Transmission of MCF-associated viruses
Both AlHV-1 and OvHV-2 appear to be transmitted by contact or aerosol, mainly from wildebeest calves (AlHV-1) and lambs (OvHV-2) under 1-year old (Mushi et al., 1981, Baxter et al., 1997 and Li et al., 1998). Incubation periods after experimental inoculation of cattle are 2–12 weeks (Plowright et al., 1975, Buxton et al., 1984 and Taus et al., 2006).
The causal viruses are passed between individuals of the reservoir hosts and from reservoir to MCF-susceptible species by the horizontal route, although vertical transmission has been inferred
from the detection of anti-MCF virus antibodies in the serum of some gnotobiotic or specific-pathogen-free lambs (Rossiter, 1981) and from recovery of AlHV-1 from a wildebeest fetus (Plowright, 1965). The principal source of free virus in wildebeest is in the tears and nasal secretions (Mushi et al., 1981). OvHV-2 viral DNA also has been detected in samples from the alimentary, respiratory and urogenital tracts of sheep (Hüssy et al., 2002). This may account for some infection of offspring occurring during or shortly after lambing or calving.
Experimental induction of MCF in cattle has been achieved using wildebeest nasal secretions containing AlHV-1 (Plowright, 1964). Infectious OvHV-2 is present in ovine nasal secretions, but appears to be difficult to isolate from this source, since the period of virus shedding is short for any given animal (Kim et al., 2003 and Li et al., 2004). OvHV-2 collected from ovine nasal secretions will infect naïve sheep (Taus et al., 2005) and also can induce MCF in cattle and bison (Taus et al., 2006).
While sheep and bison can be infected by intranasal nebulisation with 103
genome copies of OvHV-2, infection of cattle is not reliable, even at 1000-fold higher doses (Taus et al., 2005 and Taus et al., 2006). At very high doses, intranasal inoculation of OvHV-2 induced MCF-like clinical signs in naïve sheep, confirming a previous report that this carrier species can develop a mild form of MCF (Buxton et al., 1985 and Li et al., 2005a).
The MCF-susceptible species generally are thought to be dead-end hosts that do not transmit virus to other animals, which has the beneficial effect of limiting the spread of disease during outbreaks. Some transmission between infected deer has been reported, although such cases appear to be unusual (Reid et al., 1986). The reason for lack of spread between MCF-susceptible animals is likely to be that the virus replicates in a cell-associated manner in these species and cell-free virus is not produced. Malignant catarrhal fever: a review. [Vet J. 2009] - PubMed - NCBI