Obesity is a disease of civilization and a global public health problem. Its development is primarily associated with excessive calorie intake, lack of exercise, genetic factors or hormonal disorders. However, recent studies suggest that there may be other contributing factors, such as a viral infection.

In the late 1980s, American researchers discovered that in the population of obese people, up to 60% had antibodies against adenovirus 36 (HAdV-D36) in their blood, while in people with a normal BMI this percentage was significantly lower.

The virus quickly became the subject of research by teams around the world, who tried to determine its potential impact on human and animal metabolism. A groundbreaking 1992 study showed that adenovirus 36 infection in chickens caused an increase in their body weight, despite no changes in their diet. Subsequent experiments on mice and monkeys confirmed these observations, suggesting that the virus may affect metabolism and the accumulation of fat tissue.

'A few years ago, I came across information about this virus on the Internet and it really caught my attention. I decided to check what it looked like in the Polish population. We started a project to study volunteers who provided us with blood samples (and we continue it to this day). Reports from around the world were confirmed - a lot of people, especially overweight and obese ones, had antibodies against adenovirus 36', Barbara Bażanów, PhD, a professor at the Faculty of Veterinary Medicine of the Wrocław University of Environmental and Life Sciences says in an interview with PAP.

'Of course, it is not that if someone has this virus, they are or will definitely be obese, because a whole range of factors come into play. But the trend is very clear', she adds.

Since then, her team has conducted several studies on HAdV-D36. The latest study, published in Veterinary Microbiology, whose first author was doctoral candidate Aleksandra Chwirot, concerned its presence in dogs (a publication on infections in horses is in preparation).

The scientists analyzed blood samples from 118 dogs of different breeds, sexes and ages, who were patients of veterinary clinics in Wrocław and the surrounding area. The animals were not obese, but they had a tendency to gain weight.

'We were the first in the world to isolate active adenovirus 36 from the tissue of an animal that had not been previously infected in the laboratory. We then confirmed its presence under an electron microscope and with molecular techniques, thanks to which we were able to present it in the form of photographs. There can be no mistake, because the literature states that the antibodies we tested are specific for this particular virus and there are no cross-reactions that could falsify the results', Bażanów explains.

She emphasises that based on the research conducted so far, it cannot be concluded that adenovirus 36 causes obesity. 'Such statements must be approached very carefully, because no studies have confirmed 100% that obesity is caused directly by this virus, and no other factors are involved', she says.

The fact is, however, that the range of HAdV-D36 hosts is much broader than previously thought.

The expert explains that one of the key mechanisms that may explain the link between adenovirus 36 and obesity is its effect on fat cells. She says that the virus has the ability to transform adipose tissue stem cells into mature adipocytes, or fat cells, leading to increased fat accumulation in the body. In particular, it accumulates around the waist, which leads to abdominal obesity.

Interestingly, adenovirus 36 infection can also affect glucose and lipid metabolism. 'It has been noticed that infected people often have better metabolic parameters than uninfected people, because newly formed fat cells are an additional storage for cholesterol and triglycerides. For this reason, the virus is the subject of research into its potential use in the treatment of type 2 diabetes', the researcher says.

'This is a really fascinating virus, the potential of which we do not yet understand', she says.

The scientist also explains that HAdV-D36 belongs to the adenovirus family. These are very common human pathogens that live in the glandular tissue of the body and usually do not cause any symptoms. They only become active when the immune system is temporarily weakened, causing symptoms of the so-called cold, most often a runny nose, sore throat and red eyes.

'They are transmitted through droplets, which is why we think that adenovirus 36 is also transmitted this way. However, there are no conclusive studies on this subject yet. And although it has been detected in other species, no one has yet proven whether and how it would be transmitted between them', she says.

The researcher emphasises that work on adenovirus 36 is still at a relatively early stage. Scientists do not know much about it. And although many publications suggest that it affects metabolism and body weight, it is certainly neither the only nor the decisive factor leading to obesity. It cannot be said that contact with an infected person increases the risk of weight gain. 'Its effect on the body is subtle and depends on many factors. We see a correlation between infection and increased body weight, but we cannot say that it is the cause of obesity. However, it may have some effect in people who are predisposed to it', Bażanów concludes.

Katarzyna Czechowicz (PAP)

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