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Classification of Viral Pathogens into Hazard Groups

 

Micro-organisms have been classified into 4 hazard groups by the ACDP (Advisory Committee on Dangerous Pathogens) on the basis of pathogenicity to humans, risk to laboratory workers, transmissibility to the community, and whether effective prophylaxis is available.

Group 1. - An organism that is most unlikely to cause human disease

Group 2. - An organism that may cause human disease and which may be a hazard to laboratory workers but is unlikely to spread to the community. Laboratory exposure rarely produces infection and effective prophylaxis or treatment is usually available. Examples include herpesvirses, ortho and paramyxoviruses, picornaviruses, adenoviruses, unconventional slow viruses.

Group 3 - An organism that may cause severe human disease and presents a serious hazard to laboratory workers. It may present a risk of spread to the community but there is usually effective prophylaxis or treatment available. Examples include HIV, HBV, Hantaviruses, Japanese B encephalitis, Rift Valley fever, Yellow Fever, rabies.

Group 4 - An organism that causes severe human disease and is a serious hazard to laboratory workers. It may present a high risk of spread to the community and there is usually no effective prophylaxis or treatment. Examples include Lassa fever, filoviruses, smallpox, Crimean-Congo haemorrhagic fever, Russian spring-summer encephalitis, Kyasanur forest.

Where pathogens that cause severe human disease are known to infect by the airborne route, primary containment using microbiological safety cabinets and the provision of secondary containment using appropriate ventilation have been recommended. For work with bloodbourne organisms such as HIV and HBV, the use of "sharps" should be avoided. Where available, vaccination is recommended for people working with known organisms. Vaccination is essential for work with HBV, polio, rabies, yellow fever, Rift valley fever, Russian spring-summer encephalitis, Kyasanur Forest disease virus. Vaccination is recommended for measles, mumps and rubella.

Containment requirements for work on hazard group 4 specimens

Hazard group 4 pathogens include viruses that cause encephalitis and viral haemorrhagic fevers. The Health and Safety (Dangerous Pathogens) Regulations 1981 stipulate that the Health and Safety Executive must be notified 30 days in advance before a diagnostic service in relation to a listed pathogen is carried out.

Viral Haemorrhagic Fever

Clinicians should consider the possibility of a VHF in a person developing a febrile illness within 21 days of arrival from an endemic area or a laboratory worker in a containment level 4 laboratory or animal unit presenting with an unexplained fever. However, the main life-threatening but treatable microbial diseases are malaria and typhoid fever. Therefore, allowance should be made for the diagnosis of malaria and the performance of certain biochemical and haematological investigations without the need for more rigorous physical containment than necessary. The guidelines developed by the ACDP are graded depending on the level of suspicion.

Minimal Risk - these are patients who have come from cities where the risk of VHF is negligible. No laboratory work must be carried out on specimens from these patients until a blood film has been examined for the presence of malaria parasites. The blood film should be rendered safe at the bedside by fixing in formalin or methanol. If blood is sent to the laboratory, a Class I microbiological safety cabinet must be used. If malaria parasites are not demonstrated, the categorization of the patient should be reassessed, and if infection by a Hazard Group 4 pathogen is still a possibility, specimens must be handled in a Containment level 3 laboratory using a Class III microbiological safety cabinet.

Moderate Risk - these are patients where infection with a Hazard Group 4 pathogen is a distinct possibility on epidemiological grounds but the more likely clinical diagnosis s malaria. No laboratory work must be carried out on specimens from these patients until a blood film has been examined for the presence of malaria parasites. The blood film should be rendered safe at the bedside, if blood is sent to the laboratory, it must be a Containment level 3 laboratory and a Class I microbiological safety cabinet must be used. If malaria parasites are not demonstrated, the categorization of the patient should be reassessed, and if infection by a Hazard Group 4 pathogen is still a possibility, specimens must be handled in a Containment level 3 laboratory using a Class III microbiological safety cabinet.

High Risk - these are patients where infection with a Hazard Group 4 pathogen is suspected e.g. the patient has been living and working in an endemic rural area, as opposed to an urban area, in particular a medical worker, or works in a laboratory where Hazard Group 4 pathogens are in use. If the patient has worked in a laboratory where Hazard Group 4 pathogens are in use, the patient should be immediately transferred to a high security unit. No laboratory work must be carried out on specimens from these patients until a blood film has been examined for the presence of malaria parasites. The blood film should be rendered safe at the bedside, if blood is sent to the laboratory, it must be a Containment level 3 laboratory and a Class III microbiological safety cabinet must be used. If malarial parasites are demonstrated and VHF as a diagnosis is discarded after assessment by clinical and laboratory staff, specimens may then be handled in routine clinical laboratories. If malarial parasites are not demonstrated, or if malarial parasites are demonstrated, but infection by a Hazard Group 4 pathogen is still a possibility, specimens for non-virological work which is essential before the patient is transferred to a specially designated high security isolation unit must be handled in a Containment Level 3 laboratory in a Class III microbiological safety cabinet.

Laboratory Procedures for suspected Hazard Group 4 Pathogens - Specimens for Hazard Group 4 virus serology or isolation must be sent, by arrangement to the Virus Reference Laboratory, CPHL, where work will be carried out at Containment Level 4. Specimens for the diagnosis of other viral infections may be sent to other laboratories that comply with the requirements for Containment Level 4, but only with their consent. Material for transport to a Containment Level 4 laboratory must be packed according to instruction given by the receiving laboratory. When there is a likelihood of many hours delay before a patient is transferred to a unit designated for the management of VHF cases, specimens for non-virological tests which are essential for the management of the patient may be carried out in Containment Level 3 laboratory in a Class III microbiological safety cabinet. Potentially infectious material to be removed from a Class III safety cabinet for incubation or storage must first be placed in hermetically sealable containers which are disinfected with 2% glutaraldehyde or Hypochlorite before removal. Such hermetically sealed containers must not be reopened unless they have been transferred back into a Class III microbiological safety cabinet. Sealed units must be used for centrifugation, and they must be opened only in a Class III safety cabinet. All waste materials and discarded clothing must be rendered safe to handle before they are removed from the laboratory and autoclaved before disposal or recycling. Water used for hand washing must be rendered safe by either chemical or hear disinfection. A high security isolation unit for the management of VHF would have appropriate Containment Level 4 laboratory facilities on-site.

Viral Encephalitis due to Hazard Group 4 Pathogens

Some Hazard Group 4 Pathogens which cause encephalitis (e.g. Russian spring-summer encephalitis) may arise in a patient or laboratory worker. Specimens for virus diagnosis should be handled in a Containment Level 4 laboratory as above. Specimens for other clinical investigations should be handled in a Containment Level 3 laboratory in a Class III microbiological safety cabinet.  

Suspected Rabies

There is no evidence that blood from patients with clinical rabies contain rabies virus. Virus may, however, be present in saliva, tears, urine, CSF and tracheal aspirates for at least two weeks after the onset of symptoms. Therefore, haematological and biochemical investigations on blood specimens may be carried out in a laboratory that complies with the requirements of work at Containment Level 2. When non-virological investigations on any specimens other than blood are required, the work must be undertaken in a Containment Level 3 laboratory and a Class I safety cabinet should be used. Staff performing this work must be vaccinated against rabies. Gloves should be worn for all manipulations which should be undertaken with care, and where possible, without the use of needles or glassware, to avoid the possibility of skin puncture. If automated equipment is used, it should be disinfected after use by washing through with an effective disinfectant compatible with the system. Specimens for virological and antibody testing for rabies should be sent to the Virus Reference Laboratory, CPHL.  

Spongiform Encephalopathies

Creutzfeldt-Jacob Disease is a transmissible spongiform encephalopathy (TME). In recent years, there has been an increase in iatrogenic cases through injections of human cadaver derived growth hormone, cornea grafts, dura mater grafts for eardrums, and contaminated neurosurgical instruments. TME agents are highly resistant to heat, radiation and chemical inactivation. The sterilization and disinfection procedures for TME agents are far more stringent than those usually applied in hospital practice for the control of conventional bacterial, viral, and other infectious disease agents. Contaminated reusable instruments should be sterilized at 134-138oC for not less than 18 minutes (hold time) or for six cycles of 3 minutes. It is thought that the downward displacement autoclave which is commonly used in laboratory work may be less effective even at these high temperatures than the surgical sterilizer operating with a pulsed vacuum cycle. Scrapie-infected tissue stored in 10% formalin has been shown to be still infectious. The application of sodium hypochlorite containing 20000 ppm appears to be effective when used in decontaminating surfaces. 2M sodium hydroxide may be used instead for the decontamination of metal surfaces that may be corroded by hypochlorite. Phenolic disinfectants, B -propiolactone, glutaldehyde, formaldehyde, alcohols and ethylene oxide are regarded as unreliable. There is practical difficulty in disinfecting safety cabinets of TME agents because of their resistance to formaldehyde and other fumigants.

To date, there are no confirmed cases of laboratory-acquired infection. However, the long incubation period makes the cause and effect difficult to relate and two recently reported cases of CJD in ex-laboratory workers who had handled neural tissues extensively are a matter of concern. Therefore, particular caution is advised in dealing with tissue from the CNS, especially from suspected cases of CJD and GSS. Equipment used in the laboratory and post-mortem room should be subjected to stringent disinfection procedures. Work practices should be reviewed to avoid puncture wounds and cuts. Eye protection and gloves should be worn. The skull of a CJD or GSS patients should only opened in a bag. At present, level 2 containment is advised for TME agents with additional measures to guard against puncture wounds and cuts and contamination of broken skin and eyes. The following precautions are recommended by the ACDP and WHO;-

Decontamination should be carried out by hypochlorite 20,000 ppm for at least 1 hour, repeated wetting with the disinfectant is necessary over the treatment period. 2M Sodium hydroxide is also active against scrapie but may not completely inactivate high concentrations of agents, as in the case of hypochlorite, repeated wetting is necessary. Autoclaving should be carried out at 134oC for 18 minutes as a single cycle, or as six separate cycles of 3 minutes each at 134oC.

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Classification of laboratories according to use