If you’re wanting to breed from your dog or cat, are noticing changes in their behaviour, or are simply interested in reproductive physiology, you may be aware dogs and cats can have abnormal seasons. This irregularity can be caused by many conditions, but one of the more interesting are ovarian cysts. What are ovarian cysts? How do they affect your pet’s season? And what can be done about them?
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Overview of the Oestrus Cycle in the Cat and Dog
You might be surprised to hear that dog and cat oestrus cycles (their seasons) vary quite significantly from humans, and from each other. The physiology is quite complex so we will try and keep things brief. The anatomy, however, is fairly similar, made up of the ovaries located near the kidneys, the oviduct leading to the uterine tubes and uterine horns, the body of the uterus, the cervix, vagina, vestibule and vulva.
Bitch
The bitch (female dog) is mono-oestrus, meaning they have one oestrus cycle per year (on average). Their cycle is not associated with the time of year (non-seasonal). Their season typically lasts around 3 months, with a ~7m non-cycling period. A bitch enters puberty and starts cycling from 6m to 2 years old (the larger the breed, the later).
Pro-oestrus (follicular phase):
- Follicles on the ovaries containing oocytes (egg cells) mature.
- There may be oestrus behaviour and the bitch may be interested in males.
- The hormones oestradiol and follicle stimulating hormone (FSH) dominate.
Oestrus (follicular phase):
- The oocyte is released into the oviduct.
- This is caused by a surge in luteinising hormone (LH).
- During this time, the bitch is fertile and reproduction may lead to pregnancy.
Met-oestrus and dioestrus (luteal phase):
- A temporary corpus luteum forms on the ovary.
- This produces pro-gestational hormones such as progesterone to create the right environment for pregnancy.
- If the oocyte was fertilised, it will be attached to the uterine wall.
- During pregnancy, the embryo releases a signal to prevent breakdown of the corpus luteum; if the dog is not pregnant, this breaks down over time.
- Dogs are unique in that they have a ‘pseudopregnancy’ of a similar length to a normal pregnancy, where the hormone levels are similar to a real pregnancy. This may mean a non-pregnant bitch shows signs of pregnancy, such as nesting or mammary gland development.
- A pregnancy/pseudopregnancy lasts on average 65 days.
Anoestrus:
- Finally, after a dog has given birth (or pseudopregnancy ends) the ovaries become relatively inactive.
- There is no cycling until the next season, usually in a year.
Queen
The queen (female cat) is poly-oestrus, meaning they can have multiple cycles per year. Their oestrus cycle is seasonal, affected by light – when the days get longer, more light is detected by the body, and less melatonin (the hormone that encourages sleep) is produced. This stimulates gonadotropin-releasing hormone (GnRH) from the brain that stimulates the production of FSH and LH from the ovaries. In effect, light ‘switches-on’ the oestrus cycle. This means queens normally cycle in spring to give birth in summer, providing the best conditions for kittens to survive. Queens at central latitudes around the equator may have much longer cycling period. Furthermore, artificial lighting may affect queens’ cycles, resulting in irregularities.
Queens differ from bitches in a few ways:
- The oestrus cycle length is variable, from 1 to 6 weeks
- Cats are induced ovulators, meaning they require sexual intercourse to stimulate ovulation of a follicle. This means the timing of intercourse is a lot less specific than in dogs.
- A pregnancy also lasts on average 65 days, but cats do not have a pseudopregnancy period. If they are not pregnant, their dioestrus lasts around 45 days.
- Because a cat is poly-oestrus, they re-enter pro-oestrus quickly after dioestrus (from days to weeks) – this is how cats can have multiple litters per year.
- Cats only enter anoestrus after light-levels decreases (and melatonin production increases) – for cats in the Northern hemisphere, this is theoretically autumn-winter time.
Ovarian Cysts
The ovaries of dogs, cats and other species can develop various abnormal structures. Some of the most common are cysts, which are fluid-filled sacs that reside on or near the ovaries. These can be functional (they produce hormones) or non-functional. Neutered animals cannot develop ovarian cysts, provided the neutering was performed correctly.
The symptoms of ovarian cysts vary depending on if they are functional or non-functional. Some dogs and cats will have ovarian cysts with no symptoms at all.
The exact causes are generally unknown, but likely linked to hormonal changes during the season. The incidence of ovarian cysts in dogs and cats is reported around 41%, though the actual rate may vary as so many cause no disease. Surveys indicate that around 75% of all ovarian disorders in dogs are ovarian cysts. Older animals are more likely to have ovarian cysts, likely because they have had more seasons, but they have been found in animals as young as a year old.
Follicular Cyst
Follicular cysts are cysts that likely arise from failure of large follicles to ovulate – instead they remain on the ovary and grow large. They have also been known to develop after administration of certain hormones in veterinary drugs. They are the most common ovarian cyst.
Follicular cysts can produce oestradiol that prolong oestrus signs, which may be observed by the animal’s behaviour. The high level of oestradiol can also lead to thickening of the uterine lining, bone marrow suppression and associated anaemia, low platelet count and low white blood cell count, hair loss, and thickened skin. The thick uterine lining can also predispose them to pyometra, a dangerous infection of the uterus.
Luteal Cyst
Luteal cysts are likely formed when the corpus luteum does not disappear at the end of the luteal phase. These can produce progesterone, which can lead to mammary gland enlargement, behavioural changes that mimic pregnancy, prolonged anoestrus, infertility and insulin resistance (which can lead to diabetes mellitus). They can also predispose to thickened uterine lining and pyometra.
Other Cysts
Cystic corpus luteum: normal corpora lutea can develop small cysts, but these rarely cause any disease and are usually non-functional.
Cystic rete ovarii: cysts may also develop in the rete ovarii (an area of cells within the ovaries) but are also usually non-functional. Interestingly, age appears to not be a factor in the development of these cysts.
Cyst of subsurface epithelium: cysts may develop below the surface of the ovaries, but they tend to be tiny, sometimes microscopic, and rarely cause disease. It has been noted they can be painful, however.
Para-ovarian cysts: finally, non-functional cysts can develop next to the ovaries, from embryonic remnant tissue.
Other Causes of Abnormal Seasons
Although ovarian cysts can be a cause of abnormal seasons in dogs and cats (provided the cysts are functional), there are many other conditions that can cause abnormal cycling.
Young animals, particularly those having their first seasons, often have naturally abnormal seasons. They can be short length, irregular duration between seasons, or even ‘silent’ with no obvious signs of coming into season. This can make breeding from a young animal harder. Conversely, older animals can sometimes have fewer seasons or even stop cycling (though dogs and cats do not have a true menopause).
Certain infections can lead to abnormal seasons, infertility or even abortion, including E. coli and Brucella canis. Hormonal and metabolic diseases can also affect the cycles, including kidney disease, Cushing’s disease, diabetes mellitus and hypothyroidism.
Finally, exposure to hormones may affect a pet’s season in various ways. These hormones can be exogenous (externally given), from veterinary or accidentally-consumed human drugs, or endogenous (produced by the animal), such as from functional ovarian cysts or secretory cancers of the ovaries, adrenal or pituitary glands.
Diagnosis and Treatment for Ovarian Cysts
Diagnosing ovarian cysts can be difficult, particularly if there are no obvious symptoms. Sometimes suspicions can be raised if there are seasonal abnormalities, such as persistent oestrus or anoestrus. Some cysts may be seen on ultrasonography, but this can vary depending on the size of the cyst and the skill of the operator. More advanced imaging, such as CT or MRI, are better at finding smaller cysts, but requires an anaesthetic and specialist equipment. Measuring the animal’s hormone levels may give clues that point to functional cysts, such as high oestradiol or progesterone. Other blood changes, like low red or white blood cell counts, can also hint. Definitive diagnosis requires histopathology, which requires a piece of cyst tissue to be analysed at a laboratory.
Treatment of ovarian cysts has traditionally required surgical removal of the ovaries and cyst (ovariectomy), sometimes combined with removal of the uterus as well (hysterectomy). This is 100% effective and also eliminates the chance of recurrence, as the hormonal tissue that creates the most common follicular and luteal cysts has been removed. There have been some surgical attempts to drain cysts instead.
There have also been some studies that have successfully treated ovarian cysts with various hormones – these had an average success rate, and some developed pyometra which required surgical removal anyway. Furthermore, there have been no studies into the risk of recurrence. Surgery remains the treatment of choice for most vets.
Treatment is not always indicated if the cysts are non-functional.
Prevention of Ovarian Cysts
As with most reproductive diseases in dogs and cats, neutering is generally effective at preventing ovarian cysts developing – the surgical removal of the ovaries (+/- the uterus) will prevent true ovarian cysts from developing. For dogs and cats not being bred from, this should be done as early as possible (around 10m-2 years old in dogs, dependent on breed size, and 4-6m old in cats). If an animal is being bred from, this should take place early in life so they can be neutered afterwards at middle-age, again reducing the risk of the development of cysts.
Although the majority of ovarian cysts appear to be non-functional, and cause no symptoms, other reproductive diseases can be much more serious, so neutering is always recommended if breeding is not desired.
References:
Sasidharan et al (2021) Ovarian Cysts in the Bitch: An Update
Sontas et al (2011) A huge ovarian cyst in a hysterectomized bitch
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