Rabies diagnosis for developing countries

Abstract

Background: Canine rabies is a neglected disease causing 55,000 human deaths worldwide per year, and 99% of all cases are transmitted by dog bites. In N'Djaména, the capital of Chad, rabies is endemic with an incidence of 1.71/1,000 dogs (95% C.I. 1.45-1.98). The gold standard of rabies diagnosis is the direct immunofluorescent antibody (DFA) test, requiring a fluorescent microscope. The Centers for Disease Control and Prevention (CDC, Atlanta, United States of America) developed a histochemical test using low-cost light microscopy, the direct rapid immunohistochemical test (dRIT).

Methodology/principal findings: We evaluated the dRIT in the Chadian National Veterinary Laboratory in N'Djaména by testing 35 fresh samples parallel with both the DFA and dRIT. Additional retests (n = 68 in Chad, n = 74 at CDC) by DFA and dRIT of stored samples enhanced the power of the evaluation. All samples were from dogs, cats, and in one case from a bat. The dRIT performed very well compared to DFA. We found a 100% agreement of the dRIT and DFA in fresh samples (n = 35). Results of retesting at CDC and in Chad depended on the condition of samples. When the sample was in good condition (fresh brain tissue), we found simple Cohen's kappa coefficient related to the DFA diagnostic results in fresh tissue of 0.87 (95% C.I. 0.63-1) up to 1. For poor quality samples, the kappa values were between 0.13 (95% C.I. -0.15-0.40) and 0.48 (95% C.I. 0.14-0.82). For samples stored in glycerol, dRIT results were more likely to agree with DFA testing in fresh samples than the DFA retesting.

Conclusion/significance: The dRIT is as reliable a diagnostic method as the gold standard (DFA) for fresh samples. It has an advantage of requiring only light microscopy, which is 10 times less expensive than a fluorescence microscope. Reduced cost suggests high potential for making rabies diagnosis available in other cities and rural areas of Africa for large populations for which a capacity for diagnosis will contribute to rabies control.

Figure 1. Picture of a positive result by the dRIT of a Chadian rabies sample, Magnification 400×.

Figure 2. Picture of a negative result by the dRIT of a Chadian rabies sample, Magnification 400×.

Figure 3. Phylogenetic tree of representative of all African groups (1, 2 and 3) after Kissi et al. (1995) and SHBRV as a root.

Branch lengths correspond to differences between sequences (scale at the bottom of the figure). Only significant bootstraps values are shown. The samples of our study are labelled as Chad+number.