Surveillance and Management of Covert Releases

Overview

A covert release involves a concealed release of a biological agent. In these circumstances, the release will not become apparent until the first cases of disease arise.

Mitigation of the effects of a covert release will require early recognition of these cases and an understanding of the spatial and temporal links between them in order to allow rapid and targeted administration of prophylactic treatment to those exposed so that further cases and the spread of contagious organisms can be prevented. This requires clinicians to be aware both of the presentations of disease caused by likely agents of deliberate release, and of the process for activating a public health response.

Cases of disease will first appear as unusual clinical syndromes or unexpected patterns of occurrence of more common syndromes. These are described in cardinal signs and clinical tips. Assessment of these cases must include detailed history taking so that common exposures can be identified.

Recognising deliberate releases

Once unusual clinical syndromes have been diagnosed as cases of disease, naturally occurring infections need to be discriminated from infections caused by deliberate or accidental releases.

The criteria for clinically suspected cases are given in cardinal signs and clinical tips. Laboratory confirmed cases implies confirmation of a clinically suspected diagnosis by the Reference Laboratory.

Expert advice will be provided if necessary in order to confirm the occurrence of a covert release and assist with epidemiological investigations to define an exposed zone in time and space.

Anthrax

Anthrax is a rare disease. In the last 20 years there has been fewer than one case per year in the UK. These are mainly cutaneous and are due to handling hides imported from countries with endemic disease (and thus often associated with the leather industry).

Deliberate release should therefore be considered in the event of:

• Single laboratory confirmed cases of inhalation anthrax.
• Single laboratory confirmed cases of cutaneous anthrax arising in individuals who do not routinely have contact with animals or animal hides.
• Two or more clinically suspected cases of anthrax  that are linked in time and place, especially geographically related groups of illness following a wind direction pattern.

Close co-ordination with veterinary colleagues is essential: grazing animals (cows, sheep, goats) are far more susceptible to disease and have a shorter incubation period than humans. Cases of anthrax in animals may provide an early warning system. Infected animals could also act as ongoing source of potential human infection. Incident managers should ensure that appropriate veterinary advice is taken.

Smallpox

Considering that smallpox no longer occurs naturally and the high level of security in the two WHO collaborating centres, it is assumed that any laboratory confirmed case indicates a deliberate release. Early identification of suspected cases is a key component for containment of smallpox. Suspicion of smallpox may arise by a clinician and/ or electron microscope virologist, who are then responsible for immediately contacting and arranging to send specimens to the reference laboratory and for notifying their local HPU.

Botulism

Naturally occurring botulism is very rare in the UK. A single laboratory confirmed case raises the possibility of a deliberate release, particularly where cases with an unusual toxin type (ie, type C, D, F, or G, or type E toxin not acquired from an aquatic food). All cases should therefore be investigated thoroughly to exclude deliberate release. Where cases arise in the absence of a known or obvious source such as a particular food item or a water quality zone, it is important to collect comprehensive details relating to food history and use of water supplies. Local hospitals should be contacted to enquire about similar cases of illness.

Deliberate release should also be considered in the event of outbreaks of two or more cases of acute flaccid paralysis, especially where there are common geographic factors between cases (eg. airport, work location) but no common dietary exposures (ie. features suggestive of an aerosol attack).

Where a food or waterborne source of botulinum toxin is suspected, relevant samples are essential to aid identification of the source of infection.

Plague

Even a single laboratory confirmed case of plague must be regarded with a high index of suspicion of deliberate release. Even cases that occur in people who have returned from endemic areas should be investigated to ascertain that the illness did not occur due to deliberate release of Y. pestis.

In addition, a deliberate release should be considered in the event of two or more clinically suspected cases of plague that are linked in time and place, especially geographically related groups of illness following a wind direction pattern.

Rodents and cats are susceptible to plague, so close co-ordination with veterinary colleagues is essential. Infected animals could also act as an ongoing source of potential human infection.

Tularemia

Indigenous tularemia has never been recorded in the UK, but cases are occasionally imported. Person-to-person spread of tularemia has not been documented.

Deliberate release should therefore be considered in the event of any clinically suspected cases, and strongly suspected if a single laboratory confirmed case arises.

Close co-ordination with veterinary colleagues is essential since animals are also susceptible to disease, and cases of tularemia in animals may assist epidemiological investigations.

Viral haemorrhagic fever

Cases of viral haemorrhagic fever are extremely rare in the UK. Even a single confirmed case of VHF must be regarded with a high index of suspicion of deliberate release. This suspicion also applies to cases that occur in people who have returned from endemic areas. All infections should be investigated to ascertain that they have not occurred due to deliberate release of VHF agents.

In addition, a deliberate release should be considered in the event of two or more suspected cases of VHF that are linked in time and place. Expert advice will be provided in order to confirm the occurrence of a covert release and assist with epidemiological investigations to define an exposed zone in time and space.

General Principles

Defining an exposed zone

The release of a biological agent will create an exposed zone: an area and time period in which the agent has been dispersed, and which poses a risk of infection. The location and size of the exposed zone will depend on the site of release, the mechanism of dispersal of the agent, and local geographical and meteorological factors. It will also depend on the properties of the particles used to disperse the agent, for example with anthrax, fine particles may carry further following primary aerosolisation, take longer to settle, and pose a risk of secondary aerosolisation if they are subsequently disturbed. The duration of the exposed zone will depend on the time of release and the survival of the agent used.

Following a covert release, determination of the extent of the exposed zone will depend on epidemiological analysis of common exposures between cases, supported later by results from environmental testing. None of the agents most likely to be used in deliberate releases have a reliable test to indicate whether people have been exposed. Initial targeting of prophylaxis will therefore rely on thorough case histories. As further epidemiological data and environmental test results become available, the definition of the exposed zone may be refined and prophylactic treatment extended or discontinued accordingly.

Protection of healthcare staff

Once a diagnosis has been established, healthcare staff can take the appropriate precautions for clinical management of cases. However, some healthcare staff may have attended cases of unusual clinical syndromes prior to diagnosis and without the appropriate precautions. They may therefore be at risk of infection either from contaminated clothing or from contagious infections, and may need to be treated as exposed persons.

Identification and handling of exposed persons

Once an exposed zone has been defined, communication channels will be required to enable potentially exposed persons to contact health services for assessment. Procedures will need to ensure that contaminated clothing is dealt with, appropriate prophylactic treatment is provided and measures are taken to prevent secondary spread of contagious organisms. Where there has been a delay in recognising a covert release, large numbers of people could potentially have been exposed through contact with the cases infected by the initial release, and they will have to be managed accordingly.

Surveillance and case finding

Once a covert deliberate or accidental release becomes apparent, clinicians and hospitals should actively look out for further presentations of cases of disease, so that if they arise, early treatment can be started, patients can be isolated if necessary, and epidemiological details can be followed up.

As epidemiological data from cases and results from environmental testing become available, the definition of the exposed zone may be refined and extended. In this event, further exposed persons will need to be identified, contaminated clothing dealt with, and appropriate prophylactic treatment issued.

Contact tracing

In the event of release of a contagious organism, contacts of infectious cases will need to identified, traced and given prophylactic treatment and follow up arranged. This will require information about the movements of cases.

Last reviewed: 15 March 2012