Page 16 - May 2005 The Game
P. 16

Your Thoroughbred Racing Community Newspaper The Game, May 2005 17
CONGENITAL HYPOTHYROIDISM: A SILENT KILLER STALKSTHEWEST
         
                                             
              
                                          
           
                                    
                
                                              
Equine Health
Veterinary researchers in the prairie provinces are working feverishly to identify the cause of a mystery disease which reared its ugly head in 2004, killing 75% of affected foals.
Congenital hypothyroidism in foals was first described by researchers at the Western College of Veterinary Medicine at the University of Saskatchewan in 1981, but 2004 was the worst year in recent memory, with 100% of the foal crop affected on some farms ranging from Manitoba to the BC interior. Caused by decreased thyroid function in the developing fetus, it’s virtually undetectable as a pregnancy progresses · but when the foal arrives, it’s afflicted with multiple defects, including contracted tendons, a “sow mouth” (protruding lower jaw), umbilical hernia, poor muscling, and an inability to stand. Though the mare may foal on schedule or even be up to a month late, a congenital- ly hypothyroid foal will be “dysmature”, looking like a preemie with a soft, silky haircoat, floppy ears, and very lax tendons and joints which leave the toes pointing up in the air if the foal does manage to stand.
Needless to say, the prognosis for such a foal is poor, with most dying or being euthanized within a few days of birth.
At the heart of congenital hypothyroidism is an abnormal thyroid gland, which is easily identifiable on necropsy. A normal thyroid gland resembles a balloon, with a thin outer skin which is one layer of cells thick, and a centre containing colloid (the storage form of thyroid hormone). But the gland of an affected foal has a thickened, irregular outer skin and contains little or no colloid. Thyroid hormones are crucial for the growth and development of the fetus during gestation, so
low levels of circulating thyroid hormones have a predictable adverse effect on the growth of these foals.
What’s less predictable is what’s behind the thyroid abnormality. But Andy Allen, DVM, MVetSc, Ph.D., a veterinary pathologist at WCVM,
has narrowed it down not to a genetic aberration, but to the presence of nitrates in the feed or water supply of in-foal mares, coupled with inadequate levels of vitamins and minerals. The nitrate appears to interfere with, or compete with, levels of iodine in these mares, and iodine is a key player in thyroid function.
“Green feed”, a cereal crop harvested before maturity, tends to have high levels of nitrate and is sometimes fed to horses in the West; WCVM has noted a higher incidence of congenital hypothyroidism in foals from these farms, particularly if the mares were not receiving iodine supplementation. Nitrate levels also tend to be high in water affected by runoff from nearby agricultural operations (especially those fertilizing fields heavily or spreading manure) and nitrate is concentrated in hay that has been sprayed with a mould-inhibiting preservative, hay which has been drought-stressed, and hay which has endured early, harsh frosts. Unfortunately, early frost was the rule rather than the exception in most parts of Western Canada in 2004, leading Allen to believe that the 2005 foal crop may also be at risk, though data has not yet been collected. “Anecdotal evidence suggests that fetal exposure (to the disease-causing agent) even as early as the first month or two of gestation might be sufficient to lead to this disease,” he says, adding
that hay and water testing for nitrates can inform breeders of abnormally high levels before it’s too late.
Preventing congenital hypothyroidism is important not only for the future of the foal, but also for its dam: mares giving birth to these foals are 30 times more likely to suffer dystocia (difficult delivery), and some have suffered severe injuries which have put their reproductive futures at risk.
Guinea pigs are turning out to be a surprisingly good lab model for those studying ways of preventing congenital hypothyroidism in developing foals. In 2003, Allen and graduate student Dr. Genevieve d’ Amours re-created congenital hypothyroidism in the lab in baby guinea pigs, and now hope, using this more cost-effective animal model, to test a number of different scenarios to pinpoint the cause of the disease. This testing could eventually tell breeders whether iodine supplementation might offset the effects of high nitrate levels, and whether nitrate is more dangerous in feed or in water or whether nitrate is, in fact, a red herring and the blame lies elsewhere.
In the meantime, WCVM is recommending to Western breeders that they test all water and feed for nitrate levels, and supplement the diet of their broodmares with a complete vitamin/mineral supplement right from breeding to their foaling date.
by Karen Briggs
Dave Landry Photo
                                
                                
                        
                        
                       
      
                                                                                  
          
       
                                                                                          
            
                                                                    
        
                                                                      
                   
                                                                
            
                                                                                               
                     
                               
                                                                                           
                                                                               
                               
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
                                                         


































































































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