Epidemiologic evidence for clustering of equine motor neuron disease in the United States.

The research paper investigates regional variations in the occurrences of equine motor neuron disease (EMND) in the United States and attempts to determine factors that could explain these geographical distributional differences. The authors found EMND rates increase from western states towards New England, and identified factors such as age and breed which may influence its distribution, all while noting that the actual cause for regional distribution of EMND remains unknown.

Methodology

• The study utilized existing data for study population which includes 97 horses with confirmed EMND and 698 control horses diagnosed with other spinal disorders from 21 US veterinary teaching hospitals from the Veterinary Medical Data Base.
• The US horse population was divided into 21 regions and the regional incidence rates of EMND were estimated over a 10-year period, from January 1985 to January 1995.
• Using Moran’s index of spatial autocorrelation, the researchers examined whether the disease showed spatial clustering. This involved joining the 21 regions into broader areas based on the similarity of their EMND rates, using a statistical technique known as cluster analysis.
• Various potential contributing factors such as age at diagnosis, month of diagnosis, breed, and sex were assessed for their impact on the distribution of the EMND using logistic regression analysis.

Results

• The research found variations in EMND rates across the 21 regions leading to a strong pattern of spatial clustering of the disease in the United States.
• Based on similarities in EMND rates, the researchers grouped the regions into five risk regions, with the incidence gradient increasing from the western states to New England.
• No direct connections were established between analyzed factors (age, breed, and others) and the geographic variations of disease rates, although there is evidence of some confounding attributable to age and breed.
• However, the reasons behind the higher frequency of EMND in the northeastern states remained unclear.

Conclusion

• The research concludes that although factors such as age and breed might confound the distribution of EMND, they do not explain the regional clustering of the disease.
• Thus, the higher incidence of EMND in particular regions, specifically northeastern states, can’t be explained by the distribution of the analyzed factors.
• This suggests the existence of other, unidentified factors causing the disease’s regional clustering.