Hydrocephalus is a relatively uncommon disorder in horses, but in Friesians it is seen at an estimated rate of 2.5 foals per
1,000 births.1 Some researchers think the higher incidence of hydrocephalus in this breed is caused by a deformation of the jugular foramen.
If this collagen-based structure fails to develop properly, a chain of events begins that may lead to fatal hydrocephalus.
A nonfunctional jugular foramen could lead to internal jugular vein compression. This could disturb cerebral spinal fluid
and enhance its accumulation, resulting in hydrocephalus.1 In an article documenting Friesian clinical issues, Siebren Boerma, DVM, of the Equine Clinic Garijp in the Netherlands
and colleagues discuss both dwarfism and hydrocephalus and attempt to connect them genetically.1
One of the most serious clinical problems seen in the Friesian breed is megaesophagus. This problem is directly related to
the suspected collagen abnormality seen in this breed.1 Megaesophagus is a chronic dilation of the esophagus, accompanied by a lack of normal muscle tone and contractile ability
in the esophageal wall. It can be seen in all animals but is usually found at a very low rate in the general horse population.
In a study conducted between 2002 and 2007 by Boerma along with Marianne Sloet van Oldruitenborgh-Oosterbaan, DVM, PhD, DECEIM,
of Utrecht University's Department of Equine Sciences, 45 cases of megaesophagus were recorded.3 These cases were seen at either the Equine Clinic Garijp or Utrecht University. Of these 45 cases, 41 were Friesians, and
the lead researchers noted a familial predisposition among affected horses, strongly suggesting that this condition may be
Horses with megaesophagus show a variety of progressive clinical signs, including loss of appetite, salivation, muscle wasting,
mild colic and esophageal obstruction or choke. Radiography with contrast material or direct visualization via endoscopy can
identify the swollen, semifunctional esophagus and confirm the diagnosis. Chronic megaesophagus often leads to aspiration
pneumonia, which can be expensive to treat and may ultimately lead to fatal complications in the horse. These horses can occasionally
be managed, and some do well. But the far better method of dealing with megaesophagus in Friesian horses is to identify animals
affected with this condition and remove them and related family members from any future consideration as breeding animals.
Friesian horses are thought to have weakened immune systems, so many problems that affect other horse breeds only marginally
tend to be worse in this breed. For example, the incidence of retained placenta is nearly 54 percent in Friesian horses compared
with only 2 to 10 percent in the general equine population.1
Insect bite hypersensitivity (excessive skin response to the bite from various seasonal insects, predominantly no-see-ems,
or Culicoides species4) occurs in 18 percent of Friesian horses, as reported in one study.1 This hypersensitivity is an intense pruritic response commonly leading to hair loss (often extensive) and skin damage of
the mane, tail, head and ventral abdomen (Photos 1 and 2). In many individuals, this skin damage is severe enough to render
the horse unusable for prolonged periods (weeks to months) during the summer fly season. Comparatively, insect bite hypersensitivity
occurred in only 8 percent of the Shetland pony population studied in the Netherlands.1 It is now thought that this Friesian condition is a familial disease with a polygenetic background. Affected animals can
be treated with traditional methods, including medicated baths and topicals, insect control and repellant, corticosteroids
and allergy desensitization, antihistamines and anti-inflammatory drugs. Friesians with this condition will often show a response
to treatment, but recurrence is likely.
Photos 1 and 2: The caudal aspect of the left forelimb of a Friesian mare showing an area of poor hair growth, scabs and irritated
skin. This area of caudal alopecia and roughened skin is located slightly higher than in most Friesians but shows the typical
dry, flaky, rough appearance seen in these horses. Allergic reaction and insect bite hypersensitivity are two of the main
causes of these lesions. (PHOTOS COURTESY OF DR. KEN MARCELLA)