Human brucellosis can be a very debilitating disease, although the case fatality rate is generally low; it often becomes sub-clinical or chronic, especially if not recognized early and treated promptly. All ages are susceptible, and even congenital cases have been recorded. Few studies have attempted to measure infection in the general population, but a recent study in southern Saudi Arabia showed about 20% of the population had serological evidence of exposure. High-risk groups include those exposed through occupation in contexts where animal infection occurs, such as slaughterhouse workers, hunters, farmers and veterinarians.
Small common-source epidemics occur as a result of the ingestion of unpasteurized dairy products, especially soft fresh cheeses of goat or sheep origin. In temperate climates, human cases of occupational origin are more likely to be seen in the spring and summer months, corresponding with abortion, parturition and post-partum care of animals, especially small ruminants. Aerosol infections can occur, especially in abattoirs. Laboratory workers involved in diagnosis and vaccine production are also a high-risk group. Where the infection has been controlled and eventually eradicated in the livestock, there has been a very significant reduction in human cases. Therefore, in these countries, a recent history of overseas travel may be relevant.
The primary objectives of human surveillance should be to identify new human infections. This is usually reported as cases per 100 000 population. Another objective is to determine whether the infections are primarily of food-borne or occupational origin. If food-borne, are they from home produced or commercial foods? If the latter, should this be publicised and a recall made? The routine surveillance of high-risk foods is likely to be both expensive and not really provide the security that can be provided through Hazard Analysis Critical Control Point (HACCP) programmes, such as mandatory monitoring of heat treatments. A secondary objective is that human infections may lead to the identification of previously unrecognized infections in animals.
The recommended WHO Case Definition is:
Clinical: An illness characterized by acute or insidious onset, continued, intermittent or irregular fever of variable duration, profuse sweating, particularly at night, fatigue, anorexia, weight loss, headache, arthralgia, and generalized aching. Local infection of organs may occur.
Isolation of Brucella spp. from clinical specimens (note that repeated attempts may be necessary); or
Brucella agglutination titre, e.g. standard tube agglutination tests: SAT³160 in one or more specimens obtained after onset of symptoms; or
ELISA (IgA, IgG, IgM), 2-Mercaptoethanol test, Complement fixation test, Combs, fluorescent antibody test.
In small laboratories or clinics, a Rose Bengal screening test may be used. Positive results should always be confirmed by the tests listed above.
Suspected. A case that is compatible with the clinical description and is epidemiologically linked to suspected or confirmed animal cases or contaminated foods of animal origin.
Probable. A suspected case that has symptoms compatible with disease and is positive in the Rose Bengal test, but negative in blood culture and showing low titres in the confirmatory tests.
Confirmed. A suspected or probable case that is laboratory confirmed.
The above case definitions may require modification depending on the availability of medical services and laboratory resources.
Mandatory and immediate case-based reporting by all health-care providers should be required. In some countries, the provision of free treatments may provide an additional information source.
Mandatory reporting from laboratories of positive results, independent of physician reporting, can also be included and will usually increase the sensitivity of surveillance. Routine surveillance of high-risk occupational groups is also recommended, which may include collection of baseline samples for use in the case of future exposures.
Each human case should be investigated for surveillance purposes, and include demographic information as well as food history, animal contacts, type of work or activity at onset, and recent travel history. In addition, a joint investigation with veterinary colleagues is highly recommended (See section on Intersectoral Collaboration).
These would typically include:
graphs of number of susceptible, probable and confirmed cases, by month;
tables of number of susceptible, probable and confirmed cases by age, sex, month and place; and
maps showing distribution and number of suspected, probable and confirmed cases by place (e.g. county, province or state).
Performance-based indicators
number of new cases (confirmed) per 100 000 population compared with previous years or other equal time frames.
Diagnostic-based indicators include:
proportional comparisons: suspected, probable and confirmed;
number of epidemiological investigations in relation to number of confirmed cases;
comparison of sources of reports, such as physicians, hospitals, laboratories and other; and
comparison of probable sources, such as food-borne, animal contact and other.
Resource-based indicators include:
number of bacteriological tests relative to number of serological tests; and
number of culture-positive cases in relation to number of cultures attempted.
At the peripheral level, all possible sources should be included to improve the sensitivity of the system, including private physicians, other health workers, public clinics and hospitals, especially from patients presenting with “fevers of unknown origin”. Some countries have designated specialist physicians to review all suspected cases to improve specificity of surveillance data. Ideally a “zero reporting system” should be used and reports submitted at least weekly to the intermediate level.
At the intermediate level, considered here to be equivalent to district, county, province or other administrative unit, case reporting would be to both the central level and the equivalent veterinary office level. (See section on Intersectoral collaboration) All reports should be validated at this level, and epidemiological case reports completed and, if necessary, field investigations undertaken of suspected outbreaks. Data from all reporting areas should be compared and regular feedback provided to the peripheral level.
At the central level, the national epidemiological surveillance unit should:
tabulate, check and entry all reports from districts and regions,
develop at least quarterly reports,
take action on outbreaks,
produce educational material where necessary, and
liaise with the Ministry of Agriculture and with other national entities such as Inter-Ministry Zoonoses Committees or Brucellosis Advisory Committees.
Periodically, the surveillance programme at the central level should be independently evaluated to determine performance efficiency.
