A foal with Lavender Foal Syndrome demonstrating opisthotonus (a state of a severe hyperextension and spasticity in which the head, neck and spinal column enter into a complete "bridging" or "arching" position), one cardinal neurological sign of the disorder. © Dr Yael Giora
The discovery, reported in the open-access journal PLoS Genetics, is the first successful use of the entire horse genome for the mapping and discovery of a mutation.
Lavender Foal Syndrome is a seizure disorder that leads to suffering and death in foals soon after birth. Aside from multiple neurological problems, it also results in a dilute coat colour, from which the disorder takes its name.
It is a recessively inherited condition. Its occurrence is often unpredictable and difficult for horse breeders to avoid without a diagnostic test for carrier status.
The frequency of carriers in unrelated Egyptian Arabians was assessed at 10.3 per cent, and 1.8 per cent in non-Egyptian Arabians.
To pinpoint the gene, the researchers focused on a group of 36 horses. It comprised six affected foals, seven of their parents and 23 close relatives. Samples from 114 individual horses from the general Arabian horse population were available for the study.
"A single genetic location associated with the disorder was rapidly identified," the researchers said.
"Subsequent sequencing of functional candidate genes in this location revealed a single base deletion that likely causes Lavender Foal Syndrome.
"From a practical standpoint, this discovery and the development of a diagnostic test for the Lavender Foal Syndrome allele provides a valuable new tool for breeders seeking to avoid the disease in their foal crop."
The authors said breeders will be able to make more informed selection of mating pairs, avoiding the production of affected foals and potentially lowering the frequency of this mutated gene in the population without wholesale culling of valuable stock.
The researchers confirmed that all horses in the study group suffering from the syndrome were homozygous for the mutation (had received the mutated recessive gene from both parents).
Among 23 relatives of affected foals, 16 were identified as carriers of the problem gene.
A sample group of 114 Arabian horses from the general population was tested to provide a rough estimate of the frequency of the mutated gene. The researchers tound that 10.3 per cent of Egyptian Arabians (six out of 58 horses) and 1.8 per cent of non-Egyptian Arabians (one out of 56 horses) were identified as carriers.
"It is possible that this may be an over-estimation of carriers, as owners who suspect they have Lavender Foal Syndrome-carrying horses may have been more motivated to participate in the study.
"However, it is equally as likely that this figure is an underestimation as there is social stigma associated with producing Lavender Foal Syndrome foals, thus motivating breeders to hide the carrier status of their breeding stock.
"Despite strict policies regarding the confidential nature of identifying information in research projects, this still influences some breeders to avoid association with Lavender Foal Syndrome research out of fear of being rumoured to own carrier horses."
Notably, three of the six carriers identified were reported to be breeding stallions, they said.
Data from the Egyptian Arabian horse registry indicates about 850 young horses are registered each year.
"Given our estimate of the number of carriers in the population, we expect that around nine affected foals would be born in the US each year.
"This is a small number. However, rumours of carrier status can very quickly negatively impact the breeding career of high-priced stallions and lead to large economic losses.
"Prevention of the economic and emotional losses associated with lethal conditions in foals, included those affected with Lavender Foal Syndrome is a high priority among Arabian breeders."
The researchers noted that the market for Egyptian Arabian horses particularly values certain popular bloodlines.
"This leads to close breeding as owners seek to increase the percentage of this ancestry in their foal crop.
"This breeding strategy thus increases the need for vigilant prevention of recessive genetic disorders.
"The test developed here will be a pivotal tool for breeders seeking to breed within lines segregating for Lavender Foal Syndrome, yet minimise or eliminate the production of affected foals."
The researchers were Samantha Brooks, Nicole Gabreski, Donald Miller, Abra Brisbin, Helen Brown, Cassandra Streeter, Jason Mezey, Deborah Cook and Douglas Antczak.
Funding for the works came from the Morris Animal Foundation as part of the Equine Consortium for Genetic Research and the Arabian Horse Foundation. The Harry M. Zweig Memorial Fund for Equine Research and the Dorothy Russell Havemeyer Foundation also provided some support.