Information and interesting facts about zoonoses
From an evolutionary point of view, humans originate from the animal kingdom. It is therefore not surprising that there are pathogens that can infect both humans and animals. Infectious diseases caused by such pathogens are called zoonoses.
Zoonoses occur both in humans and in animals and are transmissible from animals to humans and/ or from humans to animals. A distinction is made between zooanthroponoses, whose pathogens are mainly transmitted from the animal kingdom to humans, and anthropozoonoses, in which transmission occurs mainly from humans to animals. In the case of facultative zoonoses (amphixenoses), transmission is reciprocal (see Fig. 1).
Fig. 1: Classification of zoonoses by main direction of infection
The life cycles and consequently also the transmission paths of zoonotic pathogens can be very diverse (see Fig. 2). Transmission options include smear infections, bite wounds, animal food products (e.g. meat, milk, eggs) or so-called vectors, such as mosquitoes or ticks, which act as carriers of a pathogen from one organism to another host. It is important to note that the pathogen does not necessarily cause the same symptoms in all carriers (hosts). Depending on the type of transmission, zoonoses can be divided into different groups:
- Direct zoonoses (Orthozoonoses) are transmitted by direct contact or by a mechanical vector, such as wind, from one vertebrate to another.
- Latent zoonoses are transmitted by an asymptomatic intermediate host.
- In a metazoonoses, avertebrates, such as mosquitoes or ticks, act as intermediate hosts. The intermediate host is also called a vector in this case. If the hosts are different species, the intermediate host is called a "bridge vector".
- In the case of saprozoonoses, the pathogen reservoir or certain developmental stages of the pathogen are found outside the animal kingdom, e.g. in water or soil.
- Cyclozoonoses need to change between different hosts during their development cycle, with intermediate and final hosts always being vertebrates. The number of different hosts during the development cycle is variable.
Fig. 2: Classification of zoonoses by transmission paths
Not only the transmission paths are divers but also the pathogens themselves. Zoonoses can be caused by viruses, as well as by bacteria, fungi, parasites or prions. In the following you will find an overview of the different pathogen species and exemplary some zoonotic pathogens that are being investigated within the research community of the German Research Platform for Zoonoses, among others. The respective link will take you to the website of the Robert Koch-Institute, where you can find interesting facts about the individual pathogens.
A virus is an organic structure that does not possess its own metabolism. They are therefore not considered to be actual "living beings". In order to multiply, a virus needs a suitable host cell whose replication apparatus the virus can use to make copies of itself and spread. In the following some zoonotic viruses are listed.
Bacteria are cellular organisms that belong to the prokaryotes, i.e. their genetic information (DNA) is not located in a cell nucleus, but is located in the interior of the cell (cytoplasm) in the form of a nucleotide. Bacteria occur in a wide range of different shapes and sizes. The human body is populated with a multitude of different bacteria, which mainly colonise the intestines and the skin. However, some bacteria also cause diseases. Below you will find some bacterial zoonotic pathogens:
A parasite is an organism that uses a host for food supply and possibly for longer-term habitat. In contrast to a symbiote, the host does not benefit from this colonization or can suffer damage as a result. Parasites can be eukaryotic unicellular organisms (outdated name Protozoa) as well as multicellular endoparasitic organisms called helminths (or worms). Helminths can be divided into flatworms (plathelminthes), which include trematodes and cestodes, and nematodes. Below you will find information on some zoonotic parasites.
- Toxoplasma gondii
- Plasmodium (Malaria)
Fungi are the third large group of eukaryotic organisms besides animals and plants. A parasitic colonization by fungi, a so-called mycosis, can lead to various disease patterns. Mostly it is dermatophytosis, in which primarily skin, nails and hair are affected. Below some zoonotic fungi are listed.
- Mircrosporum canis
Prions are proteins that can be present in the human/animal organism both in a normal form and in a harmful (pathological) form. Pathological structural changes can be "transferred" to other proteins. However, confirmation changes can also occur sporadically or be caused by a genetic defect. Much is still unknown about prions especially when it comes to potential treatment strategies.