Case of the Month - August 2025

Signalment and History

Seven of nine mallard ducklings (Anas platyrhynchos) housed in the same enclosure and considered to be healthy on July 28, 2025, were found dead in the morning of July 29, 2025. Gross necropsies of few of the ducklings performed at the submitting facility did not revealed any significant lesions. Two of the ducklings were submitted for full necropsy at the University of Minnesota Veterinary Diagnostic Laboratory.

Gross Findings

The entire carcasses of 2 mallard ducklings were in a state of good postmortem preservation. The duodenal loop of duckling#1 was slightly dilated and transmurally reddened. The affected segment contained blood tinged digesta. No significant lesions were detected in duckling#2.

Figure 1: The wall of the duodenal loop is reddened.

Histopathology

Both ducklings had similar intestinal lesions. The cytoplasm of enterocytes in the small intestine (e.g. ileum) contains coccidial developmental stages, including numerous trophozoites and merozoites as well as fewer meronts/schizonts and gamonts (macrogametocytes or microgametocytes). Rare oocysts were present in the lumen of the colon in duckling #2.

Figure 2: The cytoplasm of numerous (if not all) enterocytes of three neighboring small intestinal crypts contains one or multiple coccidian trophozoites (three of which are highlighted by black arrows).

Ancillary Testing

• Fecal floatation: No ova or oocysts were observed in the small intestinal contents.
• Molecular diagnostics: negative for avian influenza virus and West Nile virus
• Bacteriology: Liver: no significant growth by aerobic culture; Intestine: negative for Salmonella sp.

Morphologic Diagnosis

1. Colon and cecum, coccidiosis, severe

Etiology

Coccidian protozoa belonging to the family Eimeriidae, which includes genera such as Eimeria, Tyzzeria, Isospora, Caryospora, and Schellakia

Discussion

Intestinal coccidiosis in ducks is relatively poorly described in the current literature, particularly when compared to intestinal coccidiosis in chicken. Young domestic ducks are the most susceptible, often developing more severe disease due to their immature immune systems.

In Anseriformes (ducks and geese), two species of coccidia have been historically reported as causative agents of intestinal coccidiosis: Tyzzeria perniciosa—considered the more pathogenic—and Wenyonella sp. Clinical signs in affected birds may include sudden death, depression, diarrhea and vent soiling (sometimes blood-tinged), emaciation, and pallor. Infections with T. perniciosa are often associated with small intestinal dilation and suppurative to necrohemorrhagic enteritis.

Recurrent outbreaks of intestinal coccidiosis have been documented in mallard ducklings in the midsummer in Minnesota. As in the present case, fecal floatation analysis often yields a negative result because infections with fatal outcome are usually peracute and the result of the injury when merozoites enter and exit exorbitant numbers of enterocyte. Molecular diagnostic investigations have identified members of the family Eimeriidae but the causative eimeriid parasite could not be identified to the genus/species level.

Differential diagnoses for hemorrhagic or mucoid diarrhea in ducks include for example salmonellosis, clostridiosis, and duck viral enteritis. Histopathologic evaluation, supported by ancillary diagnostic testing such as PCR or electron microscopy, remains essential for definitive diagnosis and species identification.        

References

1. Fenton H, McManamon R, Howerth EW. Anseriformes, Ciconiiformes, Charadriiformes, and Gruiformes. In: Terio KA, McAloose D, St. Leger J, editors. Pathology of Wildlife and Zoo Animals. San Diego: Academic Press; 2018. p. 697–721. https://doi.org/10.1016/B978-0-12-805306-5.00029-8
2. University of Minnesota – Veterinary Diagnostic Laboratory: data base

Funding provided by the Minnesota Environment and Natural Resources Trust Fund.