Crimean-Congo Hemorrhagic Fever is the most widespread viral hemorrhagic fever disease.⁶ The question this article will tackle concerns the method by which the pathogen is transferred to humans, as well as potential prevention and control methods.

The most commonly known reservoir for Crimean-Congo Hemorrhagic Fever is ticks. Other animals may also be involved in the transmission of the virus to humans if they are already infected.⁵ The virus can also be transmitted between humans.

This article will cover in more detail what Crimean-Congo Hemorrhagic Fever is, the multiple ways by which the responsible virus can be transmitted to humans, and how such transmission may be controlled or prevented.

Introduction

Crimean-Congo Hemorrhagic Fever (CCHF) is a rare viral disease, similar to the Ebola and Marburg virus diseases. It was first identified in Crimea (1944), then later the virus was identified in Congo (1945), giving the disease its name.¹ As with other hemorrhagic fevers, symptoms often include the sudden appearance of intense headaches, a high fever, and weakness.

Over the past years, there have been several emergency outbreak cases.² Outbreak fatality rates are quite high, the highest being 40%.³ The sickness can last for up to two weeks, and as the disease progresses to the fourth day mark, symptoms tend to become more dangerous, introducing more serious uncontrolled haemorrhage, for example. If a patient is hospitalised, there is a 50% chance of survival.⁴ 

CCHF is also known to cause serious damage to the function of the liver.⁷ Uncontrolled haemorrhage can make the disease fatal, and the patient may have undergone multi-organ failure and shock. Where the disease is not fatal, the patient usually recovers within 10 to 20 days of symptom onset. They may experience malaise in the following months, however, the majority of patients do make a full recovery.⁵

CCHF is caused by a virus known as the Crimean-Congo Hemorrhagic Fever Virus (CCHFV) from the Orthohantavirus genus.⁶ This virus is a type of Bunyavirus. Bunyaviruses are known to cause fever, as well as target the liver, and, in the case of CCHF, the vascular endothelium.⁸ It is not yet known whether the disease has a severe pathological impact on species other than humans and potentially non-human primates.¹

As the most widespread type of viral hemorrhagic fever, CCHF is endemic to the following areas:³

• Africa
• The Balkans
• The Middle East
• Asia

The estimated number of infections per year ranges from 10000 to 15000, of which 3.3% to 5% are fatal.⁶ It is important to understand the methods by which CCHFV is transmitted to humans. Although there are currently no licensed vaccines for people infected with the CCHFV, there are ways to limit the spread through knowledge of what is causing it.⁵

Transmission pathways

Tick-to-human transmission

Primary vector: Hyalomma ticks

One fact confidently determined regarding CCHFV is its principal reservoir: ticks.⁵ Regions reporting cases have the established residence of the Hyalomma tick species.⁶ Hyalomma ticks are widespread across Africa and Asia. Spread to regions can occur via vertebrate hosts, like migratory birds and livestock. Adult ticks do prefer to feed on large animals.¹² Because of its animal-borne nature, the virus is described as “zoonotic”, meaning that it is transmitted from animals to humans.^9,10 Hyalomma marginatum ticks are also known as the main vector for this disease.¹² This means they transfer the virus from the reservoir to a host (for example, to humans).¹¹

Immature Hyalomma marginatum are mostly active between June through October, feeding on one host, before usually moulting to adults, and actively seeking and feeding on a second host. Adults are more active in the spring. Between the ticks, they can transmit the virus to each other sexually, and can transmit it to their offspring. One female tick can lay up to 7000 eggs.¹² The tick maintains the virus throughout the rest of its life cycle. As both reservoirs and vectors, they can transmit the virus to a human directly, through tick bites. It can also be transmitted to humans via contact with tick blood or tissue,⁶ which can occur through an action as simple as squashing a tick.⁵

Exposure risk

Seeing as adult Hyalomma marginatum like to feed on large animals, it is important for those working in an agricultural setting with livestock or other large animals, such as animal breeders, farmers, veterinarians, hunters and slaughterhouse workers, to be cautious of tick populations.^5,6 It is important that they are aware of the presence of ticks, especially as 100 ticks can be present on one animal.¹² Contact with ticks that are reservoirs of CCHFV can result in transmission of the virus to the person.

Certain times of the year may prove more of a risk for people, and where increased awareness is important. For example, between the years 2012 and 2021, CCHF cases in the East of Turkey were monitored. It was found that the cases began in March, peaked in July, and stopped in October.¹³ This aligns quite well with the activity of immature Hyalomma marginatum between June to October, and adult activity heightening in the spring.¹² It would be beneficial to keep key seasonal variations in mind in countries where there are established populations of these ticks.

Animal-to-human transmission

Reservoir hosts: Livestock and wild animals

It is not fully understood exactly how CCHFV is maintained through parts of the ecosystem. Reservoir hosts, like livestock, do not show any pathological symptoms related to CCHF, despite infection. Instead, they accommodate the virus for amplification.¹⁴ Reservoir hosts may be wild or domestic, such as cattle, hares and goats.⁹ This requires extra caution for those frequently in contact with such animals, particularly where the role involves contact with the animals’ blood, bodily fluid or tissue.⁵ Again, this includes veterinarians, slaughterhouse workers, farmers, and other workers. It is also that content with rodents and shrews may also initiate transmission of the virus.⁹

Risk of transmission via animal products

Cases where transmission is thought to have possibly come from the consumption of meat or animal products from infected animals have been few. The virus does not usually remain active following preparation of slaughtered meat, through the acidification process.⁶

Human-to-human transmission

Modes of transmission

Transmission of CCHFV between humans can occur from one already infected patient to another human. Due to the nature of this kind of transmission, it tends to occur in healthcare settings, where there is a risk of contact with a patient’s blood, bodily fluids or tissues. This does not make up the majority of cases, however, it is still very important for healthcare workers to avoid direct contact with infected substances like these when treating patients.⁵ The following are a couple of examples of the types of settings where this may occur.

Nosocomial infections

In the absence of required knowledge to handle cases of CCHF, healthcare workers are at a higher risk of infection.¹⁵ One example of a hospital outbreak occurred during 2023 in Kandahar, Afghanistan. Following the case of one admitted patient with CCHF, a further 48 suspected cases were thought to come from the same hospital. Amongst them were 13 hospital staff. Unfortunately, during patient examination and following procedures, safety against infection was compromised. For example, there was a lack of protective equipment use, and the samples taken from patients were not handled correctly. Visitors were permitted without restriction, and the isolation of patients was not strictly enforced. A lot can be learnt from this in future CCHF cases can be handled.¹⁵

Household transmission

Although uncommon, it may be possible for transmission to occur within the household. This includes sexual transmission.¹⁶

Other rare modes of transmission

Laboratory-acquired infections

8% of all nosocomial cases out of 158 between 1953 to 2016 occurred in laboratories. It did affect laboratory workers (6.3%). The reported nosocomial cases had a mix of transmission methods, like percutaneous and cutaneous contact, proximity and indirect contact.¹⁷

Possibility of airborne transmission

Airborne transmission is a possibility, although it has been hypothesised to explain the spread of the infection in healthcare and laboratory settings.¹⁶

Prevention and control measures

Preventing tick bites

As mentioned before, it is important to keep an eye on the seasons where Hyalomma marginatum are more active, or at seasons where the number of CCHF cases is known to rise. This is particularly applicable to those regions with an established population of Hyalomma marginatum. Staying away from areas with a high population of these ticks, where possible, will help avoid tick-to-human transmission. People can also consider using tick repellents and regularly checking their clothes and skin for ticks.⁵

Safety around livestock

It is especially important for workers who are more likely to encounter ticks to be careful to prevent transmission. This includes owners of livestock, as large animals such as cattle could have several ticks. They should consider using tick repellents on themselves, as well as handling animals with protective equipment.

Healthcare worker precautions

Healthcare workers need to utilise protective equipment to keep themselves and other patients safe. This method helps prevent hospital outbreaks. Protection from direct contact with patient bodily fluids, blood and tissue is very important in preventing human-to-human transmission of CCHFV.

Public awareness

It can make a huge difference to make sure people living in areas where CCHF has occurred know how to help control and prevent further infection. Knowing about the ticks and ways in which to avoid them, as well as ways to avoid transmission from other animals and humans, can help limit the overall spread.

Summary

Crimean-Congo Hemorrhagic Fever is the most widespread viral hemorrhagic fever disease, and there are several methods by which it can be spread. CCHFV can be transmitted to humans directly from the reservoir (ticks), from other infected animals (e.g. livestock) and other infected humans. Public awareness in areas affected by CCHF is important to protect against further transmission.