Lassa Virus
LEVELS: Highly unlikely; Moderate; NOT ASSIGNED; Reference or research laboratory required; Negligible; NOT ASSIGNED; Little; NOT ASSIGNED
| Register id | lassa_virus |
| Type | virus |
| Scientific name | Lassa mammarenavirus |
| NCBI taxid | 3052310 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-06, against criteria version 093366e352a2 |
Overview
Lassa virus is a serious human disease of West Africa that, so far as anyone has been able to show, has nothing to do with pigs. It is carried by a rodent — the multimammate rat, Mastomys natalensis — which is not native to North America, and people catch it from that rat's urine and droppings, or from each other. It causes an estimated 100,000 to 300,000 human infections a year: four in five are a mild flu-like illness, but the remainder can progress to haemorrhage, breathing difficulty, shock and death, and permanent hearing loss follows in up to a third of patients. Overall mortality is about 1%, though in an epidemic it can reach 50% among people sick enough to be hospitalised. Its place on this register is a statement about what we do not know. The SHIC factsheet says plainly that Lassa "is not a known pathogen of pigs", that transmission in pigs "is apparently not described", and that there is no description of clinical signs, immunity, diagnosis, prevention or control in swine — the virus has never been found in a pig anywhere. That is why several of its criteria carry no level at all. An empty cell here means we hold nothing on the question, not that the answer is reassuring, and Lassa is registered rather than left out precisely so that gap is on the record: SHIC publishes a swine factsheet on it, so a reader looking for it should find it and find out what is and is not known. The realistic route into the United States is an infected rodent arriving in a shipment rather than anything happening on a farm, diagnosis is confined to reference laboratories such as CDC and requires the highest level of biocontainment, and there is no licensed vaccine for humans or animals — though several are in development and early treatment of people with ribavirin works.
C1 Zoonotic potential
Likelihood of transmission from pigs to humans
Highly unlikely: Human infection with the agent has never been reported, or has been reported but is not attributed to pig or pork exposure
Basis. shic_lassa: "LASV is not a known pathogen of pigs." "LASV transmission in pigs is apparently not described." "Natural LASV infection has only been documented in humans, Mastomys natalensis, and a few other rodent species in LASV-endemic areas of Africa." Human infection is rodent-derived -- "Rodent-to-human transmission of LASV occurs via contact with body fluids, excreta, urine, tissues, or blood, as well as inhalation of infectious aerosols" -- or human-to-human.
How this level was decided
Level 1, and unambiguous. Human Lassa fever is enormously common -- 100,000 to 300,000 infections a year -- so the L1 clause that applies is the second one: reported, but not attributed to pig or pork exposure. Every documented route runs from the multimammate rat or from another person. The pig is not a host at all, let alone a source. Same shape as dengue and Murray Valley, both scored L1.
C2 Zoonotic impact
Severity of human illness caused by the agent
Moderate: Clinically important illness is common and may require medical treatment or hospitalization, but death or permanent disability is uncommon
Basis. shic_lassa: "LASV causes a mild, flu-like illness in 80% of infected humans. Others may go on to develop Lassa fever characterized by hemorrhage; respiratory distress; vomiting; facial swelling; pain in the chest, back, or abdomen; shock; and sometimes death." "Death in humans is relatively uncommon, with an overall mortality rate of about 1%. However, during epidemics, the case-fatality rate in hospitalized patients may reach 50%." "Hearing loss, which may be permanent, is a common complication occurring in up to one-third of human patients." "Zoonotic transmission results in low rates of mortality (1-2%)."
How this level was decided
LEVEL 3, AND FLAGGED. C2 scores the agent itself regardless of where exposure came from, and the standing reading is to judge a typical-to-severe bout rather than the tail -- the influenza A and West Nile precedents. On that test Lassa is level 3: the typical infection is mild and flu-like in 80% of people, the 20% that progress are clinically important and hospitalised, and overall mortality is about 1%, which is "death or permanent disability is uncommon". THE ARGUMENT FOR LEVEL 4, and it is not a weak one: permanent hearing loss in up to a third of PATIENTS is not a tail phenomenon, and a 50% case-fatality rate among hospitalised patients in an epidemic is exactly the "substantial risk of life-threatening illness" wording. I have stayed at 3 because both of those figures are denominated on patients rather than on infections, and because Eric set the tail rule against precisely this kind of reasoning on West Nile. Worth his eye.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
NOT ASSESSED — no evidence in folder. No level was assigned on this criterion. This is a statement about what the evidence folder holds, not a finding about the agent.
Corrected by Eric from L3 Routine biosecurity keeps it out. Reason: RULED NOT ASSESSABLE AND ROUTED TO KISS ANALYSIS. Against this cell he wrote only "KISS". THE PASS HAD SCORED LEVEL 3 AND HE IS OVERRIDING: the level 3 rested on the reservoir being a rodent and rodent control being routine biosecurity, which is a reasonable inference about rodents and not a statement about LASV reaching a pig. The factsheet says "LASV is not a known pathogen of pigs" and "LASV transmission in pigs is apparently not described", so there is no route to exclude and no effort to grade. Same rule he applied to influenza C and PCV4 -- where the folder cannot support the claim, the cell stays empty rather than carrying a level indistinguishable from a real one.
Basis. shic_lassa: the reservoir is "Mastomys natalensis (the multimammate rat)", which "is not native to the U.S.; however, the virus could enter the U.S. through deliberate or accidental importation of LASV-infected reservoir hosts." Prevention in people is rodent exclusion -- "avoid contact with rodents and their excretions, store food in rodent-proof containers, and keep rodents out of homes." "LASV is stable as an aerosol when relative humidity does not exceed 30%."
How this level was decided
Level 3, by the standing rule that scores the route rather than the reservoir. The route to a pig, if there were one, is a rodent walking into a building. Housing alone does not exclude rodents, so this is not level 2; a rodent control programme is squarely on the routine biosecurity list, so it is not level 4 either -- there is no filtration, treated feed or carrier testing in the picture. Consistent with reston_virus at 3. Inferred rather than supported because the factsheet describes rodent exclusion for households rather than for herds, and because no route into a pig has ever been demonstrated.
C4 Detection difficulty
Difficulty of recognizing and confirming infection
Reference or research laboratory required: A validated assay exists, but only at a national reference or research laboratory rather than at local or regional level
Basis. shic_lassa: "No specific tests have been described for LASV detection in swine." "LASV isolation must be performed in a BSL4 laboratory." "Only reference laboratories such as the Centers for Disease Control and Prevention (CDC) are capable of detecting LASV, and the symptoms in humans are non-specific." "RT-PCR is considered the gold standard test for LASV." "None of these assays are currently available for reliable use in the field."
How this level was decided
Level 3, and this one the factsheet answers in its own words. Validated assays plainly exist -- RT-PCR is called the gold standard and commercial human ELISAs are named -- so this is not level 4, where no reliable test exists anywhere. But the factsheet says outright that only reference laboratories such as CDC can detect it and that isolation needs BSL4, which is the level 3 label verbatim. The swine gap ("no specific tests have been described for LASV detection in swine") does not lift it to 4: the standing reading is that a validated assay exists and can be adapted, and C4 asks where it can be run.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. shic_lassa: "LASV is not a known pathogen of pigs." "Information on LASV infection in swine is not available." "LASV infection has not been documented in swine. There is no description of clinical signs, immunity, diagnostics, prevention, or control of LASV infection in swine."
How this level was decided
Level 1. No natural infection of pigs has ever been documented anywhere, so there is no field disease and no measurable effect on cost of production. This is the same call as dengue_virus and murray_valley_encephalitis_virus, both level 1 on the same ground. Note this is an affirmative reading rather than an absence: the factsheet states that LASV is not a known pathogen of pigs, it does not merely fail to mention pigs.
C6 Market impact
Duration of material disruption to pork and pig markets
CONFIRMED NOT ASSESSABLE. No level was assigned on this criterion, and Eric has reviewed that and ruled it correct — the folder holds nothing bearing on it. This is a statement about what the evidence supports, not a finding about the agent.
Confirmed by Eric. CONFIRMED NOT ASSESSABLE. Against this cell he wrote only "KISS", confirming the null the pass had already left. The pass nulled it under SCORING/market_impact -- no pig has ever been naturally infected with LASV anywhere, so there is no precedent to reason from and asserting level 1 would be inventing the observation. This is the ground he nulled dengue on.
Basis. shic_lassa: "Natural LASV infection has only been documented in humans, Mastomys natalensis, and a few other rodent species." "LASV infection has not been documented in swine." There is no record anywhere of LASV being diagnosed in a pig, and therefore none of any market responding to such a diagnosis.
How this level was decided
NOT ASSESSED -- no evidence in folder, and this is the standing rule applying rather than a judgment being dodged. SCORING/market_impact: "WHERE THERE IS NO PRECEDENT ANYWHERE, C6 IS NOT ASSESSED... The test is not whether the agent is exotic -- it is whether anything has ever been observed." Eric nulled dengue C6 on exactly this ground, that dengue "has never naturally infected a pig anywhere, so there is none", while confirming Zika at level 1 because pigs are naturally infected abroad with no market reaction and there was therefore something to reason from. Lassa is the dengue case: no pig has ever been infected, so no absence of market reaction can be observed and asserting level 1 would be inventing the observation. He has now ruled this way on four entries, every one of them C6. THIS MAKES THE ENTRY UNEXPORTABLE and adds it to the KISS set, which is the correct outcome rather than a cost.
C7 Treatment potential
Potential for treatment to improve outcomes
Little: Effective pathogen-directed treatment is available and works, or animals recover acceptably without it
INFERRED — reasoned, not stated. The evidence implies this level rather than stating it. The reasoning is below.
Basis. shic_lassa: "Prevention and control measures for LASV in swine are not established." "Human infections can be treated early with ribavirin to prevent severe disease. Experimental studies with favipiravir... show the drug to be promising." "the only treatment for LASV, ribavirin, must be administered very early during infection for treatment to be successful." "the emergence of LASV in swine would likely be difficult to detect and treat."
How this level was decided
Level 1, on the second clause of the standing note -- no imperative to treat, because there is no documented natural disease in pigs to treat. Same reading as dengue_virus. NOTE WHAT THIS IS NOT: it is not a claim that treatment would be worthless, and the factsheet is candid that detection and treatment in swine would both be difficult. It is that C7 scores the value of a treatment for the disease this agent causes in pigs, and no such disease has been described. Human ribavirin therapy exists and is used, which is relevant context but is not what this criterion measures.
C8 Vaccine availability
Availability of effective vaccines or bacterins
NOT ASSESSED — no evidence in folder. No level was assigned on this criterion. This is a statement about what the evidence folder holds, not a finding about the agent.
Corrected by Eric from L1 Available, or not needed. Reason: RULED NOT ASSESSABLE AND ROUTED TO KISS ANALYSIS. Against this cell he wrote only "kiss". THE PASS HAD SCORED LEVEL 1 and flagged it as sitting close to a line the standing C8 note draws hard -- that asserting a vaccine is not needed is still a claim about the disease, and cannot be made about a disease nobody has characterised. The pass argued Lassa was distinguishable from parechovirus_a because the factsheet affirmatively says LASV is not a known pathogen of pigs. He has ruled it is not distinguishable enough. WITH C3, C6 AND C8 ALL NULL, lassa_virus is unexportable and joins the KISS set.
Basis. shic_lassa: "Multiple vaccines have been described including vesicular stomatitis virus-based vaccines, live non-pathogenic strains, and reverse genetics-produced reassortant viruses. No vaccine is currently commercially available for humans or animals." "There are no licensed vaccines for LASV in humans or animals." "LASV is not a known pathogen of pigs."
How this level was decided
LEVEL 1, AND FLAGGED because it sits close to a line the standing note draws hard. The note forbids using level 1 as a safe default for an agent whose disease-causing potential is UNKNOWN -- that is why parechovirus_a, porcine_tottorivirus and the porcine ephemeroviruses carry nulls at C8. I have scored 1 rather than null because Lassa is not that case: the factsheet makes the affirmative statement that LASV is not a known pathogen of pigs and has never been documented in one, so "no vaccine is needed FOR PIGS" is a supported claim about the agent rather than an inference from silence. That is the same ground on which dengue_virus and murray_valley_encephalitis_virus were scored 1. The distinction to test if Eric disagrees: parechovirus_a IS a pig virus of unknown significance, whereas Lassa is a rodent virus that has never been shown to infect a pig at all.
Levels and evidence are generated from data/assignments/lassa_virus.yml; the overview is authored in data/overviews/lassa_virus.md. Do not hand-edit this page — corrections go in data/assignments/CORRECTIONS.yml.
Quoted material marked Basis is from Zimmerman JJ, Karriker LA, Ramirez A, Schwartz KJ, Stevenson GW, Zhang J, eds. Diseases of Swine, 12th edition. Hoboken: Wiley Blackwell, 2025 — except where another source is named in the quotation itself.