Lyme disease is the most common vector-borne disease in the United States. It is a bacterial infection spread by tick bites. Despite various claims, there is currently no evidence that Lyme disease can spread between people by blood transfusions, sexual intercourse or from mother to child.
What is Lyme disease?
Lyme disease is caused the bacterium Borrelia burgdorferi. The bacterium is spread by ticks when they bite humans. An infected bite often results in a bullseye rash, and then other symptoms include fevers and headaches. If treated with antibiotics early on, the bacteria can be eliminated within a few weeks. If the infection is not treated more severe symptoms like joint pain and problems with the nervous system can occur in some people.
How do you get Lyme disease?
B. burgdorferi is transmitted to humans through a group of ticks called Ixodes. B. burgdorferi is a member of a family of bacteria called spirochetes, which are characterised by their distinct ‘wavelike’ body shapes. According to 9 out of 11 experts, tick bites are the only way you can get Lyme disease.
Dr Randolph, an expert in parasite ecology from the University of Oxford, explains that this is because the “bacteria undergo essential biological development while within the tick vector to make them transmissible to vertebrate hosts (including humans)”. Despite this, some claims have been made that the bacteria can spread between people by blood transfusions, breast feeding, through the placenta during pregnancy or sexual intercourse.
What is the evidence for person-to-person transmission of Lyme disease? For obvious ethical reasons, scientists cannot do an experiment where we purposefully try to infect a person by different methods and see if it works. This means that the evidence we can get about Lyme disease transmission is indirect.
Epidemiological data can be collected to see if cases of Lyme disease correlate with factors like tick habitats (which they do). Epidemiological data is not the best way to find out how a disease spreads - just because there aren’t ticks in your neighbourhood doesn’t mean you cannot get a tick bite, for example you may have been travelling recently.
Another approach the spirochetes in samples of blood, semen or breastmilk. In 2015, a study found Borrelia bacteria in semen and vaginal secretions. The bacteria has also been found in blood, urine and breastmilk. These studies attracted media attention and made various unsupported claims about what their findings mean.
“It is worth noting that the presence of the bacterium in a particular tissue or fluid is not sufficient to claim that the bacterium is transmitted from it” says Dr Brunner from Washington State University. This is an important observation and relates to Dr Randolph’s point that B. burgdorferi need to undergo certain processes in the tick before it can infect humans.
To really know whether person-to-person transmission of Lyme disease is possible, we need to actually observe it happening. So far, no clinical trials have been able to do this. A clinical study found that patients receiving blood transfusions did not get Lyme disease. Multiple studies have also shown “that an adverse outcome due to maternal infection with B. burgdorferi at any point during pregnancy in humans is at most extremely rare”.
As it is not possible to do a clinical study to see if Lyme disease is sexually transmitted, the next best evidence is animal models. Multiple experiments in rats and hamsters where animals infected with B. burgdorferi were kept in close proximity with non-infected animals for periods of time have shown that Lyme disease cannot be transmitted sexually. Similar experiments also showed transmission did not occur by urine, faeces or from mother to baby.
The plot thickens.. Lyme disease is a topic which is filled with misinformation and confusion. This extends beyond just how the disease is transmitted to other aspects of Lyme disease, such as diagnostics and different subtypes of the disease (namely, ‘chronic Lyme disease’). As shown here, different types of data can be collected to investigate a scientific question.
Looking at one piece of data in isolation can lead to incorrect conclusions – just because bacteria was found in semen samples, does not necessarily mean Lyme disease is sexually transmitted.
Misinformation has the potential to have harmful effects on patients, for example by recommending therapies which do more harm than good.
The only known way to get Lyme disease is from a tick bite. There is a lot of controversy around the topic of Lyme disease, so it is useful to try to understand what type of evidence a claim is based on.
This article is based on 11 experts answers to the question: 'Can Lyme disease be transmitted from one person to another?' Check out the Metafact website to see more answers to popular science questions.