Conference 01 - 2015 Case: 03 20150909

Signalment:

4 month old Dorset cross ewe (Ovis aries)As part of an IACUC-approved experimental protocol, this lamb had a long term IV catheter in the right jugular vein. Catheter care included frequent bandage changes and cleaning of the catheter site with a chlorhexidine based solution. The catheter was replaced several times during the study, with aerobic bacterial cultures of the catheter tip when removed.

Gross Description:

At necropsy, the right jugular catheter was encased in a thick coat of fibrin with enmeshed erythrocytes, which merged into the jugular wall (throm-bophlebitis, figure 3-1). In the right caudal lung lobe, an adherent fusiform shiny tan thromboembolus, approximately 4 x 4 x 30 mm, largely occluded the lumen of the large pulmonary artery (figure 3-2).

Histopathologic Description:

Submitted tissue is right jugular vein (cross-section, opened longitudinally) and adjacent soft tissues. Adherent to the intima are two large nodular organizing septic thrombi, one with a central ovoid defect (catheter sheath). The two thrombi occlude >50% of the lumen. The thrombi contain large numbers of predominantly degenerate neutrophils and karyorrhectic debris as well as bacterial rods, often forming discrete lamellations separated by fibrin with abundant enmeshed erythrocytes. A Brown and Hopps tissue Gram stain identifies the rods as gram- negative, and they are often present in very large numbers. At the peripheral margins of both thrombi there is ingrowth of plump fibroblasts with scant immature collagenous matrix as well as new small caliber blood vessels lined by plump endothelium (angiogenesis). There is partial and incomplete re-endothelialization of both thrombi. The venous wall is largely effaced by fibroplasia, with extension into adjacent adnexa. In some sections there is focally extensive recent hemorrhage in the venous media. Histology of the pulmonary artery thromboembolus (not submitted) was similar, including central canal.

Morphologic Diagnosis:

Jugular vein: Thrombophlebitis, chronic-active, focally extensive, severe with Gram negative rods.

Lab Results:

There were multiple negative blood cultures and catheter cultures during the study (duration approximately 90 days). Two weeks prior to necropsy, the removed catheter grew a heavy pure culture of Pseudomonas aeruginosa. Additional cultures of blood surfaces at necropsy were positive for heavy pure growth of P. aeruginosa.

Condition:

Pseudomonas aeruginosa thrombophlebitis

Contributor Comment:

Inflammation or infection of the venous wall (phlebitis) is frequently complicated by thrombosis. Thrombosis results from antemortem intravascular coagulation, and must be differentiated from post-mortem clotting.(2,5) Predisposing factors are described by the classic Virchows triad of endothelial damage, turbulence or stasis of flow, and hypercoagulability. In this case, the presence of the catheter may potentially cause endothelial damage by direct physical injury, as well as turbulence by interrupting laminar flow. Both inflammation as well as endotoxin from gram-negative bacteria may activate the coagulation cascade. Resolution of thromboemboli can occur by thrombolysis, organization (fibrosis and contracture) with re-endothelialization, and/or re-canalization.(2)

Jugular thrombophlebitis can be a significant problem in ruminants, both related to catheterization as well as perivascular administration of irritating solutions (such as 5% dextrose for ketosis or calcium gluconate for milk fever). Broken off fragments from the jugular site were able to travel to the right heart before lodging in a pulmonary artery (thromboembolism). The distinction between bland and septic thromboemboli can be of critical importance, as the latter may give rise to additional foci of infection, including embolic pneumonia or nephritis.

The inherent resistance of Pseudomonas aeruginosa to chlorhexidine-based disin-fectants is well-documented.(3,4) Pseudomonas aeruginosa is a normal inhabitant of water systems, and is therefore nearly ubiquitous in distribution. The organism is a significant cause of hospital acquired infections, often with a poor prognosis related both to the resistance of the organism to treatment as well as comorbidities in the patients. Pseudomonas aeruginosa colonization of medical devices is facilitated by pili and fimbriae as well as biofilm formation, the latter making an-tibiotic treatment unrewarding.(1) Single blood cultures are frequently negative in cases of bacteremia, and frequent repeated large volume blood cultures have the best success.

JPC Diagnosis:

Jugular vein: Thrombophlebitis, fibrinosuppurative, chronic-active, focally extensive, with marked mural granulation tissue.

Conference Comment:

Conference participants were impressed by the level of detail within the organizing thrombus, including the presence of lines of Zahn. These are more characteristically present in arterial thrombi and refer to the laminated appearance of the thrombus due to alternating layers of platelets and fibrin with enmeshed erythrocytes and leukocytes.(2) Most participants misinterpreted the vessel as an artery due to the thickness of the wall. Although they did not see an internal elastic lamina to confirm the vessel as an artery, most believed the vessel wall was too expanded by mural granulation tissue to allow its visualization. Participants described the clear area within the center of the larger thrombus but did not readily associate it with a catheter, and most agreed that while they considered the possibility of bacteria within the lesion, the abundant karyorrhectic debris made identification of microorganisms exceedingly difficult.

Hemostasis was reviewed in detail during the conference including a discussion of initiating events. While turbulent blood flow, a component of Virchows triad, is clearly present with the placement of an intraluminal catheter, endothelial injury is also a very important factor in the formation of a thrombus. Endothelial injury results in the exposure of tissue factor, and other subendothelial components such as collagen, resulting in platelet aggregation and the initiation of coagulation. The release of tissue factor, located within the plasma membrane of activated endothelium, results in the initiation of the extrinsic coagulation pathway; the intrinsic pathway is initiated by collagen and other subendothelial com-ponents coming into contact with pre-kallikrein, high molecular weight kininogen and factors XII and XI (contact group of coagulation factors).(2) When tissue factor comes into contact with factor VII, it forms a complex which along with calcium, activates factor X to initiate the common pathway. While conceptually it is easier to learn and discuss the coagulation cascade as two separate pathways which combine to form the common pathway, the in vivo process is more commonly considered a single intertwined set of events that begins with the exposure of tissue factor. (2)

Participants reviewed the basic steps in the hemostatic process starting with vasoconstriction and formation of a platelet plug, followed by coagulation and formation of a fibrin meshwork, followed by fibrinolysis and finally tissue repair, and the role of platelets in this process was discussed. Platelets adhere to exposed subendothelial collagen and von Willebrands factor is released by local activated endothelium, which provides a more secure connection between the collagen and platelets via platelet receptor GPIb. Platelets then release the contents of their α-granules and produce other mediators which continue to promote hemostasis. The release of adenosine diphosphate (ADP) results in the binding of fibrinogen to platelet receptor GPIIb-IIIa, and the fibrinogen forms a scaffold as the platelets aggregate, eventually covering the defect. Factors released from the aggregated platelets, such as platelet derived growth factor, stimulate fibroblast recruitment which can eventually result in fibrosis at the thrombus location, as occurred in this case. (2)

References:

1. Laverty G, Gorman SP, Gilmore BF: Biomolecular Mechanisms of Pseudomonas aeruginosa and Escherichia coli Biofilm Formation. Pathogens 2014:3(3):596-632.

2. Mosier DA: Vascular Disorders and Thrombosis. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. Fourth ed. St. Louis, Mo: Mosby Elsevier; 2007: 61-87.

3. Nakahara H, Kozukue H: Isolation of chlorhexidine-resistant Pseudomonas aeruginosa from clinical lesions. J Clin Microbiol 1982:15(1):166-168.

4. Oie S, Kamiya A: Microbial contam-ination of antiseptics and disinfectants. Am J Infect Control 1996:24(5):389-395.

5. van Vleet JF, Ferrans VJ: Cardiovascular System. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. Fourth ed. St. Louis, MO: Mosby Elsevier; 2007: 559-611.