Epidemiology of Yersinia Infection
Strains of Y. enterolitica can be found in meats (pork, beef, lamb etc.), oysters, fish, and raw milk. The exact cause of the food contamination is not known. However, the prevalence of this organism in the soil and water and in animals offers ample opportunities for this organism to enter the food supply. The most susceptible are the very young, the debilitated, the very old, and immunocompromised individuals. Outbreaks of yersinosis have been associated with chocolate milk, tofu, and pasteurised milk.
The organism may be isolated from the patient’s faeces, blood, or vomit. Because of the difficulties in isolating yersiniae from faeces, several countries rely on serology. The organism may be isolated readily from food. Y. enterocolitica can be presumptive identified in 36-48 hours.
No infectious disease has created greater havoc than plaque. The first described pandemic in the 6th century AD is thought to have killed 100 million people. The black death outbreak in the middle ages wiped out 25% of the population in Europe. The last pandemic originated from China at the end of the 19th century. Plaque declined worldwide in the first half of the 20th century, with occasional outbreaks in Asia and Africa and sporadic cases elsewhere. Urban plaque, the epidemic form of the disease, is transmitted by fleas from rats to man. Wild plaque is epizootic among squirrels, rabbits, prairie dogs and pack rats. This reservoir leads to sporadic cases in man.
Yersinia pestis is a non-motile, short non-spore forming bacillus. Plaque is a natural disease of both domestic and wild rodents. Rats are the primary reservoir: they usually die acutely but may occasionally develop a more chronic form of infection. The disease is transmitted by fleas. The characteristic lesion of plaque is the bulboe, which is an enlarged lymph node. Pneumomia may develop in which case infection via respiratory droplets may occur.
Diagnosis can be made by direct examination of smears of fluid from lymph nodes or sputum, or by culture. IF may be used for direct examination. It is of utmost importance to treat the infection early. Y. pestis is responsive to streptomycin, chloramphenicol, and gentamycin. Formalin-inactivated and live attenuated vaccines are available and should be given to persons involved in high-risk activities.
F. tularensis is a short, non-motile bacillus. Tularaemia occurs in N. America, Europe, Russia, and Japan. It appears to be a disease of the Northern Hemisphere. Rabbits are the most important reservoirs of the disease. Man may get infected by coming into direct contact with tissues of infected rabbits or through the bite of deer flies and ticks. The organism behaves primarily as an intracellular parasite.
An ulcerating nodule often develops at the site of inoculation. The lymph nodes and blood stream may then be invaded, leading to more generalised disease. The characteristic lesion is a granulomatous nodule. The disease commonly involve the face, eyes or the oropharynx. Spread to the lungs or inhalation of the organisms may lead to pnemonic tularaemia. A typhoid-like disease may follow the ingestion of the organisms. If untreated, the disease is usually protracted and delirium and coma often develops. The fatality rate varies from 5 to 30%.
A diagnosis may be made by the direct examination of smears using fluorescent antibody. Special media must be used for culture. Streptomycin or gentamycin are the drugs of choice in the treatment of tularaemia.
Pasteurellae are primarily animal pathogens which can occasionally cause a variety of syndromes in man ranging from localised abscesses to septicaemias. They are non-motile rod-shaped bacilli, frequently with bipolar staining. Pasteurella multocida is the species most commonly encountered in human infections. It is the cause of haemorrhagic septicaemia in a variety of animals. The organism is frequently carried in the respiratory tract of healthy domestic animals, swine and rats. Human infections may take the form of (1) localised infection resulting from animal bites (cellulitis, abscess or osteomyelitis), (2) respiratory tract infections (pneumonia, empyema, lung abscess), (3) systemic infections (bacteraemia, peritonitis, meningitis). Diagnosis depends on the isolation and identification of the organism. Most strains of P. multocida are sensitive to penicillin.
The brucellae are gram-negative coccobacilli. They are aerobes with complex growth requirements. The genus contains three species which are pathogenic for man. The incubation period of brucellosis is long, often several weeks or months. The onset of symptoms is insidious, with malaise, chills, fevers, sweats, weakness, myalgia, and headache. Fever may be remittent, especially with B. melitensis (undulant fever). Lymphadenopathy and hepatosplenomegaly may occur. Bacteraemia is present in more than 20% of cases. Meningoencepalitis, osteomyelitis, endocarditis, interstitial nephritis and localised vertebral spondylitis may occur. It is difficult to establish a diagnosis in the later stages of the disease as the complaints may be very vague. Hepatic involvement is common in human brucellosis in the form of an acute diffuse hepatitis with focal necrosis or chronic small noncaseating granulomas. B. suis may cause chronic suppurative abscesses and large areas of caseation in the liver and spleen.
A patient with brucellosis commonly presents with an unexplained fever. The establishment of a definitive diagnosis requires cultivation of the organism from the blood, or from a biopsy of the bone marrow, liver, or lymph node. Specimens should be cultured in trypticase soy broth under 10%CO2. Owing to the increased use of antibiotics, the majority of the cases are now diagnosed by serology. Most strains of brucella are sensitive in vitro to the tetracylines and streptomycin.
B. abortus infects cattle worldwide. B. suis infects cattle as well as swine. Goats are important as sources of B. melitensis in Mediterranean countries. A live attenuated vaccine is available for use in cattle against B. abortus. As a result of this and other public health measures such as pasteurisation, the incidence of brucellosis has declined significantly in developed countries in recent years.