formerly thought when it was known as Lankesterella.1 Parasitism may cause rapid and fatal disease in fledgling birds. This particular
parasite is a special threat to successful species preservation of endangered
Bali mynahs (Fig. 1).
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| Fig. 1. The Bali
mynah (Leucopsar rothschildi), also known as Rothschild's starling
or Jalak Bali, is characterized by a white body, black wingtips, and
blue patches around the eyes. |
Much controversy has attended the study and classification
of Atoxoplasma sp. This genus as been variously associated with Plasmodium, Haemoproteus, Toxoplasma sp. in birds, as well as the
amphibian parasite Lankesterella sp. over the past hundred years.3 Due to its morphologic similarity to Isospora sp. it was most recently
considered a stage of that parasite’s life cycle or a new species of Isospora altogether, called Isospora serini.4 Although
some researchers favor a return to the genus Isospora, Atoxoplasma spp. are currently differentiated from Isospora sp. according to the
site of asexual amplification or merogony.5,11
Life Cycle
Following ingestion of sporulated oocysts and release
of sporocysts, the sporocysts of Isospora and Atoxoplasma invade
intestinal epithelial cells. Isospora undergoes merogony in the intestinal
epithelium, whereas Atoxoplasma enters the blood stream via the vasculature
of the small intestine.6 Atoxoplasma sporocysts subsequently
invade mononuclear leukocytes and undergo asexual division in circulating
and tissue lymphocytes, monocytes, and macrophages, as well as in intestinal
epithelial cells.6,7,8,9 The resulting merozoites form microgametes
and macrogametes. Gametogony, the sexual stage of the coccidian life cycle,
occurs in intestinal epithelial cells.5 Here the micro- and macrogametes
combine to form zygotes, which then undergo multiple fission cycles to produce
sporozoites within oocysts. These unsporulated oocysts are passed in the feces
beginning nine to ten days post-infection and continue to pass for months,
long after any clinical signs in the surviving birds have resolved (Fig. 2).1,4 The oocysts sporulate in the environment and are then infective. Oocysts may
be identified by fecal floatation, but atoxoplasma and isosporan oocysts may
be difficult to distinguish. Oocysts of both organisms contain two sporocysts
with four sporozoites each (Fig. 3).6
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| Fig. 2. Life cycle
of Isospora sp. Atoxoplasma sp. life cycle is similar
except infection of mononuclear cells occurs in blood and tissues where
the organism proliferates by binary fission (Gardiner CH, Fayer R, Dubey
JP: An Atlas of Protozoan Parasites in Animal Tissues. Washington, DC:
USDA/ARS, Agriculture Handbook # 651 p.31). |
Fig. 3. Schematic drawing
of Atoxoplasma sp. oocyst containing two sporocysts with four
sporozoites each (Gardiner CH, Fayer R, Dubey JP: An Atlas of Protozoan
Parasites in Animal Tissues. Washington, DC: USDA/ARS, Agriculture Handbook
# 651 p.20). |
Clinical Signs
Clinical signs of disease
in fledglings are nonspecific and include diarrhea, anorexia, depression,
ruffled feathers, ataxia, and
hepatic enlargement that may be grossly visible through the living bird’s
skin as "black spot." 5,7,10 Mortality rates for atoxoplasmosis
approach 80% in young passerines and the disease can devastate an aviary.7 Adult birds that are shedding oocysts often lack clinical signs, making elimination
of the parasite from aviaries difficult.7 Patency of infection
lasts for up to eight months due to the long life of macrophages in birds
and the resultant long-lived pool of merozoites.1,11 Oocysts are
very stable in the environment and are not inactivated by most disinfectants.7 Thus, this parasite is especially virulent due to the damage to intestinal
epithelium, liver, spleen, myocardium, and skeletal muscle in young birds.
Atoxoplasma also is insidious due to its long life in persistently infected
adult birds and its stability in the environment.
Diagnosis
Diagnosis of atoxoplasmosis has traditionally been via
postmortem examination and histopathology of fledglings that die acutely or
by fecal floatation on persistently infected adult birds. Identification of
atoxoplasma oocysts is notoriously difficult because of structural similarity
to those of Isospora sp. and sporadic shedding of the organism by infected
birds.5,11 Diagnosis can also be made on the live bird by peripheral
blood smear examination and by cytologic examination of liver or spleen imprints
or aspirates.9 Currently, research is underway to further evaluate
less invasive, more sensitive, and more specific diagnostic tests for atoxoplasmosis
in persistently infected adult birds as well as in sick fledglings.11
At necropsy, infected birds have hepatomegaly and splenomegaly.
Small, white foci are visible grossly on the liver and heart.5,9 The intestines may be distended and have translucent walls.7 Cytologic
and histologic specimens reveal granulomatous to lymphohistiocytic inflammation
of the heart, spleen, intestine, and liver.5,11 Macrophages may
contain atoxoplasma merozoites. This form of the organism is round to oval,
3-5 m m diameter, cytoplasmic inclusion that causes indentation of the host
cell nucleus, giving infected cells a characteristic appearance (Figs. 4-7).5,12 Monocytes
and lymphocytes containing merozoites may be seen within blood vessels on
histologic sections. In the live patient, parasitized lymphocytes can be
found in peripheral blood smears stained with a Romanowsky-type stain such
as Wright’s, Giemsa, Leishman, or Diff-Quik stains (Fig. 8).5,6,7,12 Examination of buffy coat smears increases the likelihood of finding organisms.7 Like macrophages and lymphocytes in tissues, infected circulating cells
display pink-staining intracytoplasmic merozoites causing an indented nucleus.10
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| Fig.
4. Atoxoplasma merozoites within macrophges and lymphocytes in a
section
of liver. H&E stain. |
Fig.
5. Atoxoplasma merozoites within the cytoplasm lymphocytes in a liver
imprint.
Scattered free nuclei from lysed hepatocytes also are present. Wright’s
stain. |
 |
 |
| Fig.
6. Atoxoplasma merozoites within macrophges and lymphocytes in a
section
of spleen. H&E stain. |
Fig.
7. Atoxoplasma merozoites within the cytoplasm lymphocytes in a splenic
imprint. The organisms often push into the nucleus resulting in an
indentation.
Wright’s stain. |
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| Fig.
8. Atoxoplasma merozoites within the cytoplasm of lymphocytes in
a blood
smear. Wright’s stain. |
A recently developed polymerase chain reaction (PCR)
test performed on feces, blood, or tissues of sick or persistently infected
but asymptomatic birds is now available at The University of Georgia by special
request.15 This test has proven very sensitive in initial clinical
trials and should be useful in identifying asymptomatic, infected birds and
determining the efficacy of treatment protocols.10
Treatment
Effective prophylaxis of and treatment for atoxoplasmosis
have not yet been perfected. Reduction of fecal-oral transmission can be achieved
by cleaning cages frequently, using screen or hardware cloth cage bottoms
to separate birds from infected droppings, and by frequently changing drinking
and bathing water to minimize fecal contamination.5 Primaquine
is reported to suppress tissue forms of the parasite, and sulfachlor has been
recommended to reduce oocyst shedding.9 Sulfonamides and amprolium,
administered to adults before the breeding season and again when chicks are
weaned, have also been suggested to reduce chick morbidity.5 However,
none of these treatment protocols have been very effective. The most current
recommendations to prevent and treat atoxoplasmosis in Bali mynahs are available
on-line via the Species Survival Plan protocol. These sources of information
may be consulted at the following URLs: www.worldzoo.org/vetforum/baliatox.htm or www.riverbanks.org/aig/new.htm.
As of this writing, toltrazuril (Baycox), diclazuril,
and sulfachlorpyrazine are under investigation for treatment of systemic atoxoplasmosis
and to evaluate any reduction of oocyst shedding in Bali mynahs.13 Sulfachlorpyrazine
inhibits the intestinal stages of the parasite life cycle and has been shown
to reduce or clear oocyst shedding for as long as it is regularly administered.13 Toltrazuril may reduce mortality from systemic disease, although its efficacy
does not appear to be as great as that of sulfachlorpyrazine.13 However, neither drug completely clears a bird of Atoxoplasma sp. infection.13 Diclazuril has been used in passerines other than Bali mynahs to treat toxoplasmosis;
it may also prove effective against Atoxoplasma sp.13 The
two websites cited above also provide the most current information on dosages
and treatment regimens using these latter drugs.
The Bali Mynah
Atoxoplasmosis presents a special threat in raising
Bali mynahs in captivity. Efforts to raise Bali mynah chicks in captivity
for later release in Bali Barat National Park have been seriously hampered
by the widespread occurrence of Atoxoplasma sp. in breeding programs
and by high chick mortality associated with the disease.9,14 The
free-ranging Bali Mynah is critically endangered in its native environment
in Bali, due in part to poaching activity.14 Furthermore, it is
unknown whether Atoxoplasma sp. infects free-ranging mynahs.14 Thus,
it is critical to the survival of the Bali mynah that this parasite not be
introduced into a naïve free-ranging population, where it could kill
wild-bred chicks.14 Since persistently infected birds can be completely
asymptomatic, research into noninvasive, sensitive, and specific diagnostic
tests for atoxoplasmosis has taken on a renewed urgency. The PCR test developed
at The University of Georgia College of Veterinary Medicine may provide a
reliable method to diagnose atoxoplasmosis in these infected birds. If so,
this new diagnostic test will improve disease diagnosis, control, and prevention
in captive breeding programs as well as prevent the accidental release of Atoxoplasma-infected birds into the wild. An enjoyable and informative
website that tells the story of the Bali mynah can be found at www.robharvey.com/information/bali.html.
References
1. Box ED: Isospora as an extraintestinal parasite
of passerine birds. J. Protozool 28:244-246, 1981.
2. Box ED: Atoxoplasma associated with an isosporan
oocyst in canaries. J. Protozool 17:391-396, 1970.
3. Levine H, et al: The genus Atoxoplasma (Protozoa, Apicomplexa). J. Parasitol 68:719-723, 1982.
4. Box ED: Exogenous stages of Isospora serini (Aragao)
and Isospora canaria sp. in the canary (Serinus canarius linnaeus).
J Protozool 22:165-169, 1975.
5. McNamee P, et al: Clinical and pathological
changes associated with Atoxoplasma in a captive bullfinch (Pyrrhula
pyrrhula). Vet. Rec 136:221-222, 1995.
6. Ball SJ, Brown MA, Daszak P, Pittilo RM: Atoxoplasma (Apicomplexa: Eimeriorina: Atoxoplasmatidae) in the greenfinch (Carduelis
chloris). J Parasitol 84:813-817, 1998.
7. Greiner EC, Ritchie BW: Parasites. In Ritchie
BW, Harrison GJ, Harrison LR (eds): Avian Medicine: Principles and Application.
Lake Worth, FL: Wingers; 1994: 1007-1029.
8. Swayne DE, Getzy D, Slemons RD, Bocetti C, Kramer
L: Coccidiosis as a cause of transmural lymphocytic enteritis and mortality
in captive Nashville warblers (Vermivora ruficapilla). J Wildl Dis
27:615-620, 1991.
9. Partington CJ: Atoxoplasmosis in Bali mynahs. J.
Zoo. Wildl. Med.; 20:328-335, 1989.
10. MacWhorter, P. Passeriformes. In Ritchie
BW, Harrison GJ, Harrison LR (eds): Avian Medicine: Principles and Application.
Lake Worth, FL: Wingers; 1994: 1172-1199.
11. Little SE, Kelley LS, Norton TM, Terrell SP: Developing
diagnostic tools to further our understanding of Atoxoplasma species.
Proc Assoc Avian Vet 2001; pp. 157-159.
12. Quiroga MI, Aleman N, Vazquez S, Nieto JM: Diagnosis
of atoxoplasmosis in a canary (Serinus canaries) by histopathologic
and ultrastructural examination. Avian Dis 44:465-469; 2000.
13. www.riverbanks.org/aig/new.htm
14. www.robharvey.com/information/bali.html
15. Kelley LS: personal communication, 2001.
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