Due to their working nature, Jack Russell terriers remain much as they were some 200 years ago. They are sturdy, tough, and tenacious. The head should be of moderate width at the ears, narrowing to the eyes, and slightly flat between the ears. There should be a defined but not overpronounced stop at the end of the muzzle where it meets the head, and a black nose. The jaw should be powerful and well boned with a scissor bite and straight teeth. The eyes are almond shaped and dark coloured, and should be full of life and intelligence.
The Jack Russell terrier and Parson Russell terrier breeds are similar, sharing a common origin, but have several marked differences — the most notable being the range of acceptable heights. Other differences in the Parson can include a longer head and larger chest as well as overall a larger body size. Compared to the Parson, the Russell terrier should always be longer than tall at the withers, whereas the Parson’s points should be of equal distance
Jack Russells are first and foremost a working terrier. Originally bred to bolt fox from their dens during hunts, they are used on numerous ground-dwelling quarry such as groundhog, badger, and red and grey fox. The working JRT is required to locate quarry in the earth, and then either bolt it or hold it in place until they are dug to. To accomplish this, the dog won’t bark but will expect attention to the quarry continuously.
The breed has a reputation for being healthy with a long lifespan. Breeders have protected the gene pool, and direct in-line breeding has been prevented. Jack Russells can live anywhere from between 13 to 16 years on average given proper care. However certain lines have been noted for having specific health concerns, and therefore could occur in any line or generation because of recessive genes. These issues can include hereditary cataracts, ectopia lentis, congenital deafness, patellar luxation, Legg – Perthes syndrome and von Willebrand disease.