Malignant Catarrhal Fever MCF

Malignant Catarrhal Fever

Malignant catarrhal fever (MCF) is an infectious and usually a fatal viral disease of ruminant species. The causative agent is a gamma herpes virus. There are two endemic forms of MCF with distinct geographical distributions; Alcelaphine herpesvirus 1 (AlHV-1), which is endemic in wildebeest and causes the wildebeest-associated form of MCF in cattle and other species. This is prevalent in South Africa. The other form, which is an emerging disease in India is Ovine herpesvirus 2 (OvHV-2). It infects the domestic sheep asymptomatically and but causes symptomatically conspicuous sheep-associated MCF (SA-MCF) in large ruminants, such as cattle, deer, bison, water buffalo and pigs. SA-MCF has been reported worldwide. It is sporadic disease rarely endemic as the susceptible ruminant population is the dead end host. The incidence of SA-MCF is present in all seasons.

Transmission: OvHV-2 is transmitted by contact or aerosol, from carrier lambs under 1-year old. The disease can also be transmitted by goats which also are the asymptomatic carriers of the disease. Incubation periods after experimental inoculation of cattle are 2–12 weeks. The clinical disease occurs when the susceptible host are in contact with asymptomatic sheep/goat either directly or indirectly. The respiratory tract is a common route for both entry and shedding of the virus. Infectious OvHV-2 is present in ovine nasal secretions, adult sheep can be infected by OvHV-2 via horizontal transmission through natural contact and it may intermittently shed large quantities of virus from nasal secretions which when transmitted to the susceptible cattle causes a fatal disease.

Clinical symptoms : The disease MCF occurs in different forms; 1) a head and eye form, 2) a peracute form, 3) an alimentary tract form and 4) a mild form. Typical signs of the head and eye form are opacity of the cornea, severe symptoms such as extreme dejection, agalactia, high fever (41– 41.5°C), rapid pulse rate, dyspnea due to nasal cavity obstruction, anorexia, lymphadenopathy, blepharospasm and congestion of scleral vessels. Early stage may involve nervous signs with weakness in legs, incoordination, muscle tremor and demented appearance, whereas in the final stages headpressing and paralysis are typical. The second form usually lasts for only 1-3 days and typical signs are gastroenteritis, high fever and dyspnea. The third form has several of the typical symptoms of head and eye form except that there are only minor eye changes and pronounced diarrhoea. The mild form is, rare, and occurs most commonly in experimental animals. Mild erosions may be seen on the oral and nasal mucosa Chronic cases have been described in cattle and bison whereas recovery of MCF have been reported in cattle that remained persistently infected afterwards.

Diagnosis: Due to similarity in symptoms with Bovine Viral Diarrhoea, Vesicular Stomatitis, Infectious Bovine Rhinotrechietis etc clinicians and veterinarians face significant challenges in diagnosing MCFV infection and/or disease in field conditions. Adding further to it is the requirement of sophisticated laboratory assays for confirmatory diagnosis. Polymerase Chain Reaction Test has become an important tool in MCF molecular diagnostics. In this assay the primers target a DNA fragment in the ORF 75 of OvHV-2, a gene coding for a viral tegument protein. The specificity for OvHV-2 arises from one of the primers (#556), which binds to a region of low homology between OvHV-2 and AlHV-1.This nested PCR has high sensitivity and is validated for detection of OvHV-2 DNA in infected sheep as well as in animals with clinical MCF. Recently real time PCR for OvHV-2 DNA has also been developed. Although serological test was also developed and commercial kit was available earlier but it seems to have been withdrawn due to cross reactivity and other inherent problems in the kit. Adding to recent advances to the diagnosis to the disease, real time PCR has been developed which is a very sensitive test. A multiplex PCR has also been developed which can be used to diagnose the disease in different wild and domestic ruminants in one test only. Despite numerous efforts, the virus has not been isolated world wide and therefore the demonstration of Genome by PCR is considered the gold standard for the diagnosis of the disease.

Before the introduction of molecular detection tools and tests histopathology was considered as definitive diagnostic test for MCF and still has a distinct place along with genomic detection. The pathognomic lesion is necrotising vasculitis which shows infiltration of tunica media and adventitia by lymphoblast like cells and macrophages, perivascular, mononuclear cell cuffing is seen in most of the organs and an obliterative arteriopathy is a characteristic vascular lesion.

Control and prevention: There is no treatment for the disease, symptomatic treatment rarely helps. There are no vaccines available till date. In India a mixed livestock farming system of cattle with sheep and goats, is most common which leads to increased chances of close contact of carrier animals (sheep and goat) with clinically susceptible animal such as cattle and buffaloes while housing or grazing. Since the main source of infection in these farms is the lambs, 9-12 months of age which shed virus in high quantities certain points to keep in mind for control of the disease are enlisted below:

• Segregation of the susceptible host from the reservoir hosts is most important step for control of the disease.
• If cattle and sheep are reared under natural flock conditions, the grazing areas should be separate.
• It has been suggested that grazing sheep in the morning hours and cattle in the evening hours on the same land patch may also help as the virus may be destroyed in the day’s sun as it is very heat labile.
• A program to produce sheep free of infection should be initiated in which; lambs are removed from contact with infected sheep prior to 2 months of age. This knowledge should be used by sheep producers and zoos to produce sheep free of OvHV-2.
• Zoological parks should only introduce sero-negative animals only and follow strict quarantine restrictions of newly acquired animals.

Systematic surveillance program has been initiated as a collaborative effort at ICAR- National Institute of High Security Animal Diseases and ICAR-National Institute Of Veterinary Epidemiology And Disease Informatics, with emphasis on the epidemiology of the disease in sheep and goat for identification of transmitting flocks along with vigilance for clinical cases of MCF in other domestic ruminants to ascertain the current status of the disease in the country.