The principal differential diagnoses are PPR in sheep and goats and bovine viral diarrhoea/mucosal disease and infectious bovine rhinotracheitis in cattle. Malignant catarrhal fever also resembles rinderpest except that it is non-contagious. Foot-and-month disease causes lesions in the month but these are primarily vesicular; in any case rinderpest does not cause lameness.
However, clinicians should not attempt to differentiate rinderpest from other diseases causing high fever, oral erosions, diarrhoea and death. All such cases should be reported as rinderpest suspect cases. Further differentiation is a matter for laboratory specialists.
It is important to emphasise that there is no fixed number of animals that have to be involved before an outbreak can be diagnosed as rinderpest. Outbreaks involving large numbers of animals do occur but they usually start after failure to recognise the index case. Therefore in its early stages an acute rinderpest outbreak consists of one case. In its later stages it may consist of hundreds.
In an endemic area most adult animals may be immune, having recovered from an earlier infection or having been vaccinated. Under these circumstances clinical disease will probably be largely (though not necessarily exclusively) confined to yearling animals that have recently lost their maternal immunity. Here we do not expect an explosive outbreak but rather a grumbling problem involving only a few animals at a time, usually with a sub-acute presentation. Finding such a situation requires an alert veterinary service.