STUDIES ON THE EPIDEMIOLOGY, DIAGNOSIS AND CONTROL OF PESTE DES PETITS RUMINANTS (PPR) IN PAKISTAN

Abstract

The current study reports the prevalence and distribution of peste des petits ruminants (PPR) in the small ruminant population of Pakistan. The objectives of the study were to conduct epidemiological analysis of PPR outbreaks in Pakistan, isolation and characterization of PPRV from field cases, to study the sero- epidemiology of PPR, to assess virulence of a field isolate of PPRV and to evaluate the duration of immunity and protective efficacy of homologous PPR vaccine. The epidemiological analysis of PPR outbreaks in the small ruminant population of Pakistan was conducted during 2005-07. A total of 62 outbreaks were investigated among sheep and goat flocks in the country's four provinces as well as Northern Areas, and Azad Jammu & Kashmir (AlK). The disease was confirmed by immuno-capture ELISA (Ic-ELISA), competitive ELISA (cELISA) and virus isolation. The epidemic curve was drawn in one of the outbreak, it resembled a typical propagated outbreak with peaks of primary, secondary and tertiary generations of cases spaced by incubation period of 3- 5 days. The overall cumulative morbidity and mortality was estimated as 69.18% and 2 1.79% respectively with a case fatality of 3 1.49 %. Species specifi c cumulat ive morbidity, mortality and case fatality for sheep were 5 1.54%, 16.73% and 32.46% respectively. These figures for goats were on the higher side with estimated values of 76.54%, 23.90% and 3l.22% respectively. PPR virus was isolated from six different field outbreaks. The PPRV isolates were identified by using Ie-ELISA and later by reverse transcriptase polymerase chain reaction (RT-peR). A cross sectional sero survey was conducted during 2005-06 to estimate the sero prevalence of PPR in the country. A total of 2798 sera samples were collected including goats (1979) and sheep (819) from randomly selected 152 vi llages in 27 districts. These were tested using cELISA for the presence antibodies against PPRV and true prevalence estimates were calculated by Rogan and Gladen estimator. The overall seroprevalence of PPR was estimated as 48.45%, the corresponding estimates for goats were 52.89% and 37.72% for sheep. The estimated values of basic case reproduction number (RO) and herd immunity threshold (HIT) were 2.12 and 52.89% for goats and l.61 and 37.72% for sheep population respectively. A linear relationship was found between seropositivity for PPR and age of the animal in goats. The virulence profile of a local isolate of PPRV was established in goats. The disease was experimentally reproduced in susceptible goats by three different routes i.e. subcutaneo us, intra nasal and thro ugh direct contact with chall enged animals. An acute form of disease was observed in animals chall enged subcutaneo usly and intranasally. While per acute form of disease was observed in animals kept in contact with challenged animals. The PPRV shedding by chall enged animals was detected by RT- PCR and the humoral response was studied using cELISA. All the experimentally infected goats di ed of PPR and the chall enge vi rus was isolated from lymph nodes of a dead goat. The isolate appeared to be hi ghl y virulent. The molecular characteri zation of the Pakistani iso lates of PPRV was carried out by employing RT-PCR with primers based on highly conserved sequences within F gene of PPRV and representative samples were sequenced. The sequ ence data of F gene of PPRV was analyzed for identities and a phylogenetic tree was generated. The phylogenetic tree based on 372 bp F gene sequences of PPRV clustered the present study iso lates into lineage 4 along with other Asian iso lates. The recent iso lates and a prev ious iso late from Pakistan (PAK- 2004) were found to be monophyletic and have close relationship with an Indian iso late (IND- PON) collected during 200 1. The genetic variations in the fus ion (F) protein suggested that PPR has estab li shed as an endemic infection in the country. The durat ion of immunity and protective efficacy of homologous PPR vaccine was investigated. For this purpose one hundred and five sheep and goats aging between 5 months to 7 years were divided in three groups. Fi rst group received normal recommended dose (l.0 ml) ofPPR vaccine, the second group received half dose (0.5 ml) of PPR vaccine and third group was kept as unvaccinated in contact control group. The experimental animals were sampled at defined intervals for three years. The post vaccination dynamics of anti bodies against PPR virus was studi ed. It was found that significant antibody titres persisted for 3 years post vaccination in sheep and goats vaccinated with either full dose or half dose of PPR vaccine. The treatment (vaccination) and the month of sampling appeared to be strongly associated with the cELISA percent inhibition (PI) valu es. The challenge protection studies were carri ed out. The vaccinates withstood challenge and were found completely resistant c linically and virologically to virulent PPR virus 24 and 36 months post vaccination. 2 The shedding of challenge virus was not detected 111 the ocular, nasal and oral secretions of vaccinates by using RT-PCR. The chal lenged animals remained protected and free of any clinical signs of PPR through out the observation period. The unvaccinated contro l animals developed typical clinical signs of PPR and the challenged vi rus was detected in the ocular, nasal and oral secretions of these animals . The humoral response of challenged animals was also studi ed . It was demonstrated that a single immunization with PPR vaccine (PPRV strain 75/] Nigeria) conferred so lid protection in sheep and goats for three years. Taken together, these studies have shown that PPR ex ist both in epidemic and endemic form in the country and poses a seriolls threat to rural economy and food security. This implies that strategic lise ofPPR vaccine will certainly be useful in reducing the impact of the infection in the country.