The route of infection shapes Rift Valley fever virus pathogenesis, humoral immune response, and horizontal transmission in sheep.

This study demonstrates that the route of Rift Valley fever virus inoculation in sheep significantly alters disease pathogenesis, immune response, and viral excretion, with subcutaneous infection facilitating horizontal transmission while intranasal infection leads to more severe clinical and histopathological outcomes.

The Gist

The Big Picture: A Viral "Heist" with Two Different Entry Points

Imagine the Rift Valley Fever Virus (RVFV) as a master thief trying to break into a sheep farm. Usually, this thief uses a mosquito as a getaway car to sneak in. But scientists wanted to know: Does it matter how the thief gets inside the building?

Does it matter if the thief kicks in the front door (subcutaneous injection) or tries to sneak through the ventilation system (intranasal/inhaled)? And once inside, does the thief leave a trail of evidence that allows other sheep to catch the virus without a mosquito?

This study put that question to the test using young sheep as the "building."

---

The Experiment: Two Groups, Two Doors

The researchers set up two groups of young sheep:

1. The "Front Door" Group (Subcutaneous): These sheep got the virus injected just under their skin, mimicking a mosquito bite.
2. The "Ventilation" Group (Intranasal): These sheep had the virus sprayed up their noses, mimicking breathing in infected air or droplets.

They also had a third group of "innocent bystanders" (contact animals) living in the same room as the infected sheep to see if the virus could spread from sheep-to-sheep without mosquitoes.

What Happened? The Tale of Two Infections

Even though both groups got sick, the virus acted like two different characters depending on how it entered.

1. The "Front Door" Group (Subcutaneous)

• The Speedster: The virus got in fast. These sheep started showing signs of infection (fever, virus in blood) almost immediately.
• The Early Bird: They developed antibodies (their body's security guards) quickly.
• The Spreader: This is the big surprise. Because the virus entered this way, the infected sheep started shedding the virus into the room. The "innocent bystanders" living with them caught the virus. It was like the thief left a trail of breadcrumbs that the other sheep followed.

2. The "Ventilation" Group (Intranasal)

• The Slow Burn: The virus took longer to get established. The sheep didn't show fever or high virus levels in their blood as quickly as the first group.
• The Heavy Hitter: Once the virus got going, it hit harder. These sheep got higher fevers, had more virus in their blood at its peak, and suffered more damage to their livers and brains.
• The Silent Killer: Despite the heavy damage, these sheep didn't seem to spread the virus to their roommates. The "innocent bystanders" in this group stayed healthy. It's as if the thief was busy breaking the house apart but didn't leave any clues for others to follow.

The "Damage Report"

• Liver Trouble: Both groups had some liver damage, but the "Ventilation" group had a more battered liver. Think of it like a car engine: the "Front Door" group had a quick start but ran fine; the "Ventilation" group had a slow start but the engine overheated and suffered more wear and tear.
• Brain Woes: The "Ventilation" group also had more inflammation in their brains. Interestingly, the sheep didn't act crazy or have seizures (no obvious neurological signs), but under a microscope, their brains looked like they had been through a storm.
• Kidneys: Both groups had very mild kidney stress, like a slight dehydration, but nothing major.

The Big Discovery: Sheep Can Pass It On (Without Mosquitoes)

The most important finding of this paper is that sheep can infect other sheep directly.

In the "Front Door" group, the virus spread from the infected sheep to the uninfected ones just by being in the same room. This proves that during an outbreak, you don't always need mosquitoes to spread the disease. The virus can jump from animal to animal, likely through breathing or close contact.

However, this only happened with the "Front Door" (subcutaneous) infection. The "Ventilation" (intranasal) group didn't spread it to their neighbors. This suggests that how the virus enters the body changes how it behaves and how easily it spreads.

Why Does This Matter?

1. Vaccine Design: If we want to make a vaccine, we need to know which "door" the virus usually uses. If it often enters through the nose, a nasal spray vaccine might be better than a shot.
2. Stopping Outbreaks: We used to think mosquitoes were the only way this disease spread. Now we know that if one sheep gets infected, it can infect the whole flock just by hanging out together. Farmers need to isolate sick animals immediately, even if there are no mosquitoes around.
3. Safety: The study showed that breathing in the virus (intranasal) causes more severe internal damage to the liver and brain than a mosquito bite does. This is a warning for anyone working with these animals: don't breathe in the dust or fluids!

The Bottom Line

The route the virus takes to enter a sheep changes the whole story.

• Mosquito style (Skin injection): Fast start, quick spread to other sheep, but less internal damage.
• Air style (Nose inhalation): Slow start, no spread to neighbors, but causes much more severe damage to the liver and brain.

This study helps scientists understand the "personality" of the virus better, so we can build better defenses to protect our livestock and our own health.

Technical Summary

1. Problem Statement

Rift Valley fever (RVF) is a zoonotic arboviral disease causing high mortality in ruminants and severe disease in humans. While transmission by mosquito vectors (Aedes and Culex) is well-established, the potential for horizontal transmission (direct animal-to-animal spread) without vectors remains unclear. Furthermore, while subcutaneous (SC) inoculation is the standard experimental model for RVF, the impact of the route of infection (specifically intranasal vs. subcutaneous) on disease dynamics, viral replication, tissue tropism, and the host immune response in sheep is largely unknown. Understanding these variables is critical for refining vaccine strategies and epidemiological models.

2. Methodology

The study employed a comparative experimental design using 20 young Castilian sheep (9–10 weeks old) housed in a BSL-3 facility.

• Experimental Groups:
  • Subcutaneous (SC) Group: 7 sheep inoculated with $1 \times 10^6$ PFU of the virulent RVFV strain 56/74; 3 in-contact (naïve) sheep housed in the same room.
  • Intranasal (IN) Group: 7 sheep inoculated with the same dose via mucosal atomization; 3 in-contact (naïve) sheep housed in the same room.
• Monitoring: Animals were monitored for 28 days post-inoculation (dpi).
  • Clinical: Daily rectal temperature and clinical signs.
  • Sampling: Blood (EDTA/serum), oro-nasal, and rectal swabs collected at 0, 1, 2, 3, 4, 7, 10, 21, and 28 dpi.
  • Post-Mortem: At 28 dpi, animals were euthanized for necropsy. Tissues (liver, spleen, brain, lymph nodes, etc.) were collected for histopathology and viral load analysis.
• Assays:
  • Virology: RT-qPCR for viral RNA (RNAemia), virus isolation in Vero cells, and neutralizing antibody (nAb) titers (Virus Microneutralization Test).
  • Immunology: ELISA for IgM and IgG against the nucleoprotein.
  • Biochemistry: Serum analysis for hepatic (AST, ALP, GGT, bile acids, bilirubin) and renal (urea, creatinine) markers.
  • Histopathology: H&E staining of liver and frontal cortex.

3. Key Contributions

• First Comparative Study in Sheep: This is the first study to directly compare SC and IN inoculation routes in sheep, a primary natural host for RVFV.
• Demonstration of Horizontal Transmission: The study provides robust evidence of RVFV horizontal transmission between SC-inoculated sheep and in-contact animals in the absence of mosquito vectors.
• Route-Dependent Pathogenesis: It establishes that the inoculation route significantly alters the kinetics of viremia, the severity of organ damage (liver and CNS), and the timing/magnitude of the humoral immune response.

4. Key Results

A. Disease Dynamics and Clinical Signs

• Clinical Presentation: All animals remained subclinical (no overt clinical signs) regardless of the route.
• Temperature:
  • SC Group: Shorter incubation; fever onset at 1–2 dpi (peaks up to 41.6°C).
  • IN Group: Delayed fever onset (2–3 dpi) but significantly higher peak temperatures (up to 41.9°C).
  • In-Contact (SC): Showed transient fever spikes (3–6 dpi), indicating infection.
  • In-Contact (IN): No fever spikes observed.

B. Viremia and Viral Shedding

• Onset: SC animals showed viral RNA detection from Day 1; IN animals from Day 2.
• Peak Viremia: IN animals reached significantly higher peak viremia and sustained higher levels than SC animals.
• Horizontal Transmission:
  • SC Group: In-contact sheep developed viremia (detected from Day 2, peaking Day 3–4) and infectious virus was isolated from blood.
  • IN Group: No viremia or virus isolation in in-contact animals.
• Swabs: Viral genome was rarely detected in oro-nasal swabs (only 4/7 IN animals at Day 1), and no infectious virus was isolated from swabs, suggesting that respiratory shedding may not be the primary driver of transmission, or detection was hindered by sampling limitations.

C. Humoral Immune Response

• Seroconversion (IgM/IgG):
  • SC: Earlier seroconversion (from Day 4).
  • IN: Delayed seroconversion (from Day 7).
• Neutralizing Antibodies (nAb):
  • SC: Early response (detectable by Day 4), but lower and more heterogeneous titers.
  • IN: Delayed response (Day 7), but characterized by higher, more homogeneous, and robust nAb titers that stabilized earlier.

D. Pathogenesis and Histopathology

• Hepatic Injury:
  • Both groups showed mild hepatic injury (elevated AST, ALP, GGT).
  • IN Group: Exhibited significantly higher concentrations of liver enzymes (AST, ALP, GGT), indicating more severe liver dysfunction.
• Renal Injury: Mild, transient increases in urea in both groups; no significant difference between routes.
• Central Nervous System (CNS):
  • SC Group: Mild, focal non-suppurative meningoencephalitis with low lymphocytic infiltrates.
  • IN Group: More severe lesions, including thickened meninges, dense perivascular cuffs, diffuse satellitosis, and moderate gliosis.
  • In-Contact (SC): Showed mild CNS lesions similar to the SC group.
  • In-Contact (IN): No CNS lesions.
• Macroscopic Findings: No gross lesions were observed at necropsy in any group.

5. Significance and Implications

• Epidemiological Impact: The confirmation of horizontal transmission in the SC group suggests that RVFV can spread within herds without mosquito vectors, particularly when animals are infected via parenteral routes (e.g., biting insects, iatrogenic, or mechanical transmission). This challenges the assumption that RVF is strictly vector-dependent for intra-herd spread.
• Vaccine Development: The study highlights that the route of administration dictates the immune profile. SC inoculation induces a faster but weaker antibody response, while IN inoculation induces a stronger, more robust neutralizing response but with higher viremia and CNS pathology. This is crucial for designing live-attenuated vaccines, where safety (neurotropism) and efficacy must be balanced.
• Pathogenesis Insights: The findings suggest that while sheep are resistant to clinical neurological disease, the IN route allows for greater viral replication in the liver and CNS, potentially due to a delayed but overwhelmed innate immune response (e.g., IFN-$\alpha$ suppression by the viral NSs protein).
• Diagnostic Considerations: The lack of detectable virus in swabs despite confirmed transmission suggests that current sampling strategies for horizontal transmission may need refinement (e.g., investigating ocular or urinary routes).

In conclusion, the route of RVFV inoculation is a critical determinant of disease outcome, immune kinetics, and transmission potential in sheep. The study underscores the complexity of RVF epidemiology and the need to consider non-vector transmission routes in disease control strategies.