High laboratory mouse pre-weaning mortality associated with litter overlap, advanced dam age, small and large litters

Abstract

High and variable pre-weaning mortality is a persistent problem in laboratory mouse breeding. Assuming a modest 15% mortality rate across mouse strains, means that approximately 1 million more pups are produced yearly in the EU to compensate for those which die. This paper presents the first large study under practical husbandry conditions to determine the risk factors associated with mouse pre-weaning mortality. We analysed historical records from 219,975 pups from two breeding facilities, collected as part of their management routine and including information on number of pups born and weaned per litter, parents' age and identification, and dates of birth and death of all animals. Pups were counted once in their first week of life and at weaning, and once every one or two weeks, depending on the need for cage cleaning. Dead pups were recorded as soon as these were found during the daily cage screening (without opening the cage). It was hypothesized that litter overlap (i.e. the presence of older siblings in the cage when new pups are born), a recurrent social configuration in trio-housed mice, is associated with increased newborn mortality, along with advanced dam age, large litter size, and a high number and age of older siblings in the cage. The estimated probability of pup death was two to seven percentage points higher in cages with litter overlap compared to those without. Litter overlap was associated with an increase in death of the entire litter of five and six percentage points, which represent an increase of 19% and 103% compared to non-overlapped litters in the two breeding facilities, respectively. Increased number and age of older siblings, advanced dam age, small litter size (less than four pups born) and large litter size (over 11 pups born) were associated with increased probability of pup death.

Necropsy protocol for newborn mice

“…Neonatal mortality is high in laboratory mouse breeding and underlying causes are poorly understood. The majority of losses occur in the first 3 days after birth 1 and reported average rates of mortality can differ between animal facilities, our own data shows a range between 14 and 39% for the C57BL/6J strain 2 . In farm animals important causes include hypothermia, pregnancy and parturition complications, poor nutrition and injuries 3 .…”

“…Our own data show a range between 14% and 39% for the C57BL/6J strain. 2 In farm animals, important causes include hypothermia, pregnancy and parturition complications, poor nutrition and injuries. 3 These factors are also likely to affect mouse pups, which have poor thermoregulatory capacity 4 and are often kept in trio breeding systems, where the increase in mortality when an older litter is present suggests that milk competition and trauma may be critical.…”

All the Pups We Cannot See: Cannibalism Masks Perinatal Death in Laboratory Mouse Breeding but Infanticide Is Rare

“…Of these, a substantial proportion will have died shortly after birth. Existing literature indicates highly variable pup mortality rates from less than 5% [ 2 ] up to 60% or more [ 3 , 4 ] in experimental studies with C57BL/6 mice, whereas our data from historical records analysed in two different facilities in the United Kingdom revealed perinatal mortality rates of 14 and 39%, respectively [ 5 ]. Perinatal mortality often includes loss of the entire litter [ 6 , 7 ].…”

“…Perinatal mortality in laboratory mouse breeding has far-reaching ethical and practical consequences, in that substantial numbers of animals die at a very early age and before they can be used in experiments. We have recently demonstrated that there is large variation in mortality records between facilities for the most commonly used strain, C57BL/6, that mortality rates found in practice are substantially higher than reference data for the strain [ 5 ] and that social conditions around birth affect mortality [ 6 ]. The present study corroborates and expands on these findings through a detailed analysis of pup cannibalism during the first four days after birth.…”

“…The phenomenon of litter overlap, which is the result of reproductive asynchrony between the two females of the trios, may be common in laboratory mouse breeding. In a retrospective analysis of data records from 34,949 litters, an estimated 55% of these were born into a situation of litter overlap [ 5 ].…”