MALIGNANT CATARRHAL FEVER (MCF)

Introduction

MALIGNANT CATARRHAL FEVER(MCF) is a fatal, multisystemic disease viral disease that causes a lot of damage to domestic and wildlife creatures worldwide and is characterized by fever, ocular and nasal discharge, erosive stomatitis and gastroenteritis, erosions in the upper respiratory try tract, encephalitis, keratoconjunctivitis,  cutaneous exanthema, and lymph node enlargement. The head and eye form is most common, and there is a distinctive lesion in the cornea.

 

Etiology of (MCF)

Malignant catarrhal fever (MCF) refers to two clinically and pathologically indistinguishable diseases, each linked to a distinct infectious agent with unique ecological characteristics. The disease is associated with two viruses from the genus Macavirus, belonging to the subfamily Gammaherpesvirinae within the Herpesviridae family.

• Alcelaphine herpesvirus-1 (AlHV1) is the wildebeest-associated MCF virus, primarily transmitted to cattle from blue wildebeest (Connochaetes taurinus).
• Ovine herpesvirus-2 (OvHV2) is the sheep-associated MCF virus, that spreads to cattle from sheep.

Notably, neither virus is transmitted from cattle to cattle, and both remain asymptomatic in their natural hosts—wildebeest and sheep. While AlHV1 can be cultured in eggs and tissue culture, OvHV2 has yet to be propagated in vitro. The molecular genomic structures of these viruses have been extensively studied.

A gammaherpesvirus closely related to OvHV2 has been identified in goats (caprine herpesvirus-2, CpHV2), while another variant has been isolated from deer (deer herpesvirus, DVH). The pathogenic potential of these newly discovered viruses remains uncertain. The complete genome sequences of AlHV1 (130,608 base pairs) and OvHV2 (135,621 base pairs) have been published, further contributing to the understanding of malignant catarrhal fever (MCF).

Epidemiology of (MCF)

Occurrence and Prevalence

The occurrence of malignant catarrhal fever (MCF) is linked to a variety of natural hosts, categorized into two main groups: reservoir hosts (such as wildebeest, sheep, and goats) and clinically susceptible hosts (including cattle, bison, and deer).

Alcelaphine MCF

Wildebeest-associated malignant catarrhal fever (MCF) is prevalent in many African countries, where cattle come into contact with asymptomatic wildebeest and hartebeest. The disease follows an epizootic and seasonal pattern. Cases have also been observed in zoological parks outside Africa when susceptible species are housed near wildebeest.

Sheep-Associated MCF

Sheep-associated malignant catarrhal fever (MCF) is found worldwide, typically occurring when cattle are exposed to lambing ewes, with clinical signs appearing one to two months later. Goats can also transmit Ovine herpesvirus-2 (OvHV2) to cattle, and while rare, clinical disease has been reported in goats. Occasionally, cases arise without any known recent exposure to sheep, though this is uncommon.

The morbidity rate varies, with the disease often appearing sporadically as isolated cases or small clusters over a short period. However, in rare, severe outbreaks, up to 50% of a herd may be affected, with the disease persisting for weeks or even months.

Both forms of MCF are highly fatal, with only rare cases of recovery in cattle. In addition to cattle, MCF significantly impacts farmed deer and has occasionally been reported in pigs, particularly those exposed to sheep on farms or in petting zoos.

Transmission of (MCF)

Both Alcelaphine herpesvirus-1 (AlHV1) and Ovine herpesvirus-2 (OvHV2), the causative agents of malignant catarrhal fever (MCF), spread primarily through direct contact or aerosolized respiratory secretions. Wildebeest calves (AlHV1) and weaned lambs (OvHV2) under one year old are the main sources of viral shedding. Nose-to-nose contact is the most efficient transmission method, but indirect spread via fomites is also possible. MCF-susceptible species are dead-end hosts and do not shed the virus. In North America, nearly all cattle cases are caused by OvHV2.

Alcelaphine MCF

AlHV1 is transmitted among wildebeest horizontally and occasionally in utero. Young wildebeest (up to four months old) shed the virus in ocular and nasal secretions, transmitting it to cattle through direct contact, aerosol inhalation, or contaminated pasture. Infected cattle do not shed the virus.

Transmission can occur over at least 100 meters, leading to recommendations that cattle be kept at least 1 km from wildebeest. In Kenya, peak outbreaks align with the presence of 3- to 4-month-old wildebeest, while in South Africa, outbreaks occur when wildebeest is 8 to 10 months old, suggesting an alternative infection source. High seropositivity rates have been found in sheep within wildebeest-inhabited areas.

Sheep-Associated MCF

Nearly all domestic sheep naturally carry OvHV2 without symptoms. High seropositivity rates have been found in sheep, goats, and North American wildlife like muskoxen and bighorn sheep. Clinically susceptible species, such as deer and bison, have low seropositivity rates.

Unlike AlHV1, perinatal OvHV2 transmission is minimal, and most lambs are infected after 2 to 3 months of age, with peak viral shedding between 6 and 9 months. While OvHV2 has been detected in ram semen, venereal transmission is unlikely.

The exact route of sheep-to-cattle transmission remains uncertain but is likely via inhalation or ingestion of respiratory secretions. MCF outbreaks often follow contact with lambing ewes, though shedding does not increase during lambing. Cases have also occurred when cattle were exposed only to rams or housed in separate enclosures with no direct contact.

A Canadian outbreak saw 45 of 163 bison die after a single day of exposure to sheep at a sale barn, with deaths starting 50 days later and peaking at 60 to 70 days. Despite the high mortality rate, no bison-to-bison transmission was observed.

Some MCF cases occur in cattle without known sheep contact, and infections persist on farms despite no recent exposure. While fomite transmission is unlikely due to the virus’s fragility, latent infections in recovered cattle may be a potential source, as PCR has detected OvHV2 in asymptomatic cattle and farmed deer. Stress may trigger reactivation in animals without direct sheep exposure.

Pathogenesis of (MCF)

Malignant catarrhal fever (MCF) is a highly fatal, multisystemic disease marked by lymphoid proliferation, vascular damage, and epithelial and mesothelial lesions. These pathological changes are closely associated with lymphoid cells, with CD8+ T-lymphocytes playing a major role in vascular lesions.

The vascular inflammation leads to the gross lesions characteristic of malignant catarrhal fever (MCF), including epithelial erosions, keratoconjunctivitis, and lymph node enlargement due to abnormal sinusoidal cell proliferation. The cerebromeningeal changes, often described as encephalitis, are a form of vasculitis. Additionally, synovitis, particularly in the tibiotarsal joints, is another common manifestation linked to lymphoid vasculitis.

The exact mechanisms underlying malignant catarrhal fever (MCF) remain unclear, but it is believed to result from direct virus-cell interactions or an immune-mediated response against infected cells.

Clinical findings of (MCF)

MALIGNANT CATARRHAL FEVER(MCF) has 3 main forms  of clinical findings and are

– Head and eye form

It involves the following signs

1. Blepharospasm and uveitis
2.  Congestion of scleral vessels
3.  Hard palate lesion
4. Dorsum of the tongue lesion
5. Gums below the incisors lesion
6. Commissures of the mouth lesion
7. Inside the lips lesion
8. Extreme dejection
9. Anorexia
10. Agalactia
11. High fever (41° to 41.5° C [106–107° F])
12. Rapid pulse rate (100 to 120/bpm)
13. Profuse mucopurulent nasal discharge
14. Ocular discharge with variable degrees of corneal edema

– Alimentary form

In the peracute form of malignant catarrhal fever (MCF), the disease progresses extremely rapidly, typically within 1 to 3 days, without the hallmark head-and-eye form lesions. Affected animals exhibit a sudden onset of high fever, respiratory distress, and severe gastroenteritis, often leading to rapid deterioration.

The alimentary form presents with profuse diarrhoea and mild ocular involvement, such as conjunctivitis rather than severe ophthalmia, distinguishing it from the head-and-eye form. This variation has been observed in large dairy herds in dry lot systems, where cattle had only indirect contact with sheep, as well as in experimentally infected cattle and farmed deer.

A distinctive feature of this form is an initial phase of mild illness, followed by a sudden, severe progression, a pattern particularly common in deer, where the final stage is often rapid and overwhelming.

– Mild form

The mild form of malignant catarrhal fever (MCF) is most commonly seen in experimentally infected animals but can also occur in natural outbreaks. It is characterized by a transient fever and mild erosions on the oral and nasal mucosae.

In some cases, affected animals fully recover, while others experience recurring episodes or develop chronic MCF. A distinctive feature of the chronic form is the presence of persistent bilateral ocular leukomata, which helps differentiate it from other manifestations of the disease.

Differential diagnosis of (MCF)

1. Mucosal disease
2. Infectious bovine rhinotracheitis (IBR)
3. Bluetongue
4. Sporadic bovine encephalomyelitis
5. Rinderpest (included for historical reasons)
6. Jembrana disease
7. FMD

Treatment of (MCF)

In the treatment of a viral disease, we allow the virus to complete its course of infection but we provide supportive and symptomatic and NSAIDS usually ease the discomfort.

Control of (MCF)

Isolating infected cattle is often recommended, but its effectiveness is uncertain due to the slow transmission rate and the unclear modes of spread. Since sheep are recognized as key carriers of the disease, maintaining a strict separation between cattle and sheep is advised. To prevent outbreaks, farms with cattle should avoid introducing sheep from areas where the disease has been reported.

For petting zoos, a preventive strategy involves isolating lambs early to prevent OvHV2 infection. Currently, no effective vaccine exists, and the development of one shortly remains unlikely. Attempts to immunize cattle with live or inactivated vaccines using Freund’s incomplete adjuvant have not protected against experimental or natural exposure to wildebeest-associated strains. Although vaccination can trigger high and long-lasting levels of virus-neutralizing antibodies, humoral immunity appears to play a minimal role in resistance to the disease.

An inactivated vaccine derived from the wildebeest-associated MCF virus has shown partial protection against virulent strains. The best potential strategy for immunity is believed to be the establishment of a respiratory mucosal antibody barrier, though achieving this through intramuscular or subcutaneous vaccines remains a significant challenge.

FAQ’s

1. What is MCF?
   A severe, often fatal viral disease in cattle, bison, and deer, caused by gammaherpesviruses from wildebeest or sheep.
2. Which animals are affected?
   Cattle, bison, and deer. Wildebeest and sheep carry the virus but show no symptoms.
3. How is MCF spread?
   Through contact with carrier species (wildebeest or sheep). It doesn’t spread between susceptible animals.
4. Symptoms of MCF?
   Fever, nasal/ocular discharge, mouth ulcers, corneal opacity, and neurological signs. High mortality.
5. Can humans get MCF?
   No, it’s not zoonotic.