The mink ranch of an Agricultural Experiment Station housed approximately 5000 mink. On Sunday, June 13, about 300 mink were found dead and many more were sick. By Monday, about 500 more animals had died. Unfortunately, the consulting veterinarian was not called until Wednesday morning. His previous records showed that the usual level of combined morbidity and mortality during this phase of their producing season was 0.9% per week.
These mink were all housed in one large building, divided into sections by movable walls. March was the breeding season. Kits were born in May and were weaned at about eight weeks of age. July, therefore, was a critical month because there were several categories of mink to be cared for: (1) lactating females with suckling kits; (2) females with kits on starting ration; (3) growing kits; (4) dry females; and (5) males (Table 1).
The diet of these mink consisted of meat, fish, cereal, and fat, supplemented with powdered milk, bone meal, vitamins, and minerals. They were fed once a day except on weekends, when they were fed on Saturday afternoon only. Protein feeds (raw meat or fish) usually made up 40 to 60% of the mink's diet. At the time of the disease outbreak, a feed trial was being conducted to determine the least expensive source of protein feed. As part of the feed trial, a separate ration was prepared for each of the five categories of mink. Three categories were divided into three experimental groups each (see Table 1). The protein part of the ration of each of theses groups came from a different source, the rest being the same for that category. Table 2 shows the composition of the various rations being fed in July.
The protein fraction of these rations (except the supplements) were usually frozen meat or fish. Liver was obtained in plastic bags and frozen. After thawing for a day at room temperature, these items were weighed, ground, and mixed with other ingredients of the ration. The protein supplement, dried blood, bone meal, and vitamin-mineral mix were supplied in various forms (pellets, granules, and powder) in 50-pound sacks. Oats, wheat, corn, bread, and skim milk were supplied in sacks of 50 pounds. The fat was received in barrels of various sizes and stored in a refrigerator. The rations for each category of mink were prepared separately. First, the ingredients that occurred in equal amounts for all three groups in one category were mixed. One-third of this mix was then mixed with the appropriate amount of frozen feed.
By Wednesday, July 16, 1096 of a total of 4907 mink had died, and 862 others were sick. The signs displayed were: Group 1, starvation (kits); group 2, dehydration (females) and starvation (kits); group 3, bite injuries (kits); group 4, starvation (kits); groups 5, 6, 7, 8, and 9, squinting of the eyes, paralysis of the front and/or hind limbs, dyspnea, and coma-prostration. Necropsy did not reveal any lesions.
In this outbreak, only groups of animals could be considered with respect to time of onset. It was observed that the mink in groups 6 and 7 had been the first to become ill (Sunday). Peak mortality occurred on Monday morning among mink in groups 6, 7, and 9 and later in groups 5 and 8. After Wednesday, very few mink died.
From these observations and information, it was possible to determine; (1) the most commonly occurring signs, (2) an approximate incubation period, and (3) feed items fed to each group of mink.