Text: Texte : Yann Bernardinelli

Diseases we thought had disappeared

A resurgence of tuberculosis was reported in 2021. The WHO is alarmed about delays in routine vaccinations. Even though they seem to be under control, centuries-old diseases never totally disappear. On top of that, climate change could bring back prehistoric viruses.

Tuberculosis? Whooping cough? Leprosy, typhus and syphilis? Who would have thought we would be talking about these diseases today? Most people think of these diseases as a long-gone thing of the past. Not so. Not only are they still active, but they still kill and even trigger an epidemic at any time. Since Covid-19 hit, the fight against these old diseases has waned, inciting the WHO to warn of potential resurgence.

Strangely quiet

In December, the WHO expressed concern about screening and immunisation boosters, especially for measles and tuberculosis. Switzerland has not escaped concerns, even though, paradoxically, a record fall in the number of cases was reported in 2020. Pierre-Alex Crisinel, head physician at the Paediatric Infectious Diseases and Vaccinology Unit at Lausanne University Hospital (CHUV), says that public health programmes were plainly suspended in 2020 because of the Covid-19 health crisis. They have not yet caught up. “The decrease in 2020 is due to the decline in screening, but also to social distancing measures put in place to combat the virus, which have also acted against other diseases.” The doctor fears an increase in cases as these measures are gradually lifted.

Vaccination coverage for measles was excellent before the pandemic, with almost 95% of Swiss children immunised. “This coverage must be maintained to limit the spread of the disease. Measles is extremely contagious, even more so than the Omicron variant of Covid-19.” The expert points out that immunisation campaigns for babies between 9 and 12 months old are very well accepted by parents, and vaccination coverage has increased since the awareness campaigns conducted between 2013 and 2015. “The problem is more for the previous generations. There have been waves of non-vaccination, and parents can be totally unaware of the danger. Some young adults are not vaccinated today. In the event of a measles outbreak, this population group would be at risk.” The disease could be eradicated through vaccination. “It would require a sustained effort, because the disease is so contagious. Plus, some people are not scared enough of measles. That can impede endeavours.”

Tuberculosis, on the other hand, still kills nearly 2 million people a year (1.5 million in 2020). These statistics mean it is the second deadliest infectious disease after Covid-19. Tuberculosis is caused by a bacterium that multiplies every 20 hours, i.e. at a much slower growth rate than Covid, and is typically transmitted within a family. “The problem is that several high burden countries for tuberculosis have disrupted the continuity of services essential to fighting the disease, meaning not just access to care for TB patients, but also screening for family members who have lived with them during lockdowns,” says Jesica Mazza-Stalder, a physician with the CHUV Service of Pulmonology. The BCG vaccine only protects newborns against severe forms of the disease. It therefore cannot be counted on to eradicate tuberculosis. “Screening and treatment are therefore crucial, as is access to care. The key to getting rid of it for good is proactive political policy and public health programmes.”

Lurking pathogens

As long as the pathogen (virus, bacterium, parasite or fungus) still exists, an infectious disease can always re-emerge, even if no longer carried by humans. That is what happened with the plague, for example. The bacterium Yersinia pestis, which causes the disease, can still be found in a few rats and fleas, and they can transmit it to humans. Between 1990 and 2020, the WHO recorded 50,000 cases, mainly in sub-Saharan Africa and South America, sometimes after disappearing for several decades in the countries concerned. “The plague is not a major problem as long as it is monitored. Antibiotics and public health measures do a nice job of keeping it at bay. Controlling or eradicating all diseases transmitted from animals to humans – referred to as zoonoses, of which more than 200 types exist – is a challenge, if not mission impossible. We’re seeing that with dengue and malaria, which are spread by mosquitoes,” says Thierry Calandra, Physician in chief of the Infectious Diseases Service at CHUV-UNIL.

In addition to viruses, the infectious diseases expert is also concerned about the evolution of certain bacteria. “They adapt and develop resistance to antibiotics. That’s a major challenge today.” For example, Staphylococci and Streptococci have never really disappeared, but are now classified as “re-emerging” diseases. Other re-emerging diseases include tuberculosis, typhoid fever and syphilis. There are also nosocomial infections, contracted during a hospital stay and caused by antibiotic-resistant bacteria such as Escherichia coli, Klebsiella, Pseudomonas, and Acinetobacter.

Disease and climate change

The close contact between humans and pathogens is another factor influencing the resurgence of certain viruses, or preventing them from disappearing. The best example is influenza, the virus that causes the flu. In addition to seasonal epidemics, the virus can evolve suddenly and trigger pandemics, as it did in 1918, 1957, 1968 and 2009. Then there is avian flu. “Migratory birds can transmit this influenza virus to domestic poultry, pigs or marine mammals, which can in turn pass it on to humans, if the species live too close together,” says Thierry Calandra. Cholera, caused by the bacterium Vibrio cholerae, has caused seven pandemics since 1817. Endemic and epidemic, cholera can spread rapidly when sanitary conditions are sub-standard, such as conflicts and natural disasters, and, for example, a large population of refugees are crowded into insalubrious camps.
This is also happening with dengue fever, which is spreading dangerously in the megacities of South America and Southeast Asia due to uncleanliness and overcrowding. A vaccine is available to combat this scourge and others are being developed.

Danger of frozen viruses

A growing number of experts are trying to anticipate the consequences of climate change on the balance of ecosystems, which could potentially cause diseases to re-emerge. Melting permafrost and glaciers are the most tangible fear, as viruses from the past that are still functional could re-emerge once they thaw. Several research studies have demonstrated this, including the smallpox virus found in remains of Vikings. Vaccines continue to be one of the best tools that humans have for eliminating infectious diseases. “Unfortunately, vaccines are not available for all pathogens, but the potential of vaccine technologies highlighted during the Covid-19 health crisis offers hope,” Jesica Mazza-Stalder concludes. /



Troubled waters

Disruption in the water cycle – in its flow pathways, its chemical balance and poor sanitation – can cause larvae to proliferate that are conducive to the development of pathogens. Concrete examples are the resurgence of dengue fever, malaria and schistosomiasis.

Natural incubators

The increase in global temperatures allows certain pathogens to proliferate abundantly where conditions used to be too cold or restricted their growth. The possibility exists that infectious agents frozen for thousands of years could re-emerge, although studies are still needed to assess the risks.

Conditioned host

As pathogen hosts, humans are also affected by climate issues, as

pollution weakens their immune systems. This creates the optimal environment for future infections.

Close correlation

Climate change, urbanisation and pollution create the ideal conditions for the development of new pathogens, and the re-emergence and spread of old ones.

Dangerously close

The emergence of yellow fever, viral encephalitis, bubonic plague and typhus all have one thing in common: the decrease in biodiversity. This destroys animals’ natural habitats and contributes to bringing species in closer contact with humans, which is conducive to the transmission of pathogens.