Definition
A sporadic disease, usually observed in young, growing pigs, characterized by listlessness, fever, anorexia, hemolytic anemia and, in severe cases, by icterus.
Occurrence
Mycoplasma suis infection can occur in all age groups of swine but seldom is clinically apparent except in young, growing pigs. Serologic evidence indicates prevalence of less than 15% among swine with only a small number of outbreaks recognized. It often is associated with stress and can recur in stressed animals.
Outbreaks occur more often in the summer and often are concurrent with other infectious diseases, parasitisms or management problems. M. suis is likely to be species specific although the question has not been extensively researched. The geographic distribution of the disease is unknown. Relatively few countries have reported it.
Historical information
Icteroanemia in growing swine was described in the United States in 1943. At least four publications on M. suis infection were published in the 1950s. Recognition that there are many carriers came after development of an indirect hemagglutination test in 1975.Mycoplasma suis was thought to reside in the cell membrane but a report in 2009 indicated that M. suis can trasmigrate to the interior of the red blood cells. Although M. suis rarely causes spectacular death losses, it is recognized as an occasional cause of anemia or icterus in young pigs.
Etiology
The disease is caused by M. suis (previously Eperythrozoonosis suis and for a short time proposed to be renamed as Mycoplasma haemosuis), an obligate, intracellular bacteria. The organism is usually from 0.8 to 1.0 um in diameter but may be larger during the acute stage of infection. It is predominantly coccoid-shaped but may appear rod or ring-shaped when viewed on an erythrocyte’s cell membrane.
Epidemiology
There likely are numerous carriers in the swine population. Presumably, most of these are recovered animals that had M. suis infection in a subclinical form. In utero transmission has been demonstrated experimentally. The disease has been reported to occur in an acute form in sows immediately prepartum with piglets from sows also affected.
Since the disease occurs more frequently in the summer, it is suspected that mosquitoes and biting flies, as well as the hog louse (Haematopinus suis), play a role in transmission. The use of contaminated surgical instruments (including castration knives) and needles used repeatedly during vaccination are believed to transmit the organism. Ongoing infectious and parasitic diseases are believed to play an important role in precipitating outbreaks.
Pathogenesis
Much of the information on pathogenesis is based on study of experimental infection in splenectomized pigs. In these pigs, the clinical and pathologic aspects closely resemble natural infections. Once M. suis is introduced into the blood, a febrile response occurs in about 7-8 days. During the acute phase of the disease, M. suis organisms are numerous on the erythrocytes. Many infected pigs become hypoglycemic and some have convulsions, develop coma and die. A hemolytic anemia develops gradually, often requiring a week after onset before it is clinically apparent. Hemoglobinuria does not occur because hemolysis is extravascular. The number of platelets, the packed cell volume and hemoglobin values decrease. Under field conditions many affected animals recover but remain carriers.
Clinical signs
In young, growing pigs, prominent signs include anorexia, listlessness, marked weakness, pale mucous membranes and, perhaps, icterus. Temperatures are raised initially but often are subnormal by the time the pigs are examined. In chronic cases there often is marked icterus and deaths may occur. Signs in breeding stock are similar but are less obvious and often go unnoticed.
In breeding herds, M. suis infection has been associated serologically with various reproductive problems in breeding herds. However, proof that these were caused by the disease is lacking.
Lesions
At necropsy of young pigs, lesions depend somewhat on the duration of illness. Early cases exhibit pallor, watery pale blood, and perhaps modest enlargement of the spleen. In cold climates there may be necrosis of extremities. Hog lice, or their ova, often are present.
In older pigs the same lesions are apparent at necropsy but splenic enlargement is more noticeable. There often is icterus (more obvious internally) and it may be marked. The bone marrow is red, suggestive of hyperplasia. The liver often has a brownish cast from hemosiderosis. There often are suffusions in the abdomen, thorax and pericardial sac. Lymph nodes are edematous and swollen. The heart often is thin-walled and flabby. Careful examination often reveals lesions of concurrent diseases or parasitisms.
Diagnosis
M. suis is an unlikely cause of major disease. The history, signs and gross lesions often are of value in diagnosis. Confirmation is made by demonstrating the organisms in blood smears stained with Wright-Giemsa or Diff-Quick stains. The organisms are usually found on the surface of erythrocytes and/or in the plasma. Infection with M. suis must be differentiated from other causes of anemia. A careful search for concurrent diseases should be made. Differential diagnoses include systemic salmonellosis, porcine circovirus associated disease (PCVAD), mycotoxicosis, hepatotoxicosis, iron deficiency anemia, or autoimmune hemolytic anemia.
Blood saved for smears to be examined by fluorescent microscopy should be collected in anticoagulant (heparin or acid citrate dextrose). One mL can be mixed with an equal part of 10% formalin. Smears made from this preparation can be stained with acridine orange. Blood collected during the early, acute stage of the disease has many more M. suisorganisms than blood from chronic cases. Serologic tests (indirect hemagglutination, IHA; or enzyme-linked immunosorbent assay, ELISA) and polymerase chain reaction (PCR) may be available to confirm M. suis as a herd problem, although these tests have limited availability. Infected pigs less than three months old seldom react on IHA.
Control
Tetracyclines and organic arsenicals have been used with success for prevention and treatment of M. suis infections. Intracellular organisms may not be responsive to tetracycline treatments. Organic arsenicals should be used cautiously and directions followed carefully. They should not be given initially to thirsty pigs or poisoning may result. Their use has been associated with deafness in breeding animals.
Affected pigs can be treated individually with injectable oxytetracycline.
The indirect hemagglutination test can been used as a screening test prior to purchase of breeding stock or to identify herd infection. A combination of CBC and PCR performed on whole blood can help diagnose suspected clinical infection.
There are no vaccines for the prevention of M. suis.