Porcine Hemagglutinating Encephalomyelitis in Pigs: Tremors and Neurologic Disease in Piglets

Porcine hemagglutinating encephalomyelitis, often shortened to PHE, is a viral disease of young piglets caused by porcine hemagglutinating encephalomyelitis virus (PHEV). This virus is a betacoronavirus and is the only known neurotropic coronavirus of swine, meaning it has a strong tendency to affect the nervous system. In practice, it is best known for causing either vomiting and wasting disease or encephalomyelitis with tremors and other neurologic signs.

Most clinical cases happen in piglets younger than 4 weeks, especially those born to sows without enough protective antibodies. The virus is common in swine populations worldwide, but obvious outbreaks are uncommon because many sows pass protective lactogenic immunity to nursing piglets through colostrum and milk.

When disease does occur, it can move fast. Some piglets start with repeated retching and vomiting, then become dehydrated, weak, and progressively thin. Others develop neurologic signs such as generalized tremors, hypersensitivity, backward walking, dog-sitting posture, paddling, blindness, nystagmus, or coma. Both forms can occur in the same litter.

For pet parents and small-scale pig keepers, this is an emergency because very young piglets can decline within hours to days. Early veterinary involvement helps confirm the cause, guide supportive care, and protect the rest of the group.

PHE is caused by porcine hemagglutinating encephalomyelitis virus (PHEV), an enveloped RNA virus in the family Coronaviridae. Pigs are the only known natural host, and the virus is not known to infect people. It spreads mainly through respiratory droplets and pig-to-pig contact, which helps it circulate quietly in many breeding herds.

After entering the body, the virus first replicates in tissues such as the nasal mucosa, tonsils, and respiratory tract. From there, it can move along peripheral nerves into the brainstem and other parts of the nervous system. This nerve spread explains why some piglets develop tremors, weakness, abnormal posture, and other neurologic signs.

The vomiting and wasting form is thought to happen because the virus affects the vagal sensory ganglion and the nerve supply to the stomach wall. That can interfere with normal swallowing and stomach emptying, so piglets retch, vomit, stop nursing well, and gradually waste away.

The biggest risk factor is lack of maternal immunity. Piglets born to nonimmune or newly exposed sows, especially in first-parity litters, are more likely to become clinically ill. Older pigs may be infected too, but they usually have milder or subclinical disease because age and prior exposure offer more protection.

Your vet will usually start with the signalment and pattern of disease. PHE becomes more suspicious when very young piglets, often under 4 weeks old, develop persistent vomiting, wasting, tremors, or other neurologic signs, especially if several littermates are affected around the same time.

Because other piglet diseases can also cause weakness or neurologic changes, diagnosis should not rely on signs alone. Your vet may recommend RT-PCR, immunofluorescence, or virus detection on samples from affected piglets. Merck notes that target tissues can include the brainstem, tonsils, and respiratory tract, and that virus isolation works best when samples are collected within about 2 days of disease onset.

In herd situations, your vet may also suggest necropsy on a freshly deceased or humanely euthanized piglet. This can help rule out other important differentials such as pseudorabies, teschovirus encephalomyelitis, edema disease, septicemia, or toxic causes. Histopathology may show inflammatory and degenerative changes in the medulla and peripheral ganglia.

Typical U.S. diagnostic costs in 2025-2026 often include a farm or ambulatory visit, sample collection, and laboratory fees. A porcine necropsy at a university diagnostic lab may run around $160 plus fees, and individual swine PCR tests at major U.S. labs are often roughly $25-$45 per assay, though exact totals vary by state, shipping, and how many piglets are tested.

Prevention centers on maternal immunity. Clinical PHE is much less likely when sows have already been exposed and can pass protective antibodies to piglets through colostrum and milk. That is why outbreaks are most concerning in naive breeding herds, newly assembled groups, or first-parity sows without enough prior immunity.

There is no commercial vaccine for PHEV, so your vet will focus on herd management rather than vaccination. In many settings, the goal is to maintain stable sow immunity while avoiding introduction of the virus into naive groups. Your vet may discuss gilt acclimation, monitoring replacement animals, and keeping farrowing groups as closed and consistent as possible.

Good biosecurity still matters. Limit unnecessary pig movement, avoid mixing age groups when possible, clean and disinfect equipment between groups, and isolate sick piglets or litters when practical. PHEV is an enveloped coronavirus, and fact-sheet guidance notes that disinfectants effective against other swine coronaviruses are reasonable choices when used correctly.

If an outbreak is already happening, your vet may consider herd-level steps such as testing sows or piglets, reviewing colostrum intake, and in some situations discussing immune serum from exposed sows for newborn piglets. The best prevention plan depends on your herd's size, breeding system, and whether the priority is maintaining immunity or protecting a truly naive population.