Abstract
Dear Editors, — Susan Little has written a most comprehensive and engaging article about several aspects of feline reproduction in a recent ‘Clinical Practice’ edition of JFMS. 1 We would like to add comments in relation to the ovarian remnant syndrome (ORS), and take the concept a bit further.
As Dr Little points out, ORS is a problematic diagnosis because the most likely pathomechanism is the inadvertent deposition of ovarian tissue after a routine spay. It's less stressful if you didn't do the surgery, because you can blame someone else! If you did the surgery yourself, it might be tempting to attribute ORS to ectopic ovarian tissue present due to a developmental anomaly. Either way, (i) you usually do not know which side the ovarian remnant is on, (ii) you need to be an exceptional ultrasonographer to find it (being approximately 1–2 mm in diameter), and (iii) even if you re-explore the most likely anatomical locations at laparotomy while the cat is showing signs of oestrus, the ovarian remnant still may not be located. Even a specialist surgeon may have difficulty finding all of the ectopic tissue. Laparoscopy has certain advantages in this setting (because it provides magnification) and may be the method of choice in cats and dogs if you have the equipment and expertise, as in the human arena. 2 Any delay in detecting the remnant will substantially increase the risk of mammary tumours developing in the future. Furthermore, the owner is unlikely to be sympathetic. They might even expect the surgery to be performed gratis.
In our opinion, it might be easier to use a different strategy — at least in some cases, or for some clients. The use of gonadotropin-releasing hormone (GnRH) implants to treat ORS in cats was first suggested by Romagnoli in 2004. 3 These implants were pioneered by Australian scientists Paul Schober and Tim Trigg who developed a patented bioabsorbable wafer that slowly elutes the GnRH analogue deslorelin. Deslorelin effects contraception by temporarily preventing release of pituitary (follicle-stimulating hormone [FSH] and luteinising hormone [LH]) and thence gonadal hormones (oestradiol and progesterone in females; testosterone in males). The observed effects are similar to those following ovariectomy or castration, but are reversed when the hormone content of the implant becomes depleted. As a hormone agonist, deslorelin first stimulates FSH and LH release, resulting in oestrus in females or temporary enhancement of testosterone and semen production in males. Down-regulation follows the initial period of stimulation, resulting in reduced release of FSH and LH, causing levels of the respective sex hormones to fall virtually to nil.
Early trials focused primarily on domestic dogs, 4 although there has been one controlled study in domestic cats 5 and preliminary data from a number of exotic species. Deslorelin implants are available in two formulations: 6-month (4.7 mg) and 12-month (9.4 mg) implants. Stated durations of efficacy should be considered minimums. Smaller implants may be effective in some animals for more than 6 months, and the larger ones for more than 12 months (often in excess of 18 months for cats). Data from various species have shown that an individual animal's response tends to be consistent: if an animal reverses earlier than expected, it will do so consistently.
Thus, for a young cat with ORS, rather than an investigation and treatment consisting of (i) serial determination of hormone (oestradiol, LH and progesterone) concentrations, possibly after GnRH stimulation, (ii) specialist ultrasonography and (iii) exploratory surgery or laparoscopy, it might be less expensive to use a deslorelin implant every 18 months to 2 years for the rest of the cat's life, presumably at the time of vaccination or annual wellness examinations. Deslorelin implants are distributed in Europe, Australia and New Zealand by Virbac (Fig 1), which bought Peptech the Australian company that developed the technology. The 6-month implant is currently available in Europe; the 12-month implant is going through the registration process. Unfortunately, the product is not currently available in North America or South East Asia, which is likely why it was not mentioned in Dr Little's review.
Some endocrinologists believe that it is actually deleterious to have elevated levels of LH and FSH (which are regularly seen in neutered animals, including cats), and that conditions such as adrenal hyperplasia and adrenal tumours can be prevented by long-term use of deslorelin implants. There is compelling evidence that this is indeed the case in ferrets. 6 It begs the question of whether we should be trialling Suprelorin (Virbac) in cats with adrenal tumours, as they can work so well in ferrets. 6
Finally, deslorelin implants have a place when temporary suppression of oestrus is indicated — for example, in cats that are being shown, or in queens where litters are not wanted currently. This is because GnRH implants postpone oestrus without any adverse long-term effect on fertility, and without the risk of cystic endometrial hyperplasia/ pyometra developing. More detailed information concerning this method of contraception can be found in the excellent recent review in JFMS by Sandra Goericke-Pesch. 7
Deslorelin implant currently available in Europe
Letters to the Editors
Letters commenting on articles published, or expanding on topics covered, in the ‘Clinical Practice’ editions of JFMS are welcome and will be considered on their individual merit. Please e-mail correspondence to