There are several pathogens that can cause swine respiratory disease problems. There are also secondary disease problems that are less frequent or severe, meaning diagnosis and, sometimes, control are more difficult.
Pleuropneumonia
This term refers to diseases in which the lung and the pleura (lung membrane) are involved, in contrast to diseases such as Mycoplasmal pneumonia, where only the lung is affected. Pleuropneumonia is usually more devastating than pneumonia, often associated with acute death loss. It also tends to affect older pigs, usually at finishing.
Pleuropneumonia is present in many U.S. hog operations, although it is definitely less prevalent than it was 10 years ago when the disease was the main cause of mortalities. The sharp decline in prevalence is mostly related to changes in production systems.
Originally, pleuropneumonia was effectively reduced by widespread use of depopulation/repopulation strategies that were used in the late '80s. Later, the widespread adoption of segregated early weaning (SEW) had a very positive impact on this disease.
However, outbreaks of pleuropneumonia are still relatively common, which makes understanding this disease all the more relevant.
Types of Pleuropneumonia
There are two main, similar bacterial pathogens involved in pleuropneumonia — Actinobacillus pleuropneumonia (A. pleuropneumonia) and Actinobacillus suis (A. suis).
A. pleuropneumonia can be isolated from a large percentage of healthy hogs at slaughter, which suggests the presence of the organism is too low to trigger disease.
Both Actinobacilli are very prevalent in swine herds and both cause extensive pulmonary damage, characterized by areas of necrosis (tissue death), hemorrhage and edema (fluid-filled tissues).
And, both pathogens cause disease through the secretion of a special group of toxins that target the macrophage (pathogen-eating cells). Since this cell is a primary lung defense mechanism, its' inactivation by the toxins renders the host very susceptible to rapid spread of the toxins.
Macroscopic (visible to the naked eye) lesions are very typical, usually affecting large portions of the lung and characterized by black-red discoloration and pleural adhesions. Surviving pigs, however, quickly recover and may reach slaughter with few or no lesions indicative of the disease. The best indicator at slaughter of a problem is the presence of pleuritis.
Although the Actinobacilli are not the only agents that cause pleuritis in the pig, they are the most common, so that the presence of pleuritis at slaughter needs to be investigated further.
Surprisingly, Actinobacillus is restricted to the thoracic cavity. Although it can be occasionally isolated from other organs, lesions are pretty much restricted to the lung and pleura, as well as adjacent membranes.
A. suis has been connected with a variety of lesions other than pleuropneumonia, including skin lesions reminiscent of erysipelas. But most isolations identified in diagnostic laboratories are from animals showing signs and lesions of pleuropneumonia, indistinguishable from those caused by A. pleuropneumonia.
Pleuropneumonia Treatment
Both Actinobacilli are susceptible to a variety of antibiotics, including penicillin and cephalosporins. But effective treatment can only be achieved through injectable medication, since the extensive necrosis and vasculitis (inflammation of a blood or lymph vessel) prevents oral medication from reaching the affected sites. This makes treatment expensive and difficult.
Vaccination against A. pleuropneumonia is partially effective with traditional killed bacterins, giving some protection against death loss but not against infection or growth retardation. More effective vaccines, using inactivated toxins, are available in other countries, but are not presently licensed in the U.S. There are no licensed vaccines available against A. suis.
Autogenous vaccines for A. suis have a reputation of effectiveness. But they should only be used after a careful diagnostic workup has been done, together with a realistic financial evaluation of the expected benefits.
Streptococcus suis, Haemophilus parasuis
Streptococcus suis (S. suis) and Haemophilus parasuis (H. parasuis) have become very common pathogens of modern, high-health farms and represent a major source of loss to the industry. Both organisms show many similarities, yet differ in many respects. For example:
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Both pathogens are members of the pig's normal flora, being present in practically all swine populations.
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Both pathogens have a large variety of serotypes (35 for S. suis; over 15 for H. parasuis). There appears to be limited cross-protection between serotypes, which hinders effective vaccination with killed bacterins.
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Although pig groups are infected by many different strains, only a few (usually one) strains actually cause disease in a given population. This suggests that there are virulent strains, although their difference with respiratory, non-virulent strains is not well understood.
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They can both be commonly recovered from pulmonary tissues from both affected and non-affected animals.
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Both cause systemic disease, characterized by neurological signs, lameness and breathing difficulty.
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Disease is commonly seen 4-5 weeks after weaning, suggesting that it is related to a drop in maternal antibody protection. It may also be related to Porcine Reproductive and Respiratory Syndrome (PRRS) virus, since this virus circulates in the nursery at approximately the same time.
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S. suis causes few, if any, visible lesions and is rarely linked to pneumonia or pleuritis. The most obvious lesion is vegetative endocarditis (inflammation of the heart lining).
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In contrast, H. parasuis causes extensive necropsy lesions, fluid-filled cavities and lameness. In susceptible animals, H. parasuis can also produce a pneumonia, which is visibly unidentifiable from that produced by mycoplasma.
Disease caused by these two similar pathogens is more prevalent in SEW farms than in conventional, one-site facilities. This suggests that off-site weaning is somehow related to an increased problem.
In our view at the University of Minnesota Swine Center group, this may be caused by the slower spread of organisms in off-site nurseries, since the only infection source is piglets that became infected before weaning. This delayed infection produces clinical disease when piglets that have lost their maternal immunity become infected.
In contrast, nurseries in one-site farms get infected rapidly from disease spread through adjacent grow-finish units.
Our swine group has shown that by increasing the infection rate of lactating piglets (by experimentally infecting them), disease and treatment rates are effectively lowered. This proves our working hypothesis is probably correct. This experimental method for protection shows some promise, although many of the safety issues involved have to be addressed before it can be used commercially.
As noted, the presence of PRRS virus in the group may be another contributing factor. Disease caused by S. suis is more severe following PRRS infection. However, this has not been demonstrated for H. parasuis, although clinically the two diseases are often seen together. At this point, the role of PRRS in these secondary disease problems is debatable.
‘Suis’ Disease Treatment, Vaccination
Treatment with penicillin or cephalosporins is usually effective in very early infections, but only about 50% of animals showing clinical signs are saved. Surviving animals may have considerable growth retardation, making treatment not very cost-effective.
Vaccination against S. suis hasn't been very successful. Veterinarians report better success with sow vaccination but results haven't been well documented.
In contrast, H. parasuis responds well to vaccination with vaccines prepared with homologous or like serotypes. Unfortunately, there are at least 15 serotypes of this organism, making protection with commercial, licensed products variable.
Another option with H. parasuis is to use autogenous vaccines, which can work well if properly prepared. It is very important that the right strains be selected for these vaccines. This requires submitting several isolates from non-respiratory sites for genetic typing, to allow selection of the prevalent virulent strain on the farm to be used in the vaccine.
Postweaning Wasting Syndrome
There are many reports of the appearance of Postweaning Multi-systemic Wasting Syndrome (PMWS) in Canada and Europe (See “Disease Cases Climb; Cause Remains Elusive,” Feb. 15, 2001 National Hog Farmer, pages 28-29).
This is an ill-defined condition characterized by debilitated weaned pigs that frequently show respiratory distress together with other, more generalized signs of disease. Circovirus type 2 can frequently be demonstrated by lesions, both in lungs (that have interstitial pneumonia) and especially in lymph nodes.
These signs and some of the lesions are also found in animals affected by PRRS. In fact, PRRS virus can be demonstrated in most PMWS cases. This has made it difficult to interpret the role that circovirus plays in this problem, and how much PMWS is actually different from PRRS. There are probably also some differences between countries, presumably related to the relative virulence of PRRS.
In Europe, where PRRS is a milder disease, PMWS appears to be an important and common problem. The situation is more debatable in countries that have the severe American strains of PRRS, making it much more difficult to differentiate the two diseases.
In both PMWS and circovirus, weaned pigs, usually at 4-6 weeks after entry into the nursery, present a systemic syndrome characterized by extreme weakness, enlarged lymph nodes and difficult breathing. Visible lung lesions are unremarkable; the lungs usually don't collapse and frequently have a tan coloration. Histological (tissue) observation reveals massive interstitial pneumonia, the hallmark of these viral infections.
There are no specific treatments or vaccines available for PMWS and it doesn't appear that SEW can bring the disease under control. Some benefit has been reported from adopting strict AIAO policies and rigorous sanitation. This may also explain some of the differences seen with Europe, since farms there tend to be continuous flow operations. Controlling or eradicating the PRRS infection may be the only strategy available to bring some measure of control to PMWS.
Salmonellosis
An infrequent problem in modern, high-health farms, salmonellosis, due to Salmonella cholerasuis (S. cholerasuis), can result in respiratory distress and pulmonary lesions. S. cholerasuis is a systemic organism, producing lesions in a variety of organs, including spleen, kidneys and lymph nodes (enlarged and hemorrhagic). In the lung, a patchy pneumonia can frequently be seen, with the pneumonic areas distributed throughout the lung, instead of in the anterior portions as seen in other bacterial pneumonias.
S. cholerasuis responds to some systemic antibiotic treatments. But since the organism can infect humans, there is considerable debate as to whether antibiotics should be provided. Some live, avirulent vaccines have recently been licensed for this disease. These are quite effective and represent a better option to managing the disease.
Other recommendations include improving sanitation and the use of meal feed instead of pellets. However, it is unclear how much these strategies, which are successful in controlling the non-systemic salmonella of the pig, are actually effective with S. cholerasuis.
