Jack Russell Terrier Health: An Introduction
The Jack Russell Terrier is a small, energetic, and determined working terrier developed primarily for fox hunting. The breed is celebrated for its longevity — it is not unusual for Jack Russells to reach fifteen or even seventeen years of age — and for a constitution that is generally hardier than many other small breeds. However, alongside their vigour, Jack Russells carry several inherited health conditions that owners should understand from the outset, particularly given how rapidly some of these conditions can deteriorate.
Primary Lens Luxation: The Most Urgent Health Risk
Primary lens luxation (PLL) is a condition in which the lens of the eye becomes detached from its normal position and falls either forwards into the anterior chamber (anterior luxation) or backwards into the vitreous cavity (posterior luxation). The condition is caused by a mutation in the ADAMTS17 gene and follows an autosomal recessive inheritance pattern. Jack Russell Terriers have one of the highest rates of PLL of any breed.
PLL is a genuine ophthalmic emergency. When the lens luxates anteriorly, it physically blocks the drainage of fluid from the eye, causing intraocular pressure to rise rapidly. This acute secondary glaucoma is extremely painful and causes irreversible damage to the optic nerve and retina. Without emergency surgical intervention — typically within hours of anterior luxation — permanent blindness in the affected eye is the likely outcome.
Signs of Lens Luxation
Owners should be familiar with the signs of lens luxation so they can act immediately if they appear:
- A sudden, marked reddening of the white of the eye (scleral injection).
- Visible cloudiness or a bluish haze across the eye — this can be the lens itself in an abnormal position, or corneal oedema caused by raised pressure.
- The dog squinting, rubbing at the eye, or appearing to be in pain.
- Sudden apparent loss of vision in one eye — bumping into objects, reluctance to move in low light.
If any of these signs appear in a Jack Russell, treat it as an emergency and contact a veterinary ophthalmologist or an emergency practice immediately, even in the middle of the night. The window for saving vision is narrow.
PLL Gene Testing
A DNA test for the PLL mutation is available through several accredited laboratories. This test is essential for all Jack Russell Terriers, both those used for breeding and companion dogs. The results identify whether a dog is clear (no copies of the mutation), a carrier (one copy — at reduced risk but can pass the mutation to offspring), or affected (two copies — will develop PLL, typically between three and eight years of age).
Knowing your dog's PLL status allows for more frequent eye monitoring in carriers and affected dogs, so that any sign of lens instability can be addressed before acute luxation occurs. Dogs confirmed as genetically affected should be monitored by a veterinary ophthalmologist at least annually from around two years of age.
Deafness and BAER Testing
Congenital sensorineural deafness occurs in Jack Russell Terriers, as in many white-coated or highly white-spotted breeds. The condition is associated with genes involved in pigmentation — specifically, a reduced number of melanocytes in the inner ear disrupts the normal development of hair cells responsible for hearing.
Deafness in Jack Russells can affect one ear (unilateral) or both ears (bilateral). Dogs with unilateral deafness may compensate well and their owners may not realise there is a problem. Bilateral deafness is more apparent and requires different management strategies.
The Brainstem Auditory Evoked Response (BAER) test is the gold standard for diagnosing deafness in dogs. It is a painless procedure in which small electrodes detect the brain's electrical response to sound, and it can identify both unilateral and bilateral deafness reliably. BAER testing is strongly recommended for all Jack Russell Terrier puppies used for breeding and is advisable for companion dogs where deafness is suspected.
Deaf dogs can live full and happy lives with appropriate management — training using hand signals rather than voice commands, for example — but bilateral deafness in particular requires owner awareness of safety risks, as deaf dogs cannot hear approaching traffic, other dogs, or the owner's recall.
Legg-Calvé-Perthes Disease
Legg-Calvé-Perthes disease (also called avascular necrosis of the femoral head) is an orthopaedic condition that affects small and toy breeds, including Jack Russells. It involves a disruption to the blood supply of the femoral head — the ball of the hip joint — which causes the bone to die and degrade. The underlying cause in dogs is not fully understood but is thought to have a genetic component.
The condition typically affects young dogs between four and twelve months of age, presenting as progressive unilateral hindlimb lameness. The dog may carry the affected leg, show muscle wastage of the hindquarter, and appear to be in pain when the hip is manipulated during examination. Diagnosis is confirmed with radiographs, which show characteristic changes to the shape and density of the femoral head.
The standard treatment is surgical removal of the femoral head and neck (femoral head and neck excision, or FHNE), which eliminates the painful bone-on-bone contact. Small dogs tend to do very well following this procedure, forming a functional false joint from surrounding soft tissue. Most Jack Russells return to normal or near-normal function with appropriate post-operative physiotherapy.
Patellar Luxation
Patellar luxation — dislocation of the kneecap from its normal groove in the femur — is common in small breeds, and Jack Russells are no exception. The patella may luxate medially (towards the inside of the leg, which is more common in small breeds) or, less commonly, laterally.
Severity is graded from 1 to 4. Grade 1 luxations are often identified incidentally during routine examination, cause minimal discomfort, and may never require treatment. Grade 3 and 4 luxations cause persistent lameness, abnormal gait (often described as skipping), and may require surgical correction to prevent long-term joint damage and arthritis.
Many Jack Russells live normal lives with low-grade patellar luxation, but owners should monitor for signs of lameness and have the grade reassessed periodically. Dogs with significant patellar luxation should not be used for breeding.
Spinocerebellar Ataxia
Spinocerebellar ataxia (SCA) is a progressive neurological condition that has been described in Jack Russell Terriers and Smooth Fox Terriers. Affected dogs develop a characteristic wobbling, uncoordinated gait — ataxia — typically between two and nine months of age. The condition results from degeneration of the spinocerebellar tracts and Purkinje cells in the cerebellum, which are responsible for coordinating movement.
SCA is progressive and there is no treatment. Affected dogs deteriorate over time and are usually euthanised on welfare grounds. The condition is inherited, and DNA testing is available in some countries. Affected dogs and their parents should not be used for breeding. In the UK, any puppy showing early neurological signs should be assessed promptly by a veterinary neurologist.
Caring for Your Jack Russell
Jack Russells are adaptable, clever, and extraordinarily tenacious — traits that endear them enormously to those who understand them. To give your Jack Russell the best chance of a long and healthy life:
- Test your dog for PLL status and schedule regular eye checks if they carry the mutation.
- Have BAER testing performed if there is any concern about hearing.
- Keep your dog at a healthy weight to reduce pressure on joints.
- Seek prompt veterinary assessment for any sudden eye changes — treat them as an emergency.
- Purchase only from breeders who can provide documentation of health testing.
With appropriate care and awareness, most Jack Russells enjoy excellent quality of life well into old age.