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Gram-Negative Rods Related to Animal Sources (Zoonotic Organisms)
CHAPTER CONTENTS
INTRODUCTION
Zoonoses are human diseases caused by organisms that are acquired from animals. There are bacterial, viral, fungal, and parasitic zoonoses. Some zoonotic organisms are acquired directly from the animal reservoir, whereas others are transmitted by vectors, such as mosquitoes, fleas, or ticks.
There are four medically important gram-negative rods that have significant animal reservoirs: Brucella species, Francisella tularensis, Yersinia pestis, and Pasteurella multocida (Table 20–1).
BRUCELLA
Disease
Brucella species cause brucellosis (undulant fever).
Important Properties
Brucellae are small gram-negative rods without a capsule. The three major human pathogens and their animal reservoirs are Brucella melitensis (goats and sheep), Brucella abortus (cattle), and Brucella suis (pigs).
Pathogenesis & Epidemiology
The organisms enter the body either by ingestion of contaminated milk products or through the skin by direct contact in an occupational setting such as an abattoir. They localize in the reticuloendothelial system, namely, the lymph nodes, liver, spleen, and bone marrow. Many organisms are killed by macrophages, but some survive within these cells, where they are protected from antibody. The host response is granulomatous, with lymphocytes and epithelioid giant cells, which can progress to form focal abscesses. The mechanism of pathogenesis of these organisms is not well defined, except that endotoxin is involved. No exotoxins are produced.
Imported cheese made from unpasteurized goats’ milk produced in either Mexico or the Mediterranean region has been a source of B. melitensis infection in the United States. The disease occurs worldwide but is rare in the United States because pasteurization of milk kills the organism.
Clinical Findings
After an incubation period of 1 to 3 weeks, nonspecific symptoms such as fever, chills, fatigue, malaise, anorexia, and weight loss occur. The onset can be acute or gradual. The undulating (rising-and-falling) fever pattern that gives the disease its name occurs in a minority of patients. Enlarged lymph nodes, liver, and spleen are frequently found. Pancytopenia occurs. B. melitensis infections tend to be more severe and prolonged, whereas those caused by B. abortus are more self-limited. Osteomyelitis is the most frequent complication. Secondary spread from person to person is rare.
Laboratory Diagnosis
Recovery of the organism requires the use of enriched culture media and incubation in 10% CO2. The organisms can be presumptively identified by using a slide agglutination test with Brucella antiserum, and the species can be identified by biochemical tests. If organisms are not isolated, analysis of a serum sample from the patient for a rise in antibody titer to Brucella can be used to make a diagnosis. In the absence of an acute-phase serum specimen, a titer of at least 1:160 in the convalescent-phase serum sample is diagnostic.
Treatment
The treatment of choice is tetracycline plus rifampin. There is no significant resistance to these drugs.
Prevention
Prevention of brucellosis involves pasteurization of milk, immunization of animals, and slaughtering of infected animals. There is no human vaccine.
FRANCISELLA
Disease
Francisella tularensis causes tularemia.
Important Properties
F. tularensis is a small, pleomorphic gram-negative rod. It has a single serologic type. There are two biotypes, A and B, which are distinguished primarily on their virulence and epidemiology. Type A is more virulent and found primarily in the United States, whereas type B is less virulent and found primarily in Europe.
Pathogenesis & Epidemiology
F. tularensis is remarkable in the wide variety of animals that it infects and in the breadth of its distribution in the United States. It is enzootic (endemic in animals) in every state, but most human cases occur in the rural areas of Arkansas and Missouri. It has been isolated from more than 100 different species of wild animals, the most important of which are rabbits, deer, and a variety of rodents. The bacteria are transmitted among these animals by vectors such as ticks, mites, and lice, especially the Dermacentor ticks that feed on the blood of wild rabbits. The tick maintains the chain of transmission by passing the bacteria to its offspring by the transovarian route. In this process, the bacteria are passed through ovum, larva, and nymph stages to adult ticks capable of transmitting the infection.
Humans are accidental “dead-end” hosts who acquire the infection most often by being bitten by the vector or by having skin contact with the animal during removal of the hide. Rarely, the organism is ingested in infected meat, causing gastrointestinal tularemia, or is inhaled, causing pneumonia. There is no person-to-person spread. The main type of tularemia in the United States is tick-borne tularemia from a rabbit reservoir.
The organism enters through the skin, forming an ulcer at the site in most cases. It then localizes to the cells of the reticuloendothelial system, and granulomas are formed. Caseation necrosis and abscesses can also occur. Symptoms are caused primarily by endotoxin. No exotoxins have been identified.
Clinical Findings
Presentation can vary from sudden onset of an influenzalike syndrome to prolonged onset of a low-grade fever and adenopathy. Approximately 75% of cases are the “ulceroglandular” type, in which the site of entry ulcerates and the regional lymph nodes are swollen and painful. Other, less frequent forms of tularemia include glandular, oculoglandular, typhoidal, gastrointestinal, and pulmonary. Disease usually confers lifelong immunity.
Laboratory Diagnosis
Attempts to culture the organism in the laboratory are rarely undertaken, because there is a high risk to laboratory workers of infection by inhalation, and the special cysteine-containing medium required for growth is not usually available. The most frequently used diagnostic method is the agglutination test with acute- and convalescent-phase serum samples. Fluorescent-antibody staining of infected tissue can be used if available.
Treatment
Streptomycin is the drug of choice. There is no significant antibiotic resistance.
Prevention
Prevention involves avoiding both being bitten by ticks and handling wild animals. There is a live, attenuated bacterial vaccine that is given only to persons, such as fur trappers, whose occupation brings them into close contact with wild animals. The vaccine is experimental and not available commercially but can be obtained from the U.S. Army Medical Research Command, Fort Detrick, Maryland. This and the bacillus of Calmette-Guérin (BCG) vaccine for tuberculosis are the only two live bacterial vaccines for human use.
YERSINIA
Disease
Yersinia pestis is the cause of plague, also known as the black death, the scourge of the Middle Ages. It is also a contemporary disease, occurring in the western United States and in many other countries around the world. Two less important species, Yersinia enterocolitica and Yersinia pseudotuberculosis, are described in Chapter 27.
Important Properties
Y. pestis is a small gram-negative rod that exhibits bipolar staining (i.e., it resembles a safety pin, with a central clear area). Freshly isolated organisms possess a capsule composed of a polysaccharide–protein complex. The capsule can be lost with passage in the laboratory; loss of the capsule is accompanied by a loss of virulence. It is one of the most virulent bacteria known and has a strikingly low ID50 (i.e., 1 to 10 organisms are capable of causing disease).
Pathogenesis & Epidemiology
The plague bacillus has been endemic in the wild rodents of Europe and Asia for thousands of years but entered North America in the early 1900s, probably carried by a rat that jumped ship at a California port. It is now endemic in the wild rodents in the western United States, although 99% of cases of plague occur in Southeast Asia.
The enzootic (sylvatic) cycle consists of transmission among wild rodents by fleas. In the United States, prairie dogs are the main reservoir. Rodents are relatively resistant to disease; most are asymptomatic. Humans are accidental hosts, and cases of plague in this country occur as a result of being bitten by a flea that is part of the sylvatic cycle.
The urban cycle, which does not occur in the United States, consists of transmission of the bacteria among urban rats (the reservoir), with the rat flea as vector. This cycle predominates during times of poor sanitation (e.g., wartime), when rats proliferate and come in contact with the fleas in the sylvatic cycle.
The events within the flea are fascinating as well as essential. The flea ingests the bacteria while taking a blood meal from a bacteremic rodent. A thick biofilm containing many organisms forms in the upper gastrointestinal tract that prevents any food from proceeding down the gastrointestinal tract of the flea. This “blocked flea” then regurgitates the organisms into the bloodstream of the next animal or human it bites.
The organisms inoculated at the time of the bite spread to the regional lymph nodes, which become swollen and tender. These swollen lymph nodes are the buboes that have led to the name bubonic plague. The organisms can reach high concentrations in the blood (bacteremia) and disseminate to form abscesses in many organs. The endotoxin-related symptoms, including disseminated intravascular coagulation and cutaneous hemorrhages, probably were the genesis of the term black death.
In addition to the sylvatic and urban cycles of transmission, respiratory droplet transmission of the organism from patients with pneumonic plague can occur.
The organism has several factors that contribute to its virulence: (1) the envelope capsular antigen, called F-1, which protects against phagocytosis; (2) endotoxin; (3) an exotoxin; and (4) two proteins known as V antigen and W antigen. The V and W antigens allow the organism to survive and grow intracellularly, but their mode of action is unknown. The action of the exotoxin is unknown.
Other factors that contribute to the extraordinary pathogenicity of Y. pestis are a group of virulence factors collectively called Yops (Yersinia outer proteins). These are injected into the human cell via type III secretion systems and inhibit phagocytosis and cytokine production by macrophages and neutrophils. For example, one of the Yops proteins (YopJ) is a protease that cleaves two signal transduction pathway proteins required for the induction of tumor necrosis factor synthesis. This inhibits the activation of our host defenses and contributes to the ability of the organism to replicate rapidly within the infected individual.
Clinical Findings
Bubonic plague, which is the most frequent form, begins with pain and swelling of the lymph nodes draining the site of the flea bite and systemic symptoms such as high fever, myalgias, and prostration. The affected nodes enlarge and become exquisitely tender. These buboes are an early characteristic finding. Septic shock and pneumonia are the main life-threatening subsequent events. Pneumonic plague can arise either from inhalation of an aerosol or from septic emboli that reach the lungs. Untreated bubonic plague is fatal in approximately half of the cases, and untreated pneumonic plague is invariably fatal.
Laboratory Diagnosis
Smear and culture of blood or pus from the bubo is the best diagnostic procedure. Great care must be taken by the physician during aspiration of the pus and by laboratory workers doing the culture not to create an aerosol that might transmit the infection. Giemsa or Wayson stain reveals the typical safety-pin appearance of the organism better than does Gram stain. Fluorescent-antibody staining can be used to identify the organism in tissues. A rise in antibody titer to the envelope antigen can be useful retrospectively.
Treatment
The treatment of choice is a combination of streptomycin and a tetracycline such as doxycycline, although streptomycin alone can be used. Levofloxacin can also be used. There is no significant antibiotic resistance. In view of the rapid progression of the disease, treatment should not wait for the results of the bacteriologic culture. Incision and drainage of the buboes are not usually necessary.
Prevention
Prevention of plague involves controlling the spread of rats in urban areas, preventing rats from entering the country by ship or airplane, and avoiding both flea bites and contact with dead wild rodents. A patient with plague must be placed in strict isolation (quarantine) for 72 hours after antibiotic therapy is started. Only close contacts need to receive prophylactic tetracycline, but all contacts should be observed for fever. Reporting a case of plague to the public health authorities is mandatory.
A vaccine consisting of formalin-killed organisms provides partial protection against bubonic but not pneumonic plague. This vaccine was used in the armed forces during the Vietnam War but is not recommended for tourists traveling to Southeast Asia.
PASTEURELLA
Disease
Pasteurella multocida causes wound infections associated with cat and dog bites.
Important Properties
P. multocida is a short, encapsulated gram-negative rod that exhibits bipolar staining.
Pathogenesis & Epidemiology
The organism is part of the normal flora in the mouths of many animals, particularly domestic cats and dogs, and is transmitted by biting. About 25% of animal bites become infected with the organism, with sutures acting as a predisposing factor to infection. Most bite infections are polymicrobial, with a variety of facultative anaerobes, especially Streptococcus species, and anaerobic organisms present in addition to P. multocida. Pathogenesis is not well understood, except that the capsule is a virulence factor and endotoxin is present in the cell wall. No exotoxins are made.
Clinical Findings
A rapidly spreading cellulitis at the site of an animal bite is indicative of P. multocida infection. The incubation period is brief, usually less than 24 hours. Osteomyelitis can complicate cat bites in particular, because cats’ sharp, pointed teeth can implant the organism under the periosteum.
Laboratory Diagnosis
The diagnosis is made by finding the organism in a culture of a sample from the wound site.
Treatment
Penicillin G is the treatment of choice. There is no significant antibiotic resistance.
Prevention
People who have been bitten by a cat should be given ampicillin to prevent P. multocida infection. Animal bites, especially cat bites, should not be sutured.
BARTONELLA
Disease
Bartonella henselae is the cause of cat-scratch disease and bacillary angiomatosis. Cat-scratch disease is one of the most common zoonotic diseases in the United States.
Important Properties
B. henselae is a small, pleomorphic gram-negative rod. It is a fastidious organism and will not grow on routine blood agar. It can be cultured on specialized media in the clinical laboratory.
Pathogenesis & Epidemiology
Cat scratches or bites, especially from kittens, are the main mode of transmission of B. henselae to humans. The organism is a member of the oral flora of many cats. There is evidence that it is transmitted from cats to humans by the bite of cat fleas. Exposure to cat urine or feces does not pose a risk of transmission. Person-to-person transmission of B. henselae does not play a significant role in infection.
It is a low virulence organism, and disease is self-limited in immunocompetent individuals.
Clinical Findings
In immunocompetent people, B. henselae causes cat-scratch disease (CSD). This disease is characterized by fever and tender, enlarged lymph nodes, typically on the same side as the scratch (Figure 20–1). A papule at the site of the scratch may precede the lymphadenopathy. CSD has a prolonged course but eventually resolves, even without antibiotics. A small percentage of those infected develop systemic disease, such as endocarditis or encephalitis.
FIGURE 20–1 Cat-scratch disease. Note the two enlarged, inflamed axillary lymph nodes in a patient with cat-scratch disease. (Reproduced with permission from Wolff K, Johnson R. Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)
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