Congenital Hypothyroidism with Goiter (CHG) in Dogs

Spanish Water Dogs (SWD) are prone to suffering from a condition known as congenital hypothyroidism with goiter (CHG) which is a serious disorder that if left untreated often proves fatal to puppies. Fortunately, there is a DNA-based carrier test available for certain breeds which includes the Spanish Water Dog and all dogs should be tested to reduce the chance of passing this horrible condition on to their offspring.

The condition explained

Dogs need the “thyroid” hormone to develop normally and they also need it for their metabolism. This is especially the case in puppies as they go through the rapid growth stage of development in the first weeks and months of their lives. In affected SWD puppies, their growth and development is seriously impacted right from the start. Their heads often appear larger than other puppies in a litter and they don't move around as much as their littermates either.

Sadly, the majority of affected SWD puppies succumb to the disorder or are put to sleep when they are around 3 weeks old to prevent further suffering. Should an affected puppy be nursed during the first weeks of their lives, their eyes remain shut, their coats are wiry and their ear canals stay closed instead of opening as they should. When they are around 2 weeks old, they develop a swelling under their necks which continues to grow over time. This swelling can often be confused as being strangles because it is easy to put the swelling down to a problem with a puppy’s lymph nodes and as such a misdiagnosis may be made.

Other signs of there being a problem includes the fact that certain bones fail to develop and lengthen which includes a puppy’s spine, legs and face which as a result makes it look like they are suffering from dwarfism. Should an affected SWD puppy be correctly diagnosed and treated with specific thyroid hormone medication, any abnormalities can be controlled and managed, but it would not prevent their goiters from swelling which constricts their airways making it virtually impossible for them to breath correctly. It is worth noting that even when a puppy is treated, they remain disfigured by the condition.

The cause

A lot of research has been carried out into the disorder and studies suggest that it is due to an inherited simple autosomal recessive gene. As such, for a SWD puppy to inherit the condition, they would need to have two copies of the mutated gene, one from each parent dog. A parent might not ever have shown any signs of suffering from CHG, but could be “obligate” carriers which means they can pass the defective gene onto their puppies even though they don’t suffer from the condition themselves. The chances of a puppy inheriting the disorder are as follows:

• Both parent dogs are carriers - on average puppies have a 50% chance of inheriting CHG
• If 2 carriers are mated in error - on average puppies have a 25% chance of inheriting the disorder

In short, some puppies will not inherit the defective gene whereas others might and sometimes none of the offspring inherit the recessive gene at all. With this said, any SWDs that have not been identified as being carriers"" would continue to pass the mutated gene onto their offspring therefore spreading the disorder throughout the breed if they are bred to other Spanish Water Dogs.

Testing

Congenital hypothyroidism with goiter is a condition that is caused by the lack of an important enzyme known as thyroid peroxides or TPO which is found in a dog's thyroid gland. Its job is to add iodine to thyroglobulin which is an essential protein. When the TPO gene is mutated, this essential enzyme is not produced. The research carried out into the disorder has allowed for the identification of the mutation which in turn means that a laboratory test is now available which detects the defective gene.

The test identifies the mutation in a SWD dog's DNA which can be taken through a blood test, a dog’s cheek cells and from frozen semen. This allows for carriers of the mutant allele to be identified and therefore not used for breeding purposes. The DNA test is offered through several veterinary laboratories which includes Laboklin as well as samples being sent to the Laboratory of Comparative Medical Genetics at Michigan State University.

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