Active Surveillance for Heartland virus in North Carolina: Clinical and Genomic Epidemiology

Through active surveillance at a large academic center in North Carolina, researchers identified two previously unreported cases of Heartland virus disease with varying clinical severity and distinct genomic sequences, highlighting the need for expanded surveillance to better understand the virus's true burden and pathogenesis.

The Gist

The Big Picture: Hunting a "Ghost" Virus

Imagine Heartland Virus (HRTV) as a ghost in the machine. It's a real, dangerous virus carried by ticks that can make people very sick, but it's incredibly hard to catch. Why? Because:

1. No Detective Kit: Doctors don't have a quick, store-bought test for it (like they do for the flu or strep throat).
2. The Great Imposter: Its symptoms (fever, low white blood cells, liver issues) look exactly like other common tick-borne diseases, like Ehrlichiosis. It's like a thief wearing a disguise that looks exactly like a delivery driver.

Because of this, many cases are missed. The virus is likely spreading more than we think, but it's hiding in plain sight.

The Mission: The "Net" Strategy

Since we can't easily test everyone for Heartland Virus, the researchers in North Carolina decided to use a clever fishing strategy.

Instead of trying to catch the specific fish (Heartland Virus) directly, they cast a wide net for a fish that looks similar (Ehrlichiosis). They went to a massive hospital system and looked at 800 patients who were already being tested for Ehrlichiosis because they had fevers and weird blood work.

The Analogy: Imagine you are looking for a rare, shy bird in a forest. Instead of searching for that one bird, you set up a feeder for a common bird that looks similar. If the rare bird shows up at the feeder, you catch it.

The Catch: Two Surprises

Out of the 800 people they screened, 53 fit the specific criteria for being tested for tick-borne viruses.

• The Result: They found 2 people with active Heartland Virus in their blood.
• The Significance: Before this study, North Carolina had only reported two cases in its entire history. This study found two more in just six months. It's like finding two new species of rare frogs in a pond where you thought there were none.

The Two Patients: A Tale of Two Stories

The study highlighted how unpredictable this virus can be.

1. Patient One (The "Mild Case"): A woman in her 50s felt sick with a fever and body aches. She went to the ER, got some antibiotics, and went home. She recovered, but the researchers found out she had the virus. She was like someone who caught a cold but didn't realize it was actually a specific strain of flu.
2. Patient Two (The "Severe Case"): A man in his 70s with diabetes got sick. He took antibiotics, but he didn't get better. In fact, he got worse. He started having seizures, confusion, and needed to be in the Intensive Care Unit (ICU). He was fighting for his life.
   • The Lesson: The same virus can be a minor nuisance for one person and a life-threatening emergency for another.

The Genetic Detective Work: "Family Reunion"

The researchers took the virus from these two patients and sequenced its DNA (its genetic code). They wanted to see where these viruses came from.

• The Finding: The viruses from the two patients were not closely related to each other.
• The Analogy: Imagine two people showing up at a party. You check their family trees. One is from a family in Missouri, and the other is from a family in Georgia. They aren't cousins; they are strangers who just happened to show up at the same party.
• What this means: The virus isn't just a local outbreak in one neighborhood. It's traveling around, likely carried by ticks moving long distances or people traveling to different areas. The virus is diverse and widespread.

The Tick Hunt: Looking for the Culprit

The researchers went to the area where the sick man (Patient Two) had been bitten and dragged nets through the grass to catch ticks. They found over 500 ticks, including some invasive "Asian Longhorned" ticks.

• The Result: None of the ticks tested positive for the virus.
• Why? The virus is so rare in the tick population that finding it is like finding a needle in a haystack. However, finding the Asian Longhorned tick is scary because that specific tick is known to carry a different deadly virus in Asia, and scientists think it might be able to carry Heartland Virus too.

The Takeaway: Why This Matters

This study is a wake-up call.

1. We are underestimating the problem: Because doctors don't have a quick test, they often assume it's a bacterial infection and treat it with antibiotics. If it's actually a virus, the antibiotics won't work, and the patient might get sicker.
2. We need better tools: We need to develop fast, cheap tests for Heartland Virus so doctors can catch it early.
3. Surveillance is key: You can't fight a ghost if you don't know it's there. By actively looking for it in patients who look like they have other diseases, we can start to understand how common it really is and how dangerous it can be.

In short: Heartland Virus is a sneaky, shape-shifting enemy. This study proved it's hiding in North Carolina, affecting people from mild to critical, and we need better detective tools to find it before it strikes again.

Technical Summary

1. Problem Statement

Heartland virus (HRTV) is an emerging tick-borne virus (genus Bandavirus) capable of causing severe, sometimes fatal, febrile illness. Despite its presence in the United States, the true burden of disease is likely significantly underestimated due to several factors:

• Diagnostic Limitations: There are no commercially available nucleic acid tests for HRTV; diagnosis relies on referral to the CDC for RT-PCR, IgM, or Plaque Reduction Neutralization Testing (PRNT).
• Clinical Overlap: HRTV symptoms (fever, leukopenia, thrombocytopenia, elevated liver enzymes) closely mimic bacterial tick-borne diseases (TBDs) like ehrlichiosis and spotted fever group rickettsiosis (SFGR).
• Serological Pitfalls: Standard serological testing for bacterial TBDs often yields false positives or "probable" diagnoses based on single acute titers, leading to the misclassification of viral cases as bacterial.
• Data Gaps: Prior to this study, only two HRTV cases had been reported in North Carolina (NC), and there was a scarcity of clinical specimens and genomic sequences from the region, hindering understanding of viral pathogenesis and evolution.

2. Methodology

The study employed a prospective active surveillance design at the University of North Carolina (UNC) Health System from March 27, 2024, to September 25, 2024.

• Cohort Selection:
  • Screened Cohort: 778 unique patients who underwent PCR testing for Ehrlichia.
  • Enrolled Cohort: 53 patients meeting specific inclusion criteria: fever + at least two of the following: leukopenia (WBC <4), thrombocytopenia (platelets <150), or elevated liver enzymes (AST/ALT >45).
• Laboratory Testing:
  • Viral Screening: Remnant whole blood samples from the enrolled cohort were tested via multiplex RT-PCR for HRTV and Bourbon virus (BRBV) RNA.
  • Bacterial Testing: Standard serology (IgG) and PCR for Ehrlichia, SFGR, and Lyme disease were performed.
  • Genomics: Samples positive for HRTV RNA underwent whole-genome sequencing (S, M, and L segments) using Illumina MiSeq. Phylogenetic analysis was conducted using IQ-TREE with maximum-likelihood methods.
• Vector Surveillance: Tick drags were performed at the exposure site of a confirmed case (Patient Two) in late 2024 and 2025 to screen for HRTV in local tick populations (Amblyomma americanum and Haemaphysalis longicornis).

3. Key Contributions

• First Genomic Sequences from NC: This study generated the first complete HRTV genome sequences from North Carolina, providing critical data for regional phylogenetic analysis.
• Active Surveillance Model: It demonstrated the efficacy of "piggybacking" viral surveillance on existing bacterial TBD testing workflows to identify missed viral cases.
• Clinical Spectrum Characterization: The study documented a wide spectrum of HRTV severity, from mild outpatient illness to severe encephalitis requiring ICU care.
• Vector Analysis: It investigated the potential role of the invasive Asian longhorned tick (Haemaphysalis longicornis) in HRTV transmission within the US.

4. Key Results

Epidemiological Findings:

• Detection Rate: Among the 53 enrolled patients meeting clinical criteria, 2 patients (3.8%) tested positive for HRTV RNA. No BRBV was detected.
• Misclassification: A significant portion of the screened cohort (83%) did not meet confirmed case definitions for bacterial TBDs, yet many had received doxycycline. In the enrolled cohort, 28.3% were classified as "probable" ehrlichiosis cases, highlighting the diagnostic confusion.
• Clinical Presentation:
  • Patient 1: A woman in her 50s with mild, persistent flu-like symptoms (outpatient setting).
  • Patient 2: A man in his 70s with poorly controlled diabetes who developed severe HRTV encephalitis, seizures, and required ICU admission. He had detectable viremia for 7 days (18–25 days post-exposure).

Genomic and Phylogenetic Findings:

• Divergence: The two NC cases were phylogenetically distinct from each other, suggesting independent introductions rather than local transmission chains.
  • Patient 2: Clustered with sequences from Georgia (2019–2025).
  • Patient 1: Showed segment-specific clustering; the S-segment clustered with Missouri (2017), while M and L segments clustered with New York/Midwest sequences. This raises the possibility of reassortment or infection acquired in diverse geographic locations.
• Diversity: Pairwise nucleotide diversity between the two NC patients was comparable to diversity seen across different US states (e.g., Kentucky, Missouri), indicating high viral heterogeneity.
• Amino Acid Substitutions: While several substitutions were found in the NSs (a known virulence factor), none were at positions previously implicated in pathogenesis, and no unique human-specific substitutions were identified.

Vector Findings:

• Over 500 ticks were collected near Patient 2's exposure site, including Haemaphysalis longicornis (Asian longhorned ticks).
• None of the collected ticks tested positive for HRTV or BRBV RNA, consistent with the low minimum infection rate of HRTV in wild tick populations.

5. Significance and Implications

• Underestimation of Burden: The identification of two new cases in a 6-month window, doubling the previously reported total for NC, strongly suggests that HRTV is underdiagnosed and underreported.
• Diagnostic Stewardship: The study highlights the limitations of relying solely on bacterial serology. It advocates for the implementation of real-time TBV RT-PCR assays, particularly for patients who fail to respond to doxycycline within 48–72 hours.
• Clinical Management: The prolonged viremia observed (up to 25 days post-exposure) supports a broad diagnostic window for testing, which is crucial for patients presenting late in their illness.
• Vector Surveillance: The detection of H. longicornis in the area is significant given its role as a vector for Dabie bandavirus (SFTSV) in Asia and its experimental ability to transmit HRTV. Continued surveillance of this invasive species is critical.
• Future Research: The genomic data provides a foundation for studying viral evolution, reassortment events, and host-pathogen interactions in emerging tick-borne viruses.

Conclusion:
This study confirms that HRTV is an active, under-recognized pathogen in North Carolina with a diverse genomic landscape. It underscores the urgent need for improved diagnostic algorithms that integrate viral testing alongside bacterial screening to accurately assess disease burden and guide clinical management.