Research and analysis

HAIRS risk assessment: Usutu virus

Updated 20 December 2023

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: December 2023

Version: 4.0

Reason for the assessment: Updated to reflect the latest Usutu virus epidemiology in Europe, ongoing mosquito surveillance in the UK and new detections of Usutu virus in a small number of wild and captive birds in the UK since 2020.

Completed by: HAIRS members

Non-HAIRS group experts consulted:

• Arran Folly, Animal and Plant Health Agency Vector-borne disease group
• the joint NHS Blood and Transplant (NHSBT) – UKHSA Epidemiology Unit

Date of previous risk assessment(s):

• 31 October 2012 (V1.0)
• 15 December 2016 (V2.0)
• 30 September 2020 (V3.0)

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

Summary

Usutu virus (USUV), a mosquito-borne flavivirus, was first recognised in Europe, in Austria, in wild birds in 2001. Subsequently, its range expanded across Europe. In August 2020, the first USUV infected wild birds in the UK were identified.

Since then, a small number of wild and captive birds have tested positive between 2021 and 2023, predominantly in the Greater London area. As of November 2023, no human cases of USUV have been reported in the UK.

Assessment of the risk of infection in the UK:

Probability

Low.

Impact

Very Low to Low for the general UK population.

Low to Moderate for higher-risk groups (for example, immunocompromised individuals).

Level of confidence in assessment of risk

Satisfactory.

Evidence gaps

The circulation of USUV is not well understood globally. There is a scarcity of seroprevalence data to assess the real prevalence of USUV in humans, particularly in relation to background exposure to West Nile virus as laboratory testing may not always discriminate between the 2 infections. A better understanding of USUV circulation, particularly in potential reservoirs and humans, requires a review of monitoring strategies, through the implementation of the One Health approach.

Action(s) and/or recommendations

Monitor the UK situation with bird and mosquito surveillance.

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

Continue to look for evidence of USUV in blood donors in returning travellers from higher risk areas.

Raise awareness of USUV among clinicians in known risk areas.

Raise public awareness on mosquito bite prevention measures, particularly in known risk areas.

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.

USUV is a mosquito-borne flavivirus first detected in South Africa in 1959 (1). USUV belongs to the Japanese encephalitis virus serocomplex and is phylogenetically similar to other human and animal pathogens such as West Nile virus (2, 3). USUV is maintained through an enzootic cycle involving birds (predominantly Passeriformes (mostly passerines such as blackbirds) and Strigiformes (owls)) as amplifying reservoir hosts and ornithophilic mosquito species as vectors (mainly Culex spp.) (4). Mammals, including humans, are incidental dead-end hosts. This is because viraemia in an infected mammal does not reach high enough levels to ensure transmission via mosquito bites and continuation of the transmission cycle. USUV has been identified in dogs, wild boar, ruminants including sheep and roe deer, as well as in captive exotic species (5).

USUV is an emerging infection in humans but the number of confirmed clinical infections to date is low. The first recorded human cases were in the Central African Republic in 1981 and in Burkina Faso in 2004. Since then, the vast majority of human cases have been reported in Europe, with an observed increase in reporting frequency during recent years. This may be due to raised awareness of the disease, changes in testing or screening (for example, of blood donors) or due to increased human exposure as USUV becomes established and is identified in susceptible bird, mosquito and human populations across a wider geographical expanse in Europe. 

In Europe, most human cases have been reported from Italy, where there is active circulation of USUV and an extensive USUV and WNV surveillance program in place. In 2022, 2 human cases of febrile USUV infections were detected in Italy, while USUV was also identified in samples from 4 asymptomatic blood donors (6).

In other European countries, where surveillance is not as extensive, human cases have been reported from Austria (7), Croatia (8, 9), Czech Republic (10), France (11, 12), Hungary (13), Netherlands (14) and Switzerland (15). Cadar and Simonin described how USUV antibody prevalence ranged from 0.02% to 3% among healthy blood donors in seroprevalence studies carried out in Austria, Germany, Italy, France, Hungary, the Netherlands, Romania and Serbia (5). Additionally, higher seropositivity rates (6 to 7%) was observed in more exposed populations such as forestry workers (16) and bird ringers (14). There is evidence in some countries to suggest that seropositivity rates have slowly increased over time (17).

In humans, USUV has an estimated incubation period of between 3 to 12 days following the bite of an infected mosquito. Due to limited clinical data available for USUV infection, this estimation is extrapolated from WNV infection as a reference (18). Although the full spectrum of clinical presentation of USUV infections in humans has yet to be fully described, it ranges from mostly asymptomatic or mild symptoms (characterised by fever sometimes accompanied with a rash (19)), to a rare severe neuroinvasive presentation observed in immunocompromised individuals. Seroprevalence studies in Italy indicate that the prevalence of antibodies against USUV in humans is higher than anti-WNV antibodies in areas where both viruses co-circulate, supporting the speculation that most human USUV infections are asymptomatic (20). Carletti and others summarised published reports describing a total of 49 human infections with USUV identified in central Italy as of 2018 (21). Of these, which were diagnosed using molecular, serological or virus isolation techniques, 25 (51%) were identified in healthy blood donors as part of surveillance (22, 23).

Mild clinically apparent human cases of USUV present with fever, myalgia, headache, asthenia and rash (20). Severe human cases are rare but cases of USUV related meningoencephalitis have been reported (11; 24 to 27). Immunosuppression is believed to have been a contributory factor in the development of the severe cases described so far. To date, 2 USUV associated fatalities has been reported, both in immunosuppressed individuals, from Croatia and Italy (9, 28).

Is the disease endemic in humans within the UK?

Outcome

No.

Quality of evidence

Good.

As of November 2023, no human cases of USUV have been reported in the UK.

Evidence suggests a large proportion of infections are asymptomatic. Most cases in Europe have been diagnosed in asymptomatic blood donors identified on blood screening using a PCR test for WNV, with subsequent specific USUV testing. In the UK, a similar approach has been used by NHSBT to screen returning travellers since 2012. The NHSBT service tests samples from donors who have reported a travel history to areas where WNV is circulating between May and November.

WNV is known to co-circulate with USUV, and the molecular test for WNV (currently Roche Cobas 6800/8800 Nucleic Acid Test to identify WNV linear 1 and 2) is cross-reactive with USUV, therefore a positive result would trigger further testing. Between 2012 and 31 October 2023, approximately 415,000 samples from returning travellers have been tested, with 3 reactive tests (in 2014, 2016 and 2022), but no confirmed USUV cases identified (joint NHSBT-UKHSA Epidemiology Unit data). Cases of undiagnosed encephalitis referred to UKHSA’s Rare and Imported Pathogens Laboratory (RIPL) for investigation are routinely tested for WNV on serology, and if positive have subsequent USUV testing with a research developmental PCR, and again no cases have been identified to date.

Is the disease endemic in animals within the UK?

Outcome

No.

Quality of evidence

Satisfactory.

Birds

Two studies carried out in the early 2000s looking for evidence of arbovirus infection in birds in the UK reported detection of USUV antibodies in wild and farmed birds (29, 30). As neither RNA or USUV virus was isolated from these animals and the data is not repeatable, these studies did not provide convincing evidence of USUV presence in the UK at that time. 

APHA’s surveillance for USUV in wild birds in Great Britain from 2005 to 2019 did not result in any detections of USUV. In late August 2020, USUV RNA was detected in samples from a small number of wild birds (5 blackbirds and one house sparrow) submitted to APHA’s wildlife disease surveillance programme from a site in Greater London (the index site) (31). This was regarded as the first detection of USUV in wild birds in the UK. Detections in blackbirds have occurred annually at the same site between 2020 and 2023 (32). Genomic characterisation and molecular clock analysis of the isolates detected in Greater London between 2020 and 2023 inclusive from birds, indicate that the virus is USUV Africa 3.2 lineage. Furthermore, the UK detections to date are most closely related to each other compared to publicly available mainland European isolates, suggesting that the virus is persisting year on year in the UK.

In 2023, USUV was detected in a deceased blackbird found in Cambridgeshire. This was the first USUV detection in the UK outside of the Greater London area. In addition, during 2023, USUV was detected in 2 blackbirds from the index site and 2 juvenile great grey owls which were relocated from ZSL Whipsnade to ZSL London Zoo. Both blackbirds and great grey owls are highly susceptible to USUV infection and associated disease. There is no evidence that USUV is present at ZSL Whipsnade.

Mosquitoes

A joint Public Health England (PHE) and APHA study in 2013 in North Kent tested over 1,000 Cx. modestus mosquitoes for USUV and all were negative for USUV (33). Following the first detection of USUV in wild birds in 2020, mosquito surveillance in Greater London identified USUV RNA in 4 Cx. pipiens s.l. pools (34).

Mosquito monitoring had previously been conducted at the same site in 2015, during which USUV was not detected. Sampling for mosquitoes, including host-seeking females, overwintering females, and developing larva has been conducted at ZSL London zoo since the 2020 USUV outbreak. Apart from detections in Cx. pipiens s.l. pools mentioned above, USUV RNA has only been detected in one larval pool in 2023, suggesting that transovarial transmission of USUV is occurring at the index site as a mechanism for viral persistence.

Table 1. The number of birds and Cx pipiens s.l pools that tested positive for USUV, by region and year, as of October 2023

Year	Region	Number of birds tested	Species and number of USUV positive birds	Number of Cx pipiens s.l. pools tested	Number of USUV positive Cx. pipiens s.l. pools
2020	Greater London	89	Blackbird (n=5) House sparrow (n=1)	Host seeking females (46) Overwintering females (10)	4 (host seeking) 0 (overwintering)
2021	Greater London	131	Blackbird (n=1)	Host seeking females (504) Overwintering females (15)	0 (host-seeking) 0 (overwintering)
2022	Greater London	98	Blackbird (n=2)	Host seeking females (55) Overwintering females (21) Developing larvae (18)	0 (host-seeking) 0 (overwintering) 0 (larvae)
2023	Greater London	70	Blackbird (n=2) Great grey owl (n=2)	Host seeking females (12) Overwintering females (0) Developing larvae (34)	0 (host-seeking) 0 (overwintering) 1 (larvae)
2023	Cambridgeshire	2	Blackbird (n=1)	Host seeking females (3)	0 (host-seeking)

Current evidence suggests USUV is not widely endemic across the UK, with detections predominantly localised to the Greater London area. However, USUV prevalence in wild bird populations cannot be determined due to an absence of routine surveillance for USUV. Even in Greater London, where surveillance activities have historically been focussed, there is a paucity of data to understand the true incidence in wild bird populations. Repeated detections over a wider geographical area as well as in multiple years would be required to confirm endemicity.

Are there routes of introduction into the UK?

Outcome

Yes.

Quality of evidence

Good.

Migratory birds are believed to play a critical role in introducing USUV to a new region, whereas it is likely that resident birds will be involved in more local spread once infection is present (35). USUV originally was identified in South and West Africa, therefore the likely migration routes into Europe involved birds travelling along the East Atlantic or Black Sea Mediterranean flyways. Of several spring migration birds sampled in one study for WNV and USUV, the hoopoe, ruff, common whitethroat, and song thrush all tested positive and were also associated with the trans Saharan migration routes (35). Nevertheless, the recent incursions into England, in Greater London and Cambridgeshire, may be the result of local east to west movements of birds from continental Europe out with the migration patterns.

Although phylogenetic analyses of USUV strains suggest that the initial appearance of USUV in Europe (into Austria) was introduced by migratory birds from Africa (36), short distance movement of infected wild birds is considered the most important factor for the extension of geographical range of USUV in Europe. Short distance migration of infected birds is also thought the most probable route of introduction into the UK. USUV infected birds have been reported in many Western European countries including Austria, Belgium, Croatia, France, Germany, Greece, Italy, Poland, Spain, Switzerland, the Netherlands (37) and the UK (34).

Since repeated incursions have occurred in mainland European countries (38 to 40), further detections in the UK – through overwintering or incursions – should not be unexpected and it may be possible to target specific areas where spring migrations of passerines are found.

In terms of other pathways, mosquito dispersal through wind assistance can occur over large distances. The dispersal does depend on whether the vectors are active at night or during the day, when air turbulence is greater. Nevertheless, the mosquito surveillance undertaken in Great Britain around ports and truck stops does not suggest there are large plumes of invasive mosquitoes regularly being blown over from the continent.

In terms of imports of captive birds to zoos and aviaries, there are no trade restrictions regarding WNV or USUV in birds of any species destined for collections. All birds receive a clinical inspection by a vet, 48 hours prior to travel. Pre-movement testing only requires negative tests for avian influenza or Newcastle disease. All birds must enter 30-day quarantine, but this is not vector proofed.

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

Outcome

No.

Quality of evidence

Satisfactory.

The implementation of mitigation strategies to prevent the introduction of USUV by bird migration or mosquitoes is impractical. No USUV specific plans are currently in place to deal with an incursion. However, on such an occurrence the implicated mosquito species would be identified and control measures suggested if feasible. There are options for management of mosquitoes if particular species cause nuisance biting and pose a vector risk to humans. Advice on management of aquatic habitats for Cx. pipiens and Culiseta annulata around the home and in urban areas is available (41).

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

Outcome

Yes.

Quality of evidence

Good.

The primary vector of USUV in Europe is Cx. pipiens s.l. and most of the USUV positive pools are associated with this species of mosquito (1, 42, 43). Other Culex species have been implicated (Cx. neavei, Cx. perexiguus) but neither of these occur in the UK.

Although a small number of positive pools of other mosquito species trapped in continental Europe have been found, this does not always confirm that they play a role in transmission, as many mosquito species acquire bloodmeals from birds and may be positive, but are then not involved in onward transmission of the virus to humans. Of those species that also occur in the UK, these include Ae. caspius, An. maculipennis s.l., Cs. annulata, Ae. vexans, Cx. modestus and Ae. detritus (44 to 50). Positive pools of the invasive mosquitoes Ae. albopictus and Ae. japonicus have also been found (1, 42, 43, 49, 51, 52).

Cx. pipiens s.l. is a widespread species in the UK (53). There are 2 physiological forms. The more ubiquitous typical form of Cx. pipiens is almost exclusively ornithophagic, and it is only thought to rarely bite humans. It breeds in a range of natural and artificial habitats and is common in urban and rural areas. It is a container breeding species and will exploit a range of habitats around the home as well as areas of flooded water and nutrient-rich waters. It overwinters in the adult female stage and this begins from late August, with mosquitoes entering houses and cool outbuildings. The molestus form of Cx. pipiens does bite humans but appears to be limited in distribution and is associated with underground aquatic habitats and specific sewage treatment systems in London. Benzarti and others demonstrated endemicity of USUV in local birds and Pipistrellus sp. of bats in Belgium (which are also present in the UK) with over-wintering of USUV in Cx. pipiens pools; extension of the host range in wild bird species, including water birds; ongoing virus introduction by wild birds from outside the country and ongoing USUV viral evolution (54). The authors speculate on whether bats could be an amplification host, although this hypothesis requires further investigation.  

The combination of endemicity of USUV in near-continental countries to the UK, combined with short distance infected migrant birds and possibly mosquito movements, increases the likelihood of repeated incursions of USUV into the UK by these species, which could occur potentially in the spring, summer and autumn months, when native Culex mosquitoes will be active. Together these factors may lead to autochthonous transmission of USUV in the UK via native Culex mosquitoes, as opposed to the most probable incursion route via USUV infected long-distance migrant species from Africa with a different seasonality, arriving on UK shores when local Culex mosquitoes would not be active.

Will there be human exposure?

Outcome

Yes.

Quality of evidence

Satisfactory.

While human exposure is possible, it’s unlikely to be extensive. USUV is maintained in a similar mosquito-bird-mosquito cycle as for West Nile virus (55). The most likely vector (Cx pipiens s.l.) exists in the UK, although the bird-biting biotype rarely, if ever, bites humans. Nuisance biting in the UK is a regional phenomenon (56), due to a variety of species including Cs. annulata, although the role of this mosquito and other human-biting species in USUV transmission is not clear. The invasive species Ae. albopictus and Ae. japonicus are not currently established in the UK.

Are humans highly susceptible?

Outcome

No.

Quality of evidence

Satisfactory.

Angeloni and others described that from 2012 to 2021, 104 human cases of USUV infection were reported from European countries where the virus was known to circulate. Most of the cases were asymptomatic (n=85) or had only mild symptoms (febrile fever, n=8) and rarely neurological involvement (n=12) (57). Apparent moderate to severe clinical disease is rare in human USUV cases. The recent increasing numbers of reports of asymptomatic human USUV infections as well as cases of mild to severe neuroinvasive USUV infections in humans in Europe may be due to changes in awareness, testing of blood donors and improved surveillance, but may also be an effect of increased human exposure to this zoonotic risk. 

Outcome of probability assessment

The probability of human infection with Usutu virus in the UK population is: 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.

There are no known reports of human-to-human transmission of USUV. Transmission has been hypothesised through blood products (58) and possibly organ transplantation, but the main route of human transmission is through the bite of an infected vector.

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

Outcome

Yes.

Quality of evidence

Good.

USUV is transmitted to birds by mosquitoes, particularly Culex species. Human cases are rare, and infection is assumed to occur from the bite of an infected mosquito.

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

Outcome

Yes.

Quality of evidence

Good.

The hosts of USUV are birds, particularly blackbirds, and the vectors are mosquitoes, principally Culex species. Both hosts and vectors are present in the UK.

Is the UK human population susceptible?

Outcome

Yes.

Quality of evidence

Satisfactory.

Available data suggests that human disease is rare, even in Europe where epizootics have taken place in wildlife. Asymptomatic or mild infection has been reported in healthy individuals. Although there is a paucity of data available on clinical outcomes, immunocompromised individuals may be at a higher risk of developing more severe disease.

Does it cause severe disease in humans?

Outcome

No, in the general population.

Yes, in higher-risk groups (for example, immunocompromised or individuals with co-morbidities)

Quality of evidence

Satisfactory.

While the majority of reported human cases are asymptomatic or present with mild symptoms, severe disease has been reported. Cadar and Simonin reported that over 100 cases of acute human infection with USUV have been described in Europe, including approximately 30 patients with neurological symptoms. Most cases in which neurological disorders have been observed had comorbidities of varying severity (5).

On review of the source literature, there appear to be 16 cases of USUV neurological involvement reported in Europe (see Table 2). Other cases have been discarded as they were identified in asymptomatic blood donors, or only had mild flu-like illness. The patients ranged in age from 29 to 80 years of age and included males and females. Three had no known immunosuppression or co-morbidities, 5 were immunosuppressed (malignancy or transplant) and 6 had co-morbidities such as cardiovascular disease and diabetes. Outcomes were reported for 9 of the 16 cases: 5 had a prompt recovery, 2 had a slow recovery and 2 died. These are the only fatalities associated with USUV reported in the literature to date. Both fatalities, one each from Croatia and Italy, were in immunosuppressed individuals (9, 28).

The number of cases reported to date is insufficient to determine specific risk factors associated with poorer clinical outcomes. The full clinical presentation of USUV infection needs to be better defined.

Table 2. A description of European human USUV cases in which neurological involvement has been reported between 2008 and 2022.

Year	Country	Gender / Age	Presentation	Co-morbidities and immunosuppression	Reference
2008	Italy	Male, 40	Meningo-encephalitis	Chronic liver disease	27
2008	Italy	Male, 73	Meningo-encephalitis	Chronic obstructive pulmonary disease, diabetes	27
2008	Italy	Female, 67	Acute encephalitis	Aortic and mitral valve insufficiency	27
2009	Italy	Female, 54	Acute encephalitis	Hypertension	27
2009	Italy	Female, 60s	Meningo-encephalitis	Immunosuppressed – B cell lymphoma and chemotherapy	24
2009	Italy	Female, 40s	Neurological disease with severe impairment of the cerebral functions – required intensive rehab	Immunocompromised patient following a liver transplant	25
2013	Croatia	Female, 29	Meningo-encephalitis – slow recovery.	No known immunosuppression or co-morbidities	59
2013	Croatia	Unknown, 56	Meningo-encephalitis – prompt recovery	Arterial hypertension, hyperlipidaemia, and diabetes mellitus	59
2013	Croatia	Unknown, 61	Meningo-encephalitis – prompt recovery	Arterial hypertension, hyperlipidaemia, and diabetes mellitus	59
2016	France	Male, 39	Facial paralysis – recovered in a few weeks	No known immunosuppression or co-morbidities	11
2018	Italy	Male, 60s	Mild form of encephalitis – treated with acyclovir, recovered and d/c day 10	Hypertension, diabetes mellitus and an underlying malignancy	20
2018	Croatia	Unknown, 25	‘Neuroinvasive disease’	Not reported	9
2018	Croatia	Unknown, 84	‘Neuroinvasive disease’	Not reported	9
2018	Croatia	Unknown, 60	Meningo-encephalitis – fatal	Chronic lymphocytic leukaemia	9
2018	Hungary	Male, 40s	Meningitis – recovered without neurological sequelae	No known immunosuppression or co-morbidities	60
2022	Italy	Male, 80	Fatal meningoencephalitis infection few days after symptoms onset	Hypertension, past infiltrating prostate adenocarcinoma, psoriasis	28

Is it highly infectious to humans?

Outcome

No.

Quality of evidence

Satisfactory.

See above evidence.

Are effective interventions available?

Outcome

Yes/No

Quality of evidence

Satisfactory.

The risk of contracting USUV could be reduced by preventing exposure to mosquitoes (use of repellent, long sleeves, avoiding being outside at dusk and dawn when mosquito vectors are most active) (by extrapolation from WNV). 

There are options for the management of mosquitoes if particular species cause nuisance biting and pose a vector risk to humans. Advice on the management of aquatic habitats for Cx pipiens s.l. and Culiseta annulata around the home and in urban areas is available (41). However, this may not eliminate all biting or exposure risk.

There is no human vaccine available and no specific antiviral therapy. Most cases appear asymptomatic or mild and self-limiting, and treatment for more serious presentations is supportive. 

Would a significant number of people be affected?

Outcome

No.

Quality of evidence

Satisfactory.

It appears that infection occurs only in a minority of those who are exposed to and bitten by infected mosquitoes. Although there is limited information on the incidence of mosquito biting in the UK, sustained human biting is currently considered to be a localised event (56).

Outcome of impact assessment

The impact of Usutu virus on human health in the UK is assessed as:

Very Low to Low for the general UK population

Low to Moderate for higher-risk groups (for example, immunocompromised individuals).

Annexe A. Assessment of the probability of infection in the UK population algorithm

Annexe B. 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*

Go to question 5.

No

Go to question 4. (Outcome)

*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.

No

Go to question 7. (Outcome)

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

Yes

Go to question 8. (Outcome)

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. (Outcome)

No

The probability of infection in the general UK population is considered very low.

Question 9: Are humans highly susceptible?**

Yes

Go to question 10.

No

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

**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 C. Assessment of the impact on human health algorithm

Annexe D. 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. (Outcome)

No

The impact of infection in the UK population is considered very low.

Question 4: Is the human population susceptible?

Yes

Go to question 5. (Outcome)

No

The impact of infection in the UK population is considered very low.

Question 5: Does it cause severe disease in humans?

Yes, higher-risk groups

Go to question 8. (Outcome)

No

Go to question 6. (Outcome)

Question 6: Is it highly infectious to humans?

Yes

Go to question 9.

No

Go to question 7. (Outcome)

Question 7: Are effective interventions available?

Yes

The impact of infection in the general UK population is considered very low. (Outcome)

No

The impact of infection in the general UK population is considered low. (Outcome)

Question 8: Would a significant* number of people be affected?

Yes

Go to question 10.

No

Go to question 9. (Outcome)

*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. (Outcome)

No

The impact of infection in higher-risk groups is considered moderate. (Outcome)

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.

References

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3. McIntosh BM. Usutu (SAAr 1776); ‘nouvel arbovirus du groupe B’. International Catalogue of Arboviruses. 1985; 3: 1059 to 1060
4. Clé M, Beck C, Salinas S and others. ‘Usutu virus: A new threat?’ Epidemiology and Infection. 2019; 147: E232. DOI: 10.1017/S0950268819001213
5. Cadar D and Simonin Y. ‘Human Usutu Virus Infections in Europe: a new risk on horizon?’ Viruses. 2022; 15(1): 77. DOI: 10.3390/v15010077
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