2023-09-17 22:17:55
West Nile virus is an emerging virus in Spain that we are going to hear more and more about due to its ability to seriously affect humans and cause encephalitis. The Valencian Community announced its first case detected at the end of August, and days later an 84-year-old Andalusian woman became the first fatality in Spain so far this year. But what do horses have to do with a virus that is transmitted by mosquito bites?
Although birds are the natural reservoir of the virus, it also infects other animals. Among them are horses, which have caught the attention of our research group. In a recently published study. It brings together data collected over 16 years (from 2005 to 2020) in Doñana (Andalusia).
The data reflects the horses’ exposure to the virus and analyzes how climate can influence its circulation. In addition, they show what we expect the future of this disease to be like in our country taking into account the climate scenarios that are expected in the coming decades.
Biological cycle of West Nile virus. The authors, CC BY
The virus has been spreading and increasing its incidence for about 20 years, and has reached increasingly northern latitudes. The figure below shows the map of the situation of this disease in humans in Europe in 2020. That year we suffered the worst outbreak of this disease in Spain, with 77 cases in Andalusia and Extremadura, of which 8 died. We previously had sporadic cases in 2004 (1), 2010 (2) and 2016 (3).
Most infections in humans are, however, asymptomatic. It is estimated that less than 1% end up resulting in serious illness.
Status of West Nile virus fever/encephalitis in Europe in 2020. ECDC-EU, CC BY
The virus also affects horses. Both humans and horses are considered epidemiological “dead ends”: although both can become infected and develop serious disease, they cannot infect other individuals or mosquitoes.
Some of the first evidence on the circulation of the virus in Spain was obtained in horses in Doñana, where seropositive horses have been detected. since 2005. In 2007, the first surveillance plan in Spain was implemented and, since then, equine outbreaks have been declared every year. These are found mainly in western Andalusia, but also in Extremadura and Castilla-La Mancha.
More recently there have been cases in horses in Catalonia (2018) and the Valencian Community (2020). In Spain, two different genetic lineages of the virus are currently detected. Lineage 1 circulates in Andalusia, Extremadura and Castilla-La Mancha and lineage 2, in Catalonia. The disease also affects some species of birds.
Situation of West Nile virus fever/encephalitis in Spain, with emphasis on the most important milestones that have occurred since 2014. The disease is expanding through three large areas: 1) Western Andalusia, 2) Extremadura-Castilla-La Mancha and south of Castilla-León, and 3) Catalunya-Valencian Community. The authors, CC BY
And what happens to Doñana’s horses?
The protagonists of our study are the Retuertas breed horsesa native breed that has been preserved in conditions of freedom in Doñana.
We analyzed the exposure of these horses to West Nile virus infection by measuring the presence of specific antibodies from 2005 to 2020 (with the exception of 2006). The data show that, after an initial decline (from 2005 to 2008), the seroprevalence is increasing, with ups and downs, until reaching a maximum of 25% in 2020. That was exactly the year in which the major outbreak of disease in humans occurred, next to the study area.
The seroprevalence in this population of horses reflects their exposure to virus infection, and is a fairly precise indicator to estimate the intensity of its circulation in the area. When a horse presents antibodies, it tells us that it was infected by the virus in the past and survived said infection.
Evolution of seroprevalence (presence of specific antibodies in serum) against West Nile virus in Doñana horses, from 2005 to 2020.
The authors, CC BY
What conditions make mosquitoes happy?
We also analyze the influence of various factors. Regarding the intrinsic factors of the horses, only age was related to seroprevalence. It is normal: the older you are, the more likely you are to be exposed, sometimes even in different years.
Seroprevalence against West Nile virus in horses by sex and age groups.
The authors, CC BY
Regarding environmental factors, the association of the observed seroprevalence with two climatic parameters was very interesting:
The data indicated that the higher the annual minimum temperature, the higher the seroprevalence. Likewise, the lower the annual precipitation, the higher the seroprevalence, although this also increased with the greater number of rainy days per year. That is, if the rain, although scarce, was spread more throughout the year.
Variation in the prevalence of antibodies against West Nile virus in Doñana horses as a function of annual minimum temperature (left graph) and annual precipitation (right graph).
The authors, CC BY
These results can be interpreted taking into account the most favorable conditions for the development of mosquitoes that transmit the virus.
Milder minimum (winter) temperatures will allow greater survival (and therefore abundance) of mosquitoes that act as vectors.
On the other hand, the lack of rain could favor a spatial overlap of mosquitoes and their hosts, be they birds or horses, in the few ponds with water available in the environment. If, in addition, the scarce rains are distributed over longer periods during the year, the exposure of horses and birds to mosquitoes will also be prolonged, making them more likely to become infected.
Climate change will increase the presence of the virus
In view of these results, we wonder what will happen in the future. Climate change scenarios predict, precisely, an increase in average annual temperatures and a decrease in precipitation.
Climate trends indicate greater intensity in the local circulation of West Nile virus. The graph projects the evolution of prevalence until the year 2100, with two climate scenarios regarding the control of global greenhouse gas emissions.
Predicted evolution of seroprevalence against West Nile virus in horses in Doñana and its surroundings based on the climate scenarios predicted until the year 2100. Two scenarios are shown: one more optimistic (RCP 4.5) and another more pessimistic (RCP 8.5). .
The authors, CC BY
The trend predicted by climate change models (even the most optimistic ones) is that the prevalence of the virus in horses will increase. Does this mean that human cases will increase? Not necessarily, but what it does indicate is that it is necessary to adopt mosquito control programs in inhabited areas to reduce the risks.
This work is a good example of how useful it is to apply the concept “one health” (of English, One Health) in the area of infectious diseases: animal, environmental and human health are closely related. Better control options emerge from cooperation between disciplines.
Juan Calderón, a retired researcher from the Department of Wetland Ecology of the Doñana Biological Station, participated in this article.
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