Inherited Ventricular Arrhythmia in Rhodesian Ridgebacks (RR IVA) is a disease that was originally described and named by a research team led by Dr. Kathryn Meurs at North Carolina State University in the USA.
Proper heart function and when to say arrhythmia
In order to understand the nature of the disease, we will briefly describe how the heart works, whether human or canine. The activity of the heart is controlled by periodic electrical impulses that spread through the heart muscle and cause its coordinated contraction. As the muscles of the chambers of the heart contract, blood is expelled to the organs and tissues of the body. The heart rhythm, or more precisely the heart rate, changes quite naturally depending on different situations such as physical exertion, rest or stress.
When the heart rhythm loses its regularity, we speak of heart rhythm disorders – arrhythmias. Arrhythmias are among the most common heart disorders in humans. They result from abnormal generation or conduction of electrical impulses in the heart. In most cases, these are completely minor arrhythmias, of which the affected person is not aware or is not substantially bothered, and which can only be detected by monitoring an electrocardiogram (ECG). A simple investigative method is Holter monitoring, which is a tiny box attached to the body that allows continuous ECG readings (typically for 24 hours).
Sudden death in Rhodesian Ridgebacks
Now let’s go back to the research team of Dr. Kathryn Meurs, who examined four cases of sudden death in four related Ridgebacks. Unfortunately, they were unable to explain the cause of these sudden deaths. All that was found were abnormal ECG recordings taken from holter monitoring in other dogs related to those that died suddenly. These related dogs showed what are known as Premature Ventricular Complexes (PVCs). This led to the hypothesis that ventricular arrhythmias were the cause of sudden heart failure in RR. It should be added that sudden death in early life is very rare in RR and in the vast majority of cases the cause is not identified, i.e. whether heart failure is the cause at all.
The genetic test for (RR IVA)
A research team led by Dr. Kathryn Meurs has introduced a genetic test that is predictive of ridgeback dogs with a disposition to RR IVA. A variant in the QIL1_chr.20g.543438G-A gene was tested, and recessive inheritance was assumed, however, the team noted that dominant inheritance can not be ruled out. So, the test result is either a) negative, b) positive heterozygote, or c) positive homozygote. Testing was therefore initiated, but information on the number of positive individuals was not known. Therefore, an initiative was taken to obtain testing data and the RR breeding community was able to collect testing results for 244 Rhodesian Ridgebacks (Table 1).
Table 1: Summary of DNA test results for RR IVA; data voluntarily provided by various members of the RR community.
| Negative | Positive heterozygous | Positive homozygous | Total | |
| Number | 43 | 117 | 84 | 244 |
| Percentage | 17.6 % | 48.0 % | 34.4 % | 100 % |
The above table shows that the number of positive Ridgebacks exceeds 80 %. Does this mean that all positive individuals are at risk? A survey initiated through the Rhodesian Ridgeback IVA Facebook page sought to answer this question. The survey included not only DNA test results, but also the results of Holter monitoring in dogs under 3 years of age, since according to the authors, dogs over 3 years of age are less at risk and the chance of heart rhythm disturbance or failure decreases significantly. The results are summarised in Table 2.
Table 2: Relationship between DNA test results for RR IVA and Holter monitoring results.
| DNA test results | Number of tested dogs | Dogs with arrhythmia according to Holter | Percentage |
| Positive homozygous | 54 | 13 | 24 % |
| Positive heterozygous | 48 | 10 | 21 % |
| Negative | 23 | 4 | 17 % |
| Total | 125 | 27 | 22 % |
These data show that DNA testing of RR IVA is not predictive because the test is not a reliable indicator of the risk of juvenile cardiac arrhythmia in RR, let alone sudden death. In the group of 125 dogs tested, the proportion of dogs with arrhythmias detected at the ” affected” level (>50 PVCs/24h, measured by Holter) is comparable regardless of the DNA test result. Even in dogs with a negative DNA test result, heart rhythm irregularities were observed.
Conclusion:
According to the results of studies and holter measurements, it appears that heart rhythm irregularities (arrhythmias) are common in Rhodesian Ridgebacks and occur in approximately 20 % of dogs, with no signs of major heart or other disease. The results correlate with the widespread occurrence of arrhythmias in humans, in which they are considered a common benign abnormality unless the individual suffers from other heart disease (e.g., has experienced a heart attack).
The genetic test for RR IVA focuses on a genetic variant in the QIL1 gene, which is present in >80 % of Rhodesian Ridgebacks, so we cannot speak of a mutation and can hardly speak of a risk factor. As sudden deaths are very rare in the RR, it is extremely unlikely that they could be associated with the variant in the QIL1 gene that is so abundant in the RR population.
Although rare cases of sudden death in Ridgebacks are known, it is likely to be the consequence of another major genetic factor or a hidden individual disease.
Based on the above, it appears that genetic testing for RR IVA is not reasonable and beneficial, and may be detrimental to the RR population, as it selects and reduces the number of individuals for mating with no proven health benefit.
References:
1) Clayton Heathcock https://camelotrr.com/RRIVA.pdf
2) Meurs, K. M. et al., “Ventricular arrhythmias in Rhodesian Ridgebacks with a family history of sudden death and results of a pedigree analysis for potential inheritance patterns,” JAVMA, 2016, 248, 1135-1138. (b) See also Meurs webinar, February 10, 2017
3) Rhodesian Ridgeback Inherited Arrhythmia (RR IVA); https://www.facebook.com/groups/1346498062074178
