Coinfection of Severe Fever with Thrombocytopenia Syndrome and Scrub Typhus in Patients with Tick-Borne Illness

Abstract.

Severe fever with thrombocytopenia syndrome (SFTS) and scrub typhus are the most common tick-borne diseases in South Korea. However, few studies have systematically examined the simultaneous presence of the two diseases. We found that two (4.9%) of 41 patients with suspected and confirmed SFTS had evidence of coinfection with scrub typhus. In addition, two (3.6%) of 55 suspected and confirmed scrub typhus patients were identified to have coinfection with SFTS. Our data suggest that diagnostic evaluation for coinfection in patients with tick-borne illness and empirical doxycycline treatment in patients with SFTS may be warranted in areas endemic for both diseases until coinfection with scrub typhus is ruled out.

Severe fever with thrombocytopenia syndrome (SFTS) and scrub typhus are the most common tick-borne diseases in South Korea.^1,2 Severe fever with thrombocytopenia syndrome virus (SFTSV) is a novel phlebovirus (now renamed Huaiyangshan banyangvirus) in the genus Banyangvirus, family Phenuiviridae, and order Bunyavirales,³ and was first reported in China^4,5 in 2011, and in South Korea⁶ and Japan⁷ in 2013–2014. Since then, hundreds of cases have been reported annually in South Korea.⁸ Ticks such as Haemaphysalis longicornis have been identified as major vectors for SFTSV.⁹ Scrub typhus is caused by Orientia tsutsugamushi, which is transmitted by chigger mites such as Leptotrombidium species, and thousands of cases are reported annually in South Korea.⁸

Severe fever with thrombocytopenia syndrome and scrub typhus have similar clinical manifestations: fever, myalgia, and gastrointestinal symptoms. Moreover, the two diseases have overlapping areas of prevalence and similar risk factors, such as history of outdoor activities, farming, exposure to sward, and occasionally tick or chigger bite.^5,10 In addition to the difficulty in distinguishing between SFTS and scrub typhus, coinfections are also a matter of concern in real clinical practice, in relation to laboratory evaluation and empirical doxycycline use. One case of coinfection with SFTS and scrub typhus was reported recently,¹¹ and another study found that 17 (23%) of 74 patients with initially suspected scrub typhus were eventually diagnosed with SFTS and seven (41%) of these had high O. tsutsugamushi antibody titers.¹² However, coinfections have not been systematically evaluated. We therefore investigated the frequency of coinfection of SFTS and scrub typhus.

All adult patients aged ≥ 18 years with suspected tick-borne illness in the Asan Medical Center, a 2,700-bed tertiary hospital in Seoul, South Korea, between July 2015 and October 2018 were enrolled. Of these patients, those who were diagnosed with SFTS or scrub typhus and whose blood plasma was available for further analysis were finally analyzed. Severe fever with thrombocytopenia syndrome was confirmed by detecting SFTSV RNA by reverse transcription–polymerase chain reaction (RT-PCR) analysis of a plasma sample, using a DiaStar 2× OneStep RT-PCR Pre-Mix kit (SolGent, Daejeon, South Korea), as described previously.¹³ To diagnose scrub typhus, we performed immunofluorescence assays (IFAs; SD Bioline Tsutsugamushi Assay; Standard Diagnostics, Yongin, South Korea) and real-time PCR for O. tsutsugamushi in plasma samples, as described previously.¹⁴ A diagnosis of scrub typhus was established if there was 1) a single positive IgG result (titer ≥ 1:320) or a 4-fold rise in IgG in serial samples by IFA,¹⁵ or 2) a positive real-time PCR result for O. tsutsugamushi.¹⁶ The study protocol was approved by the Institutional Review Board of the Asan Medical Center.

A total of 44 and 107 patients with suspected tick-borne diseases were confirmed to have SFTS and scrub typhus, respectively, during the study period. Most patients with confirmed SFTS and those confirmed as having scrub typhus were initially suspected to have the corresponding diseases. Of the 44 patients with confirmed SFTS, 41 (93.2%) whose plasma samples were available for further analysis, with 20 paired samples and 21 single samples, underwent scrub typhus IFA. In addition, 32 patients whose plasma samples were available for further molecular analysis underwent scrub typhus PCR. Of these 41 patients, two (4.9%) patients had evidence of coinfection of scrub typhus. During the same study period, 107 patients met the aforementioned serologic criteria and were confirmed to have scrub typhus. Of these 107 patients with scrub typhus, 55 (51.4%) whose plasma samples were available for further molecular analysis underwent SFTSV RT-PCR. Of these 55 patients, two (3.6%) patients had evidence of coinfection of SFTS. Detailed clinical characteristics and laboratory data of these four patients with coinfection are shown in Table 1. One was a 62-year-old woman (Patient 1) diagnosed with SFTS in August 2018 whose O. tsutsugamushi IgG titer increased from ≥ 1:640 at admission to ≥ 1:5120 19 days later and who was positive for O. tsutsugamushi PCR. Another was a 64-year-old woman (Patient 2) diagnosed with SFTS in May 2016 whose acute plasma sample gave a positive scrub typhus PCR outcome; however, a subsequent single plasma sample IFA gave a negative result. Another was an 81-year-old woman (Patient 3) who was diagnosed with scrub typhus in November 2015 based on an initial high O. tsutsugamushi IgG titer (1:2,560) along with a positive O. tsutsugamushi PCR and was eventually diagnosed with SFTS based on a positive SFTSV RT-PCR result, also from her plasma sample. The last was a 73-year-old man (Patient 4) diagnosed with scrub typhus in November 2016 whose O. tsutsugamushi IgG titer increased from ≥ 1:40 at admission to ≥ 1:2,560 5 days later and was also eventually diagnosed with SFTS based on a positive SFTSV RT-PCR.

Table 1.

Baseline characteristics and outcomes of the four coinfected patients

Variables	Severe fever with thrombocytopenia syndrome–suspected and confirmed patients		Scrub typhus–suspected and confirmed patients
	Patient 1	Patient 2	Patient 3	Patient 4
Season	Summer	Spring	Fall	Fall
(Month)	(August)	(May)	(November)	(November)
Age (years)	62	64	81	73
Gender	Female	Female	Female	Male
Underlying disease	HTN	None	Diabetes	HTN
	Diabetes	–	–	Primary biliary cirrhosis
	Asthma	–	–	–
Symptom duration before hospital visit (days)	6	6	10	5
Clinical manifestation
Fever, febrile period (days)	Yes (4)	Yes (5)	Yes (2)	Yes (4)
Tick- or chigger-bite wound	None	None	Yes	None
Skin rash	None	None	Yes	Yes
Bleeding events	None	None	None	None
Myalgia	Yes	Yes	None	None
Lymphadenopathy	None	None	None	None
General weakness	Yes	Yes	None	Yes
Nausea/vomiting	Yes	Yes	None	None
Abdominal pain	None	None	None	None
Diarrhea	Yes	None	None	None
Cough/sputum/rhinorrhea	Yes	None	Yes	Yes
Headache	None	Yes	None	None
Altered mental status	Yes	Yes	None	None
Initial laboratory data
White blood cell count (/μL)	1,200	1,800	11,800	9,800
Neutrophils (%)	60	64	83	80
Lymphocytes (%)	36	33	13	13
Monocytes (%)	4	2	4	7
Platelet count (/μL)	68,000	62,000	122,000	121,000
Hemoglobin (g/dL)	14	13	12	12.6
Prothrombin time international normalized ratio	1.08	0.96	1.03	1.09
Activated partial thromboplastin time (seconds)	46	39	43	31
C-reactive protein level (mg/dL)	1.06	0.2	8.5	6.2
BUN (mg/dL)	38	24	21	18
Creatinine (mg/dL)	1.69	1.13	0.8	1.09
Estimated glomerular filtration rate (mL/minutes/1.73 m2)	32	48	69	66
Aspartate aminotransferase (IU/L)	329	121	60	61
Alanine aminotransferase (IU/L)	105	38	49	58
Alkaline phosphate (IU/L)	58	46	45	182
Total bilirubin (mg/dL)	0.3	0.2	0.3	0.5
Creatinine kinase (IU/L)	1,431	2,339	89	245
Lactate dehydrogenase (IU/L)	1,641	1,837	493	356
Myoglobin (IU/L)	885	348	None	None
Central nervous system involvement	Yes	Yes	None	None
Lung involvement	Yes	Yes	Yes	Yes
Shock	Yes	None	None	None
Clinical course
Intensive care unit admission	Yes	Yes	None	None
Mechanical ventilation	Yes	None	None	None
Inhospital death	None	None	None	None
Hospital stay (days)	43	15	4	6
Treatment
Doxycycline	Yes	Yes	Yes	Yes
Ribavirin	None	Yes	None	None
Supportive care	None	None	None	None
Plasmapheresis	Yes	Yes	None	None
Convalescent plasma therapy	None	None	None	None
Azithromycin	None	None	None	None
Severe fever with thrombocytopenia syndrome virus reverse transcription–polymerase chain reaction (log copies/μL)	Positive (3.2)	Positive (4.4)	Positive (1.0)	Positive (1.2)
Scrub typhus PCR (log copies/μL)	Positive (0.54)	Positive (0.51)	Positive (0.94)	Negative
Scrub typhus gene sequencing results	Orientia tsutsugamushi strain Nishino	Orientia tsutsugamushi strain Inha-B697253-1	Orientia tsutsugamushi strain Boryong	Negative scrub typhus PCR result
Scrub typhus IgG titer initially	1:640	Negative	1:2,560	1:40
Scrub typhus IgG titer in paired sera	1:5,120	Negative	None	1:2,560

HTN = hypertension; PCR = polymerase chain reaction.

A previous Korean study¹² found that as many as seven (41%) of 17 patients with SFTS who were initially suspected of having scrub typhus had high O. tsutsugamushi antibody titers (1:2,560). However, they did not systemically evaluate coinfections with SFTS and scrub typhus, and it is difficult to rule out the possibility that a single high titer of scrub typhus antibody is the result of past or recent infection. Actually, to our knowledge, there is only one case report of coinfection with SFTS and scrub typhus based on molecular evidence for both diseases.¹¹ Because different vectors are involved in SFTS and scrub typhus, we thought that the true coinfection rate might be lower than 41%. We, therefore, systemically evaluated the frequency of coinfection of SFTS and scrub typhus using serologic tests and molecular analysis. We found that two (4.9%) of 41 suspected and confirmed SFTS patients had evidence of coinfection of scrub typhus, and two (3.6%) of 55 suspected and confirmed scrub typhus patients were identified to have coinfection with SFTS. These findings provide important information in terms of further diagnostic tests for SFTS in patients with scrub typhus and empirical doxycycline use in patients with SFTS. However, further studies are needed to elucidate questions concerning the mechanism of coinfection, such as whether it involves bites by different vectors or a bite from a common vector. We note that in China, SFTSV has been detected by RT-PCR in Leptotrombidium scutellare, a vector for scrub typhus.¹⁷

A rapid diagnosis of SFTS, with relatively high sensitivity, is usually made by RT-PCR for SFTSV in blood samples. However, for scrub typhus, rapid molecular diagnosis has some limitations in routine clinical use due to the relatively low bacterial loads in sera. Some workers have proposed performing PCR from buffy coat samples, but the preparation of such samples requires technical expertise. Therefore, a diagnosis of scrub typhus usually depends on serologic examinations such as IFA. However, a single IFA result does not support a definitive diagnosis, and a dynamic change in the IFA titer between paired sera is required.¹⁸ In this context, our data suggest that empirical doxycycline use is warranted until laboratory evidence rules out coinfection.

This study has some limitations. First, about half of the patients with confirmed SFTS did not undergo paired scrub typhus IgG serology tests because paired samples had not been stored or because they were not collected initially. In addition, about half of the patients with confirmed scrub typhus did not undergo further SFTSV RT-PCR because samples had not been available. Therefore, some selection bias may have been introduced. Second, in some patients, scrub typhus PCR was performed after doxycycline administration and may have yielded false-negative results. Third, some may be confused with our previous data¹⁴ reporting one false-positive PCR result (Patient 2 in Table 1 in the current study) for scrub typhus in 15 SFTS patients and one false-positive RT-PCR result (Patient 4 in Table 1 in the current study) for SFTS in 21 scrub typhus patients. Actually, the 41 patients with SFTS in the current study included the 15 patients with SFTS enrolled in the previous study,¹⁴ and the 55 patients with scrub typhus in the current study included 21 patients with scrub typhus enrolled in the previous study.¹⁴ However, we have described these findings as false-positive scrub typhus PCR and false-positive SFTSV RT-PCR with the specificity of 95% for scrub typhus PCR and 95% for SFTSV RT-PCR, respectively, because we did not have the concept of coinfection at the time of our previous writing. Fourth, some may argue the challenges in diagnosing scrub typhus using serologic methods. Although we used the serologic diagnostic criteria according to the 2007 Korea Center for Disease Control and Prevention guidelines,¹⁵ a single high IFA titer is insufficient for a definite diagnosis because a single positive IFA result may suggest past infection with scrub typhus.¹⁹ So, further well-designed studies with more strict serologic criteria for scrub typhus will reveal the true prevalence of coinfection in patients with suspected tick-borne illness. Last, the theoretical possibility of crossreactivity between the antibodies to SFTS and scrub typhus should be mentioned. However, to our knowledge, there are no reports of such crossreactivity.

In conclusion, our data suggest that about 3–4% of SFTS and scrub typhus patients are coinfected with scrub typhus and SFTS, respectively. Therefore, in areas endemic to both diseases, empirical doxycycline treatment may be warranted until coinfection with scrub typhus is ruled out in SFTS patients, and further diagnostic tests for SFTS in scrub typhus patients are necessary.