That sounds like a real ugly bug 

Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
That would explain the sudden loss.That sounds like a real ugly bug![]()
In light of your significant investment in these ducks and the similarity of these possible causes I think a necropsy would be super helpful, as casportpony suggested. More helpful than usual even.Riemerella anatipestifer Infection
This bacterial disease of ducks is also known as Pasteurella anatipestifer infection, infectious serositis and New Duck disease. Anatipestifer infection causes high mortality, weight loss and condemnation. In the acute form, listlessness, eye discharge and diarrhea are commonly seen. Ducks show incoordination, shaking of the head and twisted neck. Birds are commonly found on their backs, paddling their legs. Typical lesions found in dead birds are infected air sacs, membranes covering the heart and liver, and meningitis. Preventive management and vaccination are effective means of control. Penicillin, enrofloxacin and sulfadimethoxine-ormetoprim (0.04-0.08% in feed) are effective in reducing mortality.
Colibacillosis
This common infection of poultry caused by Escherichia coli., causes reduced hatchability, infection of the yolk sac (omphalitis), a septicemia (bacterial invasion of bloodstream) in ducks 2-8 weeks of age and salpingitis and peritonitis in breeder ducks. In market ducks, E. coli. infection produces lesions very similar to those seen in Riemerella anatipestifer infection (see above). Good sanitation and management are important preventive measures. Sulfadimethoxine-ormetoprim (0.04-0.08%) and chlorotetracycline (0.044%) in feed are helpful in controlling this disease.
Toxins
Ducks are particularly susceptible to certain toxins, and in some cases strikingly more than chickens or turkeys. Therefore, duck caretakers must be especially diligent in preventing ducks from consuming or being exposed to these toxins.
Aflatoxin poisoning
Molds (fungi) that grow on cereal grains and oilseeds before and after harvest produce a number of toxins that are particularly harmful to ducks. By far the most toxic of these substance is a group of toxins called aflatoxins. Aflatoxins are produced by the molds Aspergillus flavus and Aspergillus parasiticus. Ducks are highly susceptible to these toxins. Very small amounts will cause high mortality. Wet harvest conditions encourage the growth of this mold.
Botulism
Ducks that have access to stagnant ponds or other areas where decaying organic matter (animal carcasses, in particular) is found may consume toxins produced by the bacterium Clostridium botulinum. This happens when temperature and other conditions are right for the growth of this anaerobic spore-forming bacterium. Botulism causes a progressive flaccid (limp) paralysis of the neck (limberneck), legs and wings. Affected ducks usually die in a coma within 24-48 hours.
Castor bean poisoning
Incidents of high death losses in wild ducks, due to consuming castor beans (Ricinus communis) have been reported in Texas. Castor beans contain ricin, a toxalbumin known to cause toxicity in humans and domestic animals.
Rapeseed meal
Some older varieties of rapeseed meal contain erucic acid and goitrogens at levels high enough to be harmful to poultry. Ducks are much more sensitive to erucic acid than are chickens and turkeys. Genetically improved varieties of rapeseed (Canola) contain much lower levels of these toxins. However even Canola meals should first be tested in ducks before their use in duck feeds on a large scale.
Insecticides, rodenticides
Duck keepers should take care not to use insect sprays or rodent poisons, that are known to be harmful to ducks, in areas accessible to ducks. Some insect sprays are highly toxic to ducks, such as parathion and diazinon. Always read the directions on the insecticide container carefully before using around ducks. Rat poisons that contain Warfarin, an anticoagulant, if consumed by ducks, can cause them to bleed to death.