Whether we like their company or not, sometimes it is better to be wary of animals. Did you know that 60% of infectious diseases and 75% of emerging diseases described in humans were of animal origin? We are talking about zoonoses. They are transmitted from animals to humans – or vice versa – by direct or indirect contact. The latest, Covid-19, continues to have an impact on our daily lives. What other known zoonoses are there? How are they transmitted? Can we protect ourselves from it? We take stock.
Definition: what is a zoonosis?
Zoonoses are infectious diseases that can be linked to a multitude of pathogens: viruses, bacteria, parasites, fungi or prions. As stated above, they represent a significant proportion of newly discovered diseases.
Some diseases, like HIV, start out as a zoonosis, but then mutate into strains only found in humans. Other zoonoses can cause recurrent outbreaks, such as Ebola virus disease and salmonellosis. Others, like Covid-19, have the potential to cause global pandemicsexplains the World Health Organization (source 1).
It is important to remember that the appearance of these diseases is not new: zoonoses already existed in the Neolithic, when humans began to settle and develop livestock and agriculture -and therefore to domesticate animals-. What has changed, however, is the frequency with which these diseases appear.
Covid, Ebola, bird flu, leptospirosis… What are the main diseases (list)?
Exists more than two hundred known types of zoonoses, which can be classified according to their mode of transmission and the pathogen in question (bacteria, viruses, fungi, parasites or prions).
The best known bacterial zoonoses:
- Plague ;
- Cat-scratch disease;
- Lyme’s desease;
The best-known viral zoonoses:
- anger ;
- bird flu ;
- Lassa fever;
- Hepatitis A;
- hepatitis E virus (HEV);
- equine encephalitis;
- Nipah virus infections;
- Ebola virus disease;
The most well-known parasitic zoonoses:
The best known fungal zoonoses:
The best known prion zoonoses:
This is mad cow disease, the cause of Creutzfeldt-Jakob disease in man.
The modes of transmission are varied. Viruses, bacteria and other viruses can find their way :
- by direct contact between an animal and a human beingas is the case of rabies or bird flu;
- by indirect contact, through the consumption of food such as eggs, milk or meatas is the case of salmonellosis or toxoplasma);
- through an insect or arachnidas is the case with Lyme disease or chikungunya;
- through the environment (water or soil contaminated by excrement or dander).
Once transmitted to humans, certain zoonotic pathogens can also mutate and be transmitted directly from human to human. This is the case, for example, of certain coronaviruses.
The increasing occurrence of zoonoses is explained in particular by increased human pressure on the environment. asked by Inserm magazine In April 2021, Claire Lajaunie, a researcher in environmental law at the Population, Environment and Development laboratory in Marseille, points out many issues (source 2):
- The extent of agricultural land, associated with deforestationwhich increases the risk of contact between wildlife, domestic animals and humans.
- The intensification of industrial livestockwhich increases the risk of spreading a pathogen between animals.
- Trafficking and sale of wild animalswhich could increase human exposure to the pathogens carried by these beasts.
- and global warming, linked to greenhouse gas emissions, which could modify the area of presence of wild species and vector insects, such as mosquitoes.
But according to Éric D’Ortenzio, an epidemiologist at ANRS-MIE (the Inserm agency in charge of coordinating research on infectious diseases) also interviewed by the magazine, other factors are involved:
- The intensification and globalization of trade in goods, animals and peoplethat facilitate the introduction and spread of pathogens.
- intensive urbanizationwhich implies the proliferation of animals susceptible to promoting zoonoses in cities.
- And the wars/conflictsresponsible for population movements and the weakening of the health systems of the affected countries.
Many zoonoses could potentially be discovered in the next few years. Researchers even fear that with all these extenuating circumstances, various agents of animal origin may manage to cause epidemics as large as those linked to the Covid-19 pandemic.
Which animal transmits more diseases? What about cats and dogs?
The most frequent vectors of zoonoses are arthropods, that is, insects (mosquitoes and flies, for example) or mites (such as ticks). the domestic animals (our dogs, our cats), as well as our petse (horses, pigs, poultry, cows, etc.) can also spread these diseases. without counting wild animals like rats, foxes, camels, llamas, bats, etc.
Some species of animals are calledspecies-reservoir : participate in the reproduction cycle of a pathogen that can then be transmitted to other species of animals. The rabies virus, for example, infects bats (reservoir species), but then infects many species, such as dogs, foxes, or rats, which can then infect humans. These vector species are calledintermediate hosts.
The symptoms of these zoonoses can be as varied as their causes. They can have repercussions:
- in the digestive system (such as salmonellosis or campylobacteriosis);
- in the respiratory system (such as bird or swine flu and coronavirus infections);
- in the liver (as is the case with the hepatitis E virus);
- in the nervous system (like rage);
- and in several other organs simultaneously.
Who are the people most at risk of zoonoses?
The severity of each disease varies greatly depending on the virulence of each pathogen, but also depending on the people infected. Certain audiences are said to be “at risk” of developing a severe form of various zoonoses:
- immunocompromised people;
- people on immunosuppressive therapy;
- young children and the elderly
- and pregnant women.
Also, some professionals are more exposed through their activities. This is particularly the case:
- Butcher shop;
- sewage workers;
- and laboratory workers.
Zoonoses represent a major public health problem worldwide due to our close relationship with animals in different contexts (agriculture, domestic animals and the natural environment), summarizes the World Health Organization.
And specify that they can too”disrupt production and trade products of animal origin for food or other purposes”.
The management of a zoonosis depends mainly on the pathogen that causes it, and not all of them can be detailed in this article. Treatments usually include:
- a possible disinfection of wounds ;
- the application of antifungal gels and ointments ;
- administration of anti-infective drugs
- me specific antibiotic therapy (amoxicillin, cephalosporin, doxycycline, azithromycin, ciprofloxacin, rifampicin, gentamicin, etc.).
The fight against the appearance of zoonoses requires the application of drastic prevention measures, but also through Research. Researchers have been intensifying their investigations for several years, particularly for identify infectious agents circulating in wild animals, develop diagnostic tools, anticipate the development of appropriate treatments and vaccines.