Search for Equus caballus papillomavirus type 2 in tissues of asymptomatic horses in southern brazil.

Researchers tested tissues from 54 clinically normal horses in southern Brazil for a papillomavirus linked to equine genital cancers. They found viral DNA in one horse, suggesting the virus can be present without visible disease and may circulate silently.

What the study asked and why it matters

• Question: Is Equus caballus papillomavirus type 2 (EcPV-2) present in the genital mucosa of horses that show no clinical or microscopic evidence of genital disease?
• Motivation: EcPV-2 has been associated with equine genital squamous cell carcinomas (SCCs), but its occurrence in healthy horses—and therefore its silent circulation and transmission potential—has been poorly characterized, especially in Brazil.
• Relevance: Detecting EcPV-2 in asymptomatic animals would support the possibility of subclinical infection and help refine surveillance and control strategies to reduce cancer risk and viral spread.

Study design and methods

• Population and setting: 54 horses from southern Brazil without clinical or histological signs of genital lesions, comprising:
  • 40 necropsied animals (systematically examined post-mortem).
  • Penile mucosa specimens from 14 horses collected at a local abattoir.
• Specimen assessment:
  • Histology: Tissue sections examined microscopically to confirm absence of genital lesions.
  • Molecular detection: Conventional PCR targeting a 306 bp fragment of the EcPV-2 L1 gene (a conserved capsid gene commonly used for papillomavirus screening).
• Confirmation and characterization:
  • Sequencing of the PCR amplicon to verify viral identity.
  • Phylogenetic analysis to assess relatedness to known EcPV-2 isolates.
• Outcome measures: Presence of EcPV-2 DNA and any histological evidence of lesions in sampled tissues.

Key findings

• Prevalence signal: EcPV-2 DNA was detected in 1 of 54 horses (1.85%).
• Case details: The positive sample came from a necropsied male that died of an acute intestinal disease, not genital pathology.
• No lesions: Microscopic examination found no histological lesions in any tissues from the EcPV-2–positive animal.
• Sequence confirmation: The amplicon sequence matched EcPV-2 and clustered phylogenetically with known EcPV-2 isolates, validating the detection.
• Novelty: This is the first report of EcPV-2 DNA in the genital mucosa of an asymptomatic horse in Brazil.

Interpretation and significance

• Subclinical carriage: Detection in a clinically and histologically normal horse supports the existence of silent, subclinical EcPV-2 infections.
• Epidemiologic implications: Clinically normal animals may serve as reservoirs, potentially contributing to viral maintenance and transmission within horse populations.
• Cancer linkage: Given EcPV-2’s association with genital SCC, understanding its prevalence in healthy horses is important for clarifying pathways from infection to disease and for risk assessment.
• Uncertainty of infectivity: PCR positivity indicates viral DNA presence but does not establish active replication, shedding, or transmissibility.

Context within the broader literature

• Global picture: Outside Brazil, EcPV-2 has been implicated in equine genital SCC and detected in both tumors and some clinically normal mucosa, with variable detection rates across studies and settings.
• Regional contribution: This study extends the geographic evidence base to Brazil and specifically documents subclinical detection in this region’s horse population.
• Magnitude caution: The observed point estimate (1.85%) comes from a small, specific sample and should not be overinterpreted as the population prevalence.

Strengths of the study

• Rigorous case definition: Inclusion of horses free of clinical and histological genital lesions enhances confidence that the detection reflects asymptomatic status.
• Molecular confirmation: Sequencing and phylogenetic analysis substantiate PCR findings and reduce concerns about non-specific amplification.
• Multiple sources: Inclusion of necropsy and abattoir cases broadens the sampling frame within the target region.

Limitations and sources of uncertainty

• Sample size: Only 54 animals were tested, limiting precision and yielding wide uncertainty around the detection rate.
• Sampling bias: The inclusion of penile mucosa from abattoir animals may skew the sample toward males and may not represent the broader population (including mares and different age groups).
• Cross-sectional snapshot: A single time point cannot determine infection duration, persistence, or progression.
• Detection vs. disease activity: Conventional PCR detects DNA but does not quantify viral load or indicate active replication; no in situ localization was reported.
• Genomic scope: Targeting a single 306 bp L1 fragment may miss divergent variants or very low-copy infections.
• Geographic generalizability: Findings apply to southern Brazil and may not extrapolate to other regions or management systems.

Practical implications

• Surveillance design: Future epidemiologic surveys should include clinically normal horses to capture subclinical carriage and refine prevalence estimates.
• Breeding management: Because transmission likely involves direct mucosal contact (e.g., during natural breeding) and potentially fomites, biosecurity and hygiene practices merit evaluation as evidence accumulates.
• Cancer prevention research: Integrating viral detection into SCC risk studies may clarify how often subclinical EcPV-2 precedes lesions and which cofactors influence progression.

Recommended next steps for research

• Larger, multi-regional prevalence studies that include mares, stallions, and different age groups, with standardized sampling of genital mucosa.
• Longitudinal cohorts to assess acquisition, persistence, clearance, and the temporal relationship between EcPV-2 detection and lesion development.
• Quantitative and localization tools (e.g., qPCR, RNA assays, in situ hybridization) to measure viral load and activity in tissues.
• Whole-genome sequencing of EcPV-2 from diverse hosts and sites to map genetic diversity and potential oncogenic determinants.
• Transmission and shedding studies during natural breeding and assisted reproduction to inform evidence-based biosecurity.
• Multifactorial analyses incorporating host factors (immune status, genetics), environmental exposures, and co-infections to explain variability in outcomes.

Bottom line

• The detection of EcPV-2 DNA in the genital mucosa of an asymptomatic horse in Brazil provides initial, sequence-confirmed evidence of subclinical infection in this region and underscores the need for broader, longitudinal studies to define prevalence, transmission dynamics, and links to genital cancer risk.