Enzootic infections are the viruses and the diseases caused by them that affect a population of animals in a particular area, season, and climate. They seem to appear in a population with a constant and relatively high frequency. For example, such enzootic infection is the influenza virus in the local populations of swine, birds, and mosquitoes.

We have turned to the publications on enzootic infections. For the review, we have performed a systematic search in the scientific literature database Scopus and have built a map of publications based on their reference lists (Figure 1). Proximity in this map and belonging to the same cluster mean that the papers cite the same publications, therefore the papers are likely to consider similar issues. The map is built using VOSviewer software.

 The papers split into seven clusters:

• navy, top left: epidemiology and the life cycle of animal viruses,
• purple, center: animal-to-human transmission,
• yellow, bottom right: genetic modifications,
• blue, center and top right: swine diseases and infections,
• orange, bottom left: arboviruses and chikungunya virus,
• light blue, bottom left: the West Nile virus,
• gray, top center: retroviruses.

Cluster description

In the description of clusters, we present both the works that are the most noticeable on the map and the recent popular research.

Navy cluster: epidemiology and the life cycle of animal viruses

This cluster contains publications on the epidemiology of enzootic infections, their spread, and possible consequences. For example, as Coura and Dias (2009) show, in the tropics, some diseases are transmitted through triatominae, or conenose bugs, – it is especially characteristic of developing countries with poor housing and sanitation conditions.

The papers here also describe the genetics and molecular structure of enzootic viruses and their life cycle. The publication by Caimano et al. (2007) and the review by Radolf et al. (2012) explain how the Lyme disease virus adapts to the organism of its host. Schoeman and Fielding (2019) review the studies on the coronaviral envelope and show its importance for the virus reproduction in the infected organism. Other research covers the origins of the viruses and the strategies for preventing their spread.

Purple cluster: animal-to-human transmission

Here, the studies are located about the ways of enzootic disease transmission to humans. The central and the most cited article has been published during the 2009 swine flu pandemic. It discusses the hypothesis that in swine, the genetic material of avian, swine, and human influenza viruses is mixing. The paper describes such “hybrid” viruses of the swine flu A (H1), which became enzootic in North America in the late 1990s and also affected humans (Shinde et al. 2009). Most of the other publications mention the transmission pathways for enzootic viruses also from swine (through food) and from mosquitoes (through the blood).

Yellow cluster: genetic modifications

The cluster contains research on the genetic diversity of viruses, detection of the new viral strains, as well as cell processes caused by the virus in the host organism. For instance, Kincaid, Burke, and Sullivan (2012) discuss the bovine leukemia virus and explain which its genetic characteristics lead to the emergence of a tumor.

Blue cluster: swine diseases and infections

Everything that is connected with the swine diseases is located here. Stärk (2000) reviews the risk factors that lead to the spread of respiratory diseases in swine. The primary pathogens of enzootic infections are also described that occur worldwide and cause major economic losses to the pig industry (Maes et al. 2008).

Orange cluster: arboviruses and chikungunya virus

Arboviruses infect arthropods and from them are transmitted to wild animals and birds. Then, in turn, the infection is often transmitted to livestock and people, which causes large-scale epidemics in tropical urban centers (Weaver et al. 2018; Weaver and Reisen 2010). There are more global consequences of local arboviruses. Some viruses are successfully controlled at their places of origin, but due to the mobility of people, they begin to spread to other places and cause epidemics there. This was the case, for example, with the West Nile virus (Gubler 2001) and the Zika virus (Atif et al. 2016).

One of the arboviruses, chikungunya, is transmitted to humans from infected mosquitoes. It causes a disease that is characterized by high fever and severe joint pain. The virus is believed to be enzootic in most of Africa, and historical evidence indicates that it has spread to other parts of the world from this region (Powers et al. 2000; Powers and Logue 2007).

Light blue cluster: the West Nile virus

The cluster is entirely dedicated to the West Nile virus, which has spread from tropical regions around the world and causes fever. It was probably brought to North America through tourism or commerce, and the first incidence cases were discovered in New York in 1999 (Petersen and Marfin 2002). By 2005, the virus had already caused more than 10,000 cases of serious illness and 400 deaths in humans in the United States, as well as thousands of fatal infections in horses (Turell et al. 2005). The epidemics it caused are partly because mosquito transmitters, which usually feed on the blood of birds, sometimes switch to humans (Kilpatrick et al. 2006).

Gray cluster: retroviruses

Retroviruses are associated with a variety of diseases, including many malignancies and immunodeficiency disorders. Maeda, Fan, and Yoshikai (2008) present a review of how retroviruses induce cancer tumors.

Interestingly, some retroviruses take root in the organism of their hosts during evolution: their genome enters the DNA of hosts and is transmitted between generations. Such retroviruses are called endogenous and in some cases protect the body from external retroviral infections. The co-evolution of retroviruses and the host organisms, as well as the mutual adaptation of endogenous and external retroviruses, are described in Arnaud et al. (2007) on the data coming from sheep.

Data source: Scopus bibliographic database. The search was made by titles, abstracts and keywords of publications using the term enzootic infections. The search resulted in approximately 4000 publications, the 2000 most cited once were selected for analysis.
Search query:
TITLE-ABS-KEY ( enzootic* AND *virus* OR *infect* OR strain )