Evaluation of Theileria equi vertical transmission rate and routes in a cohort of asymptomatic mares and their foals.

Evaluation of Theileria equi vertical transmission rate and routes in a cohort of asymptomatic mares and their foals studied the frequency and pathways of mother-to-foal transmission of blood parasites Theileria equi and Babesia caballi, which cause equine piroplasmosis, in healthy mares and their newborn foals.

Background and Objectives

• Equine piroplasmosis is a tick-borne parasitic disease affecting horses, primarily caused by two species: Theileria equi and Babesia caballi.
• The disease is of concern due to its effects on equine health and the possibility of mother-to-foal (vertical) transmission, which can sustain the parasite’s presence in horse populations.
• This study aimed to assess the frequency (rate) and the potential transmission routes of vertical transmission of T. equi and B. caballi in a large cohort of 179 clinically healthy mares and their foals.
• The study focused on asymptomatic animals to understand silent carriers’ role in parasite transmission to offspring without evidence of clinical disease in the mothers.

Methods

• Sampling occurred in 179 mare-foal pairs, with blood samples taken from foals within 72 hours after birth.
• Serological testing using Indirect Immunofluorescent Antibody Test (IFAT) was done on a subset of 107 mare-foal pairs to detect antibodies as indicators of past or present infection.
• Species-specific nested PCR testing was conducted on all blood samples (358 total from mares and foals), as well as samples from placenta (24), umbilicus (6), and colostrum (18) to detect parasite DNA.
• Blood smear evaluations were used to visually confirm viable T. equi parasites in foal blood samples.
• Genotyping of T. equi DNA was performed, specifically analyzing the 18S rRNA gene to identify parasite strains present in positive samples.

Key Findings

• Seroprevalence data:
  • For T. equi, 59.8% of mares and 54.2% of foals showed positive antibodies, indicating widespread exposure or infection in the population.
  • For B. caballi, seroprevalence was 42.1% in mares and 40.2% in foals.
• PCR results:
  • 30.2% of mares were PCR-positive for T. equi, confirming active parasite presence.
  • Only 2.2% of mares were PCR-positive for B. caballi, indicating a much lower active infection rate.
• Vertical transmission rates:
  • No vertical transmission of B. caballi was observed (i.e., no foals were PCR-positive for this parasite).
  • Four foals (7.4% of PCR-positive mares for T. equi) were PCR-positive at birth, indicating vertical transmission of T. equi.
  • These foals showed viable T. equi parasites on blood smears, although they were asymptomatic for neonatal equine piroplasmosis; one foal developed acute renal failure, possibly linked to infection.
• Detection of parasite DNA in reproductive tissues and fluids:
  • T. equi DNA was found in placenta, umbilicus, and/or colostrum samples from mares positive by PCR.
  • However, detection of parasite DNA in these tissues/fluids did not consistently correlate with foal infection status (i.e., some foals were not infected despite parasite DNA present in maternal tissues).
  • One carrier foal was born without detectable T. equi DNA in the corresponding placenta sample, suggesting alternate or undetected transmission pathways.
• Genotyping results:
  • The 18S rRNA genotype E of T. equi was present in infected foals, placenta, and colostrum, suggesting the transmitted strain was consistent across these sample types.

Conclusions and Implications

• Vertical transmission of T. equi occurs at a relatively low but notable frequency (7.4%) in asymptomatic mares and their foals.
• No evidence of vertical transmission was found for B. caballi in this cohort.
• The presence of parasite DNA in placenta, umbilical cord, and colostrum suggests multiple potential routes for transmission, including possibly colostral (via mare’s first milk) passage, but further research is needed to confirm these pathways.
• As vertical transmission can lead to silent infections in foals without obvious clinical signs at birth, this route might contribute to maintaining endemic equine piroplasmosis in herds.
• Understanding transmission routes is important for developing control measures to reduce disease spread and protect equine health, especially in breeding populations.

Future Directions

• Further studies should investigate the role of colostrum in transmitting T. equi, including whether viable parasites or parasite DNA can infect foals through nursing.
• Research into mechanisms enabling parasite survival and transmission through placental and umbilical tissues may provide insights into blocking vertical transmission.
• Examination of clinical outcomes and long-term effects of vertically transmitted T. equi infections in foals will help assess disease impact.
• Development of sensitive diagnostic tools to detect early infections in foals to enable timely veterinary interventions.