by Dr. Nathan Hitchcock, Gayndah Veterinary Surgery, 26/4/2019
Apparent failures of BEF (Three Day Sickness) vaccine in cattle.
We have had a number of queries lately about perceived failure of the BEF vaccine in cattle. Most of these questions relate to producer’s suspicion that the virus has somehow changed, and the vaccine appears less effective as it was. These suspicions are based on events where previously vaccinated animals have been affected by the virus. The assumptions that the vaccine is not as effective as it was are not true, but there are good reasons for this apparent deterioration of efficacy that have nothing to do with the virus changing.
There are a lot of factors involved in this, and in general there is a good explanation for most of the perceived failures. Rigorous scientific work has been done to determine if, in fact, the virus mutated and if the vaccine is still effective. All of this work consistently points to a stable, unchanging virus, and that the vaccine should still be as effective as it ever was.
To address a perceived failure of vaccine, the first questions that must be answered are:
- Was the vaccine used as per manufacturers recommendations? (2 x doses, 2 weeks to 6 months apart, followed by annual boosters, under the skin – NOT in the muscle).
- Was the vaccine mixed correctly?
- Was the vaccine stored as per manufacturers recommendations? (Refrigerate, do not freeze, use within 6 hours of mixing)
- Was the injecting equipment and operator competent – in other words, did every animal get a 2mL dose under the skin?
In more than 50% of queries we receive, one or more of these issues can be identified as a factor. We have had situations where producers have injected the diluent and forgotten to mix the actual vaccine into the diluent. Many producers give one dose of vaccine at weaning and never give a second dose. Some animals receive the first two doses and are never treated again. Some operators inject in the muscle instead of under the skin. Some administration guns are faulty and do not actually inject all the dose. Some vaccine handling and storage can only be politely described as inadequate. Some operators will put a long needle through one piece of skin and out the other side, then squirt the dose on the ground. All these factors can cause an apparent failure of immunity, and some can be very hard to identify in hindsight (eg. errors of administration technique).
If all these issues are addressed adequately, and there is still a perceived vaccine failure, we need to consider the relationship between the virus, the insect vectors that spread the virus, the weather, and the animals.
Weather conditions are the main factor in development of large outbreaks. When conditions have been dry the concentration of insect vectors will be low, spread of the virus stops, and the disease largely dies out. After a period of time the immunity of young animals that have never been exposed to the disease or vaccine will be non-existent, and older animals will have lower level of immunity as their antibody levels decline (in the absence of stimulation by disease or recent vaccination). With the onset of wet weather insect numbers rise sharply, the virus begins to spread geographically, and a vicious spiral develops of increasing numbers of infected cattle infecting lots of insects which infect more and more cattle. We refer to this situation as a high viral load, or challenge. When immunity is low it is often adequate to protect against a low challenge, but in a high challenge the total numbers of virus particles being injected into an animal by the biting insects may be more than the total number of antibodies in that animal’s entire body. In this situation immunity is overwhelmed by the high challenge and clinical disease will develop.
A lot of producers do not vaccinate all their herd. Often only the bulls are vaccinated, and the rest of the herd is left unprotected. This leaves the herd open to develop an enormous viral load over a short period of time, overwhelming the immunity of the vaccinated, high value animals. This situation also applies to vaccinated herds downwind of unvaccinated herds.
Fortunately when inadequate immunity is overwhelmed in a situation like this, the affected animal’s immune system already knows how to make more antibodies, and will do so much sooner and in larger numbers than an animal that has never had immunity stimulated by vaccine or previous infection. This results in faster recovery, less permanent damage and very good survival. This has led to the farmer’s myth that only one dose of vaccine is required initially. There is an element of truth in this myth, especially in dry years, but the immunity will not be adequate to protect the animal in the face of a large viral challenge.
Some years a vaccinated herd will appear to have 100% protection against BEF virus, and in other years it will appear that protection is poor. In reality the vaccine is never 100% effective. When vaccinated herds are compared with unvaccinated herds (assuming that all the factors relating to the actual handling, storage and administration of vaccine are correct) the vaccinated herds will have:
- Almost no deaths.
- Very few severely affected affected animals (1% of 1,000 animals is still 10 animals, while nearby unprotected herds may have as much as 50% affected).
- Very little (or nil) long term effects in affected animals (eg. downers and chronically lame ill-thrift animals).
Affected animals in vaccinated herds (compared to equivalent unvaccinated herds):
- Recover faster
- Do not lose as much weight
- Make up a much smaller % of the total herd
- Do not have lingering effects of the disease
Almost all questions we receive about vaccine failure relate to situations where:
- Individual vaccinated animals (eg. Bulls) are surrounded by unvaccinated animals, and the immunity is overwhelmed,
- Herds have not been vaccinated in line with the manufacturer’s recommendations, allowing low levels of immunity to develop,
- Vaccinated herds are surrounded by unvaccinated herds, resulting in huge viral load,
- Vaccinated herds with an apparent high infection rate (when compared to infection rates in the same herd in other years) have a much lower infection rate, mortality and symptom severity compared to nearby unvaccinated herds at the same time.
The best way to respond to an apparent vaccination failure is NOT to stop vaccination. In the long term this will increase losses. An apparent failure means the vaccination program has been inadequate for the season, and the viral challenge was too high. The correct response is to adjust the vaccination program; either vaccinate high value animals more often, or vaccinate a larger % of the herd.
The benefits of a vaccination program extend further that just prevention of mortality. Mortality prevention is the most obvious, but probably not the most significant, effect of a vaccination program in dollar value. Animals affected with BEF that survive have lost weight quickly for several days or weeks, then replace that weight slowly, if at all. This weight is usually lost during wet weather when pasture is at its best, meaning that although a small percentage of days in the year are affected, a large percentage of the potential weight gain for the year is compromised. This effect can be significant. For example, take a herd of 400 animals worth $1,000 each (400kg @ $2.50/kg). Losing 4 animals per year to BEF (1% mortality) equates to a loss of $4,000 in deaths. Assuming that 10% of the herd will be affected but survive if not vaccinated, and that each affected animal loses 50kg, then takes 4 weeks to recover that 50kg, we see a loss of 50kg ($125) plus loss of weight the affected animals should have gained through that time (300g/day weight gain x 30 days = 9kg, or $22.50). We have a total loss of $147.50 per affected animal, x 40 animals affected = $5,900 loss of production. These figures are not an exaggeration, and in fact the % of the herd affected can be much higher than 10%. Between deaths ($4,000) and weight loss ($5,900) this herd has lost around $10,000 in an average BEF outbreak, and outbreaks usually occur at least yearly. These loss figures do not take into account losses of high value stud animals, abortions caused by the disease (where a significant portion of that animal’s production for the entire year is lost), and costs of treating affected animals (drugs and labour). Using a figure of $10 per dose of vaccine, the entire herd can be vaccinated for $4,000 per round (plus other costs: labour, mustering, etc), giving a dollar return of about 2 or more for 1. In almost all situations of cattle farming in the North Burnett an effective BEF vaccination program is a sound, logical investment in herd management.