Digestive tract infections by flukes cause mortality in ducks in Quebec
Subsamples of black ducks, northern pintails, and mallards submitted to the laboratory for analysis.
An episode of massive mortality of dabbling ducks was reported in Baie Lavallière (Saint-Anne-de-Sorel, southern Quebec). In total, 130 black ducks, 16 mallards, 10 northern pintails, and one green-winged teal were found dead between March 23 and 27 of this year.
Several specimens were submitted for analysis at the Quebec regional centre (CWHC / CQSAS) to determine the cause of the observed mortalities. The ducks submitted were all in very good nutritional condition and showed distension of the intestines and caeca by a large quantity of hemorrhagic liquid often containing aggregates of fibrin. In some areas, the intestinal mucosa was hemorrhagic and covered with fibrin. Numerous small whitish ovoid structures barely visible to the naked eye were present in the intestinal content. Examination of these structures under binoculars confirmed that they were trematodes (flatworms). Acute hemorrhagic fibrinonecrotic typhlocolitis/enteritis was identified as the cause of death of these birds. Avian influenza virus was not detected by molecular method in these birds.
A) Caeca (arrows) of a black duck showing marked dilatation as well as hemorrhagic and pale areas on the serosa. B) Opened caeca filled with hemorrhagic liquid content in which numerous small-sized trematodes (arrows) are observed. Clusters of fibrin (f) cover the mucosa.
Different species of intestinal trematodes are known to potentially cause mortality in ducks. In Quebec, the species Sphaeridiotrema globulus and Cyathocotyle bushiensis are the most frequently described. The morphology of the parasites observed in these cases suggests a combined infection by these two species of trematodes (molecular identification in progress). Experimental studies have demonstrated that a duck can consume a lethal dose of parasites over a 24-hour period and die within 3 to 10 days following the onset of infection. These parasites can therefore be responsible for the acute mortality episode observed here. According to the timeline of events, the first observations of black ducks in Baie Lavalière were made on March 19, 2024, when the bay was newly ice-free following spring thaw. It can therefore be suggested that the ducks became infected upon their arrival at this site. It is worth noting that this mortality episode lasted less than a week; no mortality was observed during a site visit on April 3. Although the number of black ducks at the site had greatly decreased, the number of northern pintails had greatly increased. This observation suggests that black ducks are more susceptible to these parasites than northern pintails. It is also possible that the vast majority of infected snails were consumed by the first birds to arrive at the site.
Microscopic images of trematodes found in the intestines of affected ducks. A) Adult trematode with morphology suggestive of the species Cyathocotyle bushiensis (to be confirmed). B) Adult trematodes (arrows) with morphology suggestive of the species Sphaeridiotrema globulus (to be confirmed).
Typhlocolitis/enteritis due to trematodes is a condition regularly reported in waterfowl in North America, especially in diving ducks such as scaups, and in coots. Mortalities associated with these two parasites are also documented in dabbling ducks, especially at the end of summer. In Quebec, several episodes have been reported since the 1960s. Nevertheless, to our knowledge, the intensity and timing (spring) of this mortality episode are rather unusual.
These parasites are transmitted to ducks (the definitive host) via intermediate hosts (snails). The faucet snail (Bithynia tentaculata), an invasive species native to Europe, plays a predominant role in the transmission of these parasites, and thus in the ecology of this condition. This species of snail, which was first documented in North America in Lake Michigan in 1871, is now present in the Great Lakes and several tributaries, including the St. Lawrence River. Some observations suggest that this gastropod has experienced an increase in abundance and distribution range in recent decades. The “invasive” nature (therefore non-native) of these trematodes and their intermediate host could explain, at least in part, this apparent imbalance in the parasites’ relationship with these maladapted North American definitive hosts. Furthermore, the high infection intensity observed in the submitted ducks could indicate an unusually high exposure to faucet snails in the last days. The relatively low water level present at this site this year may have favoured duck exposure to snails, as they were less deep and therefore more accessible to dabbling ducks. Additionally, it appears that the abundance of faucet snails may be favoured by increased temperature and eutrophication (nutrient increase) of the water. Therefore, we may wonder how the often problematic relationship between these parasites and their hosts will be affected by the global changes observed in the ecosystem.
It should be noted that these parasites pose no risk to humans and domestic mammals.
Stéphane Lair, RCSF-Quebec (CQSAS) with the collaboration of Matthieu Beaumont (ECCC) and Paul Messier (SABL)
