Introduction

Crimean-Congo Haemorrhagic Fever (CCHF) is a disease caused by a virus that is spread by tick bites. This virus is called the arbovirus Crimean-Congo haemorrhagic fever virus (CCHFV). It is transmitted by a special type of tick known as the Hyalomma tick. The virus can cause symptoms of fever, headache and dizziness, sometimes accompanied by nausea and diarrhoea. The fatality rate is high, at 30%, therefore, we need to understand the environmental factors that lead to this disease.¹ The distinguishing medical features include leukopenia (low white blood cell count), elevated levels of creatine kinase, which is an indicator of muscle damage, and thrombocytopenia (low blood platelet count). Aspartate aminotransferase levels are also increased.²

This leads to bleeding problems and serious organ complications, typically within 5 days of infection. The main regions affected include Africa, Asia and the Middle East. The high prevalence in Asia has led to CCHF being referred to as Asian Ebola. The first large known outbreak occurred in the Crimean Peninsula.³ Environmental factors such as temperature and humidity, alongside ecological and human factors, all influence the habits and habitats of ticks and therefore, the spread of this virus.⁴ We will explore the mechanisms behind CCHF and the environmental factors that affect the spread of this virus. We will also look at management strategies and future directions.

Climate factors

There are a few climate factors which influence the spread of CCHF. Some studies have shown that in some countries, tick activity is higher in the spring and summer months because of the higher temperatures. Ticks also tend to reproduce more and lay more eggs at higher temperatures. On the other hand, tick numbers seem to be reduced in very humid conditions, suggesting that ticks can’t thrive in high humidity.^4,5 However, in some regions with low humidity due to low rainfall, there are fewer available water sources for tick breeding, so there are some conflicting factors when it comes to whether high or low humidity is better for ticks.

Land surface temperatures are a key factor for the spread of CCHF, with higher temperatures leading to increased prevalence in many countries, likely due to the higher tick activity.⁶ Optimum conditions seem to be warm temperatures with fairly high humidity and rainfall.⁷ Since climate factors heavily influence the activity and distribution of ticks, climate change has had an impact on this. For example, ticks in Europe have become more active in winter than in previous years due to warmer conditions. Birds carrying these ticks have also migrated due to global warming and climate change, and with continued climate change, the distribution is likely to be subject to more changes. This leads to an increased CCHF risk in warming regions to which ticks have been migrating, such as Spain and other parts of Europe.⁸

Ecological factors

A wide array of ecological factors also contributes to the spread of CCHF. Tick dispersal is greatly affected by bird migration, which has caused an increased presence of the tick type that carries CCHFV in the Mediterranean. Other animals, such as horses and livestock, are important as they can also carry these ticks. Ticks can adapt to different climates and

can live on many different animals, including dogs; hence, it is important to protect these animals that come in close contact with humans from the ticks too. Some countries have introduced practices to prevent this, such as Turkey with their livestock.⁹ 

Seasonal migratory patterns of certain birds also impact tick migration, as they can transfer ticks over a great distance. Birds occupy sites temporarily during migration, which allows ticks in these sites to use the migratory birds as hosts; therefore, seasonal bird movements are key to CCHF spread.¹⁰ 

Biodiversity is another considerable factor; lower biodiversity in many cases increases disease risk and the likelihood of other tick-borne infections, such as Lyme disease. This is because high biodiversity means that there are more animals that aren’t good hosts for ticks, which can have a dilutional effect on tick populations. So, low biodiversity can, in some instances, mean that there are fewer dilutional hosts and, therefore, a greater concentration of animals that are good tick hosts.¹¹ 

Finally, land changes influence CCHF prevalence; for example, some hosts, such as monkeys, shed ticks in forests, and when these forests are destroyed and replaced with human housing, the ticks will use humans as hosts instead of the monkeys that used to live there.¹² As another example, some grassland is not habitable for ticks, and when urbanisation, in which grassland is replaced by suburban areas has increased CCHF prevalence, because suddenly the area becomes habitable to ticks since there are now appropriate hosts living there (humans). This is an issue in countries such as India, in which land-use patterns are changing rapidly.¹³ On the other hand, some grassland is habitable for tick species; therefore, certain animals that graze in open fields are more susceptible to being tick hosts, such as camels.¹⁵

Human-related environmental factors

Human factors and interventions change tick spread patterns too and therefore the spread of CCHF. People who work in agriculture and are close to livestock that host ticks have an increased chance of tick bites due to proximity.¹⁴ Moving livestock when animals are sold in markets helps ticks to spread, therefore, agricultural practices are a key human factor.¹⁶ Urbanisation and land-use changes, such as in India and other countries, increase human interactions with ticks.

As tick habitats are intervened with, ticks tend to occupy humans and livestock instead of their natural animal host, which increases CCHF incidence.¹² Humans travelling with hosts, such as dogs, spread ticks.¹⁰ Some goods are prone to tick infestation, and movement of these goods leads to ticks and CCHF being dispersed; hence, travel and

trade are risk factors for CCHF. Free movement on some border regions leads to more countries being exposed to Hyalomma ticks and CCHF.¹⁸

Prevention and control strategies in response to environmental factors

There are a few different ways to manage the spread of CCHF. Surveillance and mathematical models for tick populations and the transmission of CCHF are effective at elucidating regions that have an increased risk for the disease. Transmission can be measured under different independent variables and conditions, such as meat handling.¹⁹ Cases and assumptions regarding tick habitats can be measured using ecological and climate data as ticks are more suited to certain climates as aforementioned. Using different metrics such as biodiversity can be utilised to predict the likelihood of CCHF outbreaks.²⁰

Education and raising public awareness are very effective ways to prevent outbreaks. Campaigns are effective in raising awareness, particularly in poorer countries, as the effects are more pronounced. Those from a lower socio-economic background are more susceptible to CCHF thus educating them on the risks is important, educating pharmacists and other healthcare professionals that are more likely to be infected is crucial as they come into close contact with infected patients.²¹ Educating other groups that have a high risk of tick bites such as farmers and people that travel regularly will lower CCHF cases and allow people to change their lifestyle to avoid infection. 

Managing tick populations through different measures, such as using acaricides, is somewhat effective in reducing tick populations for some species, although some tick populations are building a resistance to the pesticide.²² Trying to reduce tick habitats by managing urbanisation effectively and allowing certain areas to have grassland where ticks cannot breed is another important strategy to manage tick populations and CCHF instances.¹³ Minimising interactions between humans and ticks through various strategies such as livestock management, reducing breeding opportunities and ensuring those working in agriculture wear protective equipment will reduce CCHF prevalence.²²

It is integral to manage public health regulations globally, particularly in regions where CCHF is common such as in India to ensure that this disease becomes less prevalent over time. Public health implications differ greatly depending on the country regarding entry and exposure likelihoods, particularly in Europe and other regions where this has not historically been a concern.

Summary

To summarise, the prevalence and infection rates of Crimean-Congo Haemorrhagic fever depend on a multitude of environmental factors, such as temperature and humidity, which affect tick distributions. As well as this, ecological factors such as bird migration, animal populations and biodiversity are key in influencing
infection rates and spread. Human practices such as agricultural methods, urbanisation and trade routes lead to higher tick and human interactions, therefore, human factors are key for CCHF spread and should be managed appropriately. There is a need for more research and the development of new interventions in order to reduce CCHF spread, such as by introducing campaigns to manage tick populations and raise awareness. There are countries in Europe, such as Spain and France, that have a more suitable climate for CCHF spread due to global warming.²³ International cooperation and awareness will inevitably be integral in decreasing CCHF likelihood, which should help to decrease the spread and make this disease less likely in the future, following strategies to manage the contributing environmental factors.