Chilblain‐like lesions during the COVID‐19 pandemic: a serological study on a case series

Dear Editor, The outbreak of chilblain‐like lesions (CLL) coincidentally to the COVID‐19 pandemic is a topic of great concern.¹–⁶ SARS‐CoV‐2 has been hypothesized as the aetiological agent of CLL, on the basis of the temporal correlation between the ‘burst’ of skin manifestations and the viral pandemic. However, the relationship between CLL and COVID‐19 remains unclear, as researchers have failed to confirm the SARS‐CoV‐2 infection in these patients using real‐time reverse‐transcription polymerase chain reaction (RT‐PCR) from nasopharyngeal swabs, which seldom tested positive.¹–⁵ Indeed, the most recent articles on the prevalence of the SARS‐CoV‐2 infection in CLL reported a positive swab test in only about one‐fifth of these patients.⁴, ⁵ In a few cases, serological tests were used, but these initially available tests were largely unreliable, as they had been rapidly developed and placed on the market with limited validation on clinical samples.

We collected data on 16 patients presenting with CLL, who were all negative to the nasopharyngeal swab. We then decided to recall our patients to perform a recently developed rapid chromatographic immunoassay (RCI) for the qualitative detection of specific antibodies to SARS‐CoV‐2, using the SD Biosensor COVID‐19 IgM/IgG Duo assay (SD Biosensor Inc., Yeongtong‐gu, South Korea). The SD Biosensor’s package insert claims a sensitivity of 99·10% and a specificity of 95·09%, established in a retrospective, multicentre, randomized, single‐blinded study on 504 specimens, if used more than 14 days after the onset of symptoms. In total, 12 of our patients agreed to undergo the RCI test; their characteristics are summarized in Table 1. All of them were tested at least 14 days after presentation of skin disease (mean value 51·3 ± 25·8), according to the manufacturer’s instructions. In 11 of 12 the presence of IgM or IgG was not detected. The sole patient with positive IgG was the only one with a history of exposure to a confirmed infected cohabitant. Four patients were also tested for other common viral infections, without significant results.

Table 1.

Age, sex, disease, onset of symptoms, details of tests and history of exposure in the described population

Pt	Age (years)/sex	Areas of involvement		Date of onset	Interval (days)a	SSR	Serology	History of exposure	Other viruses tested
		Hands	Feet
1	19/F	No	Yes	11 March	70	Neg	Neg	–	–
2	15/F	No	Yes	8 March	72	Neg	Neg	Schoolmate with COVID‐19 confirmed by swab	CMV Neg, EBV latent infection
3	16/M	Yes	No	1 April	55	Neg	Neg	Son of a nurse	–
4	14/F	No	Yes	27 March	48	Neg	Neg	Nurse’s daughter (nurse tested negative)	CMV Neg, PB19 Neg, EBV latent infection, enterovirus Neg
5	12/F	Yes	Yes	8 April	15	Neg	Neg	Police officer’s daughter	CMV Neg, PB19 Neg, EBV latent infection; enterovirus: IgG Neg, IgM undefined results
6	15/M	No	Yes	15 April	36	Neg	Neg	Public health employee’s son	Enterovirus Neg
7	15/M	No	Yes	27 March	14	Neg	IgM (Neg), IgG (Pos)	Sister with COVID‐19 confirmed by swab	–
8	12/F	No	Yes	27 February	81	Neg	Neg	–	–
9	12/M	No	Yes	10 February	90	Neg	Neg	–	–
10	9/F	Yes	No	4 May	18	Neg	Neg	Both parents are nurses (tested negative)	–
11	11/F	No	Yes	3 April	55	Neg	Neg	Sister of patient 12	–
12	12/F	No	Yes	28 March	61	Neg	Neg	Sister of patient 11	–

CMV, cytomegalovirus; EBV, Epstein–Barr virus; Neg, negative; PB19, Parvovirus B‐19; Pos, positive; Pt, patient; SSR, SARS‐CoV‐2 swab result (polymerase chain reaction). ^aInterval between onset of disease and serological test.

This is the first study in which patients with CLL have been investigated with both RT‐PCR nasopharyngeal swab and serological RCI, after a sufficient time lapse to develop antibodies against SARS‐CoV‐2. Indeed, higher levels of IgM and IgG have been found in the second and third weeks of illness, then IgM begins to decline and almost disappears by week 7, while IgG persists. Their long‐term persistence still remains unknown, although data suggest a serological profile similar to that of SARS‐CoV, which shows detectable IgG for up to 24 months.⁷

Therefore, while swabs are known to be associated with an elevated number of false‐negative results, serological tests may represent a reliable instrument to identify past infections. We observed a high prevalence of negative serological results in CLL. These data, associated with the high prevalence of negative RT‐PCR tests as seen in the literature,⁴, ⁵ raise two hypotheses.

Firstly, SARS‐CoV‐2 is not the aetiological agent of CLL. In this case, we should consider another viral agent, and the presence of SARS‐CoV‐2 may therefore be only a coincidental finding. However, a viral outbreak during another viral pandemic seems improbable. One of our cases presented with clinical onset of CLL some days before the first Italian recognized case of COVID‐19. This could support the hypothesis of a different aetiological agent. On the other hand, the literature suggests that the SARS‐CoV‐2 virus had already appeared in northern Italy between the second half of January and early February 2020.⁸ We also considered that these may be true chilblains, but this seems unlikely, as our patients had no personal history of chilblains or rheumatological diseases and were forced to stay home during lockdown, avoiding external cold weather. Furthermore, our regional meteorological station registered similar or higher temperatures during February, March and April 2020 compared with the mean temperatures of the past 13 years.

The second theory is that SARS‐CoV‐2 is the aetiological agent of CLL, but does not induce detectable IgM and IgG. In this case we can put forward two more hypotheses: (i) the viral load was not enough to induce antibody formation; and (ii) the host developed an early and robust interferon type I response, muting early viral replication and not permitting the development of detectable IgM and IgG.¹ However, this would be in contrast with the formerly proposed hypothesis of delayed immune‐mediated reaction to the virus in genetically predisposed patients.¹

The main limitation of our serological study is the lack of independent validation of the available tests on the market, despite excellent clinical performance in real life of the used assay (personal experience).

In conclusion, while our study confirms that acral skin lesions are not a specific marker of SARS‐CoV‐2 infection,⁴ it is necessary not only to expand our observation by serologically testing a larger number of patients with CLL, but also to investigate aetiological agents other than SARS‐CoV‐2.

Acknowledgments

We thank the patients described in this manuscript, all of whom gave their written informed consent to publication of their case details. The authors thank their colleagues who contributed to collecting the cases (Roberta Maria Pandolfi, Gabriella Pravatà, Maria Cristina Sicher, Chiara Mattiucci); Alessandra Iadicicco and Erica Dolzan, who performed the serological tests; and Paolo Lanzafame for his critical review. A special acknowledgment also goes to Dr Serena Giacomin, president of the Italian Climate Network, and Dr Filippo Orlando, who provided us with the meteorological data.

Author Contribution

Laura Rizzoli: Conceptualization (equal); Writing‐original draft (equal); Writing‐review & editing (equal). Lucia Collini: Data curation (equal). Michela Magnano: Investigation (equal); Writing‐original draft (equal); Writing‐review & editing (lead). Stefania Termine: Data curation (supporting). Rosario Barcelli: Investigation (equal); Methodology (equal). Salvatore Domenico Infusino: Investigation (equal); Writing‐original draft (equal). Paolo Bauer: Investigation (supporting); Validation (supporting); Visualization (supporting). Giulia Rech: Conceptualization (equal); Formal analysis (equal); Writing‐review & editing (equal). Carlo Rene Girardelli: Writing‐review & editing (supporting). Riccardo Balestri: Supervision (equal); Writing‐original draft (lead); Writing‐review & editing (lead).