The rapid assessment consists of confirming an outbreak of VHF and estimating its geographical distribution, assessing the impact on health, and determining the existing response capacity and immediate needs.
Confirming an outbreak of VHF and estimating its geographical distribution
Initial case definition
As for all potential epidemics, this is best determined in advance, as part of emergency preparedness. Simple, viable case definitions should be developed for suspect, probable, and confirmed cases of VHF.
Examples of case definitions for VHF are:
· Suspected case: acute fever with either jaundice, or cutaneous and internal bleeding, accompanied by shock; in the case of dengue the rash should also be mentioned.
· Probable case: a suspected case with at least two of the following signs: severe myalgia and headache, conjunctivitis, rash, shock, proteinuria, death, where the person has had contact with a possible source of transmission.
· Confirmed case: a suspected or probable case with one of the following: virus isolation from blood or tissue; detection of viral antigen or genome in blood, tissue or other body fluid; presence of specific IgM antibody in titre high enough to indicate recent infection.
In a rapid assessment, it may be difficult to distinguish yellow fever from other haemorrhagic illnesses or diseases such as malaria. However, to maximize case detection at this early stage, it is often necessary to use a broad case definition such as “jaundice, fatal or non-fatal” to identify suspected cases.
Confirming the increase in the number of cases
(See Chapter 2)
Case-finding and estimating geographical distribution
(See Chapter 2)
It is important to recognize that there could be many asymptomatic or mild cases who are hospitalized with a non-specific febrile illness. To be thorough, VHF and yellow fever case-finding efforts should not be limited to infectious wards but include other hospital departments and health facilities.
Collection of specimens
Because the definitive diagnosis of a VHF can only be made by serology or virus isolation, it is essential that appropriate specimens be collected during the rapid assessment.
Key considerations in specimen collection are as follows:
· Essential information should be included with specimens (locality, name of patient, age, sex, date of sampling, date of disease onset, and summary of clinical and epidemiological findings).
· All specimens should be collected in sterile containers.
· All specimens must be considered potentially infectious and dangerous. Therefore, stringent safety precautions should be observed.
· For every patient, a specimen of whole blood should be collected without anticoagulant for virus isolation or antibody detection.
· Do not freeze whole blood or liver specimens: separate sera if specimens are to be frozen.
· All sera and cerebrospinal fluid (CSF) specimens should be frozen for preservation during transport. For virus isolation, specifically, specimens should be stored ideally on liquid nitrogen or dry ice.
· Specimens are best hand-carried from peripheral areas to the central level.
· Use non-breakable containers (plastic, screw-cap) with absorbent material to contain any leakage, and double outer containers. Follow International Air Transport Association (IATA) regulations for air transport of specimens.
The specimens required for laboratory analysis and confirmation are as follows:
- whole blood from patients who have been sick less than seven days (do not separate sera from blood clots unless laboratory workers can be protected against infectious aerosols);
- convalescent sera from patients at least 14 days after onset (sera should be carefully separated from blood clots);
- for suspect yellow fever cases, liver specimens should be taken at postmortem with a biopsy needle (these should be divided in two - one placed in 10% buffered formalin and the other treated in the same way as a whole blood specimen - not frozen without anticoagulant);
- skin snips preserved in formalin from fatal cases of suspect VHF.
To verify the clinical diagnosis and identify the causative virus, it is advisable to transport specimens to WHO collaborating centres for urgent analysis.
Assessing the impact on health
Collecting information on a representative sample of cases
When the cause of a VHF outbreak is unknown, careful interviewing and physical examination of suspect, probable, and confirmed cases is extremely important.
These early clinical findings provide clues as to the type of virus and source of infection.
As a minimum, gather information on:
- name, age, sex, residence, date of onset, and of reporting;
- signs and symptoms, severity of illness, treatment given, and response to treatment; and
- presence of risk factors, e.g. history of contact.
Useful information on the mode of transmission can be gained by investigating the contacts of identified index cases. It is also important to ask about exposures to infected animal hosts (e.g. contact while slaughtering livestock).
The definition of a “primary” or “close” contact is one or more of the following:
- has shared the same place (for working or travelling), the same room or meals, had occasional face-to-face contact during the period of communicability of a severe, classical or mild form of the disease;
- has given care, handled the patient’s belongings, participated in autopsy or burial preparations without special protection; or
- has travelled from an area where VHF transmission is endemic.
The definition of a “possible” contact is:
- was a close contact of a case during a period in which she or he possibly was not yet contagious (e.g. persons hospitalized in the same ward).
Whatever the method chosen, the characterization of the contact should include a clarification on the index case: was he or she suspect, probable or confirmed?
Analysing the information
The information should be analysed in terms of time, place, and person (See Chapter 2).
Assess vectors present
One rapid assessment priority is to determine whether vectors that may transmit VHF or yellow fever are present in the affected area. It is not the purpose of a rapid assessment to carry out a detailed entomological survey, but rather to ask the following questions.
· Are vectors present in the affected area? If so, what are they?
· Are they known to bite humans?
· Are there breeding sites? If so, how extensive?
The answers to these preliminary questions are critical to deciding on the need for further entomological studies and control measures for vectors and natural hosts.
Assess disease in other vertebrate hosts
· Are there unexplained deaths in monkeys in the affected area? If so, where and when did they occur?
· Are there unexplained deaths or abortions in livestock? If so, where and when did they occur? (Particularly relevant for Rift Valley fever.)
Assessing local response capacity and immediate needs
Local response capacity and immediate needs should be assessed to determine the type and quantity of external support required.
Local epidemic surveillance
· Are there sufficient trained personnel, vehicles, and communications support to maintain adequate surveillance? Is outside technical help needed?
· Is there a need for animal studies (e.g. sentinel herd surveillance) or further entomological investigations?
Response capacity of local health services
· What steps have local health officials taken to organize epidemic response? Is there a plan of action, standardized reporting procedures, and trained staff?
· Are hospitals equipped to carry out safe barrier nursing measures? (Check bed nets, gloves, disinfectants, masks, and gowns.)
· What is the local cold chain capacity? Trained vaccinators? Jet injectors? Vehicles? Stocks of syringes? Yellow fever vaccine stocks in country?
· Do medical, nursing and laboratory personnel need further training on case detection and safe patient management?
· What links have been established with key community members (e.g. for allaying panic in case of outbreaks, for general health education and improved surveillance and case detection)?
· What vector control equipment, pesticides, and larvicides are available?
· Has a strategy been developed for dealing with press inquiries?
Determine immediate needs
To determine immediate needs the following questions should be addressed.
· Is there an outbreak of VHF which has led or could lead to a large number of cases?
· If so, are external resources needed to contain it?
If the answer to both questions is “yes”, then an emergency response is needed.