Studies of the epidemiology and risk factors involved in the pathogenesis of congenital chondrodystrophy of unknown origin in Australian beef herds

Abstract

A form of congenital chondrodystrophy of unknown origin (CCUO), often referred to in Australia as acorn calf disease, was reported irregularly in beef cattle in south-eastern Australia until 2001, with the exception of an extended outbreak in NSW in 1991 and two isolated reports from Western Australia in 1992. A notable increase in reports of affected calves since 2002 raised serious concern about the cost of this condition to affected producers, and about the possibility of CCUO being an emerging condition. The case definition of a calf with CCUO used throughout this project was a calf from a clinically well dam showing signs of disproportionate dwarfism at birth including limb deformities (bowed legs, shortened limbs, enlarged joints or arthrogryposis), superior brachygnathia (domed head and/or dished face) and spinal deformities (lordosis, kyphosis and scoliosis). This project was a direct response to an industry request for research to progress the understanding of the epidemiology of CCUO in Australia, in particular its geographic and temporal distribution and risk factors associated with CCUO occurrence. A nation-wide survey of rural veterinarians combined with data from published reports provided the most complete description of the spatio-temporal distribution of CCUO in Australia to date. The data demonstrated an increase in CCUO occurrence since 2002, with reports each year involving multiple farms. While the majority of these farms were in south-eastern NSW and north-eastern Victoria, a trend towards more cases outside this focus region was evident, with reports from areas previously thought to be CCUO-free in Western Australia and South Australia. This survey identified some cases not included in official records, indicating that some farmers affected by CCUO do not report the birth of affected calves. Under-reporting of CCUO means that even this study does not fully depict the level and distribution of affected farms in southern Australia. The estimated direct income loss for farmers arising from the 1081 CCUO calves reported for the period 2002 to 2007 totaled AU$818 315, with an average loss per affected calf of AU$757. A case-control study involving 46 affected farms from the focus region of south-eastern NSW and north-eastern Victoria showed all affected mobs from 2002 to 2007 calved during spring, and found significant associations between the birth of CCUO calves in a mob and four management and environment variables that link either directly or indirectly with poor maternal nutrition during gestation. The birth of CCUO calves was more likely when: • dams had grazed in paddocks with inadequate amounts of pasture • dams had grazed in a paddock with the main pasture type being native or mixed pasture compared to improved pasture • dams had grazed in a paddock with predominately hilly to steep terrain • dams were given a supplement feed (variable included in one final model). This last finding was contrary to expectation but most likely reflects that mobs fed a supplement were those experiencing the most extreme nutritional stress and that the supplement provided was insufficient to meet requirements for normal foetal development. Similar to the results for mobs, analysis of the case-control data to examine associations between the birth of CCUO calves in a paddock and the environment and soil characteristics of the paddock found significant associations with the two environment variables listed above related to main pasture type and paddocks with predominately hilly to steep terrain. This further confirmed the direct or indirect link with poor maternal nutrition during gestation. The results for paddocks also found a significant association between increased soil potassium and the birth of CCUO calves in a paddock. This finding was unexpected as potassium has not been shown to affect other trace minerals in vivo. However, a dietary excess of potassium has been shown to produce mild metabolic alkalosis and affect calcium homeostasis. At this point the potential role for potassium in the development of CCUO calves is not understood. Notably the investigation of soil variables in this case-control study found no association with soil levels of the suspect trace minerals manganese and zinc, or with any other mineral such as iron, that might alter the availability of these trace minerals. Association of the birth of CCUO calves with periods of drought on the 46 study farms was investigated using time series analysis based on historical climate records. CCUO occurrence was associated with drought five months before calving, corresponding to the 2nd to 4th months of gestation for spring-calving herds on the 46 farms during 2002 to 2007. In this study drought was defined as a three-month period in which rainfall was within the first decile (that is, the lowest 10% on record) for a given location. An on-farm trial conducted to investigate the effect of administration of an injectable multimineral supplement (including manganese, copper, zinc and selenium) at intervals during gestation on the birth of CCUO calves found no difference in the proportion of CCUO calves between treatment (22% of calves born had CCUO) and control (20%) groups, and no significant difference in dam liver manganese, zinc or copper levels between groups, although each decreased in both groups over gestation. The evidence gathered by this project shows that CCUO in Australia is: • an emerging condition reported since 2002 on an annual basis with outbreaks involving ten or more farms per year during five of eight years from 2002-2009 • predominately a condition reported in beef herds in south-eastern NSW and in north-eastern Victoria; however, a feature of emergence is a trend toward more reports outside this focus region • associated with maternal nutritional deficiency during gestation that is related to drought during the 2nd to 4th months of gestation • a condition that occurs more frequently than is recorded in official records due to non-reporting by some affected farmers. Although aspects of this project were constrained by the quality of case records, these conclusions definitely apply to affected farms in south-eastern NSW and north-eastern Victoria where CCUO occurrence is of concern to the producer. These findings suggest that industry, government and researchers in southern Australia should either maintain or introduce activities that provide beef producers with information to support recognition, reporting and prevention of CCUO, and continue targeted research to further improve the understanding of factors contributing to CCUO occurrence and of the pathogenesis of this condition.