Malignant Catarrhal Fever in Cows: Symptoms, Causes, and Prognosis

Malignant catarrhal fever, often shortened to MCF, is a severe viral disease of cattle and other hoofed animals. It is caused by a group of ruminant gammaherpesviruses. In cattle, the disease is usually sporadic, meaning it often affects single animals rather than sweeping through an entire herd. Even so, when it appears, it is a medical emergency because the illness is commonly fatal.

The virus causes widespread inflammation of blood vessels and tissues throughout the body. That is why affected cows can develop a mix of eye, mouth, respiratory, digestive, and neurologic signs. A cow may start with fever and depression, then quickly progress to cloudy eyes, nasal discharge, oral ulcers, diarrhea, weakness, or sudden death.

In the United States and much of North America, the form seen most often is sheep-associated MCF, linked to ovine herpesvirus 2 carried silently by sheep. Sheep usually do not look ill, which can make the disease especially frustrating for producers and pet parents managing mixed-species properties.

Because MCF can resemble other serious cattle diseases, your vet will usually recommend prompt isolation, testing, and a careful review of recent contact with sheep or other susceptible species.

MCF in cattle is caused by infection with certain ruminant gammaherpesviruses. The most important cause in North America is ovine herpesvirus 2 (OvHV-2), the virus behind sheep-associated malignant catarrhal fever. Sheep are the main reservoir host. They usually carry the virus without becoming sick, which means healthy-looking sheep can still be part of the exposure history.

The virus is spread mainly through nasal secretions and likely by aerosol over short distances, though longer-distance spread has also been documented under some conditions. Lambs are especially important because they tend to become infected when young and may shed more virus than adult sheep. Shared airspace, close pen contact, and contaminated feed or water areas can all increase risk.

Less commonly, MCF can also be associated with other reservoir species, including wildebeest and goats, depending on geography and management setting. In practical terms, most cattle cases on mixed farms are investigated first as possible sheep-associated disease.

A key point for pet parents and producers is that cattle are usually considered dead-end hosts. In other words, a sick cow is not thought to be the main source of spread to other cattle. The bigger concern is the original exposure source and whether other susceptible animals are still being housed near carrier species.

Diagnosis starts with your vet's exam and herd history. MCF is often suspected when a cow has a combination of high fever, corneal cloudiness, oral erosions, nasal and eye discharge, diarrhea, and enlarged lymph nodes, especially if there has been contact with sheep or lambs. Because these signs can mimic other major cattle diseases, your vet will usually build a list of rule-outs right away.

Laboratory confirmation is important. Testing commonly includes PCR for MCF viruses such as OvHV-2 on EDTA blood or fresh tissues, along with necropsy and histopathology if the animal dies or is euthanized. Typical tissue changes include widespread inflammation of blood vessels and lymphoid tissues. Fresh samples from lymph node, spleen, kidney, liver, lung, or brain may be recommended depending on the case.

Your vet may also test for look-alike conditions such as bovine viral diarrhea, infectious bovine rhinotracheitis, bluetongue, or other ulcerative and febrile diseases. That is one reason early veterinary involvement matters. A prompt diagnosis helps guide isolation, herd management, and realistic decision-making.

If MCF is strongly suspected, ask your vet what samples are most useful before treatment, transport, or carcass handling changes the testing plan.

Prevention is centered on separating cattle from reservoir species, especially sheep and lambs. There is no commercial vaccine for MCF, and there is no treatment that reliably prevents death after clinical disease begins. That makes management and biosecurity the most important tools.

If your property keeps both cattle and sheep, ask your vet to help design a realistic separation plan. Risk is higher with close contact, shared barns, shared fence lines, common feed and water areas, and housing cattle near young lambs, which are important virus shedders. Lambing and weaning periods deserve extra caution.

Good prevention steps include separate enclosures, separate equipment when possible, avoiding shared indoor airspace, and careful planning for animal movement onto the farm. If a suspicious case appears, isolate the cow promptly and review recent contact history with your vet. In some settings, long-distance separation may be recommended for highly susceptible species.

Mixed-species farms can still work, but they need thoughtful management. Your vet can help match the prevention plan to your land, herd structure, and budget so the approach is practical as well as protective.