What are zoonoses?
Zoonoses are defined as infectious diseases transmissible from humans to animals and animals to humans. Three different types of infections are distinguished:
- Zooanthroponoses: the pathogen is predominantly transmitted from animals to humans.
- Anthropozoonoses: Here the transmission is predominantly from humans to animals.
- Amphixenoses: Also called facultative zoonoses - here transmission occurs in both directions.
Both the life cycles and the transmission routes differ significantly in zoonotic pathogens. Possibilities of transmission are smear infections, bite injuries, animal food or carriers such as mosquitoes or ticks, which are also called vectors. Depending on the mode of transmission, zoonoses are divided into different groups:
- Direct zoonoses: transmission occurs through direct contact or by wind.
- Latent zoonosis: Transmission occurs through an asymptomatically infected intermediate host.
- Metazoonosis: Transmission occurs through an invertebrate vector such as a tick.
- Saprozoonosis: The pathogen is found in water or soil - i.e. outside the animal kingdom.
- Cyclozoonosis: Several vertebrates act here as intermediate and final hosts - so there is a longer development cycle.
The pathogens themselves can also differ. The infection can be caused by viruses, bacteria, fungi, parasites or prions. Prions are certain infectious disease-causing proteins which, in contrast to other pathogens, do not possess DNA or RNA.
According to studies, about a quarter of the global loss of forest landscape is due to the conversion of forest land into commodities such as soy, palm oil, or wood fiber. The rate of commodity-related deforestation has also not declined since 2001. The detriments to ecosystem functions such as carbon sequestration are well known - but the loss of disease regulation during deforestation has not been well studied. A link between infections and forest conversion was shown in a recently published meta-analysis in Southeast Asia. According to the study, the increasing spread of zoonotic-borne diseases such as dengue fever and chikungunya is associated with the conversion of land to commercial plantations such as rubber and oil palm. Abandoned farmland may also be a risk for zoonotic diseases such as Lyme disease in North America and Europe or typhoid fever in Taiwan.
The study, published in March 2021 in the journal Frontiers in Veterinary Science, examined on a global scale whether the loss and gain of forested landscapes is correlated with outbreaks of vector-borne and zoonotic diseases.
This involved analysing global trends between changes in forest cover over recent decades and epidemics of human infectious diseases using the GIDEON database. This database provides data sets on infectious diseases and has been used in several other studies. As well as the general This recorded exactly 3,884 outbreaks of 116 zoonotic diseases and 1,996 outbreaks of 69 vector-borne infectious diseases between 1990 and 2016.
The relationship between increasing areas of oil palm and epidemics of human infectious diseases was also additionally analyzed. Information on forest cover and population demographics was taken from the World Bank, and data on oil palm area was taken from the so-called FAOSTAT database of the Food and Agriculture Organization of the United Nations.
The researchers found that an increase in the incidence of zoonotic and vector-borne infectious diseases was associated with deforestation - in tropical countries- and reforestation - in the temperate zone.
A strong causality was shown between deforestation and epidemics such as malaria and Ebola in tropical countries such as Brazil, Bolivia, Democratic Republic of Congo, Indonesia, Myanmar, Malaysia and Peru. Regions in temperate climates - including the US, China and Europe - showed links between reforestation and diseases such as Lyme disease, which is transmitted by ticks. Surprisingly, a causality between oil palm and outbreaks was analyzed in China and Thailand (i.e., low deforestation). However, according to the analysis, these areas were susceptible to mosquito-borne diseases such as yellow fever and Zika fever.
Although the study provides an overview of the relationship between global deforestation and outbreaks of zoonotic and vector-borne diseases, and provides evidence that reforestation and plantation construction may also contribute to infectious disease epidemics, it remains not easy to clearly classify the individual influences of forest loss and conversion, demography, and human intervention.
In any case, the results highlight the significance and importance of forests for biodiversity and human health. Furthermore, the researchers call for an international governing body to ensure the conservation and regulation of forests and diseases, and thereby a healthy planet and healthy people.