INTRODUCTION
Vaccination, initially with inactivated virus, was considered a possibility for the control of ND at the time of the apparent emergence of the virus. However, after the 1933 outbreak in England, an attenuated live vaccine was produced which was called strain H. Later, the naturally occurring USA isolates of low virulence, Hitchner B1 (HB1) and La Sota, became the most used veterinary vaccines throughout the world. Fifty years or more have passed since vaccine was first used to protect village poultry against ND. (Placcidi and Sentucci, 1952). During this time, a wide variety of types of vaccine have been developed. Many, but not all, have been tested on village poultry. It is the purpose of this chapter to present an overview of the different kinds of vaccine available. It is not the intention to recommend a particular vaccine, but rather to try and outline the relative advantages and limitations of each, with particular reference to its use in the village situation and giving examples of how the different vaccines are employed.
The principle of vaccination against a viral disease is well-known: to elicit an immunological response against the virus in a way that does not cause the disease. The simplest way to do this is to take the virus, kill it, and then inject it into the bird. This is an inactivated vaccine. Another approach is to select a naturally occurring virus that is not virulent enough to cause serious disease, and infect the birds with this virus. This is a live vaccine. This latter approach can be taken further by taking a non virulent natural virus and selecting a clone from the virus population with desirable properties, such as lack of vaccinal reactions, or heat tolerance. This is a cloned live vaccine. Finally, it is possible to genetically engineer a vaccine by, for example, taking part of the genetic material of the virus that codes for a surface antigen, and inserting this into another, different, virus to produce a recombinant vaccine.
These different approaches to vaccination have been applied to ND. There are three types of vaccines used for ND: live lentogenic, live mesogenic and inactivated vaccines. Live lentogenic vaccines are usually derived from field viruses that have been shown to have low pathogenicity for poultry but produce an adequate immune response. Typical vaccine strains are HB1, La Sota and F strain and some viruses from the asymptomatic enteric pathotype, which are usually based on the V4 or Ulster 2C viruses. However, these viruses have been frequently subjected to selection pressures by manufacturers in order to improve their immunogenicity or to enable their use by a particular method of application.
INACTIVATED VACCINES
Inactivated vaccines are produced by growing a ND virus in eggs, and then treating the infective allantoic fluid with an inactivating agent, such as formalin or betapropiolactone. An adjuvant, such as mineral oil, is usually then added to make the inactivated virus more immunogenic. Since the vaccine is no longer capable of replication or spread, it has to be injected individually into every bird needing vaccination. It is normally injected into the back of the thigh muscle (sometimes the breast muscle is used), using 0.3 or 0.5 ml per bird. This requires some training, and cannot be done by every keeper of chickens without prior demonstration. Inactivated vaccines produce very high levels of antibodies against NDV, and provide good protection against the virulent virus.
In intensive poultry production, inactivated vaccines are usually applied after an initial priming vaccination with a live vaccine. In village poultry, however, good results in the absence of an initial vaccination with live vaccine have been reported (Bell et al., 1990). The reason for this is probably, as serological surveys have shown where they have been carried out (Bell and Mouloudi, 1988), that antibodies to the virus are already present in the village poultry as a result of previous infection by the wild virus.
Inactivated vaccines have been used extensively in village poultry, for example, in a successful project in Burkino Faso (Verger, 1986). Although inactivated vaccine gives good protection, it is relatively expensive to produce. It also carries a slight risk to the user of accidental self-injection. While inactivated vaccines are, to some extent, heat sensitive, they are much less so than conventional live vaccines which makes transporting them to villages more feasible.
LIVE VACCINES
Live vaccines differ from inactivated vaccines in that they can replicate in the host. This is both an advantage and a disadvantage. It is an advantage in that it is not necessary to vaccinate every bird individually; the vaccinal virus can spread on its own from one bird to another. It is, however, a disadvantage in that, since an infection with a live virus is involved, this may result in clinical signs because of the innate virulence of the vaccine virus or by exacerbating other organisms that may be present, especially in the respiratory tract. The severity of this reaction depends therefore on the particular vaccinal strain used (Westbury et al., 1984) and the presence or otherwise of concurrent infection with other pathogens.
Another advantage of live vaccines compared to inactivated vaccines, is their ease of application as they can be applied to the drinking water or with an eye-dropper.
Although NDV has essentially only one serotype, there is a wide difference in the pathogenicity of different strains, ranging from those that cause virtually no signs to those that kill within a few days. These have been classified, in order of increasing pathogenicity, into asymptomatic enteric, lentogenic, mesogenic and velogenic strains. The majority of live vaccines are derived from asymptomatic enteric or lentogenic strains, although some vaccines derived from mesogenic strains are still in use.
Conventional lentogenic vaccines
The level of vaccine reaction is an important consideration for intensive commercial poultry and because HB1 has very mild vaccinal reactions, it has been widely used for initial vaccination of intensive poultry. In a controlled trial in village poultry, HB1 provided effective protection against ND (Bell et al., 1990). La Sota produces moderate vaccinal reactions, especially in immunologically naive birds and is not usually recommended for primary vaccination. In theory, La Sota would also be unsuitable for vaccinating a multi-age population, including young chicks which is inevitably seen in the village situation. This is because the virus spreads and it is not practical to isolate the adults from the chicks. In practice, the degree of reaction from La Sota as a primary vaccine depends on the residual level of antibodies, which could protect the birds from vaccinal reactions, and on the extent of other concurrent infections, such as Mycoplasma spp, pathogenic E. coli, or infectious bursal disease virus and other respiratory viruses. In intensive systems, vaccination using spray delivery systems which produce small particle sizes, may also exacerbate the vaccine reaction.
Some lentogenic vaccines have been cloned by taking a single infectious virus and growing a homogenous population from it, with the aim of selecting a virus which gives less vaccinal reactions than a La Sota-like virus, while retaining its superior immunogenicity compared to a HB1-like virus. An example of this kind of vaccine is "clone 30".
All conventional live vaccines have the disadvantage of needing to be kept at low temperatures to maintain their efficacy. This is not a problem for intensive poultry production in an industrial setting, but the maintenance of the "cold chain" during distribution can be very difficult in village settings, particularly where there is high ambient temperature.
Another problem that is often encountered when using commercial vaccines in the village situation is that they are sold in vials containing 1 000 or 500 doses, many more than the average village farmer needs. In fact, the packaging is a major component of the cost of manufacturing them, because a vial containing a smaller number of doses would not necessarily reduce the cost proportionally.
Oil adjuvant, normally used with inactivated vaccines to improve immunogencicity, has also been tested with live vaccines and found to improve immunogenicity (Peleg et al., 1993), but this combination has not been tested with village chickens.
Heat tolerant vaccines
Some asymptomatic enteric viruses have been noted for their greater heat resistance than more conventional lentogenic viruses. This property has been enhanced by selection and cloning in the laboratory to produce heat tolerant vaccines. These have a distinct advantage in the village situation because it is possible to transport the vaccine without a cold chain. The most extensively used vaccine has been the NDV4-HR vaccine, which was pioneered in Malaysia, where a significant proportion of the village poultry was eventually covered by this vaccine (Ibrahim et al., 1992). The application was in feed, which, because of its thermostability, it was possible to pre-coat with the vaccine. Theadvantage of this method is that it is not necessary to catch the chickens before vaccinating them. The same vaccine has also been tried in other countries in South East Asia, but not always with the same success as in Malaysia. Tests of its application on a variety of foodstuffs have produced variable results (Spradbrow, 1992). The vaccine was also tested in some African countries, but applied by eye-drop and gave good protection against the virulent virus (Saglid and Spalatin, 1982; Bell et al., 1995). Given the difference between African and Asian feeds, the variety of feeds within Africa, and the variable results with some feedstuffs in Asia, it seems that application of this type of vaccine is best done by eye-drop. It can also be argued that the additional security provided by the vaccine is an incentive to invest in some form of housing, in which case catching the chickens is no longer a problem.
More recently, a similar vaccine to NDV4-HR, called I-2 (Bensink and Spradbrow 1999), has been made available for local production in non-industrialised countries, which has the significant advantage of low cost. In trials in Ghana, Mozambique, Tanzania and Vietnam village chickens, vaccinated with strain I-2, were protected against artificial and field challenge with virulent virus (Amakye-Anim et al., 2000; Dias et al. 2001; Tu et al., 1998; Wambura et al., 2000).
Mesogenic vaccines
Mesogenic strains have long been used for vaccination in the village situation. These produce severe vaccinal reactions in an immunologically naïve population, and the use of this kind of vaccine is not advisable in situations where chickens are without any immune protection against the virus. Normally mesogenic vaccines, such as Komarov (Saifuddin et al., 1990) and Mukteswar (Alexander, 1997) are used as secondary vaccines after a primary vaccination with a lentogenic vaccine.
RECOMBINANT VACCINES
NDV has two surface glycoproteins, fusion [F] and haemagglutinin/neuraminidase [HN]. The genes coding for either of these can be inserted into a different kind of virus to make a recombinant vaccine. For example, the fusion gene inserted in herpes virus of turkeys produced a vaccine which gave good protection against virulent NDV (Morgan et al., 1993). One advantage of this technique is that the host virus may have better stability than NDV. Another advantage is that antigens for multiple different pathogens can be inserted into the same host virus to produce a single vaccine against several different diseases. Perhaps the most significant advantage for field use is that it is possible to monitor the response to the vaccine independently of the wild virus but in its presence, and conversely, it is possible to detect antibodies against the wild virus in the presence of vaccination. This is done by using an enzyme-linked immunoabsorbent assay (ELISA) that uses a purified antigen, and comparing the results with those of an ELISA using a whole virus antigen. For example, Makkay et al. (1999) prepared an ELISA using only nucleocapsid protein of NDV as antigen. This detected antibodies against wild virus, but not antibodies against a recombinant fowl pox virus expressing HN glycoprotein. A parallel ELISA using whole virus as antigen detected antibodies against the vaccine.
A disadvantage of recombinant vaccines is that where they have been developed commercially the cost is high.
VACCINE APPLICATION
Mass administration methods
In intensively developed commercial poultry industries, an important cost of vaccination is the administration. For this reason, mass application methods have been developed, primarily for live vaccines. Various forms of equipment are manufactured to generate coarse sprays, which allow mass application with minimum adverse reaction, although in some circumstances, mass application by fine sprays and aerosols are employed (Kouwenhoven, 1993).
Application of live vaccines via drinking water is still employed in some areas, although in the commercial sectors, this gives some problems in preparing and cleaning the drinking water system, with a tendency for a less than uniform uptake (Kouwenhoven, 1993).
Administration to village chickens
Vaccine administration to village chickens is not comparable to intensively reared commercial chickens as the birds are rarely housed and seldom in large numbers. However, labour is usually available at little or no cost which means that individual methods of vaccine administration are feasible.
Eye-drop administration
Application of the vaccine by eye-drop methods is probably the most effective for live lentogenic vaccines (Fig. 1). It ensures that the vaccine reaches the individual bird and, as a consequence, titres obtained are usually uniform throughout the flock.
Correct dilution of the vaccine is critical. If eye-droppers are being used, they should be calibrated beforehand (see Alders and Spradbrow 2001a). In the absence of suitable eye-droppers, it is also possible to use the tip of a feather or a syringe (preferably a 1 mL syringe) to administer the drop. However, these two options should be seen as last resorts as they are inaccurate and cause considerable wastage of vaccine. Most live ND vaccines require re-vaccination at 3-4 monthly intervals.
Eye-drop administration provides good protection because the vaccine passes to the Harderian gland just behind the eye, which in chickens is a key organ in the development of the immune response.
Figure 1 Eye-drop administration. When using an eye-dropper, hold it in a vertical position. Eye-droppers are calibrated according to the size of the drop that forms when the dropper is held in a vertical position (Alders and Spradbrow 2001a).
Administration of the vaccine via drinking water
Vaccination by placing the live vaccine virus in the drinking water is easier than application to individual birds, but it provokes a lower level of immunity than eye-drop administration, has less uniform uptake and requires more frequent application. The vaccine should be given twice, initially 2-3 weeks apart, with re-vaccination at least every three months.
It is important to:
remove drinking water from the chickens for 1-2 hours before the administration of the vaccine.
mix the vaccine with a volume of water that the chickens will be able to drink during one hour (usually 5-7 mL of water per bird).
always use fresh and clean water.
It is important not to:
use metal water receptacles.
use disinfectants to clean water receptacles as they will inactivate the vaccine virus.
use treated tap water. If there is only access to treated tap water, it is advisable to let the treated tap water stand overnight to allow the chlorine to dissipate, or add one teaspoon of powdered milk per 10 litres of water to neutralise the effects of the chlorine.
place water receptacles containing vaccine directly in sunlight or in hot areas.
allow other animals access to the vaccine. It should be restricted to chickens.
In rural areas, it is best to give the drinking water in the morning just as the chickens are released from the chicken house. In areas with abundant surface water, chickens find their own source of drinking water and vaccination via water is not appropriate.
Administration via feed
Oral vaccination of chickens with thermostable vaccines (i.e. NDV4-HR and I-2) has been successful in some developing countries. Good veterinary services, local availability of suitable grains and recovery of virus from the grain are important considerations for successful oral vaccination. One problem with food-based ND vaccination is the low recovery of virus from some grains (especially maize), a consequence of either binding or inactivation. This method should be thoroughly tested before being used widely in the field. The vaccine must be given more often when administered via feed, making it more expensive, and survival rates in the face of an outbreak are lower than those achieved by eye-drop administration. Food used in any vaccination campaign should therefore be recommended by the Veterinary Authority. 7-10 grams of food per bird should be well mixed with the corresponding number of doses of appropriately diluted vaccine. With most grains, 1 ml. of fluid will efficiently moisten 10 grams of grain. The treated food is best given in the morning as the birds are leaving the roost. The vaccine should be given twice, initially 2-3 weeks apart, with re-vaccination at least every 2-3 months.
Administration via injection
Inactivated ND vaccines are administered by intramuscular or subcutaneous injection only (in the breast or the leg). Inactivated vaccines should be allowed to reach ambient temperature (approximately 28°C) and the contents should be well shaken prior to use. If stored in a cool, dark location, an inactivated vaccine may retain its activity for 1-2 weeks outside a refrigerator.
Inactivated vaccines are more effective in chickens which have previously received a living vaccine. Re-vaccination is usually done every 6 months.
Accidental injection into the vaccinator of inactivated vaccines based on emulsions formed with mineral oil can cause a serious localised reaction. These usually require incision and washing. Expert medical advice should be sought at once, and the doctor must be informed that the vaccine was a mineral oil emulsion.
In many parts of Asia, mesogenic strains (for example, Mukteswar) of the ND virus are used and can be administered by injection only. This vaccine should be used in birds over eight weeks of age and following a primary vaccination with a lentogenic strain such as F strain.
DISCUSSION
The inactivated and recombinant vaccines have the advantage of not inducing vaccinal reactions. The heat tolerant clones produce almost no vaccinal reactions whereas the other live vaccines produce slight to moderate reactions, depending on the vaccine strain and the immune status of the population vaccinated.
Inactivated vaccines are, however, the most difficult to apply and whether using needle and syringe or automatic equipment, training is necessary before any injection technique is mastered.
For transportability, heat tolerant vaccines appear to be best and can be transported to even remote villages under high ambient temperatures without a cold chain. The inactivated vaccine is second best, having a better heat tolerance than the conventional live vaccines.
In choosing a vaccine for use in the village situation (Bell, 2001), one factor to take into consideration is previous experience with that type of vaccine. There has been extensive village experience in the use of both heat tolerant and inactivated vaccines. Live mesogenic vaccines have also been used in villages, particularly in Asia. The other live vaccines, with the possible exception of some clones, have, at least, been formally tested in villages.
Finally, cost is an important factor. All the live vaccines are relatively cheap, and can be even cheaper if they are produced locally. Inactivated vaccines are more expensive and recombinant vaccines are likely to be very expensive when produced commercially.
The choice of vaccine and how to administer it depends not only on the preceding factors, but also on the conditions in each region, such as the structure of veterinary services, previous experience, the population distribution, the communication infrastructure and the climate.
