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The susceptibility of mule and muscovy ducks to "blackhead" disease caused by Histomonas meleagridis was studied, using an experimental intracloacal inoculation. Turkeys were used as controls. Morbidity, mortality and body weight gain were recorded regularly during the experiments. A direct examination of the caecal content was made to determine the absence or presence of the parasite. Gross and microscopic lesions were observed on days 7, 14, 21, 28 and 35 post infection to evaluate any clinical histomoniosis in ducks and to appraise the histomonad's carriage. A scoring system was developed both for gross and histological lesions of the caecum and liver. Infected mule and muscovy ducks (n = 83) never developed any clinical signs of histomoniasis. Weight gains of infected mule and muscovy ducks were similar to those of uninfected ducks. In 67% of the ducks (56/83), it was possible to demonstrate the parasite in the caecal content throughout the experiment. Typical macroscopic caecal lesions were observed in five of the ducks between days 7 and 21 post infection, with a caecal necropsy main lesion score (MLS = 1.6) less severe than that in turkeys (MLS = 2.9). Only caecal histological lesions occurred in six of the cases. Therefore, ducks do not seem to be a susceptible host for "blackhead" but may act as carrier animals for H. meleagridis. The virulence was apparently not changed, since 67% of turkeys (10/15) infected with the caecal content of positive ducks displayed classical signs of blackhead disease. Even if H. meleagridis alone does not represent a substantial danger in the duck production, its infectivity should to be taken into account in the transmission to more susceptible species.

An electron microscopic study of the endogenous development of Eimeria mulardi Chauve, Reynaud and Gounel, 1994 was carried out in mule ducks which are hybrids of the domestic duck (Anas platyrhynchos) and the muscovy duck (Cairina moschata). All of the endogenous stages were seen within the nucleus of the host cell. Merozoites arose from ectomerogony and three mutually similar merogonies were noted. The asexual stages were found in leukocyte-like cells in the lamina propria of the jejunum, ileum and caecum, while the gamonts developed in glandular epithelial cells in the same part of the intestine.

An experimental Eimeria mulardi coccidiosis was reproduced in mule ducks. A single dose of toltrazuril (7 mg per kg) was administered at two different endogenous stages: Day 2 (first meronts) or Day 5 (last meronts and gamonts). The efficacy of the drug was assessed on four criteria (body weight, oocyst counts, macroscopic lesions, and presence of the meronts and gamonts). To have a curative effect, a single dose of toltrazuril should be administered early on. However, a single treatment on Day 5 had a prophylactic effect.

A new species of coccidium. Eimeria mulardi n. sp., has been described from the mule duck. The endogenous cycle is studied in experimentally infected ducks. A least two generations of meronts were observed, mainly in the lamina propria of jejunum and ileum and the gametogony occurred principally in glandular cells of the intestinal tract, except in duodenum. All the endogenous stages were localized in the nucleus of the cell.

A coccidium was isolated from mule duck farms, the sporulated oocysts of which most closely resemble that of Isospora anseris, a goose coccidium. By experimental infection, these oocysts were proved to be able to develop in the goose and in several duck species, but unable to develop in the chicken. No pathogenicity was noted in any species.

The authors isolated 2 Isospora species from ducks: Isospora mandari and Isospora sp. This was the first report of the genus Isospora from the duck in France. One of the 2 isolated species (Isospora sp) has not been described in the duck before.