A Patient with Severe Fever with Thrombocytopenia Syndrome, Activated Partial Thromboplastin Time Prolongation, and Transient Antiphospholipid Antibodies

Abstract

A prolonged activated partial thromboplastin time (APTT) is observed in patients with severe fever with thrombocytopenia syndrome (SFTS) and is one of the risk factors for severe disease. The mechanism underlying a prolonged APTT is largely unknown. The presence of antiphospholipid (aPL) antibodies in various viral infections has been documented but never reported in a patient with SFTS. We herein report the first SFTS patient with APTT prolongation and concurrent transiently positive aPL antibodies (lupus anticoagulants and anticardiolipin antibodies) with no coagulation factor deficiency.

Introduction

Severe fever with thrombocytopenia syndrome (SFTS) is a tickborne emerging hemorrhagic fever caused by Dabie bandavirus, also called SFTS virus, which belongs to the genus Bandavirus (formerly called Phlebovirus) in the family Phenuiviridae (formerly Bunyaviridae). SFTS was first reported in China in 2009 (1,2), and since then, SFTS patients have been identified in several other countries (mainly Japan and South Korea, plus Vietnam, Taiwan, Myanmar and the US) (3-5). In Japan, the annual number of reports of SFTS patients has ranged from 40 to 83 cases (6) and the overall mortality of SFTS patients in Japan and China has ranged from 6.3% to 30.0% (3,6).

One of the characteristics and indicators of severe disease in SFTS patients is a prolonged activated partial thromboplastin time (APTT) without prolongation of the prothrombin time (PT) (7-9). It was reported that SFTS patients with APTT prolongation ≥60 seconds had a 4-fold higher mortality rate than those with less marked prolongation (8). Hemorrhagic manifestations resulting from thrombocytopenia and prolonged APTT, including gastrointestinal bleeding and pulmonary hemorrhage, and multiple organ dysfunction resulting from cytokine storm are reported as the cause of death (7,8). Hyponatremia and increased serum creatinine are additional prognostic factors for critical outcomes (7,8,10). However, the mechanisms underlying the prolongation of the APTT in SFTS patients remain largely unknown.

We herein report our treatment of an SFTS patient with a prolonged APTT and transiently positive antiphospholipid (aPL) antibodies (lupus anticoagulants and anticardiolipin antibodies).

Case Report

A 77-year-old previously healthy Japanese woman visited a hospital after a 2-day course of fatigue and diarrhea. She lived in a mountainous area in a western part of Japan and worked in agricultural fields. She was taking no regular medication, including anticoagulants, and had no history of thrombosis, bleeding, or miscarriage. At her admission to the hospital, her body temperature was 38°C, and she had difficulty moving by herself. A tick was found on her right thigh, and reverse transcription polymerase chain reaction (PCR) for Dabie bandavirus in her blood showed positive findings. The diagnosis of SFTS was confirmed, and she was transferred to our hospital on Day 5.

Her height and weight were 135 cm and 32.4 kg, respectively. She was ill-appearing, and vital signs were remarkable: respiratory rate 20 breaths per minute, oxygen saturation (SpO₂) 97% on 1 L per minute supplemental oxygen with a nasal cannula, body temperature 37.8°C, and Glasgow Coma Scale E3V4M6. Her circulation was stable. Her physical examination did not reveal a rash, petechiae, lymphadenopathy, mucosal bleeding, bruises or edema.

The initial laboratory investigation showed significant leukopenia and thrombocytopenia [white blood cell (WBC) count 1.9×10⁹/L, neutrophils 1.7×10⁹/L, lymphocytes 0.15×10⁹/L, hemoglobin 13.5 g/dL, mean corpuscular volume 81 fL, platelet count 76×10⁹/L, immature platelet fraction (IPF) 9.4% (reference range (ref.) 0-6.8%), mean platelet volume 11.9 fL (ref. 10.2-11.5 fL), platelet distribution width 14.7 fL (ref. 12.3-15.2 fL)]. Elevated IPF did not suggest bone marrow suppression, so we concluded that the leukopenia and thrombocytopenia were caused by SFTS.

The following findings were also abnormal: coagulation screen findings [PT 0.99, international normalized ratio (INR), APTT 74.8 seconds, fibrinogen 260 mg/dL, fibrin degradation products (FDP) 13.1 μg/mL, D-dimer 5.8 μg/mL] and the liver function [aspartate transaminase (AST) 77 U/L, alanine transaminase (ALT) 32 U/L, and lactate dehydrogenase (LDH) 379 U/L]. C-reactive protein was slightly elevated at 0.34 mg/dL, and electrolytes (sodium 130 mmol/L, potassium 3.4 mmol/L, chloride 96 mmol/L) were slightly low. The patient's creatine kinase (CK; 133 U/L) and kidney function (serum creatinine 0.50 mg/dL) were within normal limits.

To determine the cause of the prolonged APTT with a normal PT, we performed mixing tests on Day 2 of the patient's admission at our hospital. Mixing tests are a way to investigate the cause of an abnormal coagulation screen result, especially a prolonged APTT with a normal PT. In this test, we mixed the patient's plasma with normal plasma to assess whether the cause of the coagulation abnormality was due to coagulation factor deficiency or inhibitor.

The patient's APTT mixing test did not show a typical pattern in the immediate mix, but it demonstrated an approximately straight pattern and a longer APTT in the incubated mix (Fig. 1). This finding suggested the presence of coagulation factor inhibitors, including aPL antibodies (11). The index of circulation anticoagulant (ICA) was calculated to interpret the result; it was 13%, which was inconclusive (12). On Day 5 we measured coagulation factor activities that are involved in only an intrinsic pathway. The results are summarized in Table 1. The coagulation factor activities were not decreased, so we considered aPL antibodies as a possible alternative cause of the patient's prolonged APTT. Lupus anticoagulants [dilute Russell Viper Venom Time (dRVVT)] and anticardiolipin immunoglobulin G (IgG) antibodies were positive, and anti-β2-glycoprotein I (β2-GPI) IgG antibodies were negative (Table 2). The APTT reagent used for lupus anticoagulant test was HemosIL^Ⓡ dRVVT (Werfen, Barcelona, Spain).

Figure 1.

The results of the activated partial thromboplastin time (APTT) mixing test. The incubation time for the incubated mix was two hours.

Table 1.

The Result of Coagulation Factor Activity Testing (Intrinsic Pathway).

Activity	Day 5	Day 100	Ref. range
Factor VIII	>120%	>120%	60-140%
Factor IX	72%	101%	60-140%
Factor XI	84%	72%	73-136%
Factor XII	94%	84%	46-156%

Table 2.

The Transition of Antiphospholipid Antibody Titers and APTT.

	Day 13	Day 100	Ref. range
Lupus anticoagulant, dRVVT	1.4	1.3	<1.3
Anticardiolipin IgG antibody	12.6 U/mL	8.4 U/mL	<10
aβ2GPI IgG antibody	<1.2 U/mL	<1.2 U/mL	<3.5
APTT	71.4 s	36.6 s	24-34

dRVVT: dilute Russell viper venom time, IgG: immunoglobulin G, aβ2GPI: anti-beta2-glycoprotein I, APTT: activated partial thromboplastin time

Disseminated intravascular coagulation (DIC) has also been described in SFTS patients (9). Although this patient did not meet the diagnostic criteria for DIC, the increase in the FDP and D-dimer values in our patient may have been related to a hypercoagulable state resulting from SFTS. FDP and D-dimer levels decreased to 4.2 μg/mL and 1.8 μg/mL, respectively, on Day 6 without anticoagulation therapy.

The patient received conservative treatment. She did not require blood transfusion. The lowest WBC and platelet count values were 1.9×10⁹/L on Day 2 and 38×10⁹/L on Day 4. Her fever resolved to normal temperature on Day 6. She recovered gradually and was transferred to the previous hospital on Day 16, when her WBC count was 5.4×10⁹/L, neutrophils 3.6×10⁹/L, lymphocytes 1.6×10⁹/L, hemoglobin 10.8 g/dL, mean corpuscular volume 87 fL, platelet count 241×10⁹/L and APTT still prolonged at 52.8 seconds (Fig. 2). The changes in the serum viral load of D. bandavirus are shown in Table 3.

Figure 2.

The changes in the patient's platelet count and activated partial thromboplastin time (APTT) during her hospitalization.

Table 3.

The Result of Dabie bandavirus Viral Load.

Day	1	5	12
Dabie bandavirus	25,492 copies/	3,881 copies/	Undetectable
(SFTS virus) viral load	5 μL of RNA	5 μL of RNA

SFTS: severe fever with thrombocytopenia syndrome

We followed the patient's coagulation screen, aPL antibodies and coagulation factor activities 100 days after her admission. The aPL antibody status turned negative, and the PT (INR) and APTT also became almost normal at 1.1 and 36.6 seconds, respectively. She did not meet the diagnostic criteria for antiphospholipid syndrome because she had not had any thrombosis or pregnancy-related morbidities in the past, and the positive aPL antibodies were transient (13).

Discussion

SFTS is a potentially life-threatening disease, and its bleeding tendency is caused by both thrombocytopenia and coagulation disorder. Suggested causes of thrombocytopenia are circulating platelet antibodies and phagocytosis in spleen (3), but the mechanism underlying coagulation disorder, including APTT prolongation with normal PT, is not known. DIC and inflammatory cytokines are potential causes (3,9), but prolonged APTT without PT prolongation is not typical. Our patient had SFTS with APTT prolongation and transiently positive lupus anticoagulants and anticardiolipin antibodies. The presence of aPL antibodies is also observed in other viral infections, such as hepatitis C virus, human immunodeficiency virus, cytomegalovirus, varicella zoster virus, Epstein-Barr virus, adenovirus, parvovirus B19, and severe acute respiratory syndrome coronavirus 2, but it has never been reported in D. bandavirus (SFTS virus) infection, to our knowledge (14-17). It has been postulated that aPL antibodies may be induced by an immune response to β2GPI-like phospholipid-binding viral and bacterial products in infection (14,17,18). This is the first report of the coexistence of aPL antibodies and a prolonged APTT in a patient with SFTS.

APTT prolongation usually suggests a coagulation factor deficiency or the presence of antibodies for coagulation factors, such as aPL antibodies. One report described an 85-year-old man diagnosed with SFTS who showed factor XI deficiency in addition to APTT prolongation at 50.5 seconds (19). If the prolongation of a patient's APTT is due to coagulation factor deficiencies, plasma-derived or recombinant coagulation factors can be candidates for the management of the patient's bleeding tendency (20,21). We therefore investigated the possibility of coagulation factor deficiency, but our SFTS patient had no such abnormality. In her case, lupus anticoagulants and anticardiolipin antibodies were weakly positive at the diagnosis but turned negative three months later. She did not have any history of thrombotic events or spontaneous abortion. She therefore did not meet the diagnostic criteria for antiphospholipid syndrome (APS) (revised Sapporo criteria) (13). While SFTS patients can develop bleeding tendencies, thrombotic events are not common, and this patient also did not have thrombosis.

Various implications concerning the identification of aPL antibodies have been considered. A systematic review classified patients with positive aPL antibodies into three groups: (i) those who met the diagnostic criteria of APS; (ii) those who developed thromboembolic manifestations but did not meet the APS diagnostic criteria, e.g. patients with temporary positivity for aPL antibodies or an inadequate follow-up period; and (iii) patients with transiently elevated aPL antibodies without thromboembolic or pregnancy-related events (16). Our patient's case would fall into the third group, but this does not mean she had thrombophilia. In addition, to our knowledge, thrombosis has not been reported as a complication in SFTS patients. The clinical importance of aPL antibodies in this group therefore remains unclear. Nevertheless, since the results of our patient's mixing test indicated the presence of aPL antibodies with no evidence of a coagulation factor deficiency, we cannot deny the possibility that the patient's APTT prolongation may have been due to lupus anticoagulants. Although APTT prolongation and positive lupus anticoagulants were clinically significant, the titer of lupus anticoagulants was weak. Therefore, the mechanism underlying the bleeding tendency and prolongation of APTT in SFTS may be multifactorial.

Conclusion

We described the first case of SFTS with a prolonged APTT and transiently positive aPL antibodies (lupus anticoagulants and anticardiolipin antibodies). Further prospective studies and case series will be necessary to accumulate clinical data of SFTS cases with APTT prolongation and aPL antibodies, which may lead to a deeper understanding of the pathogenesis of SFTS and better management of SFTS patients.

The authors state that they have no Conflict of Interest (COI).