Sheep Associated Malignant Catarrhal Fever in cattle
Amir Amin Sheikh
Division of Veterinary Physiology and Biochemistry, F.V.Sc & A.H., R.S.PURA, SKUAST-Jammu, Jammu and Kashmir
Introduction
Malignant catarrhal fever (MCF) is a fatal lymphoproliferative disease of cattle and other ungulates caused by the ruminant Gamma-herpesviruses, Alcelaphine herpesvirus-1 (AlHV-1) and Ovine herpesvirus- 2 (OvHV-2). Two endemic forms of MCF with distinct geographical distribution exist worldwide. Alcelaphine herpesvirus-1 (AlHV-1) naturally infects wildebeest and causes wildebeest associated MCF (WA-MCF) in cattle in regions of African sub-continent. The Ovine herpesvirus-2 (OvHV-2) infects all varieties of domestic sheep as a sub-clinical infection and causes sheep associated MCF in susceptible ruminants in most regions of the world. These viruses cause inapparent infection in their reservoir hosts (wild-beest for AlHV-1 and sheep and goat for OvHV-2) but fatal lymphoproliferative disease when they infect MCF-susceptible hosts (Bison for AIHV-1 and cattle, water buffalo, bateng, antelopes and pigs for OvHV-2).
Sheep associated malignant catarrhal fever is an emerging important disease and it is particularly significant in the Indian context where mixed farming is common practice. In India, there is mixed livestock farming system of cattle with sheep and goats which leads to increased chances of close contact of carrier animals with clinically susceptible animals. In India, the detection of cases of SA-MCF in cattle and OvHV-2 infection in sheep during the last decade has established the presence of the virus in native sheep of the country. This disease is on the list of notifiable disease to the World Organization for Animal Health (OIE). Due to recent clinical cases of sheep associated malignant catarrhal fever in cattle in India and subsequent detection of OvHV-2 in these animals, this virus has caught attention in India also.
Etiology
The disease is caused by Ovine herpes virus-2 (OvHV-2) which is enzootic worldwide in domestic sheep and transmitted to a wide variety of domestic and wild ruminants, including pigs. This virus is belonging to family- Herpesviridae, subfamily – Gammaherpesvirinae and genus-Macavirus.
Host range
Sheep are the original host but the infection may naturally be transmitted to cattle, bison, deer and pigs. Goats can also act as a source of infection for cattle. There is a wide spectrum of species-susceptibility to OvHV-2-induced disease, ranging from relatively resistant cattle species and cases are usually sporadic, to much more susceptible deer species, bison and water buffalo to extremely susceptible Bali cattle and deer.
Transmission
The OvHV-2 is transmitted by contact or aerosol, mainly from less than a year old lambs. Lambs are infected usually at 3-6 months of age by aerosol transmission from other individuals within the flock and begin to actively shed virus at 6-9 months of age. Shedding decreases at 10 months with adults shedding at a much lower rate than adolescents. 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.
Clinically susceptible species acquire the virus through inhalation, although ingestion of virus-laden secretions from contaminated feed or water. Colostrum and milk samples from infected ewes are strongly positive for OvHV-2 DNA, which suggests that mammary secretions could serve as an important source of infection for newborn lambs. Semen of rams and suggested an important role for sexual transmission of this disease.
Incidences and Epidemiology in India
Overall prevalence of OvHV-2 infection in the cattle in the Kashmir region was estimated to be less than 1 per cent. Captive female bison died within three days of development of clinical symptoms from Bannerghatta National Park, Bangalore. Investigated outbreak of sheep associated malignant catarrhal fever in crossbred cattle in a village of Andhra Pradesh has been reported between December and January 2013–14. Also it has been reported that the prevalence of OvHV-2 infection in sheep population of Karnataka was 24.44 %.
Pathogenesis
The pathogenesis of MCF consists of three components:
- T-lymphocyte hyperplasia in lymphoid organs and accumulation of these cells in non-lymphoid tissues
- Epithelial degeneration/necrosis and hyperkeratosis
- Vasculitis
Clinical signs
Incubation period of the disease ranges from 11-73 days. Clinical signs of MCF in cattle are presented in distinct forms of clinical disease: peracute form, head and eye form, alimentary form, neurological form and cutaneous form.
Per-acute form
This form lasts for 1–3 days. There is usually high fever, dyspnoea and an acute gastroenteritis.
Head and eye form
Typical signs of head and eye form are opacity of the cornea, with a narrow grey ring at the corneo–scleral junction shown in fig. 1, 2 and 3. The characteristics of this form include high fever (40–41 °C), mucopurulent nasal discharge, dyspnea due to nasal cavity obstruction, lymphadenopathy and blepharospasm. The muzzle and nares are usually encrusted and the superficial lymph nodes are often markedly enlarged in cattle.
|
|
|
The alimentary form
The alimentary form has several of typical symptoms of head and eye form except that there are only minor eye changes and pronounced diarrhea. The oral mucosa is often hyperemic and may contain multifocal or diffuse areas of necrosis shown in fig. 4. Erosions may be found at the tips of the buccal papillae.
 |
|
| Fig. 4: Inflamed, discolored and eroded oral papillae |
Neurological form
Early stages of neurological form involve nervous signs with weakness in one leg, in-coordination, muscle tremor and demented appearance. The final stage of this form includes head pressing, nystagmus and paralysis which are are typical signs.
Cutaneous form
In this form skin is erythematous or ulcerated and hardened; scabs may develop particularly on the perineum, udder and teats shown in fig. 5 and 6. Horn and hoof coverings may be loosened or sloughed. The joints may be swollen and milk production often drops.
 |
|
 |
|
||
Post mortem lesions
Gross findings at post-mortem examination include petechial haemorrhages on the tongue, buccal mucosa, in the gastrointestinal and respiratory tracts and urinary bladder. Commonly, there are raised pale foci on the surfaces of the kidneys and these may extend into the cortex. There is general enlargement of lymph nodes.
Diagnosis
Diagnosis of SA-MCF poses significant challenges to veterinarians due to multi-systemic involvement of the disease and symptomatic resemblance to many other diseases in the field and its complex pathogenesis. A complete history of the affected animal along with clinical symptoms and its contact with carrier species, grazing habits, calving, type of rearing etc. are important to arrive to a tentative diagnosis. However, confirmatory diagnosis is based on laboratory tests.
Histopathological examination
Samples will be obtained from the lungs, heart muscle with carotid artery, brain (cerebrum, cerebellum, midbrain), lymph nodes, tonsil, liver, kidneys, adrenal glands, ureter, tongue, bile duct, bronchial wall, blood vessels and eye (cornea, lens, choroid).
Histopathological examination demonstrated fibrinous oedema and necrosis in the lymph nodes, vasculitis in blood vessels of the brain and kidney. Mononuclear infiltration in the adrenal glands, necrotic liver and chronic cholangitis and infiltration of the lymphocytes in the portal areas.
Serological test
Antibody of OvHV-2 can be detected by using AIHV-1 as the source of antigen. Domestic sheep consistently have antibody that can be detected by immunofluorescence, ELISA or immunoblotting. Complement fixation test and competitive inhibition enzyme linked immunosorbent assay can also be used.
Differential diagnosis
The clinical differential diagnosis of MCF should include bovine viral diarrhea/mucosal disease, foot and mouth disease, blue tongue, rinderpest and infectious bovine rhinotracheitis but the stated diseases do not show ocular (kerato-conjunctivitis), enlargement of lymph nodes and signs of encephalitis
Treatment
The recent availability of derivatives of acyclovir compounds that inhibit replication of herpesviruses shows promise in potential treatment regimens. In order to prevent secondary bacterial infection drugs like oxytetracycline and sulphadimidin, procaine penicillin,ceftiofur sodium can be administered through parenteral route. Supportive treatment with flunixin meglumine and vitamin A has been suggested. Corticosteroid may be used as a life saver.
Prevention and control
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. 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.
- 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.
- Zoological parks should only introduce only sero-negative animals.
- The distance of separation between sheep and susceptible species should be as great as possible that should be at least 1000 meters if not further.
Conclusion
With spread to a large part of the world, MCF has become one of the emerging diseases of ruminants. Due to its sporadic nature and overlapping clinical signs it produces, the disease is now receiving due attention in our country. With development of PCR based assays and their improved versions, it has now become easier to know the status of this disease. Currently, there is no policy in place related to the control of MCF. However segregation of the susceptible host from the reservoir hosts is most important step for control of the disease. A program to produce sheep free of infection should be initiated. Zoological parks should introduce only sero-negative animals.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3832689/
https://www.pashudhanpraharee.com/theileriosis-in-goat-sheep/
