As mentioned previously, thanks to their limited popularity and successful, informed breeding programs, Harriers are fortunate enough to not suffer from a great deal of genetic diseases. Actually, these dogs are considered one of the healthiest registered breeds. Occasionally, genetic conditions pop up and it is for this reason that breeders still advocate running genetic screens on adults and puppies; breeding individuals with genetic problems would increase the incidence of the genetic problems and the Harrier would no longer enjoy its healthy reputation. Though they haven't been seen in many years, eye problems have been diagnosed in some Harrier puppies and so it's best if you get your Harrier screened for the most common eye problems in dogs.
If your Harrier hasn't been screened, then be on the lookout for signs that could indicate an eye problem. Monitor your dog for squinting, any redness or inflammation, discharge and/or cloudiness in the eyes, an overflow of tears, sunken or bulging eyes, and/or irritability. The third eyelid of dogs should be invisible in a healthy canine; if the third eyelid is visible, this could be an indication of an eye condition. Also be on the lookout for behavior that could indicate an impairment or total loss of vision. Dogs can usually adapt very well to vision problems in familiar environments; if you see your dog confused and/or disoriented in new environments, or if you change the landscape of your home, this could be an indication of vision problems. Many of these signs could merely be signs of an allergic reaction to something, but it's better to have your dog checked by the vet to rule out any possible problems with his eyes.
Cataracts are the most common problem seen in the Harrier breed (they are probably one of the most common problems seen in dogs in general). The eye contains a structure called the lens, which is a transparent layer that has the function of focusing light stimuli that will later be translated into images; the lens MUST remain transparent for vision to be normal. In certain instances, either due to injury or genetic predisposition, the lens loses its transparency and becomes cloudy, or opaque; without a transparent lens, light cannot pass and be focused inside the eye, and so vision is impaired. This opacity is caused by the deterioration of the very delicately balanced biochemical properties that characterize the lens; excess water moves into the lens and insoluble proteins build up. Cataracts can affect one eye or both and can take up a small or large percentage of the lens; most often, surgery can correct the problem.
Another eye problem seen in Harriers, and again in many other dog breeds, is Progressive Retinal Atrophy. This disease involves the deterioration of the nerve receptors in the retina, responsible for gathering images into a cohesive whole and sending the information on to the brain. There is no treatment for PRA and total blindness is most often the end result of this disease, unfortunately. Thankfully, screens do exist for PRA and a number of other eye problems, so potential problems have a good chance of being caught early.