A national livestock census has put the goat population in Nigeria at 34.5 million [1] . The keeping of goats in any community is related to the value attached to their production. Goats are mainly kept for meat [2] . Goat dung is also used as manure to improve the crop yield of local farmers. However, diseases and other factors in- cluding poor management limit the production of this group of ruminants. Chief amongst these diseases is the Peste des petits ruminants [3] [4] . Peste des petits ruminants (PPR) is a highly contagious disease of small ru- minants caused by a virus in the genus Morbillivirus, Family Paramyxoviridae. The infection is responsible for high morbidity and mortality in sheep and goats and some wild small ruminant species. Recently, severe out- break of the disease has been reported among dromedary camels in Sudan [5] . The huge number of small rumi- nants reared in the endemic areas makes PPR a serious disease threatening the livelihood especially of poor far- mers. In order to protect susceptible animals from the scourge of PPR, a homologous attenuated PPR vaccine has been developed and is commonly used for prevention and control of the disease [6] and recently the efficacy of a candidate thermostable PPR vaccine has been demonstrated in small ruminants in Nigeria [7] . In spite of rigorous vaccination exercise against PPR, outbreaks of the disease continue to occur among vaccinated and nonvaccinated animals [8] . Hence, the need to boost the immune response of goats to vaccination by employing the simultaneous administration of an immunostimulant and PPR vaccine. The use of immunostimulatory agents such as aluminium salts [9] [10] and Levamisole [11] [12] has been reported to help in stimulating the immune system, encourages and sustains the immune system and its response to a vaccine without having any specific antigenic effect in itself [10] . Levamisole hydrochloride, a synthetic phenylimidazothiazole is used primarily as a potent anthelminthic [13] . It was first introduced in 1966 [14] and later found to have immunostimulatory properties [15] . It has been used as an immunostimulator to stimulate the immune system either alone or in combination with other substances in the treatment of cancer [16] and to improve weight gain [17] . It has also been used as immunopotentiator in the treatment of lepromatous leprosy and as adjuvant [18] [19] . Due to the devastating effect of PPR virus infection and occasional failure of PPR vaccine efficacy, it has become necessary to determine the effect of Levamisole on the immune response to vaccination. This study was designed essen- tially to determine the immunostimulatory effect of Levamisole when used in combination with homologous PPR vaccine in goats.

Results of virus neutralization test indicated seroconversion by significant rise in PPR virus antibody titre over time among animals in groups A, and B. However, no significant change in antibody titre was observed among members of groups C and D throughout the period of the experiment. Geometric mean titre (GMT) values of the reciprocal of PPR virus neutralizing antibody titre for all the groups including the pre-vaccination antibody and weekly post vaccination antibody values are presented in Table 1. Seromonitoring of vaccinated goats revealed a considerable increase in antibody titre (≥3-folds) one week after vaccination in groups A (pre-treated and vac- cinated) and B (vaccinated only). However, at the 4th week post vaccination, there was significant (p < 0.05) drop in GMT of antibodies among the group B goats (Table 1). Group C (pre-treated only) and group D (control) recorded no significant rise in antibody titre throughout the period of seromonitoring (Table 1). Hematological analysis revealed that total leucocyte count (TLC) increased significantly overtime in groups A, B and C. How- ever, there was no significant change in TLC in the control group throughout the period of the experiment (Ta- ble 2). The weekly changes in TLC in groups A, B and C were statistically significant (p < 0.01). But weekly change in group D was not significant (p > 0.05) (Table 2). The differential leucocyte count (DLC) pattern showed that goats in group A (vaccinated and pre-treated), had no significant change in the weekly neutrophil values overtime. However, significant changes (p < 0.01) were recorded in the weekly values of lymphocytes, monocytes and eosinophils (Table 3). Generally, the post-vaccination DLC values were significantly higher than the pre-vaccination values among goats in group A (Table 3). There was a significant increase (p < 0.01) in lymphocyte values overtime among the group B goats. Generally, the postvaccination DLC values were signifi- cantly higher than the prevaccination values (Table 3). Similarly in group C (pre-treated only), the post vaccina- tion values were significantly higher than the prevaccination values (Table 3). The difference is more marked with lymphocytes and eosinophils when compared with other leucocytes. In group D (control group), the differ- ence in total leucocyte was not significant (p > 0.05) between prevaccination and postvaccination values. How- ever, occasional differences (p < 0.05) in DLC were noted at different periods (Table 3). Apart from animals in group C, where considerable increase in BMI was recorded, there was no significant change (p > 0.05) in BMI values between experimental groups as well as between prevaccination and postvaccination values throughout the period of experiment (Table 4).

Note: n: Sample size; PRE-VAC: Pre-vaccination; 1 WK POSTVAC: One week post vaccination; 2 WKS POSTVAC: Two week post vaccination; 4 WKS POSTVAC: Four weeks post vaccination.

Note: n = Sample size; PRE-VAC: Pre-vaccination; 1 WK POSTVAC: One week post vaccination; 2 WKS POSTVAC: Two week post vaccination; 4 WKS POSTVAC: Four weeks post vaccination.

PPR continues to be a major socio-economically important disease of small ruminants in Nigeria. The disease is characterised by high morbidity and mortality and is among the greatest threats to a successful livestock produc- tion in many parts of the world where it exists [4] . The simplest and most logical preventive measure against in- fectious diseases is vaccination and this practice is considered by far the most humane and cost effective method of combating the spread of diseases [25] . However, in tropical areas disease outbreaks have been reported among vaccinated and non-vaccinated animals due to vaccine failure as a result of rapid vaccine deterioration and poor immune response of goats to vaccination. This necessitates the need to boost the immune response of goats to vaccination by employing the simultaneous administration of an immunostimulant and PPR vaccine. Levamisole has been used as an immunostimulant in humans and animals [26] . It has beneficial effects on host defense mechanism and restores depressed immune responses in both animals and humans [27] .

This study reveals that experimental goats in group A (pre-treated with Levamisole and vaccinated with PPR vaccine) had high levels of seroconversion following vaccination with possibility of better protective immunity over a long period of time when compared with those in group B, that received vaccine only. Significant change in GMT of the reciprocal of neutralizing antibody was observed beginning from second week post-vaccination in group A and this pattern was observed throughout the period of experiment. This observation is in agreement with the report of [28] where it was observed that Levamisole had an immunostimulating effect on the immune response of sheep to blue tongue vaccination. There was no seroconversion among members of unvaccinated but Levamisole treated group C indicating that Levamisole alone does not initiate the production of antibodies against PPR virus. The study also revealed that levamisole enhances the production of immune cells. This is demonstrated in groups A and C where lymphocytes, monocytes and eosinophil were significantly increased following administration of Levamisole. Bozic et al. [19] demonstrated that Levamisole exerts its immunosti- mulatory activities in pigs against colibaccillosis by recruitment and activation of cells that participate in cell- mediated immunity. In addition, Kumar et al. [17] observed increase in lymphocyte following administration of Levamisole possibly as a result of increase in the function of lymphoid organs such as the thymus and spleen resulting in increased proliferation of the bone marrow stem cells. The observed pattern of leucocyte change could have been as a result of increase in the function of the lymphoid organs due to Levamisole. Its effect when combined with PPR vaccine as in goats in group A, resulted in high seroconversion and significant change in leucocyte value. The lack of significant change in body mass index (BMI) among animals in all the groups could be attributed to the short duration of the study. This is in disagreement with [17] who observed that Levamisole improves weight gain in animals. It is possible that animals in groups A and C would improve in weight gain if the study is extended for more than four weeks. The study could not be extended beyond the experimental pe- riod because of its preliminary nature and limited funds. Besides, since the study was carried among small hold- er livestock, owners could not allow the extension of the study period and challenge experiments. Nonetheless this study has provided evidence for the usefulness of Levamisole in enhancing the development and sustaining humoral immune response following vaccination of goats with PPR vaccine. Levamisole appears safe when used in humans or when animal products from treated animals are consumed by humans. The drug has been used in humans for diseases related to imbalances in the regulation of immune responses or deficiencies of the immune system, including autoimmune diseases, chronic and recurrent diseases, chronic infections and cancer. It has beneficial effect on host defense mechanisms and restores depressed immune responses in animals and humans [29] .

Levamisole appeared to have an immunostimulating effect on the response of goats to PPR vaccination. The study has provided experimental evidence that Levamisole can be used as an immunostimulant in combination with PPR vaccine to prolong and maintain high antibody titre in vaccinated animals by increasing the production of cells that participate in immune protection. Further studies on cell mediated and humoral immune response of vaccinated goats over extended period, would provide evidence for the increased mobilization of immune cells and duration of immunity in animals pretreated with Levamisole before vaccination.

The authors thankfully acknowledge the technical assistance of David Bukbuk, Musa, Joshua and Monilade. The senior author was a postgraduate (MSc Microbiology) student in the Department of Veterinary Microbiology and Parasitology.