1.1 BACKGROUND OF STUDY
There has been a great challenge in recent times in
antiviral therapy of poultry diseases. Fewer substances are available for
treatment of antiviral infections when compared with the large amount of
antibiotics available for treatment of bacterial diseases in poultry; (Esimone
et al., 2007). At the same time, the frequency of viral resistance to the
relatively few antiviral agents are increasing; (De Clerq, 1993).
One of the most important reasons for lack of success in
developing antiviral drugs is due to the nature of infection of viral agents
which totally depend upon the cell they invade for their multiplication and
survival (Vanden Berghe et al., 1986; Vlietnick et al., 1997). Since many of
the existing disinfectants and antiseptics fail to kill all pathogenic viruses,
the demand for new antiviral agents is great and needs all possible approaches
toward the development of new antiviral drugs.
The research in the use of phytochemicals as antimicrobials
for treatment of certain animal diseases is of paramount interest to scientists
and the poultry industry, (Hammer et al., 1999). Plants and plant products
present some hope to scientists, serving as an alternative avenue to discovery
from current mainstream approach of attempting to design narrow spectrum drugs
for specific molecular targets, (Cowan, 1999). While numerous reports exist on
the antimicrobial effects of these plants against some bacteria, only very few
reports exists on the antiviral activity of these plants.
Medicinal herbs have the potential for addressing multiple
targets with minor side effects, development of low resistance due to selective
pressure of infective agents and cost effectiveness, (Cowan, 1999). The
antiviral properties are rarely studied using laboratory based assays to
establish their efficacy in traditional medicine, (Esimone et al., 2005).
Conventional techniques for evaluating antiviral agents
include; Invitro and invivo techniques. Invitro techniques include, plaque
inhibition/reduction assay, virus yield reduction assay, inhibition of virus
induced cytopathic effect, inhibition/reduction of synthesis of virus specific
polypeptides, immunological assays for detecting viral antigens and viral
enzyme inhibition assays, (Fernando et al., 2008). The invivo methods include
the use of ferrets, laboratory mice, cotton rats and chicken for measuring a
number of parameters indicating the extent of inhibition of infection
The embryonated chicken egg system is a standard method for
the propagation and isolation of egg adapted viruses. Antiviral agents have
successfully been screened using embryonated chicken egg as media for both
virus cultivation and inhibition assays, (Heartl et al., 2004).
Newcastle disease is one of the most significant diseases of
importance in the poultry industry in Nigeria, with a morbidity of upto 100%
and mortality ranging between 80 – 100% in severe cases, (Kouwenhoven, 1993).
The survived cases of this disease fail to gain the presumed body weight in
case of meat production and show drastic decrease in egg production during the
course of the disease in case of laying birds. Onset of disease is usually
sudden with flock showing somnolence, diarrhoea, respiratory distress, drop in
egg production, severe prostration and neurological signs which manifest as
paralysis of the legs, twitching of the muscles, torticollis and circling
movement. Disease is highly contagious and persists in poultry flock. This has
posed a major concern for the poultry industry in Nigeria which has
necessitated a search for suitable antiviral agents for these diseases.