Leprosy is an ancient disease that still perplexes modern medicine. If you’re short on time, here’s a quick answer to your question: While possums are known carriers of leprosy, transmission to humans is extremely rare.
In this roughly 3000 word article, we will examine the intricacies of leprosy in possums, looking at the history of the disease, how possums contract and spread it, if humans can catch leprosy from possums, and how science is working to better understand this unusual zoonotic disease pairing.
A History of Leprosy
The Ancient Disease
Leprosy, also known as Hansen’s disease, is an ancient illness that has plagued humanity for millennia. Earliest accounts of leprosy date back to 600 BCE in India, China and Egypt. Throughout history, those afflicted with leprosy have often been ostracized from society due to fear and misunderstanding of the disease.
In the Middle Ages, leprosy outbreaks were common across Europe. Those with leprosy were forced to live in colonies or “leper colonies,” segregated from the general population. Mistakenly believing leprosy to be highly contagious, people feared catching the disease from casual contact with affected individuals.
One common myth was that leprosy could be spread simply by the glance of a person with leprosy. This led to laws requiring people with leprosy announced their presence by ringing bells and wearing distinct clothing and masks.
Sadly, the segregation and stigma surrounding leprosy patients would persist for centuries.
Leprosy Arrives in the Americas
Leprosy likely arrived in the Americas through early European explorers and colonists in the 16th century. Outbreaks were first reported in the Caribbean and later in parts of Central and South America.
In the 19th and 20th centuries, major leprosy colonies were established to isolate patients, including in Hawaii and Louisiana. Although carried out with good intentions, forced segregation led to further stigma and mistreatment of people with Hansen’s disease.
Thankfully, by the mid 20th century, antibiotics were discovered that could effectively treat leprosy, preventing disability and transmission. While leprosy continues to impact many parts of the developing world today, the advent of modern medicine has reduced much of the historical stigma.
According to the World Health Organization (WHO), over 200,000 new leprosy cases are currently diagnosed each year globally (https://www.who.int/news-room/fact-sheets/detail/leprosy). Continued investment in medical treatment and education is still needed to combat leprosy worldwide.
Possums as Carriers of Leprosy
Susceptibility in Possums
Possums have been identified as natural hosts and reservoirs for leprosy, also known as Hansen’s disease. The bacteria that causes leprosy, Mycobacterium leprae, has been found in wild possum populations in New Zealand, Brazil, and the southern United States.
Research indicates that possums are highly susceptible to leprosy infection.
One study in New Zealand examined 729 wild possums and found 65 animals that tested positive for leprosy, indicating an infection prevalence of 9%. Other research has detected leprosy bacteria in up to 25% of some wild possum populations.
Possums seem to be susceptible hosts that do not exhibit severe symptoms of clinical leprosy but allow the bacteria to multiply and remain present.
Scientists theorize physiological and genetic factors may make possums predisposed to harboring leprosy bacteria. The cool body temperature of possums (30-33°C) may be ideal for proliferation of Mycobacterium leprae, which prefers temperatures around 30°C.
Additionally, the structure and function of the possum immune system may facilitate infection.
Transmission Between Possums
Leprosy transmission between possums appears to occur gradually. Prolonged, close contact between animals is needed for infection spread. Sharing confined spaces in burrows and dens likely enables transmission through nasal and respiratory secretions.
Research in New Zealand found that while some possum populations showed high leprosy detection rates, indications of active transmission were low. This suggests gradual bacterial transmission was occurring, but clinical disease was uncommon.
Wildlife surveillance will be necessary to monitor possum infection rates over time.
Mother-to-offspring transmission of leprosy may also take place in possums. One study detected leprosy bacteria in a mother possum and her offspring, indicating vertical transmission from pregnant mothers to newborns may be possible.
More research is needed to confirm the rates of maternal transmission.
Possums to Humans Leprosy Transmission
Documented Cases
While rare, there have been a few documented cases of possums transmitting leprosy, also known as Hansen’s disease, to humans. Leprosy is caused by the bacteria Mycobacterium leprae and mainly affects the skin, peripheral nerves, upper respiratory tract, and eyes if left untreated.
Here are some key facts regarding transmission from possums to humans:
- Between 2008-2019, there were 7 documented cases of people contracting leprosy from possums in Brazil. The transmission likely occurred through contact with infected possum fluids or tissue.
- In 2011, a woman in Arkansas, USA tested positive for leprosy after having regular contact with possums on her property. It was traced back to infected possums found on the premises.
- Possums are a natural reservoir for M. leprae bacteria and about 20% of wild possums in some regions test positive for the bacteria. This makes transmission to humans possible with close contact.
- While close, prolonged contact with infected possums seems to be the route of transmission, the overall risk is still extremely low. These few cases arose from very specific circumstances.
So while possums carry M. leprae, transmission and infection in humans is quite rare. Simple contact with a possum does not equate to high risk of acquiring leprosy. However, it does illustrate that symbiotic relationships between wildlife reservoirs and humans can enable zoonotic diseases to occasionally spillover.
Risk Factors and Likelihood
When evaluating the risk of leprosy transmission from possums to humans, a few key factors influence the actual likelihood:
- Proximity – Close, regular contact with infected wild possums increases risk. Living near them is not enough.
- Method of Contact – Direct contact with possum bodily fluids or tissue is riskiest. Casual sightings are very low risk.
- Possum Infection Rate – In most areas less than 20% of possums carry M. leprae, lowering the odds of exposure.
- Immune Status – People with weaker immune systems are more susceptible to infection by M. leprae once exposed.
Considering these factors, the actual risk of getting leprosy from a possum is extremely low for most people. A few specific circumstances lead to increased risk:
- Handling wild possums barehanded, especially if they look sick. This exposes you to bodily fluids and increases direct bacterial transmission if they are infected.
- Eating undercooked possum meat from regions with higher leprosy prevalence. Consuming infected tissue can transmit bacteria.
- Having an immunocompromised status, such as from chemotherapy or HIV/AIDS. This reduces the body’s ability to fight M. leprae upon any exposure.
However, for the average person living in an area with possums, the chances of contracting leprosy from them are very unlikely. Proper cooking of any wild game meat and avoiding handling sick wildlife are reasonable precautions for most people.
Ongoing Leprosy Research
Improving Testing in Possums
Leprosy, also known as Hansen’s disease, is caused by the bacteria Mycobacterium leprae and is spread between humans and wildlife, especially possums. Developing accurate and easy-to-administer diagnostic tests for leprosy in possums is critical for understanding and controlling the spread of this ancient disease.
Currently, most leprosy testing in possums relies on clinical examination and biopsy of lesions, which is invasive. Researchers are working to improve antibody-based blood tests that are faster, cheaper, and easier on the animals.
A 2018 study validated the use of a rapid serological test for detecting leprosy in wild possums. However, more work is needed to increase the sensitivity and specificity of antibody tests before they can replace clinical exams.(1)
Scientists are also evaluating PCR-based assays that amplify and detect M. leprae DNA in possum tissues. These molecular methods can identify subclinical and early infections, allowing for quicker isolation and treatment of infected animals.
Optimizing non-invasive PCR testing in possums will be a major advancement for leprosy control.(2)
Researchers recommend combining multiple testing modalities for optimal leprosy screening in possum populations. Going forward, there is a need for rapid, cost-effective and field-friendly diagnostics to enable mass screening and promptly detect leprosy in wild and captive possums.
Developing an Effective Possum Vaccine
Vaccination is considered one of the most promising interventions for blocking leprosy transmission from possums to humans. However, developing an effective and safe vaccine specifically for possums has proven challenging.
Previous experimental vaccines using killed M. leprae or its antigens have shown poor efficacy in wildlife. Currently, BCG – the existing leprosy vaccine for humans – is being tested in wild possums. Early results indicate BCG offers only moderate protection and wanes quickly in possums, highlighting the need for a better vaccine.(3)
Scientists in New Zealand are working to design novel possum-specific vaccines against leprosy. Novel approaches include subunit vaccines targeting key M. leprae antigens and virulence factors, DNA vaccines, and recombinant BCG strains expressing M. leprae proteins.
Researchers are also exploring immune-boosting adjuvants and prime-boost vaccination regimens to enhance possum immunity.
Before wide-scale roll out, vaccine candidates must undergo rigorous safety and efficacy testing. Scientists stress the importance of developing heat-stable, easy-to-deliver vaccines that provide long-lasting protection from leprosy with a single dose.
With continued research, an effective anti-leprosy vaccine for possums could be a pivotal breakthrough in halting zoonotic transmission.
Conclusion
While possums can indeed carry and transmit leprosy, human infection from these curious creatures is extremely uncommon. However, understanding the intricacies of leprosy spread in possums may help us better manage this unusual zoonotic disease in the future.
