This week saw one of the strangest scientific discoveries in the last few months. An article was published on 5th November in the New England Journal of Medicine detailing a case study of a HIV positive man presenting with fever and weight loss who, after several months of unsuccessful treatment, passed away from unknown causes1. Extensive post mortem analysis found malignant tumorous masses, in his lungs and lymph nodes, of non-human origin. Whilst this is beginning to sound like the opening plot of a cheesy horror film, the cells were found to originate from a species of tapeworm, known as Hymenolepis nana (or Dwarf tapeworm), endemic to the region. To my knowledge, this is the first documented case of zoonotic transmission of cancer. Cancer is typically a non-infectious disease, however, there are some notable examples of transmission of tumorigenic cells between individuals of the same species.
Transmission of tumours from human to human is not uncommon. Patients who receive organ donations regularly inherit previously unidentified tumours from their donors2. The progression of the tumours is aided by the necessity for transplant patients to take immuno-suppressive drugs to prevent organ rejection. However, the circumstances don’t have to be as extreme as an organ transplant. Children are capable of contracting cancer in utero from their mother3. The high level of genetic similarity between mother and child, and the lack of immunological education in the early foetus makes vertical transmission possible albeit rare. Perhaps the most bizzare example of human to human transmission is the case of a surgeon who became infected with the tumour of a patient he was operating on after sustaining an injury during the operation4.
Sharing of tumours can also be seen in other species. In both dogs and Tasmanian devils transmission of cancer cells between individuals is highly common. In the case of dogs the cells are transmitted sexually, whilst in Tasmanian devils common face biting behaviour allows for the transfer of malignant cells. Due to incredibly low genetic diversity within both species the transmitted cells are not recognised as foreign and therefore do not trigger the host’s immune system5.
But how did our Colombian man become host for tape worm tumours? Well, his HIV status appears to be the key factor. He had been diagnosed as HIV positive nearly eight years prior to his death and was potentially infected many years before that. At this late stage of infection his ability to generate an immune response would have been severely compromised. This had serious consequences, firstly it may have caused the tapeworm to develop abnormal growths normally limited in healthy people. Secondly, it prevented his body from recognising and responding to the invasive cells that broke free from the worm and disseminated throughout his body. Had the HIV virus not made him severely immunocompromised his immune cells would have rapidly engulfed and destroyed the invading worm cells.
So, should we start thinking of cancer as an infectious disease? In the developed world the low exposure to parasitic worms and the relatively low HIV rate mean the chances of a similar case occurring in say London or Rome are almost non-existent. However, in the developing world similar cases may be highly prevalent with these events previously being unidentified due to limitations in local healthcare. Identifying zoonotic cancers may be crucial when considering treatment options in the future. Furthermore, the potential to develop tumours adds a further dimension of risk to infection with parasitic worms.