Previous Page Table of Contents Next Page

DIFFERENTIAL DIAGNOSIS


· Nairobi sheep disease - no hepatitis, not in newborn lambs

· Bluetongue - mouth and foot lesions (coronitis)

· Heartwater - serous fluids in body cavities; neurological signs

· Ephemeral fever - recumbency and rapid recovery

· Wesselbron - rare viral disease, less severe than RVF

· Toxoplasmosis, leptospirosis, brucellosis, Q fever, salmonellosis - basic diagnostic methods for differentiation

· Peste des petits ruminants - high mortality in lambs

· Foot-and-mouth disease - neonatal mortality and abortions in small ruminants

Single cases of RVF can be confused with many viral diseases, which cause sudden death in sheep and produce generalized lymphadenopathy and petechial and ecchymotic haemorrhages throughout the carcass. However, RVF manifests itself in a dramatic fashion with the following:

The diseases that may be manifested in this manner are described below.

Nairobi sheep disease causes abortions, high fatality rates and produces gastroenteritis in sheep and goats. This does not show a higher pathogenicity for neonates, however, which is a feature of RVF, and while it does cause abortions, these and the clinical disease are of a more sporadic nature. Deaths are usually in older animal age groups, and the carcasses have similar haemorrhages, but there is no hepatitis.

Bluetongue causes a febrile disease often with diarrhoea, but also causes muzzle oedema and mouth lesions, which will be obvious in some cases. Hyperaemia and erosion of the buccal mucosae, lameness and coronitis with skin hyperaemia will assist in making a clinical differentiation. Sudden deaths at the viraemic stage produce generalized petechial and ecchymotic haemorrhages, which on postmortem appear similar to RVF. There is no hepatitis.

Heartwater can cause sudden death with lymphadenopathy and generalized haemorrhages throughout the carcass. There is no hepatitis and usually the fluid in the serous cavities will be excessive and obvious. Neurological signs can be seen. Brain smears can be prepared to make a definitive diagnosis.

Ephemeral fever produces a clinical syndrome in dairy cattle that is very similar to RVF. There is a sudden onset of fever of a similar nature to RVF but generally more severe. The dysgalactia that occurs is the same, together with nasal and ocular discharges. However, the muscle weakness and recumbency, which are a feature of ephemeral fever cases, do not occur with RVF. Ephemeral fever does not produce any disease in sheep, goats or young cattle.

Wesselbron virus has been confused with RVF in South Africa, where it appeared to produce similar lesions and occurred in similar circumstances. This has not presented a problem elsewhere in Africa, nor in subsequent epizootics in South Africa.

Toxoplasmosis, leptospirosis, brucellosis, Q fever and salmonellosis all feature as possible differential diagnosis for RVF. However, they are not present in such an explosive manner over large areas simultaneously. They are not associated with rainfall, nor do they produce such high neonatal mortality. Good supportive laboratory competency is required to make a diagnosis.

Diagnosis of RVF

RVF antigen detection

RVF antibody detection

The International Office of Epizootics (OIE) Manual of standards for diagnostic tests and vaccines contains guidelines on the collection of samples and the diagnostic techniques for diagnosis of RVF infection.

RVF should be suspected if there is a sudden onset of large numbers of abortions in cattle, sheep, goat or camel populations associated with a high neonatal mortality and the presence of liver lesions. Cases of disease in people associated with the affected animals also assist in making a tentative diagnosis. A provisional diagnosis may be based upon the clinical picture, climatic and ecological factors such as the presence of huge mosquito populations, together with the explosive nature and onset of the disease.

Laboratory confirmation of RVF

RVF is a member of the human haemorrhagic fever group of viruses, such as Ebola and Crimean Congo haemorrhagic fevers. These viruses present a serious hazard to all personnel handling infected carcasses, blood and other tissues both in the field abattoir and in the laboratory. For this reason it is recommended that field veterinarians and laboratory personnel be vaccinated against RVF, if this is possible. The handling of RVF infected material should only be carried out under P-2/P-3 conditions or with type II biosafety cabinets and HEPA filtered respirators, where the security of the staff can be assured. For this reason, the appropriate diagnostic procedures are dependent upon the facilities available.

Diagnostic tests

There are two types of test. The first is to identify or isolate the RVF virus or antigen and the second to demonstrate the presence of rising titres of RVF specific antibody or IgM. The test system chosen will depend upon the facilities available that can be safely used.

Detection of RVF virus/antigen

Control of infection in case of haemorrhagic fever in African hospitals

WHO

Detection of specific antibody to the RVF virus

*These tests may detect low titre cross-relationships with other Phleboviruses, but high titre positives will be specific, i.e. 1/160 to 1/320 or greater.

RVF virus antigen in cell cultures 24 hours post inoculation with specimens for diagnosis

GLYN DAVIES

Specimen collection for RVF diagnosis

Refrigerate but do not freeze.

All tissue samples should be transported preferably in a phosphate buffered saline/glycerol suspension. Samples in buffered formolin may be transported in unfavourable conditions for many days without any deterioration. See the OIE Manual of standards for diagnostic tests and vaccines for further details.

What samples should be collected in an outbreak?

At an outbreak site, where sheep, cattle or camels are aborting and there are deaths in neonates, it is suggested that the following samples be collected:

What information is required?

The following basic information should be collected:

Previous Page Top of Page Next Page