Hydranencephaly in Calves: Severe Brain Malformation and Neurologic Signs in Cows

Hydranencephaly is a severe congenital brain malformation seen in some calves. In this condition, much of the cerebral hemispheres does not develop normally and is replaced by fluid-filled spaces. The brainstem and some deeper structures may still be present, which is why a calf can sometimes be born alive, breathe, vocalize, and even show basic reflexes.

Affected calves often have serious neurologic problems from birth or soon after. They may seem blind, weak, slow to learn where the udder is, or unable to nurse effectively. Some can stand and walk, but their behavior is abnormal because the parts of the brain needed for normal awareness, vision processing, and coordinated responses are missing.

This is not an infection the calf "caught" after birth. It develops before birth, usually after damage to the fetal brain during pregnancy. In cattle, hydranencephaly is most often discussed alongside other congenital defects such as porencephaly and arthrogryposis, especially when viral infection of the pregnant dam is involved.

For pet parents and producers, the hardest part is that there is no way to restore the missing brain tissue. Your vet can help confirm what is going on, rule out other causes of neurologic disease, and talk through realistic care options for the calf and herd.

Hydranencephaly develops when the fetal brain is damaged during gestation. In cattle, one of the best-known causes is in utero viral infection. Veterinary references describe hydranencephaly in ruminants after fetal exposure to viruses such as Akabane virus and related Simbu serogroup viruses. Merck also notes that bluetongue virus can cause hydranencephaly or porencephaly in calves, leading to ataxia and blindness at birth.

These viruses do not affect every pregnancy the same way. The outcome depends heavily on when during gestation the dam is infected, because the developing brain is especially vulnerable during certain windows. If infection happens while the cerebrum is forming, the fetus may survive but be born with major malformations.

Merck also notes that, more rarely, bovine viral diarrhea virus and some other infectious agents have been associated with hydranencephaly in calves. In some cases, a congenital brain defect may be suspected but the exact cause is never proven, especially if no testing is done on the calf, dam, placenta, or herd mates.

For herd-level prevention planning, your vet may think beyond the individual calf and ask whether there were other abortions, stillbirths, twisted limbs, weak calves, or neurologic newborns in the same breeding season. That pattern can be an important clue that an infectious fetal cause is involved.

Diagnosis starts with a farm call and neurologic exam. Your vet will look at the calf's mentation, ability to stand, suckle reflex, menace response, vision behavior, and other cranial nerve and postural findings. Because several newborn calf problems can look similar, your vet will also consider differentials such as hypoxic birth injury, meningitis, septicemia, trauma, metabolic disease, and other congenital defects.

History matters. Your vet may ask about the dam's pregnancy dates, vaccination program, insect exposure, abortions or malformed calves in the herd, and whether multiple calves were affected. In some cases, herd-level blood testing or targeted infectious disease testing may be recommended to look for evidence of fetal viral exposure.

A definitive diagnosis often depends on advanced imaging or postmortem examination rather than exam findings alone. In field settings, many cases are strongly suspected based on the calf's signs and herd history. If the calf dies or humane euthanasia is chosen, necropsy is often the clearest way to confirm hydranencephaly and to collect tissues for diagnostic testing. Veterinary diagnostic lab fees for calf necropsy commonly start around $150-$175 for neonates at some US labs, but total billed cost can be higher once transport, farm call, sample handling, and additional testing are included.

Because this condition is severe and irreversible, diagnosis is not only about naming the defect. It also helps your vet advise you on prognosis, welfare, biosecurity, and whether the herd needs a broader reproductive disease workup.

Prevention focuses on protecting pregnant cows from fetal infections and reducing herd-level reproductive disease risk. Work with your vet on a vaccination plan that fits your region, herd type, and breeding calendar. Because some viruses linked to congenital defects are insect-borne, timing of breeding and calving season can matter, especially in areas with heavy midge or mosquito activity.

Good prevention also means paying attention to patterns. If your herd has abortions, stillbirths, weak calves, twisted limbs, or blind newborns, tell your vet quickly. A single malformed calf can be tragic. Several in one season can point to a broader reproductive problem that needs investigation.

Biosecurity matters too. Isolate new arrivals when appropriate, review vaccination status before breeding, and keep accurate breeding and calving records. Those records help your vet estimate when fetal exposure may have happened and which infectious causes are most plausible.

Not every case is preventable, and not every malformed calf will have a confirmed cause. Still, early veterinary involvement gives you the best chance to protect future pregnancies, reduce repeat losses, and make informed herd decisions.