1 May 2017
Elisabetta Mancinelli
Job Title
Several hamster species are commonly kept as pets – and as practising vets, we should look with interest into research investigations and case presentations as results may be of clinical interest and applicable to our everyday job.
Syrian hamsters are nocturnal, solitary, and originally from south-eastern Turkey and northern Syria. Several coat variations exist, and they are probably the most common hamster species kept as pets. They are also used as research models.
Ocular disease is a relatively common presentation in hamsters and several reports exist in the literature of conjunctivitis, keratoconjunctivitis sicca, eyelid melanoma, retinal dysplasia and Harderian gland abscess (Mangkoewidjojo and Kim, 1977; Schiavo, 1980; Williams, 2007; Zaffarano et al, 2015). Their small size can greatly complicate examination and further investigations, and sometimes preclude use of certain tests commonly performed in other species, delaying an appropriate diagnosis. Studies that help establish reference ranges for specific ophthalmologic tests for healthy animals may, therefore, be of clinical interest.
Rajaei et al (2013) determined values for the phenol red thread test (PRTT) in 16 healthy adult Syrian hamsters. PRTT is a quantitative tear film test less commonly performed compared to the Schirmer tear test (STT), but potentially useful in exotic species for several reasons. PRTT requires only a small volume of tears (1μL); therefore, the test can be repeated several times at relatively short intervals (one to five minutes). Because the thread remains in the eye for only a short period (15 seconds), it takes less time to perform it, compared to STT, and it induces minimal sensation (Biricik et al, 2005). Normal values have been established for other exotic species, including rabbits, guinea pigs, chinchillas, rats, mice, Amazon parrots and other Psittaciformes.
For the present study, the hamsters were gently, physically restrained without sedation, and a complete physical and ophthalmologic examination performed, including PRTT, fluorescein test and slit lamp biomicroscopy.
For PRTT, the lower eyelid of each hamster was pulled out and the folded head of the cotton thread placed into the ventral conjunctival fornix for 15 seconds. The thread, impregnated with pH-sensitive phenol dye (which changes from yellow to red when it absorbs tears that are slightly alkaline) was then removed, and the part of the thread that had changed to red was immediately measured in millimetres.
The mean ± SD PRTT values for the study population were 6.8mm/15s ± 2.5mm/15s with a range from 3mm/15s to 11.5mm/15s. A significant difference was found between sexes (as seen in guinea pigs, dogs and horses) as the mean PRTTs in males were 5.1mm/15s ± 1.2mm/15s, whereas the mean PRTTs in females were 8.5mm/15s ± 2.3mm/15s. No statistically significant difference was present between right and left eyes.
Broadwater et al (2010) suggested this may be related to the larger size in male dogs compared to females. Female Syrian hamsters are heavier than males, but, in the present study, weight did not have a significant effect on PRTT results. Therefore, the reasons behind this difference remain unknown.
Syrian hamsters notoriously have a low tear production, which is, nevertheless, adequate to protect their ocular surface. This may be a fluid conservation mechanism in a desert-dwelling species (Ofri et al, 1999; Rajaei et al, 2013).
Rajaei et al (2016) performed a series of quantitative tests in 40 healthy adult Syrian hamsters (21 males and 19 females) to obtain reference values for tear production, horizontal palpebral fissure length (HPFL; the distance between the inner end of the ocular caruncle and the temporal canthus was measured with a digital calliper), eye blink frequency (during a five-minute period with no handling) and intraocular pressure (IOP) in this species.
The animals received a complete physical and ophthalmoscopic evaluation, including direct and indirect ophthalmoscopy, fluorescein staining and slit lamp biomicroscopy. Tear production was measured with PRTT, modified Schirmer tear test (mSTT) and endodontic absorbent paper points tear test (EAPPTT). PRTT was performed by placing a 3mm folded head of a phenol red cotton thread impregnated with pH-sensitive phenol dye in the ventral fornix of each eye for 15 seconds.
The mSTT strips were obtained by longitudinally dividing standard (35mm in length and 5mm in width) commercial STT strips aseptically with a scalpel blade and stainless steel ruler to yield two strips 35mm in length and 2.5mm in width. Forceps were used to insert an mSTT strip in the ventral conjunctival fornix, where they were left for one minute. Strips were removed and the wet portion measured.
EAPPTT used standardised endodontic absorbent paper points, which are commonly used in dentistry because of their highly absorptive and drying properties. Because they allow carriage of medications (such as antiseptics and disinfectants), they can assist in collection of samples for microbiological culture or be used as an alternative method for tear film measurement (one standardised absorbent paper point is inserted in the ventral conjunctival fornix of an eye for one minute then the wet portion is measured by use of digital callipers graduated in millimetres).
IOP was measured by use of rebound tonometry. For this test, animals were grasped by the nape of the neck, between a thumb and forefinger, while maintaining a grip on the tail and supporting the animal’s body against the palm of the other hand to avoid any pressure to the eyelids or neck. Correlations between test results and bodyweight were also evaluated.
All tests were performed between 4pm and 6pm to minimise possible variations associated with diurnal changes. Mean ± SD values for IOP, PRTT, EAPPTT, mSTT, HPFL and blink frequency were 4.55mmHg ± 1.33mmHg, 5.57mm/15s ± 1.51mm/15s, 4.52mm/min ± 1.55mm/min, 2.07mm/min ± 0.97mm/min, 5.84mm ± 0.45mm, and 1.68 blinks/min ± 0.43 blinks/min, respectively. For all variables, values did not differ significantly between the right and left eye or between males and females. No correlation was also present between measured variables and bodyweight.
The desert, or Roborovski, hamster is the smallest species in the hamster family (4cm to 5cm in length and 20g to 25g in weight) and originates from western and eastern Mongolia, and northern China. It was originally imported into the UK in the 1960s by the London Zoo (Keeble, 2009) and is one of the lesser-known laboratory animal models. Distinctive features are white eyebrows, sandy gold coat with no dorsal stripe and a white belly (Keeble, 2009).
Knowledge of the reproductive system and physiology of this species is limited, but of interest as it is well known in other rodent species the oestrous cycle can affect circadian rhythm parameters. For example, activity levels increase during days of preoestrus and oestrus in golden hamsters and rats, likely as a strategy for increasing the probability for physical contact between males and females, thus increasing mating success (Richards, 1966; Fritzsche et al, 2000; Wollnik and Turek, 1988).
Therefore, a study was undertaken to investigate the time course of the vaginal and behavioural oestrous cycles of the desert hamster by means of vaginal smear cytology, serum luteinising hormone (LH) levels, wheel-running activity patterns and behavioural pairing tests (Scheibler and Wollnik, 2013). A total of 59 females and 28 males were used in the experiments and the males were placed next to females to maintain the females’ cycles.
The authors found the oestrous cycle of this hamster species lasted between four and six days, following a pattern rather similar to that described for other rodent species. However, an important finding was the existence of five oestrus stages based on the presence of different cell types and their relative quantities on vaginal smears.
The fifth cycle stage, between preoestrus and oestrus, had not been previously described for the golden hamster, the rat, the Mongolian gerbil, nor for any other species of the genus Phodopus (this may also be due to the methodologies used). This fifth stage, named early oestrus, differed from preoestrus by a complete lack of leukocytes, a predominance of epithelial and cornified cells in the vaginal cytology, and a dramatic increase of serum LH levels (this rise is known to indicate ovulation in hamsters, other mammals and rats; Krinke, 2000) likely associated with increased fertility.
The early oestrus stage lasted four to six hours, was not observed in every female, but, when present, affected the length of the whole oestrous cycle.
With early oestrus, the preoestrus stage lasted 14 to 18 hours and the total length of the oestrous cycle length was 4 days. Without early oestrus, the preoestrus was prolonged to 18 to 36 hours and the oestrous cycle length varied between 4 to 6 days.
Vaginal cytology, with twice daily samples, proved invaluable in differentiating the oestrus phases in the desert hamster and smear showed more similarity with that of gerbils (as the presence of mucus was a characteristic feature of the dioestrus stage in desert hamsters and gerbils not described in vaginal smears of golden hamsters) and rats than that of any other hamster species.
From a behavioural point of view, desert hamsters showed only subtle oestrus-correlated changes in wheel-running activity. For example, they failed to show the characteristic scalloping of activity onset (an earlier onset of activity during oestrus as seen in rats), but showed prolonged activity during early oestrus. As seen in the golden hamster, sexual behaviour was most pronounced during oestrus, but aggressive behaviour was observed during each stage of the oestrus cycle.
In general, females should reduce aggressive behaviour to mate, but this is not the case for all rodent species, including the desert hamster (Takahashi, 1990).
