Bovine Malignant Catarrhal Fever (MCF)

Also known as

Malignant head catarrh, Snotsiekte, Catarrhal fever, Gangrenous coryza

Description

Malignant catarrhal fever (MCF) is an acute, highly fatal infectious disease caused by a virus that affects ruminants and sometimes swine. It is mainly a disease of domestic cattle, water buffalo, Bali cattle (banteng), American bison, and deer. MCF has also been seen in a variety of captive ruminants in zoos.

In some species, such as bison and some types of deer, MCF is acute and highly lethal, often affecting large numbers of animals in a group. With occasional exceptions, the disease in cattle is generally more sporadic and affects single animals.

MCF is typically fatal; however, there are a few outbreaks in which several animals are affected, with evidence of recovery and mild infections in some cases.

The main carriers of the virus are domestic sheep and wildebeest; this disease occurs worldwide. MCF has been a major problem in farmed deer operations, and in recent years has become a threat to the commercial bison industry.

The disease is systemic, and lesions may be found in any organ, although severity and frequency varies greatly. The principal lesions are inflammation and necrosis (death) of respiratory, alimentary, or urinary mucosal linings.

It often begins as an erosive stomatitis (sore mouth) and gastroenteritis (inflammation of the intestinal lining), with erosions in the upper respiratory tract, keratoconjunctivitis (inflammation of the cornea and conjunctiva of the eye), encephalitis, cutaneous exanthema (skin rash) and lymph node enlargement.

Some cases merely show chronic hair loss and weight loss.

Acute MCF cases caused by ovine herpesvirus-2 and alcelaphine herpesvirus-1 are very similar. The disease course may range from very acute (sudden and severe) to chronic. Cases in deer and bison are often very acute with sudden death. Deer and bison that survive for a few days usually develop bloody diarrhea, bloody urine, and corneal opacity (cloudy eyes) before dying, with high fever (106°–107°) and depression.

Other signs may include inflammation of nose and throat with copious mucus discharge; erosions and discharges containing mucus and pus from the upper respiratory tract, eyes and mouth tissues; swollen lymph nodes; lameness; and central nervous system signs (depression, trembling, lack of response, stupor, aggressiveness, or convulsions).

Various body systems may be affected in different individual animal in the same outbreak, partly depending on how long they survive. On average, the time to death in European cattle breeds is somewhat longer than in deer, bison, water buffalo, and Bali cattle.

In cattle, swollen lymph nodes and severe eye lesions (inflammation of all surfaces of the eye, inflammation in the front chamber of the eyeball, corneal cloudiness) are more frequent, and bloody diarrhea and bladder inflammation less frequent than in deer and bison.

Corneal cloudiness is a common clinical sign of MCF in cattle. Skin lesions (redness, oozing, cracking, crust formation) are common in cattle that do not die quickly. As many as 25% of the herd may experience chronic disease, and sometimes the disease comes and goes.

Mortality rates in clinically affected animals generally approach 95%. Sometimes the survival rate in a herd might be higher, although the survivors can rarely return to normal production.

A mild chronic form of MCF has been reported in some poor-doing yearling cattle that tested positive for MCF but this is very uncommon. These cattle are suspected to have underlying persistent BVDV infection but prove negative when tested for BVD virus, and further testing reveals MCF antibodies.

In recent years it’s been discovered that many of the clinically normal cattle in affected herds are latently infected with the MCF virus.

Signs

• Inflammation,
• Ulceration, and discharge from oral and upper respiratory mucous membranes;
• Eye lesions,
• Nervous system disturbances,
• Dermatitis;
• Depression;
• High fever;
• Complete loss of appetite;
• Eyes severely affected with corneal opacity causing blindness;
• Avoidance of bright light;
• Copious nasal discharges of pus and mucus;
• Crusting of the muzzle;
• Enlargement of all lymph nodes;
• Oozing dermatitis affecting the inner thigh and udder/teats;
• Sensitivity to touch especially around the head;
• Aggressiveness;
• Seizures during later stages;
• Death in five to 10 days after onset of signs

Cause

Malignant catarrhal fever (MCF) is generally caused a by virus transmitted from pregnant or recently-lambed sheep or goats to cattle although several months may elapse between the contact with sheep and clinical disease in cattle.

The actual method of transmission to cattle remains unknown. Cattle do not transmit MCF to each other and this particular virus (ovine herpesvirus 2) does not cause much problem in sheep or goats.

MCF in cattle results from infection by one of several members of a group of closely related ruminant gammaherpesviruses (Rhadinovirus genus). The principal carriers and their viruses are sheep (ovine herpesvirus-2), wildebeest (alcelaphine herpesvirus-1), and goats (caprine herpesvirus-2).

Another strain of unidentified origin has caused MCF in white-tailed deer. Nearly all clinical cases in cattle are caused by the sheep or wildebeest viruses.

These viruses are maintained within the sheep and wildebeest populations. Lambs are infected at 3 to 6 months of age by other individuals within the flock and begin to actively shed virus at 6 to 9 months of age. Adult sheep shed the virus at a much lower rate than adolescents.

Wildebeest calves, in contrast, are infected before birth by their dams, and actively shed virus until 4 to 6 months of age. Transmission is by transfer of virus-laden nasal secretions by direct contact or airborne routes.

Transmission from sheep to cattle has been demonstrated at fairly long distances and at distances of up to 3 miles in bison. In Africa, most wildebeest-associated MCF is seen around the time of wildebeest calving; however, sheep-associated MCF does not follow the same pattern.

Ewes do not shed the virus in placental tissues or secretions and do not show more frequent shedding around lambing time. Seasonality of the disease may be due to climatic influences on virus survival and age-related shedding patterns in lambs.

The causative viruses exist in nature as subclinical infections in other ruminant species (sheep, goats) that serve as carriers. The disease is becoming a significant source of economic loss in several ruminant species, particularly confined bison. The vast majority of MCF cases in the US are caused by ovine herpesvirus 2 which exists as a widespread subclinical infection in sheep.

The severity of sheep-associated virus outbreaks in cattle depends on factors such as herd numbers and density, closeness of contact, and amount of shed virus available for transmission. Cases are seen sporadically in European breeds of cattle (Bos taurus), because they are a relatively resistant species.

By contrast, Bali cattle, bison, and some but not all deer species are highly susceptible. Bison are about 1,000 times more susceptible to infection than cattle. In bison exposed to large numbers of adolescent sheep, losses can be devastating. The incubation period is variable and ranges from 14 to more than 200 days from initial exposure.

Animals that survive have lifelong infection. Some susceptible species, including cattle and bison, may be latently infected. Recrudescence (reappearing) of latent infections is possible and may be the cause in cases with no known history of contact with carriers.

Diagnosis of MCF is based on clinical signs, lesions found at necropsy, and laboratory confirmation. Other diseases that might sometimes look like MCF include bovine viral diarrhea/mucosal disease, rinderpest, infectious bovine rhinotracheitis (IBR), and East Coast fever (theileriosis).

When central nervous system signs are prominant, MCF can resemble rabies and tickborne brain infections. A history of contact with a carrier species (sheep, goats, or wildebeest) can be helpful, although recrudescent cases can occur without such history.

Prevention

No vaccine is currently available. Sheep can be free of this virus if they are weaned early and isolated from the rest of the flock. The only other effective control strategy is separation of carriers from susceptible species (keeping cattle and bison away from sheep and wildebeests).

When large numbers of potent shedders are present, such as in lamb feedlots, distances of up to a mile (between the sheep and other species) may be necessary to protect highly susceptible species such as bison.

Treatment

No treatment has been found to provide any consistent benefit, but stress reduction of subclinical or mildly affected animals may help. Affected cattle do not recover and should be euthanized for welfare reasons.