Systemic Illness Caused by Corynebacterium pseudotuberculosis

CHAPTER 34 Systemic Illness Caused by Corynebacterium pseudotuberculosis



Suzanne M. Pratt


Corynebacterium pseudotuberculosis is an uncommon but serious cause of systemic illness in horses. Mortality rates are as high as 40% in horses treated with antimicrobials, and systemic infections are fatal in horses that are not treated. Development of systemic disease involving the liver, lungs, kidneys, fetus, pericardium, mesentery, diaphragm, mediastinum, and uterus have been reported in horses from areas with endemic C. pseudotuberculosis disease. C. pseudotuberculosis infections are considered endemic in the western and southwestern regions of United States, especially California and Texas. In these regions, external subcutaneous abscess formation is the predominant form of disease. Recent outbreaks of C. pseudotuberculosis abscesses in Colorado, Wyoming, Utah, and Kentucky, where the disease was previously rare, suggests an expanding geographic distribution. Diagnosis of internal C. pseudotuberculosis infection can be difficult because of the insidious onset and nonspecific nature of clinical signs and clinicopathologic abnormalities. Although fewer than 10% of horses with C. pseudotuberculosis infection develop systemic disease, an increased index of suspicion is appropriate for systemically ill horses with a history of an external C. pseudotuberculosis abscess. Horses that reside on property where other horses have had external abscesses are considered at increased risk, and not all horses with systemic infections manifest external abscesses before becoming ill. Successful treatment and resolution of internal infection involves long-term antimicrobial treatment.


C. pseudotuberculosis is a gram-positive, pleomorphic, soil-borne bacterium with a worldwide distribution. The pathogenesis of C. pseudotuberculosis infection is incompletely understood, but insects are suspected to play a role as mechanical vectors. Haematobia irritans, Stomoxys calcitrans, and Musca domestica have been identified as potential vectors. It is speculated that the bacteria enter the horse through bites or abrasions in the skin with subsequent lymphatic or hematogenous dissemination. C. pseudotuberculosis has a cytotoxic surface lipid that facilitates intracellular survival and abscess formation. The bacterium produces phospholipase D exotoxin, which may promote spread of infection through increasing vascular permeability. The exotoxin may also enhance survival and multiplication of the organism via complement depletion and inhibitory effects on phagocytic cells.


There is no apparent breed or sex predisposition, although in one study of systemic infections in horses, a majority of study horses were mares, raising concern that mares may be more susceptible. The apparent sex predilection in that study may have been an artifact of the small number of horses in the study or a reflection of management practices. Horses living on pasture may have greater exposure to insect vectors and might not be as closely observed as stabled horses. The median age of horses with internal infections is 7 to 9 years, slightly older than for horses with external abscess formation only. External abscesses develop at greatest frequency from September through November in California. Detection of internal infections occurs slightly later, with peak occurrence from November though January. This apparent 2-month difference may represent a true delay in development of internal infections compared with external abscesses or may be a result of delayed owner recognition and seeking of veterinary care. Recognition of this systemic illness, especially in horses with concurrent external abscesses, can be difficult.


Systemic infection of C. pseudotuberculosis most commonly affects the liver, lower portion of the respiratory tract, and kidneys. Abortions are also associated with systemic C. pseudotuberculosis infection. Splenic infection has been suspected based on ultrasonographic findings but has not been definitively diagnosed with microbacterial culture. Most cases involve infection of a single organ system; however, more than one third of cases have two or more affected systems. Pneumonia, with infection of an abdominal organ system, usually the liver, is the most common combination of organ systems involved.



DIAGNOSIS



Clinical Signs


The most common clinical signs in horses with systemic C. pseudotuberculosis infections are nonspecific and include inappetence, lethargy, weight loss, respiratory signs and abdominal pain. Vital signs are variable depending on duration and severity of disease, organs affected and prior medical therapy. Horses tend to be mildly febrile, moderately tachycardic and mildly tachypneic. Most horses have a concurrent external C. pseudotuberculosis abscess and therefore also have ventral or limb edema. The presence of a concurrent external abscess may cause similar clinical signs, especially mild fever, and may therefore mask evidence of systemic infection and delay further diagnostic investigation.



Clinicopathologic Data


In one study, the most common clinicopathologic abnormalities were hyperproteinemia due to hyperglobulinemia and hyperfibrinogenemia and leukocytosis due to mature neutrophilia (Table 34-1). These findings are nonspecific for a chronic inflammatory or infectious process, and horses with external abscesses can have similar clinicopathologic findings. Most horses are not anemia, although some may have mild anemia of chronic disease. In some horses, mild anemia may be masked by dehydration. Low numbers of severely affected horses develop disseminated intravascular coagulation (DIC). Diagnosis of DIC is based on a combination of findings, including prolonged bleeding at venipuncture sites, petechial hemorrhages, thrombocytopenia, and abnormal coagulation test results (e.g., prothrombin time, partial thromboplastin time, fibrin degradation products, and antithrombin III activity). DIC develops secondary to many systemic disease conditions and septic processes and is generally associated with a poor prognosis.



Table 34-1 Representative Values of Abnormal Clinicopathologic Findings in Horses with Internal Corynebacterium pseudotuberculosis Infection

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The serum synergistic hemolysin inhibition (SHI) test is a reliable aid in the diagnosis of systemic C. pseudotuberculosis infections. Horses with internal infection have reciprocal serum titers greater than 512 (see Table 34-1), although rare false-negative results may occur. Horses having only external abscesses may also have serum SHI titers of 512 or greater, but 40% will not have a high titer. The SHI test is useful in identifying C. pseudotuberculosis as the cause of systemic illness, but a complete clinical evaluation is necessary to establish evidence for an internal infection in horses with concurrent or recently resolved external abscesses.

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Systemic Illness Caused by Corynebacterium pseudotuberculosis

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