Dense, dark red to purple-red spots are scattered over the pig’s entire body, and it is not eating or drinking at all. This is an emergency warning sign that calls for heightened vigilance. This combination of symptoms usually means that the pathogen has entered the bloodstream, triggering a systemic inflammatory response or microvascular damage, rather than being a simple problem on the skin’s surface.
In this situation, the most important action at the scene is not to rush to medicate, but first to perform a simple test: press firmly on a spot on the skin with a finger and observe whether the color fades. The result can help quickly distinguish the nature of the condition.
Does not fade when pressed: points to hemorrhagic or embolic lesions
If the spots do not blanch when pressed, it indicates significant bleeding or necrosis in the subcutaneous capillaries, with blood components leaking out of the blood vessels. These cases are usually more serious and require prompt investigation for several major diseases.
Classical swine fever is the first condition to be on alert for. The body temperature of affected pigs rises to 40.5–42°C and remains elevated. The petechiae on the skin are pinhead-sized and densely distributed, and do not blanch at all when pressed. In severe cases, the entire skin may appear cyanotic. Classical swine fever spreads rapidly; unvaccinated herds may experience a concentrated outbreak within 1–3 days, with an extremely high mortality rate.
The signs of African swine fever (especially mild/attenuated strains) are more subtle. The body temperature may be only mildly elevated (39.5–41°C). The spots on the skin may appear only on the edges of the ears, the abdomen, and the extremities, as scattered pale-red marks that blanch slightly or show little blanching when pressed. This differs in appearance from the widespread petechiae seen in acute classical swine fever. If an affected pig is also depressed but still able to stand, has a progressive decline in feed intake, and has dry, hard feces with mucus, it should be treated as highly suspicious and laboratory testing should be performed promptly to rule it out.
Acute septicemic streptococcosis can also cause dark-purple ecchymoses on the skin that do not blanch when pressed, but it may progress more rapidly. Affected pigs often develop large, confluent purple patches under the neck and abdomen and on the inner sides of the limbs. Some pigs show neurological signs before death, such as opisthotonos and paddling movements of the limbs. In some cases, death occurs just 6–12 hours after the onset of illness.
The skin lesions associated with porcine circovirus-associated dermatitis and nephropathy syndrome are also diagnostically useful. Its spots are usually slightly raised papules, round or irregular in shape, with the center gradually darkening. They do not blanch under pressure, and the lesions are most commonly seen on the hindquarters, abdomen, and back. Compared with the acute septicemias mentioned above, circovirus disease runs a relatively longer course. Pigs may show progressive weight loss and reduced appetite rather than suddenly becoming completely anorectic.
Blanching under pressure: indicative of congestive, erythematous lesions
If a spot blanches when pressed and turns red again after the pressure is released, this indicates that the lesion is mainly due to vasodilation and congestion and has not yet progressed to actual hemorrhage. Acute swine erysipelas is the most typical example of this kind of presentation.
In swine erysipelas, body temperature rises sharply to 42–43°C. Affected pigs stop eating, vomit, and develop bright-red to dark-purple patches on the skin; blanching under pressure is a characteristic sign. The “diamond skin” lesions of the subacute form—diamond-shaped or square-shaped raised areas—also blanch under pressure. Swine erysipelas is highly sensitive to penicillin-class drugs. If the diagnosis is correct, signs of a decrease in body temperature and a return of appetite can often be seen fairly quickly after medication is given. This differs markedly from the treatment response in viral diseases or streptococcal septicemia.
Priority order and recommended actions for on-site investigation
In the absence of laboratory facilities, investigation and response can be carried out in the following order of priority.
Step one: Isolate immediately to prevent spread. Regardless of the cause, this pig is already in a severe systemic infection state and poses a risk of shedding pathogens into the environment. It should be moved immediately to a separate isolation pen and handled by dedicated personnel, with contact with other pigs avoided.
Step two: Narrow down the possibilities using key clinical signs. In addition to the blanching test, measure rectal temperature. A high fever (above 42°C) with blanching points first to erysipelas; a high fever without blanching requires distinguishing between classical swine fever and streptococcal infection, while also remaining alert to African swine fever; a low-grade fever or no high temperature combined with non-blanching raises the possibility of a low-virulence African swine fever strain or porcine circovirus.
Step three: Avoid using potent fever-reducing drugs indiscriminately before the cause is clear. For viral diseases such as classical swine fever and African swine fever, potent fever-reducing drugs (such as metamizole) can only suppress the temperature temporarily; they cannot eliminate the pathogen and may also mask the condition and interfere with the body's immune response. When the cause is unknown, prioritizing fluid replacement (oral rehydration salts or intraperitoneal injection) to maintain strength and correct dehydration is more practically meaningful than simply reducing the fever.
Step four: Testing for high-risk diseases is an unavoidable part of the process. If a sick pig has diffuse non-blanching spots accompanied by refusal to eat or drink and depression, collect anticoagulated blood (from the tail root) or lymph node tissue for testing as soon as possible, while isolating the pig and providing supportive care, to rule out African swine fever and classical swine fever. This is not only a matter of responsibility toward the individual sick pig, but also a way to protect the farm's biosecurity baseline.