Dynamic study to evaluate post exposure prophylaxis against rabies in canines

Introduction

Rabies is an alarming infectious as well as zoonotic disease because of its high incidence, world-wide distribution, treatment costs, mortality rate and high disability-adjusted life years (DALY) score. Out of 60,000 human rabies victims reported globally, 20,000 rabies deaths are only from India and it accounts for 59.9 percent of deaths in Asia and 35 per cent of deaths globally with over 3.7 million DALY score (WHO, 2018) [9] . The disease is still underreported and neglected due to lack of awareness, resource and integrated surveillance. It is almost invariably fatal once clinical signs occur, as a result of acute progressive encephalitis. Although a number of carnivores serve as natural reservoirs, dogs are the main source of human infections and pose a potential threat to more than 3.3 billion people worldwide (WHO, 2010) [8] . In India, risk of exposure was high because of the massive dog population of about 11.67 million with an estimated dog: human ratio of 1:36 in the year 2012 as evidenced by Department of Animal Husbandry and Veterinary Services (19 th Livestock census by Government of India) and it was estimated that 97 per cent of human rabies are due to dog, followed by cats (2%) and 1% by jackals, mongoose and others (Cliquet et al., 2007) [1] . The probability of success of rabies vaccination in dogs depends on the type of vaccine used, number of boosters and age of vaccination. Even though prophylactic vaccinations are being done regularly by mass vaccination of domestic dog, the effects of these vaccinations are not being assessed. So, it is imperative to know whether dogs in a representative population have antibody titres above the protective level (≥ 0.5 IU/ml) after the prime vaccination and also to know whether the antibody titres are maintained above the protective level until the time of annual revaccination. Considering these views and overall significance of rabies in India, the current study has been undertaken to ascertain antibody titre against rabies in dogs complying with standard post exposure prophylaxis schedule.

Materials and Methods

The present study was carried out in dogs brought at Immunization Unit, Madras Veterinary College Teaching Hospital, Chennai between September 2017 and November 2018. The longitudinal rabies vaccine efficacy study was also conducted to elucidate the frequency and magnitude of rabies virus neutralization antibody (RVNA) in post-exposure prophylaxis (PEP). The rabies vaccine (Rahsharab, Indian Immunologicals) produced with BHK 21 cell substrate, inactivated by β-propiolactone and adsorbed on aluminium phosphate with the potency of greater than or equal to 2.5 IU per dose was used for immunization in this study. In this study, the dogs were immunized with inactivated rabies vaccine subcutaneously and the blood samples were collected on days 0, 3, 7, 14, 28 and 90 post vaccination (dpv) for PEP and sera were obtained after centrifugation at 2000 rpm for 30 mins and stored at -20 ͦ C until tested for rabies antibody quantification. FITC-labelled monoclonal rabies antibodies (M/s Merck Millipore, Germany)-Concentrated blend (5 mL) and Anti-Rabies Nucleocapsid Conjugate (M/s Bio rad, USA)-lyophilized (3 mL) were used. The serum samples were subjected to Rapid Fluorescent Focus Inhibition Test (RFFIT) to estimate the functional virus neutralizing antibodies against rabies in the test serum and the procedure was carried out as per Smith et al. (1996) [7] .

Results and Discussion:

The mean titre values ranging between 1.38 and 14.35 IU/mL was found from dpv 0 to dpv 90 (Figure 1) of 18 dogs, only four dogs had a protective titre on dpv 0 of post exposure prophylaxis study. The percentage of dogs with adequate antibody concentration (more than 0.5IU/mL) was 94.44 per cent at dpv 3 and 100 per cent at dpv 7 with mean antibody titre of 6.51±1.98 and 15.74±4.50 IU/mL, respectively. At dpv 28, dogs elicited peak immune response with mean titre of 26.61±6.30 IU/mL of 18 dogs recruited, four dogs were not return on dpv 90. The statistical analysis revealed a decrease (p<0.01) in mean antibody titre (14.35±2.36 IU/mL) on dpv90 (Table 1) but antibody persistence and the percentage of dogs with adequate antibody concentration was remained higher than the protective level. All animals which received PEP were observed for 120 days and none of the animals showed clinical signs during the observation period. Out of 18 dogs, four dogs (22.22%) showed a protective tire on dpv 0 of post-exposure prophylaxis. This pre vaccination borderline protection of dogs might be due to earlier exposure to very mild antigen or regular prophylactic vaccination as reported by Fekadu (1988) [4] . The rest of 14 (77.7%) dogs showed titre values less than 0.5 IU/mL ranging from 0.07 to 0.4 IU/mL. All post vaccinal sera except one showed protective titre on dpv 3. This was correlated with Manickam et al. (2008) [6] who challenged dogs intramuscularly with virulent rabies virus brain suspension and injected anti-rabies vaccine and found RVNA after 3 days of post vaccination with peak titre on dpv28. The percentage of dog with adequate antibody concentration were 100 per cent at dpv7, with peak antibody concentration on dpv28 (26.61±6.30 IU/mL) and waning of same on dpv90 (14.35±2.36 IU/mL). Similar observations were documented earlier by Sunilkumar et al. (2016) [10] and Minke et al. (2009) [11] . Whereas, Shi et al. (2017) [12] observed peak immune response at dpv42 with mean of 12.29±9.751 IU/mL. Irrespective of bite site, age, sex, previous vaccination history, all dogs showed protective titre by dpv 7 which support the use of potent cell culture inactivated rabies vaccine for PEP. But, the initial elevation of antibody response in vaccinated animals was important to neutralize the virus. Our findings were also supported by Manickam et al. (2008) [6] who showed that a high and sufficiently protective titer was produced after 5 vaccinations up to 28 days. But some studies conducted earlier by Clark & Wilson (1996) [3] included a booster vaccination on the day 90. Though neutralizing titre appeared to be waning in our study, it was likely that the immunity thus stimulated will last for a much longer period. Also, the increased potency of ≥ 2.5 IU/dose veterinary rabies vaccines was expected to contribute to better PEP results. PEP vaccination of dog by employing Essen regime (0, 3,7,14 and 28 day) was found to be protective in all vaccinated animals which was accordance with the observation of Basheer et al. (1997) [2] . But, Hanlon et al. (2002) [5] documented that rabies post-exposure treatment (PET) with vaccine alone protocol without immunoglobin was unable to provide protection against disease following severe exposure to rabies virus. Although many PEP regimens have been proposed in human, it is still a controversial topic in animals. But, the Essen regime of PEP schedule of human beings is followed for animals also in veterinary clinical practice with a hypothetical idea of dumping antibody in the central nervous system by crossing the blood brain barrier through a series of vaccination regimen.

Conflict of Interest Not available

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References

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