Malignant catarrhal fever

Description

Malignant catarrhal fever (MCF) is an infectious viral disease in ruminants, including bison and sheep. MCF is caused by a group of herpes viruses.

In North America, MCF is caused by ovine herpes virus-2 (OHV-2), which infects sheep with no ill effects and may spread to susceptible species. Bison tend to be very susceptible, and deaths are frequent in exposed animals.

How to report

All suspected or confirmed cases must be reported to the Office of the Chief Provincial Veterinarian (OCPV) within 24 hours:

Hours: 8:15 am to 4:30 pm (open Monday to Friday, closed statutory holidays)
Phone: 780-427-3448
Toll free: 310-0000 before the phone number (in Alberta)
After business hours: 1-800-524-0051
Fax: 780-415-0810

Clinical signs

MCF refers to the disease caused by the OHV-2 virus in susceptible species such as bison. OHV-2 can spread from sheep to bison, causing MCF disease.

OHV-2 infection in sheep

Sheep display no signs of infection or disease. While all sheep should be presumed to be carriers of the OHV-2 virus, it does not cause disease in sheep. Generally, lambs are born virus-free. But by 5 to 6 months, almost all lambs are carriers. This implies that lambs are infected through contact with the adult members of the flock.

OHV-2 infection in bison

Bison tend to be very susceptible to OHV-2, and deaths are frequent in exposed animals.

The incubation period for MCF – the time between exposure and clinical disease – can be from 7 to 25 weeks.

Stress, such as transport, handling and confinement, appears to play a significant role in the development of the disease. Stressed bison are more likely to be affected by the virus.

Clinical signs of disease last 1 to 7 days, and the outcome is always fatal.

Bison with MCF may show the following clinical signs:

• sudden death
• severe depression, weakness
• reluctance to eat or drink
• separation from the herd
• fever
• nasal discharge
• clouding or ulceration of the eyes
• respiratory problems such as coughing, shortness of breath, open-mouth breathing
• painful or difficult urination
• erosions in the mouth and upper respiratory tract
• diarrhea or dysentery (bloody diarrhea)

Diagnosis

Other diseases such as bovine viral diarrhea (BVD), Johne’s Disease and Salmonellosis can be mistaken for MCF. Therefore, all suspect cases should be confirmed with proper laboratory diagnostic tests.

Exposure to the virus can be confirmed through the detection of antibodies in a blood sample. This test does not distinguish between the different MCF herpes viruses, therefore its results can be difficult to interpret.

Polymerase chain reaction (PCR) and other molecular testing can detect viral genetic material. In live animals, MCF is diagnosed by performing the PCR test on a blood sample. In dead animals, MCF is diagnosed by detecting typical microscopic lesions in the carcass and is confirmed using PCR tests on the tissues. The PCR test does not work well when tissues are decomposed.

Where it’s found

MCF occurs worldwide. Although its generally sporadic, herd outbreaks have been reported.

In Africa, the wildebeest carries alcelaphine herpes virus-1 (AHV-1) which causes MCF in susceptible species on that continent. In North America, MCF is caused by OHV-2, which infects sheep with no ill effects and spreads to susceptible species like bison.

There is also a herpes virus that causes MCF in white-tailed deer. There are likely other undiscovered members of the MCF virus group in ruminant populations that may or may not cause disease or interfere with diagnostic tests.

How it spreads

Among sheep

The virus is shed primarily in the sheep’s nasal and eye secretions and transmitted through either direct or indirect contact. The significance of mechanical transmission – the spread of the virus on boots, clothing or vehicles – is unknown.

From sheep to bison

Scientific opinion varies on how bison become infected with OHV-2. Although most outbreaks of MCF are associated with exposure to sheep, outbreaks of MCF in bison have been reported with no known contact with sheep.

Spread of the virus between bison does not appear to occur easily.

Risk to humans

There is no risk to humans.

Prevention and control

Prevention

Keep sheep as far away from bison as possible

Because sheep are important carriers of the virus, bison should not be grazed near sheep. Losses of up to 50% of some bison herds have occurred following exposure of bison to sheep, even with short contact at sales barns. A buffer zone is advised to reduce contact between the 2 species.

OvHV-2 is fragile in the environment, surviving for only a few days depending on the humidity and temperature, so sheep and bison can graze the same pastures but over separate seasons.

Ensure transport haulers have not recently transported sheep.

Minimize stress in bison and sheep

Since stress appears to play an important role in the development of the disease, minimizing stress in both bison herds and sheep flocks may help reduce the incidence of MCF. Extra caution should be taken during times of high stress such as birthing and weaning.

Vaccination

Currently there is no MCF vaccine available. This is due to:

• limited knowledge of the virus
• the small size of the bison industry that makes it unattractive for pharmaceutical companies to conduct research
• previous failures in manufacturing a vaccine for the wildebeest MCF (AHV-1)

Treatment

There is no effective treatment for MCF. Isolation of affected animals is usually recommended.

Biosecurity

Biosecurity refers to practices designed to prevent, reduce or eliminate the introduction and spread of disease. Biosecurity practices tailored to each operation minimize the introduction and transmission of disease. Find out more about Biosecurity and livestock.

Resources

What is Malignant Catarrhal Fever (fact sheet)