Research and analysis

HAIRS risk assessment: Dengue

Published 6 February 2024

About the Human Animal Infections and Risk Surveillance group

This document was prepared by the UK Health Security Agency (UKHSA) on behalf of the joint Human Animal Infections and Risk Surveillance (HAIRS) group.

HAIRS is a multi-agency cross-government horizon scanning and risk assessment group, which acts as a forum to identify and discuss infections with potential for interspecies transfer (particularly zoonotic infections). Its work cuts across several organisations, including:

• UKHSA

• Department for Environment, Food and Rural Affairs (Defra)

• Department of Health and Social Care (DHSC)

• Animal and Plant Health Agency (APHA)

• Food Standards Agency (FSA)

• Public Health Wales (PHW)

• Public Health Scotland (PHS)

• Department of Agriculture, Environment and Rural Affairs for Northern Ireland (DAERA)

• Welsh Government

• Scottish Government

• Public Health Agency of Northern Ireland

• Department of Agriculture, Food and the Marine, Republic of Ireland

• Health Service Executive, Republic of Ireland

• Infrastructure, Housing and Environment, Government of Jersey

• Isle of Man Government

• States Veterinary Officer, Bailiwick of Guernsey

Information on the risk assessment processes used by the HAIRS group can be found on GOV.UK.

Version control

Date of this assessment: January 2024

Version: 1.0

Reason for the assessment: Increase in reports of autochthonous dengue cases in European countries and the geographical spread of competent vectors.

Completed by: HAIRS members and external experts

Non-HAIRS members consulted:

• Alexander Vaux, Principal Medical Entomologist, UKHSA’s Medical Entomology and Zoonoses Ecology team

• Emmanuel Ezeoke, Senior Scientist, UKHSA’s Emerging Infections and Zoonoses team

• Philip Veal, Consultant Epidemiologist, UKHSA’s Travel Health and International Health Regulations team

• Iman Mohamed, Senior Scientist, UKHSA’s Travel Health and International Health Regulations team

Summary

Dengue is a disease caused by dengue virus (DENV), a single-stranded RNA virus belonging to the Flavivirus genus. Dengue is primarily a mosquito-borne infection transmitted by the bite of an infected female Aedes aegypti or Aedes albopictus mosquito. The geographic distribution of DENV and its mosquito vectors has changed in recent years, with recent locally acquired cases being reported from European countries including France, Italy and Spain. This qualitative assessment was completed to assess the current risk DENV presents to the UK public.

Transmission of DENV in the UK is contingent on 2 factors:

• the presence of infected humans
• the presence of competent mosquito vectors

There is no current evidence of local transmission of DENV in the UK. All human cases reported in the UK have been acquired as a result of travelling to endemic areas, the majority to South and South East Asia, the Caribbean, Eastern Africa and South and Central America.

Whilst sporadic incursions of competent vectors have been detected through active surveillance programmes in the UK, there is no current evidence of established populations of these vectors required for DENV transmission.

Assessment of the risk of infection in the UK

Probability

Currently, the probability of infection is considered Very Low for the UK population, given the absence of established (evidence of reproduction and overwintering) competent mosquito vector populations in the UK required for DENV transmission.

Impact

The impact on the UK population is considered Very Low.

Level of confidence in assessment of risk

Good. 

The quality of evidence used in this assessment is considered mostly good, and thus the confidence of the risk assessment output is good (high quality evidence from multiple reliable sources and verified, expert opinion concurs).

Actions and/or recommendations

Continue to monitor the UK situation with ongoing invasive mosquito surveillance and incidences of mosquito nuisance.

Continue to monitor the situation in the UK and European countries for increasing reports of autochthonous cases or changes in pathogenicity for human infections.

Whilst a cross-agency contingency plan exists in England for the early detection and eradication of invasive mosquito incursions, this should be expanded to provide appropriate actions in response to confirmed establishment of mosquito populations at a UK level.  

If competent mosquito species become established, each imported case in that area would need to be followed up with local mosquito control.

This remains an evolving situation. The HAIRS group will continue to monitor and review new evidence as it becomes available.

Step 1. Assessment of the probability of infection in the UK human population

This section of the assessment examines the likelihood of an infectious threat causing infection in the UK human population. Where a new agent is identified there may be insufficient information to carry out a risk assessment and this should be clearly documented. Please read in conjunction with the Probability algorithm.

Is this a recognised human disease?

Outcome

Yes.

Quality of evidence

Good.

Dengue is a disease caused by dengue virus (DENV), a single-stranded RNA virus belonging to the Flavivirus genus. Dengue is primarily a mosquito-borne infection transmitted primarily by the bite of an infected female Aedes aegypti (commonly known as the yellow fever mosquito) or Aedes albopictus (Asian tiger mosquito) mosquito (1). An Aedes mosquito acquires the virus by feeding on a viraemic host (for example an infected human). The virus within the mosquito undergoes an extrinsic incubation period of approximately 10 days at 27°C before being transmitted to a new susceptible host during subsequent bloodmeals. Although rare, other modes of transmission have been reported including vertical transmission (from mother to child in utero) (2, 3), blood transfusion, and organ and tissue transplantation from viraemic donors (4).

The main mosquito vector globally is Aedes aegypti but other mosquito species, including Aedes albopictus, have been implicated in virus transmission (5) and are the primary vectors in Europe. Humans are known to be the main reservoir host maintaining epidemic cycles of dengue. Whilst DENV is capable of infecting several animal species (for example bats, non-human primates and birds (6)), their role as amplifying reservoirs is uncertain, and thus dengue is considered vector-borne, not zoonotic.

DENV is the most common arbovirus transmitted from mosquitoes to people, and its incidence has increased more than 30-fold in recent decades alongside the geographical expansion of DENV and certain invasive Aedes mosquitoes that act as vectors (7). Dengue is, until recently, found in tropical and sub-tropical countries, mostly in urban and semi-urban areas where the Aedes mosquito breeds. In 2023, and as of 2 October, 4.2 million DENV cases and over 3,000 associated deaths have been reported globally (8). However, the global burden of dengue is difficult to estimate due to under-reporting of cases in many countries (due to the asymptomatic nature of most infections). Brady and others estimated that approximately 390 million DENV cases may occur each year, of which 96 million manifest clinically (any severity of disease) (9).

There are 4 antigenically-related DENV serotypes (DENV-1, 2, 3 and 4) which are genetically diverse, with several genotypes and clades of each serotype that have been associated with dengue disease (10). A fifth DENV serotype, DENV-5, was described in 2015, having been isolated from a human case in 2013. However, this serotype has not been reported as the cause of subsequent human outbreaks since its first isolation (11). DENV serotypes often co-circulate within the same geographical area, and have been known to co-infect humans, resulting in varying degrees of disease severity (12). Whilst up to 80% of human cases of dengue will be asymptomatic, and recover in one to 2 weeks, approximately 5% of cases can develop severe infection requiring hospitalisation for treatment (13). Illness in humans is characterised by an abrupt onset of fever often accompanied by severe headache and pain behind the eyes, muscle pain, joint pains, nausea, vomiting, abdominal pain and loss of appetite (13). Severe dengue (also known as dengue haemorrhagic fever) involves haemorrhagic symptoms (bleeding) and organ failure, sometimes leading to death. Secondary infection with a different DENV serotype may increase the risk of developing severe disease (14, 15), which tends to occur in countries where dengue is endemic. Severe dengue is rare in travellers returning from dengue endemic countries (16).

In the UK, there is no human vaccine available against DENV, and no specific antiviral therapy. Treatment for more serious presentations is supportive. 

Is the disease endemic in humans within the UK?

Outcome

No.

Quality of evidence

Good.

There is no current evidence of local transmission of DENV in the UK. Where data is available, most cases reported in the UK have been acquired as a result of travelling to areas with known dengue circulation, the majority to South and South East Asia, the Caribbean, Eastern Africa and South and Central America (see Table 1).

Table 1. The number of travel-associated dengue cases reported in England, Wales and Northern Ireland between 2018 and 2022, by the most common world region of travel (as assigned based on the United Nations world region classifications) (note 1)

World region of travel	2018	2019	2020	2021	2022	Total
Southern Asia	185	281	10	65	248	1,667
South-Eastern Asia	136	218	23	3	52	1,253
Caribbean	9	70	20	5	24	306
Eastern Africa	15	40	7	3	23	165
South America	3	19	8	-	20	143
Central America	8	24	5	-	15	105
Total	356	652	73	76	382	-

Note 1: some cases may have travelled to more than one region, and thus may be included more than once across regional counts

Recent reviews suggest that Asia and the Americas are the main regions from which travellers worldwide acquire dengue. Asia is considered the main source of dengue importation into Europe and the United Kingdom (17). During 2022, France and Spain were the only European countries to report autochthonous dengue cases (18). In France, 65 autochthonous cases (9 transmission events) were reported, which exceeded the number of cases reported during the entire 2010 to 2021 period (48 autochthonous cases). In 2022, 6 transmission events took place in areas where dengue transmission had not previously been documented (south-western France, Corsica) (20). Spain’s Ministry of Health reported a confirmed and a probable case of dengue following travel to the island of Ibiza in February 2023. Both cases were residents of Germany, who had visited Ibiza in August and October 2022, respectively, before becoming unwell, implying that Ibiza may have been where their exposure took place. Aedes albopictus has been established in Ibiza since 2014 (20).

In 2023, and as of August, most dengue cases reported globally were from the Region of the Americas, notably Brazil and Peru – the latter experiencing one of the largest dengue outbreaks in history. The unusually high number of cases reported in 2023 has been partially attributed to warm and rainy weather associated with tropical cyclones, and the coastal El Niño effect, which has created optimal conditions for Aedes mosquito breeding (21, 22). In Europe, and as of October 2023, autochthonous dengue cases were reported from Italy (72 cases), France (41 cases) and Spain (3 cases) (23).

Is the disease endemic in animals within the UK?

Outcome

No.

Quality of evidence

Good.

There is no current evidence of local transmission of DENV in the UK. Whilst DENV antibodies have been detected in several animal species in endemic areas (for example bats, non-human primates and birds (6), the evidence is circumstantial and their role as amplifying reservoirs is uncertain, and thus dengue is not considered a zoonosis. Humans are known to be the main reservoir host maintaining epidemic cycles of dengue.

Are there routes of introduction into the UK?

Outcome

Yes.

Quality of evidence

Good.

Transmission of DENV in the UK is contingent on 2 factors:

• the presence of infected humans
• the presence of competent mosquito vectors

The number of travel-associated human dengue cases reported in England, Wales and Northern Ireland have fluctuated annually with notable declines during 2020 and 2021, most likely associated with a reduction in global travel during the COVID-19 pandemic. Most human cases of dengue have no or mild symptoms (13), and thus would not require hospitalisation for their care; in other words, being managed in the community. It is therefore possible for human-to-mosquito transmission of DENV to occur where competent mosquito vectors and viraemic human cases co-exist. Duong and others reported how asymptomatic, viraemic human cases were markedly more infectious to mosquitoes than clinically symptomatic patients (24). However, in the UK, there is currently no evidence of established competent mosquito vectors (for example Aedes albopictus), nor evidence to suggest that the arrival of competent vectors would immediately increase the chances of autochthonous human cases of dengue, as likely endemic transmission areas are limited by climatic variables (25, 26).

The exotic invasive mosquitoes responsible for DENV transmission in Europe in recent years are Aedes albopictus and Aedes aegypti. The former has been reported in many European countries and continues to become established in new regions annually, notably in France (27). In 2013, Aedes albopictus was established in 8 European Union and European Economic Area countries, with 114 regions being affected. In 2023, the mosquito is established in 13 countries and 337 regions (18) and has been detected in many more European countries. Aedes aegypti was previously widely established across the Mediterranean, but it largely disappeared through extensive control efforts. It is now showing evidence of re-establishing in Eastern Europe, and also established on Madeira Island, Portugal, where it has been involved in DENV transmission (28).

The main route of importation into Europe and subsequent spread has been via the trade in used tyres (25). However, both Aedes albopictus and Aedes aegypti have been found to colonise new areas via main highway routes, having moved across regions in vehicles. It is therefore considered likely that these species may be increasingly found in the UK, introduced via road and rail links with continental Europe (29). However, the number of haulage and commercial vehicles entering the UK from European countries has reduced in recent years (30), which may decrease importation risk. 

In 2014, one male Aedes albopictus was found in Merseyside, England (31). This detection presented no apparent disease risk to public health, and a follow-up survey found no evidence of an established population. This was the third time that this species had been reported in the UK, having previously been reported in 1865 and 1919.

In September 2016, Aedes albopictus was detected in the UK for the first time, with a small number of Aedes albopictus eggs found in one location in Kent, England, close to the motorway system (32). It is likely that these were laid by adult mosquito(es) imported from continental Europe in vehicles. Ae. albopictus mosquitoes were subsequently found at different sites in Kent (annually from 2016 to 2019) and London (2019) without evidence of establishment.

There were no non-native mosquito detections during surveillance from 2020 to 2023, even though surveillance was scaled up post 2020. In 2023, eggs of Aedes aegypti were detected at a freight storage facility near an airport in Southern England, likely to have been imported through air-freight from Africa or Asia. No additional detections of larval or adult Aedes aegypti were detected during follow-up trapping activities.

There is no current evidence of established populations of the competent vectors for DENV in the UK, despite active surveillance programmes run by UKHSA entomologists in collaboration with Port Health and local authorities. 

Are effective control measures in place to mitigate against these routes of vector introduction?

Outcome

Yes.

Quality of evidence

Good.

The focus of current UKHSA mosquito surveillance has shifted from seaports and airports to motorway services stations in southern England to detect Aedes albopictus being imported through ferry ports and the Eurotunnel in vehicles. This sampling strategy is being widely used across Europe to monitor for the extension of Aedes albopictus distributions. More recently, additional surveys have been conducted in key urban centres that are considered through modelling as at higher risk of supporting established populations.

If or when Aedes albopictus or Aedes aegypti are found in the UK, a combination of habitat manipulation (removal of container habitat) and control (larvicides) are implemented in order to reduce or eradicate the population. A cross-agency contingency plan for invasive mosquitoes has been developed which outlines proposed actions in the event of detecting non-native invasive mosquito species in England (33).

As the mosquito becomes more established in the rest of Europe, the likelihood of mosquito establishment in the UK increases as incursions increase. In the event of established competent mosquitoes, there may be a requirement for local mosquito control in the vicinity of imported human cases of DENV. This strategy to minimise autochthonous cases is currently instituted in France, Italy and Spain. However, local UK climate differs from these countries, making local transmission less suitable but still possible. Temperature is an important factor affecting Aedes mosquito vector competence. Generally, temperatures between 20°C and 26°C are most suitable for DENV transmission to occur (34, 35).

Whilst there are no measures in place to prevent potentially DENV infected humans travelling to the UK, the risk of infection in travellers can be reduced through raising awareness on mosquito bite prevention (such as using mosquito repellents, wearing long-sleeved clothing and trousers and sleeping under mosquito nets). Upon clinical suspicion, rapid diagnostic testing and effective treatments are available in the UK for individuals who present with symptoms compatible with DENV infection and have recently returned from a country where DENV is known to circulate. Additionally, enhanced surveillance is currently being improved to ascertain the burden of disease in this sub-population within the UK.  

Outcome of probability assessment

The probability of a locally-acquired human infection with DENV in the UK population is considered Very Low.

Step 2: Assessment of the impact on human health

The scale of harm caused by the infectious threat in terms of morbidity and mortality depends on spread, severity, availability of interventions and context. Please read in conjunction with the Impact algorithm.

Is there human-to-human spread of this pathogen?

Outcome

No.

Quality of evidence

Good.

Dengue is a systemic viral infection that is most commonly transmitted between humans via mosquitoes. Other modes of transmission, such as vertical transmission (from mother to child in utero), through blood transfusion, or organ and tissue transplantation from viraemic donors are exceptionally rare and have been infrequently reported in the literature, none of which were in the UK (36 to 38).

Is there zoonotic or vector-borne spread of this pathogen?

Outcome

Yes.

Quality of evidence

Good.

DENV is vector-borne via particular Aedes mosquitoes. See above.

For zoonoses or vector-borne disease, is the animal host or vector present in the UK?

Outcome

No. Incursions of the competent vectors have been reported, although there are no known established populations within the UK.

Quality of evidence

Good.

Although the first evidence of a competent vector incursion was detected in 2016, with subsequent incursions in 2017, 2018 and 2019 (32), no Aedes albopictus or Aedes aegypti mosquito population establishment occurred. Given the proximity of Aedes albopictus mosquitoes in France to the UK and the ability of the species to colonise new regions via road networks, then future incursions should not be unexpected (39 to 41). Climatic conditions in the UK are currently permissible for mosquito vector establishment in parts of southern England (26, 42). Climate models suggest that, if introduced, Aedes albopictus could establish itself over most of England and southern Wales by the 2060s (25, 42).

Outcome of impact assessment

The impact of DENV on human health for the UK population is considered Very Low, given the current absence of established populations of the competent mosquito vectors in the UK.

Annexe A

Figure 1. Assessment of the probability of infection in the UK population algorithm

Assessment of the probability of infection in the UK population algorithm: accessible text version

Outcomes are specified with (Outcome) beside the appropriate answer.

Question 1: Is this a recognised human disease?

Yes

Go to question 3. (Outcome)

No

Go to question 2.

Question 2: Is this a zoonosis or is there a zoonotic potential?

Yes

Go to question 4.

No

The probability of infection in the UK population is considered Very Low.

Question 3: Is this disease endemic in humans within the UK?

Yes (note 2)

Go to question 5.

No

Go to question 4. (Outcome)

Note 2: This pathway considers reverse-zoonosis of a pathogen already in circulation in the human population.

Question 4: Is this disease endemic in animals in the UK?

Yes

Go to question 8.

No

Go to question 5. (Outcome)

Question 5: Are there routes of introduction into animals in the UK?

Yes

Go to question 6. (Outcome)

No

The probability of infection in the UK population is considered Very Low.

Question 6: Are effective measures in place to mitigate against these?

Yes

The probability of infection in the UK population is considered Very Low. (Outcome)

No

Go to question 7.

Question 7: Do environmental conditions in the UK support the natural vectors of disease?

Yes

Go to question 8.

No

The probability of infection in the UK population is considered Very Low.

Question 8: Will there be human exposure?

Yes

Go to question 9.

No

The probability of infection in the general UK population is considered Very Low.

Question 9: Are humans highly susceptible? (note 3)

Yes

Go to question 10.

No

The probability of infection in the UK population is considered Low.

Note 3: Includes susceptibility to animal-derived variants

Question 10: Is the disease highly infectious in humans?

Yes

The probability of infection in the UK population is considered High.

No

The probability of infection in the UK population is considered Moderate.

Annexe B

Figure 2. Assessment of the impact on human health algorithm

Assessment of the impact on human health algorithm: accessible text version

Outcomes are specified by (Outcome) beside the appropriate answer.

Question 1: Is there human-to-human spread?

Yes

Go to question 4.

No

Go to question 2. (Outcome)

Question 2: Is there zoonotic or vector-borne spread?

Yes

Go to question 3. (Outcome)

No

The impact of infection in the UK population is considered Very Low.

Question 3: For zoonoses or vector-borne disease, is the animal host or vector present in the UK?

Yes

Go to question 4.

No

The impact of infection in the UK population is considered Very Low. (Outcome)

Question 4: Is the human population susceptible?

Yes

Go to question 5.

No

The impact of infection in the UK population is considered Very Low.

Question 5: Does it cause severe disease in humans?

Yes

Go to question 8.

No

Go to question 6.

Question 6: Is it highly infectious to humans?

Yes

Go to question 9.

No

Go to question 7.

Question 7: Are effective interventions available?

Yes

The impact of infection in the general UK population is considered Very Low.

No

The impact of infection in the general UK population is considered Low.

Question 8: Would a significant (note 4) number of people be affected?

Yes

Go to question 10.

No

Go to question 9.

Note 4: This question has been added to differentiate between those infections causing severe disease in a handful of people and those causing severe disease in larger numbers of people. ‘Significant’ is not quantified in the algorithm but has been left open for discussion and definition within the context of the risk being assessed.

Question 9: Are effective interventions available?

Yes

The impact of infection in higher-risk groups is considered Low.

No

The impact of infection in higher-risk groups is considered Moderate.

Question 10: Is it highly infectious to humans?

Yes

Go to question 12.

No

Go to question 11.

Question 11: Are effective interventions available?

Yes

The impact of infection in the UK population is considered Moderate.

No

The impact of infection in the UK population is considered High.

Question 12: Are effective interventions available?

Yes

The impact of infection in the UK population is considered High.

No

The impact of infection in the UK population is considered Very High.

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