In the 1980s, a remarkable number of young Americans exhibited symptoms of Parkinson's disease. Whatever could be the cause? Researchers quickly found an answer to this question. The feature these patients had in common was their excessive use of heroin. The resulting brain disorder was sparked by a by-product which is released when this hard drug is being synthesised.
The only positive aspect to come out of this is that, ever since, scientists have had a good 'model' for research into Parkinson's disease. The end of this debilitating disease may not yet be in sight, but progress has been made in dealing with the symptoms of Parkinson's, including this new insight. Medication is now also available to suppress the symptoms. Clear scientific advances are being made in the fight against Parkinson's, although we still do not have a drug to cure it, and marmosets play a crucial role in this process.
Comparable immune system
At BPRC, we are studying the early preliminary stages of Parkinson's disease. We use our marmosets to do so, monkeys officially referred to as common marmosets. Marmosets have much in common with humans, including a comparable immune system. This makes them a suitable animal model in the search for therapies for serious immunological diseases, like multiple sclerosis (MS) and Parkinson's. Our research always involves healthy monkeys, since it would otherwise be impossible to know which other interactions are transpiring in the body. In addition, we consider it ethically unsound to use unhealthy animals in studies.
Keeping distress to a minimum
The by-product in question is administered to the animals in small quantities. This slow and gradual approach is a conscious choice, intended to keep distress for the marmosets to a minimum and further refine the trial model, in line with the three Rs (i.e. reduction, refinement and replacement of experiments involving animal testing). Distress refers to a lack of welfare in the broadest sense of the word. This could involve pain and illness, but also stress. Despite the fact that 'distress' is a very broad concept, we do keep track of the level of distress experienced by our animals with a scoring system. If we were to administer the by-product in one go, the distress would be greater, which we wish to avoid.
The notion that marmosets lose a lot of weight in the study is a misunderstanding. Our publicly available application to conduct this study only stated that they could lose weight. However, in the manner we are conducting the study, their weight remains stable. If they did lose 10 or 15 grams (while weighing 300 g), we would provide supplementary food or remove them from the study. However, this is purely hypothetical. In other words, the fact that something could occur, does not necessarily mean that it will. This is precisely why we monitor the animals so closely: to be able to prevent this from happening.
Early stage of the disease process
Yes, the monkeys do display symptoms of Parkinson's disease. This is indeed the intention, but because we only want to study the early stage of the disease process, these symptoms are never severe. They certainly also are not 'out for the count'. Early on, they are almost entirely unaffected. They can eventually reach a point where they are apathetic, yet extremely alert, which is exactly like patients with Parkinson's. They may also tremble slightly and move stiffly, which is the exact same effect that Parkinson's disease has on humans, which is what makes this such a suitable model.
How long a monkey will be used in a Parkinson's study depends on what is being researched. The longest study until now was three months. Although we would like to follow the monkeys for a longer period, compensation occurs after about ten weeks, as it also does in human patients. They 'normalise' and the body recuperates temporarily, so that they can live for a while longer without symptoms, despite the damage to their brain.
The impact of Parkinson's
A time comes that the disease resumes its course. However, we do not allow this to happen to our monkeys, as we euthanise them after the study. Their brains then become extremely valuable to the further research. Their importance is reflected in the fact that there is still no drug that can cure Parkinson's disease. Anyone who knows someone with Parkinson's, or used to know someone, realises just how dreadful this disease is and what its impact is on a patient's immediate environment: on their children, partner, relatives and friends.
No alternative methods to conduct research are available either, nor does it seem likely that such a method will appear any time soon. If we were to end primate studies in the Netherlands today, this would continue at the same pace in other countries - or perhaps even at a higher one – in countries like the US and China. And if this biomedical research were to be ended everywhere, the study of this disease would also end. In that case, no drug will be developed and we will have to accept that Parkinson's patients will remain Parkinson's patients. In addition, the number of patients will only continue to rise and patients will suffer the consequences of this disease for longer, since we know that more people are now reaching old age and the human lifespan is increasing.
This is a disturbing prospect for Parkinson's patients, for their loved ones and for society. At BPRC, we are wholeheartedly making every effort to discover means of combating this serious disease.