By Stephen Champion, BSc (Hons) BVM&S MRCVS,
Veterinary Surgeon, CHAMPIONVET.com
Written for Farming Scotland Magazine, July 2001

In the second of two articles (the first published in the last Winter edition of Farming Scotland), Stephen examines the “Pasteurella” part of multicomponent sheep vaccines, such as Heptavac-P Plus. An understanding of the diseases from which these vaccines protect sheep, and the way in which they work, is beneficial when planning their use in strategic preventative medicine and flock health.

Pneumonia in sheep is very common and the most common form of acute pneumonia is pasteurellosis, which is caused by the bacterium Pasteurella. It is the biggest killer of growing and adult sheep in the UK (other than Foot and Mouth disease!). It is a disease that occurs sporadically, without warning, and commonly kills one in ten animals when an outbreak occurs. It can cause an “annoying” number of deaths over a period of days or weeks, where the decision to gather the flock to treat, or to leave well alone, is not always easy.

The bacteria have recently been classified into two distinct, but closely-related species. Pasteurella haemolytica (or the A biotype) is associated with a disease called pneumonic pasteurellosis, occurring in all ages of sheep. Adult sheep are often affected by pneumonic pasteurellosis , although the most severe form involves septicaemia in young lambs, where other organs and not just the lungs are affected. The disease occurs usually in the Spring and Summer. Some affected sheep will be found dead without having been ill, whilst others will be dull and breathing heavily. It should be remembered however, that not all animals that puff and pant have pneumonia! Warm weather, or other diseases that cause a fever, can create the same symptoms.

The second species, Pasteurella trehalosi, causes a distinct disease called systemic pasteurellosis in hoggs, which occurs most frequently from September to December. Typically a number of sheep are found dead over a few days, when feeding on rape, turnips or aftermath.

A unusual aspect of this bacteria is that it can be found in up to 95% of healthy sheep, living in the tonsils and throat. From this starting point, pasteurellosis can affect even the healthiest animal, when stress factors lower its resistance to disease and allow the bacteria to take hold in increasing numbers.

Much is talked about stress in humans and animals in the modern world in which we live. The fact is that stress acts on the body to produce hormones that actually lower the number of protective white blood cells in our bodies. This predisposes our bodies to disease and the same is true of sheep.

Environmental and management stressors such as dipping for scab, castration of male lambs, dosing with wormers, gathering in warm weather and moving to a higher plane of nutrition (especially with reference to P. trehalosi) can cause outbreaks of pasteurellosis. Warm, wet weather and cold, wet windy weather can predispose. A climate not unfamiliar to us in Scotland!

However, by far the most important predisposing factors to pasteurellosis are infections with other bacteria and viruses. The most important and significant of these are parainfluenza type III (known as PI3 virus), adenovirus, sheep pulmonary adenomatosis (known commonly as SPA or jaagsiekte) and Mycoplasmas. Tick-borne fever can also play an important part in instigating disease.

Control of Pasteurellosis

Treatment of pasteurellosis using antibiotics can be very effective, the usual antibiotic chosen by your veterinary surgeon being long-acting oxytetracycline (with many trade names). It has been used, often very successfully, for treatment of individual sheep, for treatment of large numbers of sheep, or for prevention. It is no doubt of great value during an outbreak but is generally of little value in the control of the disease. This is because pasteurellosis often is sporadic and unpredictable in onset, and often lasts for only a short time. It should be emphasised that a reliable diagnosis by your veterinary surgeon, at the very least to ascertain that pneumonia is present, is mandatory before widespread treatment is instigated.

Safe management practices are still likely to be the most economic methods of control. Attention should be drawn to any practices that are likely to stress large numbers of sheep at one time. For example, gathering should be kept to a minimum and should be done calmly when the weather is appropriate and not too hot. There should be well-planned strategy in place to avoid sheep with a heavy worm burden, likely to reduce resistance to disease. Alternatives to dipping can be explored, such as pour-on products where appropriate. Changes of diet for young sheep can be done gradually and not when other stressors are present, such as variable weather conditions. Shelter should be provided during bad weather. Finally, measures can be taken to avoid the build up of other diseases on a flock level, such as SPA, by appropriate selection and culling of affected animals.

There is no doubt that vaccination against pasteurellosis offers a safe, reliable and practical means of control.

Vaccination as a Means of Control

The two species of pasteurella are further divided into a number of serotypes, approximately 17 in all. Furthermore, it was found in the 1960’s that immunity was serotype-specific, and vaccines were developed to protect against a number of serotypes commonly found in sheep. In 1982, newer vaccines, the multicomponent vaccines became available, such as Heptovac-P and Ovivac-P. These protect against both Clostridia and Pasteurella. (For a full account of clostridial disease, refer to the article in the Winter edition.)

Not all serotypes of pasteurella are found in Britain, but one type called A2 has been consistently dominant over many years. This also happens to be the most difficult to protect against in sheep! Thus it has been important to always include this serotype in vaccines, amongst other important ones. However, it would be really neat to be able to cross protect against other serotypes, without having to include all these in a vaccine, which may overload the immune system!

This has been accomplished in the new generation of pasteurella vaccines, named Heptovac-P Plus and Ovivac-P Plus. It was found at the Moredun Research Institute in Edinburgh, that pasteurella bacteria contain structures called Iron-Regulated Proteins (IRPs). These IRPs help the bacteria to regulate the metal iron from the sheep’s body for its own use and are common to many different serotypes. Hence, when a vaccine containing IRPs is injected into a sheep, the sheep will make antibodies against many different serotypes, but all with IRPs in common. This is cross-protection, and means you stand a better chance or protecting your sheep against many serotypes that may crop up and cause disease.

A Brief Synopsis of Vaccination Procedure

Hoggets, gimmers and ewes should be given two doses of Heptavac-P Plus, 4 to 6 weeks apart, finishing before tupping, followed by the same vaccine 4 to 6 weeks before lambing. In subsequent years, the pre-lambing dose should be repeated to maintain immunity and boost collostrum levels. On farms where ewe losses are a problem during Winter, a single dose of Ovipast-P Plus should be given 2 to 3 weeks before the usual problem period.

Lambs from birth to weaning. Generally lambs can be vaccinated from 3 weeks old with Ovivac-P Plus. (In theory they should gain protection against pasteurellosis for the first 4 weeks of life from the ewe’s collostrum, if she has been properly vaccinated.) If, however, losses due to pasteurellosis is a problem in very young lambs, vaccination can be started with a pasteurella-only vaccine (Ovipast-P Plus) from 2 weeks old, followed by a second vaccination 3 weeks later.

Stock hoggs before wintering. Give 2 doses of Ovivac-P Plus, 4 to 6 weeks apart and ideally a single dose of Ovipast-P Plus (pasteurella only) in the spring. Rams can be vaccinated with the hoggs and given a single booster before periods of stress. One should ascertain that hoggs have been vaccinated before they are purchased and brought onto the farm as this is the period when deaths often occur.

Finally, it should be remember that sheep vary in their response to any given vaccine, even if administered exactly as recommended. For this reason a small number of sporadic deaths may still occur from pasteurellosis and clostridial diseases in vaccinated flocks.

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