Molecular identification of Hypoderma sinense cutaneous myiasis in a child from a cattle-raising pastoral region of China: a case report

Abstract

Background

Human cutaneous myiasis caused by Hypoderma sinense is rarely reported and may be mistaken for common bacterial skin disease.

Case presentation

A 3-year-old boy from a livestock-raising pastoral region of China presented with multiple painful subcutaneous nodules that had not responded to topical antibacterial treatment. Marked eosinophilia, elevated total IgE, and linear ultrasonographic structures raised suspicion of parasitic infestation. A larva was removed intact from the posterior neck lesion through a pre-existing punctum under local anesthesia. Whole-genome shotgun sequencing followed by de novo assembly generated a 16,888-bp mitochondrial contig containing a complete COI barcode region, supporting identification of the larva as H. sinense. At the one-month follow-up, the pain and subcutaneous nodules had resolved.

Conclusions

This case highlights the importance of considering cutaneous myiasis in children from livestock-raising regions with painful, treatment-resistant subcutaneous nodules and demonstrates the diagnostic value of ultrasonography, minimally invasive larval extraction, and molecular identification.

Background

Myiasis refers to infestation of humans or other vertebrates by dipterous larvae, which feed on living or necrotic tissues, body fluids, or ingested material. Clinically, human myiasis is commonly classified into cutaneous, wound, and cavitary forms; cutaneous myiasis is the most frequently encountered form and includes furuncular, migratory, and wound-associated presentations [1]. Furuncular myiasis typically presents as a boil-like painful nodule with a central punctum or discharge, whereas migratory myiasis may cause creeping or shifting subcutaneous lesions. These non-specific manifestations can mimic common pediatric skin disorders, including furunculosis, infected epidermoid cysts or insect bite reactions, and diagnosis may be delayed when larvae are not directly visible [2].

The genus Hypoderma comprises botflies whose larvae mainly parasitize cattle, yaks, and other ruminants. In humans, Hypoderma infestation is accidental and may present as cutaneous, ocular or, rarely, neurological disease.1,3 Hypoderma sinense (H. sinense) has been established by morphological and molecular evidence as a livestock-associated species affecting cattle and yaks in China, particularly in the Qinghai–Tibet Plateau and adjacent pastoral regions [3,4]. Human infestation by H. sinense remains rarely documented. Moreover, historical taxonomic confusion with closely related Hypoderma species and the lack of routine species-level identification may have limited recognition of this parasite in human cases. [4,5]

Accurate species identification is clinically and epidemiologically important. Molecular identification using mitochondrial markers, especially the mitochondrial cytochrome c oxidase subunit I gene, has been widely used for species-level identification of dipterous larvae and other parasites [6]. With the increasing availability of high-throughput sequencing, shotgun sequencing can further support sequence-based taxonomic identification when conventional morphological identification is insufficient.

Here, we report a case of cutaneous myiasis caused by H. sinense in a 3-year-2-month-old boy from a cattle-raising pastoral region in western Sichuan, China. The child was initially treated for presumed furunculosis, but the lack of response to topical antibacterial therapy, together with his epidemiological background, prompted further evaluation. Ultrasonography revealed findings suggestive of parasitic infestation and helped guide a conservative approach rather than blind or wide incision. After local infiltration with lidocaine, the larva protruded through a pre-existing skin opening and was removed intact with minimal tissue injury. Sequencing-based mitochondrial contig analysis subsequently supported identification of the larva as H. sinense. This case illustrates a stepwise diagnostic transition from a common pediatric skin infection to a livestock-associated zoonotic parasitic infestation with species identification supported by molecular analysis.

Case presentation

A 3-year 2-month-old boy presented to our dermatology department with painful subcutaneous nodules on the trunk, posterior neck, and right scapular region for more than one month. The first lesion appeared on the right chest without an obvious trigger and was associated with intermittent pain. Similar lesions subsequently developed on the posterior neck and right scapular region, accompanied by local erythema.

The patient had previously visited another hospital, where the lesions were diagnosed as furunculosis. Topical fusidic acid cream was prescribed, but the symptoms did not improve. One day before presentation to our hospital, a small opening developed at the center of the posterior neck lesion, with a small amount of pale-yellow exudate. There were no prodromal or systemic symptoms during the disease course. The patient lived in Aba Tibetan and Qiang Autonomous Prefecture, Sichuan Province, China. Cattle, sheep, and other domestic animals were frequently present around the child’s home, and flies were common in nearby animal-rearing areas.

On physical examination, the patient was in good general condition, and systemic examination was unremarkable. Dermatological examination revealed three palpable subcutaneous nodules on the posterior neck, right chest, and right scapular region; the posterior neck lesion showed a central punctum.

Laboratory tests showed marked eosinophilia (54.1%; absolute eosinophil count, 10.03 × 10^9/L) and elevated serum total IgE (740.347 IU/mL), suggesting a parasitic or hypersensitivity-related process in the context of poor response to antibacterial therapy.

High-frequency ultrasonography performed with a 7–15 MHz linear probe revealed a linear slightly hyperechoic structure in the posterior neck lesion, measuring approximately 1.3 × 0.2 cm. In the right scapular region, a similar linear slightly hyperechoic structure measuring approximately 1.1 cm in length was detected in the deeper soft tissue, close to the superficial muscle layer. In the right chest wall, ultrasonography showed a well-defined heterogeneous hypoechoic nodule in the subcutaneous tissue, measuring approximately 0.9 × 0.3 cm, with detectable internal blood flow. The linear structures in the posterior neck and right scapular lesions raised suspicion of larval infestation, whereas the right chest wall nodule was non-specific (Fig. 1A–B).

Fig. 1.

Ultrasonographic findings. A Right chest wall lesion showing a heterogeneous hypoechoic nodule measuring approximately 0.9 × 0.3 cm. B Right scapular lesion showing a linear, mildly hyperechoic structure measuring approximately 1.1 cm in length

Given the visible punctum in the posterior neck lesion, the larva was extracted through the existing opening under local anesthesia. After routine disinfection and infiltration with 2% lidocaine at the base of the lesion, the larva protruded through the punctum and was removed intact using vascular forceps (Fig. 2A–B). This minimally invasive approach avoided unnecessary wound enlargement and reduced local tissue trauma. Only the larva extracted from the posterior neck lesion was available for molecular identification. The other two lesions had no accessible skin openings, and blind or extensive exploration was therefore avoided to minimize unnecessary tissue injury in this young child.

Fig. 2.

Clinical lesion and extracted larva. A Clinical appearance of the posterior neck lesion showing a furuncle-like subcutaneous nodule with a central punctum. B Larva extracted intact through the pre-existing skin opening. Molecular analysis supported its identification as Hypoderma sinense

Because additional unextracted larvae could not be excluded, the patient received empirical oral albendazole at 400 mg once daily for 3 consecutive days. Oral cefaclor was administered at 20 mg/kg/day in three divided doses for 7 days for possible secondary bacterial infection. At the one-month follow-up, both pain and subcutaneous nodules had resolved.

Molecular identification and sequence analysis

The extracted larva was rinsed with sterile normal saline, and tissue from the middle segment was collected for molecular analysis. Before CTAB extraction, the larval tissue was mechanically disrupted using a tissue grinder to facilitate disruption of the cuticle and release of the internal tissues, followed by proteinase K digestion at 65 °C for 30 min. Genomic DNA was then extracted using the cetyltrimethylammonium bromide (CTAB) method. Whole-genome shotgun libraries were prepared using the BGI Optimal DNA Library Prep Kit (BGI-Shenzhen, China) and sequenced on the DNBSEQ-T7 platform (BGI-Shenzhen, China), generating PE150 paired-end reads. No COI-targeted PCR or independent confirmatory PCR was performed, and no COI-specific primers were used for species identification.

To remove host-derived sequences, raw reads were aligned to the human reference genome hg19 using BWA-MEM v0.7.18 [7]. Unmapped reads were retained and assembled de novo using MEGAHIT v1.2.9 [8]. The assembled contigs were screened against a local COInr invertebrate COI barcode database using BLASTn v2.16.0 [9], and candidate mitochondrial contigs were further validated by online BLASTn searches against the NCBI nucleotide database. The COI barcode sequence used for species identification was recovered in silico from the assembled mitochondrial contig rather than generated by targeted PCR amplification.

Among the assembled contigs, contig k141_17052 was 16,888 bp in length and contained a complete COI barcode region recovered directly from the de novo-assembled mitochondrial contig. Screening against the local COInr invertebrate COI barcode database showed that this COI region was highly similar to Hypoderma sinense. Online BLASTn analysis of the COI barcode region further showed that the top matches corresponded to H. sinense, supporting species-level identification of the extracted larva (Fig. 3). Mapping of the retained non-human reads back to this contig showed complete coverage and a uniform high sequencing depth, with no abnormal deletion detected. Together, these results supported the identification of the larva as H. sinense at the species level.

Fig. 3.

BLASTn validation of the COI barcode sequence. Online BLASTn analysis of the COI barcode sequence showed that the top matches corresponded to Hypoderma sinense, supporting species-level identification of the extracted larva

Discussion and conclusions

Species of the genus Hypoderma, commonly known as warble flies, are obligate parasites of domestic and wild ruminants, whereas human hypodermosis is an accidental zoonotic myiasis that may present with migratory or furuncle-like subcutaneous lesions. Rare intracerebral involvement has been reported with H. bovis, whereas H. tarandi may cause ophthalmomyiasis in reindeer-herding regions [10,11]. H. sinense is a livestock-associated species affecting cattle and yaks in China. Because humans are aberrant hosts, larval migration and development may be atypical, and the normal life cycle observed in ruminants is generally not completed.

Against this background, we report a human case of H. sinense cutaneous myiasis from northwestern Sichuan, with species identification supported by molecular analysis. Pediatric cutaneous myiasis in China has been reported mainly in pastoral settings, and most reported cases have involved H. bovis or H. lineatum. In contrast, human infestation by H. sinense remains rarely documented. Species-specific prevalence data for H. sinense in Aba Prefecture are limited. However, an earlier survey cited in an FAO review reported an overall Hypoderma larval infestation rate of 84.2% among 1,149 yaks examined in Sichuan Province, with H. sinense among the species identified [12]. A later survey of 25,769 yaks in eastern Qinghai Province reported infestation rates of 82.2–98.7% in young yaks and 42.4–50.6% in adult yaks [13]. Although conducted in different periods and not specific to H. sinense or Aba Prefecture, these studies suggest that hypodermosis has historically been common among yaks in the eastern Qinghai–Tibetan Plateau. Livestock hypodermosis can also cause substantial economic losses through hide damage and reduced growth, milk yield, and meat production [12,13]. In pastoral communities, frequent contact with affected livestock may increase the likelihood of accidental human exposure.

The present case is best interpreted as a rare accidental infestation associated with livestock exposure rather than evidence that human infestation is common or that human-to-human transmission occurs. The precise route of infestation could not be established because no direct contact with an adult fly, attached eggs, or a visibly infested animal was documented. Plausible mechanisms include accidental oviposition by an adult Hypoderma fly on the child’s hair or exposed skin, followed by penetration of newly hatched larvae, or indirect transfer of newly hatched larvae from the coat of an infested animal during close contact. Given the frequent presence of livestock and flies in the child’s living environment, either route was possible, but neither could be confirmed.

Clinically, the presentation was misleading. The lesions were initially diagnosed as furuncles, which was understandable because furuncular myiasis can resemble bacterial skin infection. However, multiple painful subcutaneous nodules, poor response to antibacterial treatment, a central punctum, marked eosinophilia, elevated total IgE, and livestock-related exposure made ordinary furunculosis less likely. High-frequency ultrasonography provided further lesion-level evidence: the linear structures in the posterior neck and right scapular lesions were suggestive of larvae, whereas the heterogeneous hypoechoic nodule in the right chest wall was non-specific.

The three nodules were located at anatomically separate sites and were clinically considered to represent distinct lesions. The separate linear ultrasonographic structures in the posterior neck and right scapular lesions further suggested that multiple larval foci were more likely than a single continuously migrating larva. However, only the larva extracted from the posterior neck lesion was molecularly identified, and no specimens were obtained from the other two lesions. Therefore, although multiple larval lesions were considered the more likely explanation, sequential swellings caused by migration of a single larva could not be completely excluded.

The immune profile provided additional diagnostic support. Marked eosinophilia and elevated total IgE were consistent with a type 2 immune response associated with tissue-invasive parasites, although these findings were not specific to H. sinense. Together with the furuncle-like nodules, exposure history, and imaging findings, they supported tissue parasitism rather than ordinary bacterial infection. Other parasitic causes of subcutaneous nodules were also considered. Gnathostomiasis typically causes recurrent migratory swellings, sparganosis commonly presents as a slowly enlarging subcutaneous nodule, and subcutaneous dirofilariasis generally presents as a solitary nodule. Extraction and molecular identification of the larva as H. sinense excluded these alternative diagnoses for the posterior neck lesion.

Ultrasonography also helped guide removal of the posterior neck larva through the pre-existing punctum under local anesthesia, avoiding unnecessary wound enlargement. Following extraction, oral albendazole at 400 mg once daily for 3 consecutive days was administered empirically because additional unextracted larvae could not be excluded and albendazole was more readily available in our clinical setting; ivermectin could also have been considered.

Accurate species identification is clinically and epidemiologically important. Morphological examination using taxonomic keys is generally the first step in determining larval identity and developmental stage, but it requires preserved diagnostic structures and specialized entomological expertise [4,5]. In this case, the specimen was fully consumed during molecular testing, leaving no residual material for formal morphological identification or instar determination. Species identification therefore relied on sequence-based analysis.

Sequence-based analysis supported the identification of the extracted larva as H. sinense, reducing the risk of misclassification and providing a verifiable record of human infestation by a livestock-associated species rarely reported in humans. Previous studies have demonstrated the usefulness of mitochondrial markers for distinguishing H. sinense from related Hypoderma species and investigating their distribution in livestock populations.¹⁴ These findings extend the available evidence regarding the host range and geographic occurrence of this uncommon parasite.

The present case has several limitations. First, the absence of livestock, fly, and environmental samples precluded attribution of the infestation to a specific source. Second, species identification was based on a de novo-assembled mitochondrial contig and sequence-database comparison without independent COI-targeted PCR or Sanger sequencing confirmation. Although the contig showed complete read coverage and uniform high sequencing depth, confirmation using an independent molecular method would have further strengthened the species-level identification. Third, only the larva extracted from the posterior neck lesion was available for molecular analysis, and neither formal morphological identification nor instar determination could be performed. Consequently, the presence and species identity of larvae in the other two lesions, the number of larvae involved, and the relationship among the three lesions could not be definitively established. Nevertheless, the sequencing-based identification of H. sinense in the extracted larva provides a rare and verifiable record of this livestock-associated parasite in a human host.

In conclusion, cutaneous myiasis should be considered in children from livestock-raising regions who present with painful, treatment-resistant subcutaneous nodules. Ultrasonography can provide early evidence of larval infestation and help guide minimally invasive management. For rare human Hypoderma infestations, molecular identification is particularly valuable because it supports species-level identification and contributes to a more accurate understanding of livestock-associated parasites at the animal–environment–human interface.

Abbreviations

BGI

Beijing Genomics Institute

BLASTn

Basic Local Alignment Search Tool for nucleotide sequences

bp

Base pairs

BWA-MEM

Burrows-Wheeler Aligner maximal exact matches

COI

Cytochrome c oxidase subunit I

COInr

Cytochrome c oxidase subunit I non-redundant database

CTAB

Cetyltrimethylammonium bromide

DNA

Deoxyribonucleic acid

H. sinense

Hypoderma sinense

hg19

Human genome build 19

IgE

Immunoglobulin E

MEGAHIT

Metagenome assembler for large and complex metagenomics data

NCBI

National Center for Biotechnology Information

PE150

Paired-end 150-bp sequencing

Author contributions

Q.J. and S.H. contributed equally to this work. Q.J. and S.H. were responsible for the acquisition and organization of clinical data, literature review, case analysis and interpretation, and drafting of the manuscript. Q.J. and S.H. also prepared the initial versions of Figs. 1–3 and revised the figures according to the journal requirements. S.La., S.Lo., and X.J. contributed to the interpretation of clinical findings and molecular identification-related data, assisted with the preparation and verification of Figs. 1–3, and substantively revised the manuscript. Y.L. contributed to the conception and design of the case report, supervised the work, critically revised the manuscript for important intellectual content, verified the final presentation of the data and figures, and managed the submission process as the corresponding author. All authors read and approved the submitted version of the manuscript and agreed to be accountable for their own contributions and for the accuracy and integrity of the work.

Funding

No funding was received for this study.

Availability of data and materials

The raw sequencing data generated in this study are publicly available in the NCBI Sequence Read Archive under BioProject PRJNA1503541 and SRA accession SRR39879377.

Declarations

Ethics approval and consent to participate

This study was approved by the Ethics Committee of Chengdu Women’s and Children’s Central Hospital, School of Medicine, University of Electronic Science and Technology of China. This study was conducted in accordance with the Declaration of Helsinki. Written informed consent for participation was obtained from the patient’s legal guardian.

Consent for publication

Written informed consent for publication of the clinical details and images was obtained from the patient's legal guardian.

Competing interests

The authors declare that they have no competing interests.