Hello, Vetaheader! We are back with a brand new newsletter. This month, it’s all about guinea pigs (and a little bit about rabbits)! 

First, we have a retrospective study on epidemiological, clinical, radiographic, echocardiographic findings and outcome in client-owned guinea pigs with cardiac disease. Diagnostic of cardiac conditions on this species is challenging, as clinical signs are non-specific; VHS and reference ranges have been determined (you can find a paper about it here), but have not been validated in guinea pigs with cardiac disease.

Medical records from 2010 to 2021 were reviewed, and a total of 80 cases were found. Prevalence of cardiac disease was 2.8% (80/2864 patients), and no sex, age, or body weight predisposition was found. Clinical signs included dyspnea (46/80), lethargy (18/80), and anorexia (10/80). The most common physical examination finding was heart murmur (10/80); however, it is common to have normal cardiac auscultation in guinea pigs with cardiac disease, which may be due to their fast heart rate

Radiographic abnormalities included subjective cardiomegaly (37/67), pleural effusion (21/67), and increased lung opacity (40/67). VHS could not be measured in 19/67 of lateral and 28/67 VD due to pleural effusion or increased lung opacity masking heart margins. There was an overlap of VHS values between this study and what has been reported for healthy animals, which means studies are needed to determine accuracy.

The most common echocardiographic diagnosis was cardiomyopathy (30/80), categorized as restrictive (11/30), hypertrophic (10/30), or dilated (9/30). Other cardiac diseases included cor pulmonale (21/80), pericardial effusion (18/80), congenital heart disease (6/80), acquired valvular disease (3/80), and cardiovascular mass (2/80). Congestive heart failure was present in 36 of 80. 

Median survival time from diagnosis was 2.5 months, and animals that died from heart disease had a significantly shorter survival time than those that died from a noncardiac disease.

The take home message is: prevalence of heart disease in guinea pigs was 2.8%, with cardiomyopathy (restrictive, hypertrophic, dilated), cor pulmonale, and pericardial effusion being most common. No sex, age, or body weight predisposition was found. More work is needed to evaluate accuracy of VHS in detecting cardiomegaly.

Second, we have a paper on de novo reference values for the thyroid hormones TT4, fT4 and TSH in healthy pet rabbits and guinea pigs alongside a TRH-stimulation test. Total thyroxine (TT4) and free thyroxine (fT4) reference values have been previously published for these species, but the studies have been performed on laboratory animals, with small sample sizes. Moreover, a TRH-stimulation test should be implemented to evaluate if the TSH ELISA kit reliably measures different serum concentrations of TSH – in a stimulation test, the hypothalamus-pituitary-thyroid axis’ directly responds to TRH injection, then TSH is secreted from anterior pituitary gland and an increase in TSH levels occur in serum. Additionally, serum concentration of TT4 and fT4 increase after TSH stimulation of thyroid.

Plasma samples from healthy pet rabbits (n=41) and guinea pigs (n=73, TT4 and fT4; n=51, TSH) were evaluated using chemiluminescence assay (CLIA; for TT4, fT4) and enzyme-linked immunosorbent assay (ELISA; for TSH) to establish reference values. A thyrotropin releasing hormone (TRH)-stimulation test (n=4; for each species) was conducted to validate the TSH ELISA test kits. 

The results showed no response of total TT4 in rabbits after TRH-stimulation (30 min); in guinea pigs, there was an increase in TT4 after TRH stimulation (120 min). This may be due to  rabbit blood being collected too early.

In guinea pigs and rabbits, age was a significant predictor for TT4, suggesting that with every month of life, the TT4 decreases by 0.02 ug/dL and 0.01 ug/dL, respectively. Sex also affected fT4 in guinea pigs, with values being lower in castrated males versus intact males.

Maximum TSH secretion is triggered by preceding TRH release circadian rhythm in early evening hours – all samples were taken at contrasting time of day in this study, so only an average TSH can be assumed (samples over the course of the day would be needed to get really accurate numbers). 

It is important to note that there is a lot of assay variability – therefore, references intervals established by different methodologies are not interchangeable!

The take home message is: new reference ranges for thyroid levels in guinea pigs and rabbits were established. Due to multiple methodologies and possible differences between labs, there may be issues when using this clinically.

Lastly, we have a retrospective on the occurence of spontaneous splenic hemangiosarcoma in pet guinea pigs. The etiology of hemangiosarcoma is unclear, but genetics and exposure to toxic substances or ionizing radiation are associated with occurrence in humans, dogs, cats. Cutaneous and visceral forms have been described, and visceral occur predominantly in spleen, heart, and liver (most common location in small mammals), and tend to rupture or metastasize to liver, diaphragm, omentum, pancreas, mesentery, and lungs. Splenectomy is the treatment of choice – it has been described in guinea pigs, and is performed in same way as dogs/cats.

Medical records ranging from 2007 to 2021, from four referral centers, were reviewed. Of 3994 guinea pigs examined, five were included in the study (prevalence index of 0.125%). The median age at the time of diagnosis was four years (range 4-6 years). 

The most common clinical sign was dysorexia (4/5). Abdominal palpation revealed space occupying mass in the area of the spleen in all animals, which was confirmed by ultrasonography. Sex may be a predisposing factor, with 3/5 being females in the cases described – women are more predisposed than men as well.

One patient was euthanized after surgery, while the others died two weeks, six months, 15 months and 49 months after the procedure. However, comorbidities were present in these cases, which may have affected median survival time.

In this study, the presence of metastasis cannot be ruled in or out, as there were not radiographs performed in all cases, and there were no necropsies available.

The take home message is: hemangiosarcoma in the spleen of guinea pigs is uncommon, but can occur. Age and sex may be predisposing factors, and there may be multiple comorbidities present in these patients.

Hope you find it useful and, as always, I will see you next month!

Sincerely,