October 1999
My old page still refers to “West Nile-like virus”, the language used while the New York outbreak was being investigated. It followed the alerts, mosquito findings and bird surveillance as the story unfolded. Last updated in February 2006, it remains a record of that period. [2]
West Nile became established across much of the Americas after its recognition in New York. Europe also has a longstanding history of transmission. Italy’s current figures belong to that continuing story; they do not mark the sudden arrival of a previously unknown virus. [3]
Humans are not the main host
The usual maintenance cycle is between birds and mosquitoes, particularly Culex. An infected mosquito feeds on a bird; another mosquito acquires the virus from an infected bird. Some mosquitoes also bite people and horses. Humans and horses generally develop too little virus in their blood to sustain the mosquito transmission cycle and are considered incidental, dead-end hosts. [3]
That changes how we should think about surveillance. A hospital patient is one possible signal of a cycle already operating outside the hospital.
About 80% of human infections cause no symptoms. Most symptomatic infections produce a febrile illness. Fewer than 1% of infected people develop neuroinvasive disease—meningitis, encephalitis or inflammation of the spinal cord causing weakness or paralysis. Older people and those with impaired immunity are at greater risk. [4]
For the person who develops encephalitis, “less than 1%” offers little comfort. Survivors can have lasting neurological disability. [4]
The birds may know before we do
Mosquito testing and surveillance of birds and horses can reveal viral activity before recognised human cases accumulate. The lead time depends on the setting, the species sampled and the quality of surveillance. A positive animal sample does not automatically predict a human outbreak. [3]
Temperature and weather influence transmission, but mosquitoes, bird hosts, habitat and human exposure also matter. For Malaysia, the useful lesson from Europe is to connect these observations. Its case numbers cannot simply be transferred to our climate, vectors or population.
This is One Health: recognising that human health, animal health and the environment are connected. It becomes practical when a veterinary finding reaches the clinician investigating unexplained encephalitis, and the clinician’s findings reach the teams sampling mosquitoes and birds.
Malaysian clues go back decades
The Malaysian trail begins well before my webpage. A report published in 1970 described isolation of a virus then identified as Kunjin from Culex pseudovishnui-group mosquitoes in Sarawak. A later sequencing study lists this isolate as dating from 1966. [5] [15]
Kunjin is recognised as a subtype of West Nile virus, although later genetic analysis placed the historical Sarawak isolate in a distinct Malaysian group. It should not be equated with the lineage 2 sequences reported in the newer Malaysian studies. [15]
The newer findings span people, birds, mosquitoes and other animals. The table separates publication year from sampling year: these studies document earlier detections, not the level of viral activity in Malaysia in 2026.
| Published | Sample and dates | Finding | Interpretation |
|---|---|---|---|
| 2014 | People 2012–2013 | WNV IgG detected by ELISA in 9/742 Orang Asli participants. | Possible prior exposure; no confirmatory neutralisation testing. [6] |
| 2020 | Wild birds 2016–2017 | WNV RNA in 16/105 tested swabs; antibody reactions in 29/155 sera. | Selected resident and migratory birds; sequences clustered within lineage 2. [7] |
| 2020 | Macaques and bats 2014–2017 | Antibody reactions in 24/81 macaques; RNA in 5/41 bats. | No macaque RNA detected; antibody findings lacked neutralisation confirmation. [8] |
| 2023 | Mosquitoes 2017–2018 | RNA in 35/285 pools comprising 2,635 mosquitoes near Kuala Gula and Kapar. | Positive pools do not measure individual mosquito infection prevalence. [9] |
| 2023 | Domestic mammals 2017–2019 | RNA in nasopharyngeal swabs from 7/91 horses; antibody reactions in cattle and goats. | Selected samples; antibody findings lacked neutralisation confirmation. [10] |
The bat finding is particularly interesting. In the 2020 Animals study, five bats were RNA-positive, yielding six positive swab sequences that clustered within lineage 2. The macaque evidence was antibody screening; confirmatory neutralisation testing was not performed. [8]
So where are the patients?
Malaysia already has a specific DVS West Nile Fever veterinary protocol, numbered PVM 1(23):1/2024. It acknowledges human and animal seropositivity and RNA findings in equids and migratory birds, and says that no clinical WNF cases had been reported at the time of the document. It sets out surveillance, early warning and control arrangements for equids and birds. [11]
Malaysia is therefore not starting from zero. In our review of publications up to 30 September 2026, we found no published reports of confirmed human West Nile disease in Malaysia. This does not establish that no infections have occurred; the Orang Asli antibody findings are separate from confirmed clinical cases.
Why has a clear human disease burden not emerged from the evidence reviewed here? There may be little clinically significant disease, infections may be predominantly silent, or some illness may not be recognised. Local ecology and viral characteristics could matter. The available studies do not tell us which explanation dominates.
“Rare” and “rarely looked for” are different possibilities. We should investigate them rather than choose the more dramatic answer.
The clinician and the laboratory
A Malaysian patient with fever, headache and confusion needs a broad assessment. Dengue, Japanese encephalitis, herpes simplex and other causes of meningoencephalitis may be relevant. West Nile belongs in the discussion when the clinical picture, travel history or evidence of local viral activity supports it.
The clinical picture can raise suspicion, but cannot confirm West Nile disease. Fever with headache, muscle aches and sometimes a rash is nonspecific. Meningitis may cause neck stiffness and sensitivity to light. Encephalitis may bring confusion, tremor, involuntary jerks or poor coordination; acute flaccid myelitis can cause rapid limb weakness, reduced reflexes and respiratory muscle involvement. These features should prompt consideration of WNV alongside other causes, especially with relevant mosquito exposure, travel or local viral activity. [4]
CDC recommends testing for WNV-specific IgM antibodies in serum and/or cerebrospinal fluid. IgM can suggest recent infection, although it may persist; IgG alone does not diagnose the cause of an acute illness. Antibodies to related viruses can interfere with interpretation. A neutralisation test, which assesses whether antibodies block viral infection in the laboratory, may help distinguish WNV from dengue or other related viruses. [12]
Early negative IgM may require later sampling. PCR can help, especially in immunocompromised patients, but its sensitivity is limited in many immunocompetent patients; a negative result does not exclude infection. In Malaysia, the clinician should discuss specimens and confirmatory options with the relevant laboratory and public-health team rather than assume that a test is routinely available everywhere. [12]
IMR’s diagnostic test list lists West Nile virus RT-qPCR for acute neurological syndrome through the Virology Unit, IDRC, IMR, NIH Setia Alam, by consultation only. Specimen and container requirements are determined after consultation. Local availability of WNV IgM and neutralisation testing still needs confirmation. [16]
From chikungunya to West Nile
In Chik…What?, I argued for shared surveillance, laboratory capacity and environmental information, while recognising that different vectors and diseases need different interventions. West Nile gives that argument a specific focus: signals from animals and mosquitoes may help us act before human disease becomes conspicuous. [13]
What would that look like? A confirmed finding in local mosquitoes or animals could prompt a targeted alert to nearby clinicians and laboratories. An unexplained neurological case could prompt appropriate WNV testing and a review of animal and vector findings in the area. Results would return to all the teams involved, with their location, date and confirmation status.
This is a proposed way to strengthen existing work. The DVS protocol provides a veterinary starting point. Targeted repeat sampling at sites with earlier detections could help establish what is happening now. [11]
A finding sitting in a journal is useful evidence. A finding reaching the right people in time is useful surveillance.
For the public, prevention remains familiar: suitable repellents, protective clothing, screens and mosquito control matched to local vector habitats. There is no licensed human WNV vaccine or specific treatment with established benefit; care is supportive. Confusion, new weakness or other neurological symptoms with fever require urgent assessment. Where active transmission is identified, blood-safety authorities also need to consider donor protection measures. [14]
Watching before the outbreak
In October 1999, I followed an outbreak on the other side of the world. By then, a Sarawak mosquito isolation report had already been in the literature for almost thirty years.
Today, Europe is counting human cases, and Malaysian researchers have added evidence from birds, mosquitoes and other animals. The unanswered question is how those findings relate to disease in people here.
We have enough evidence to keep watching, and enough uncertainty to watch carefully.
The lesson from 1999 was to watch the outbreak.
The lesson now may be to watch before the outbreak.