Rabies Virus Neutralizing Antibodies in Unvaccinated Rabies Occupational Risk Groups in Niger State, Nigeria

Aims: To determine the presence of rabies virus neutralizing antibodies (rVNA) as well the potency
of the rVNA in rabies occupational risk humans in Niger State of Nigeria.
Study Design: Cross-sectional study.
Place and Duration: Research was conducted at the Department of Veterinary Public Health,
Ahmadu Bello University, Zaria, Nigeria and Rabies Unit, Centers for Disease Control and
Prevention, CDC, Atlanta, USA, between May, 2012 and March, 2013.
Materials and Methods: A total of 185 human volunteers were recruited from rabies risk
occupational groups who filled a structured questionnaire on their previous bite history and
vaccination status, between May and July, 2012. A 2 ml each of blood from volunteers was collected
and centrifuged at 3000 rpm for 10 minutes and sera separated into pre-labeled vacutainers.
Standard Rapid fluorescent focus inhibition test (RFFIT) was used to detect the presence of rVNA in
the sera. Further end point titration of the rVNA positive human sera was conducted to determine the
potency.
Results: The results indicated that, detectable titre of rVNA was recorded in 16.4% (23 of 140)
viable human sera screened. Although from the questionnaire survey, 21.7% (5 out of the 23
positives) responded to have been vaccinated over ten years prior. At least 3 of the respondents (1
dog butcher and 2 dog meat consumers) who responded not previously vaccinated had some
neutralizing antibody titre range of 0.65 – 0.7 IU/ml which is above the minimum protective titre
(0.5IU/ml) recommended by WHO. Similarly, 3 respondents (2 veterinarians and 1 animal health
personnel) who responded to have been previously vaccinated (> 10 years earlier) yet had a high
titre range of 0.5 – 5.4IU/ml. The highest specific rate for rVNA of 25% each was seen amongst the
dog butchers and pet owners followed by hunters (20%) and dog meat consumers (14.8%). Up to
125 (67.6%) of the volunteers do consume dog meat with only 12 (9.6%) of them being dog butchers
who source dogs for slaughter from households within and outside their territories.
Conclusion: Although the WHO minimum protective titre of rVNA is 0.5 IU/ml, the presence of
relatively high titres amongst these risk groups in this report is an indication of a serious public health
threat. This study recommends the vaccination of rabies high risk groups and further screening of
rabies occupational risk and non risk groups in the study area and Nigeria at large. This study
concludes that there is serological evidence of rVNA in the serum of vaccinated and unvaccinated
rabies occupational risk groups in Nigeria. Rabies occupational risk groups are at risk of contracting
rabies without overt symptoms of rabies, but serological response with some reasonable titres that
may or may not be protective.