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Update on the diseases associated with the PCV2: clinic, diagnosis and control

The PCV2 has been associated, in an ongoing way and throughout time, with more pathologies called, collectively, diseases associated with the PCV2 (PCVDs): PCV2 systemic disease, PCV2 lung disease, PCV2 enteric disease, PCV2 reproductive disease,PCV2 subclinical infection and porcine dermatitis and nephropaty syndrome.

Porcine circovirosis or, as it has been recently suggested to call it, systemic disease due to circovirus type 2 (PCV2-SD), is a relatively recent disease that was described for the first time in Canada in 1996 by Drs. John Harding and Edward Clark. Nevertheless, we know, thanks to retrospective studies, that the infection due to PCV2 was already present in pigs since 1962 in Germany, and that the disease was present in Europe since the mid-nineteen eighties.

Table 1. Diseases associated with the PCV2: terms proposed recently, replaced terminology and diagnostic criteria.

PCVD (acronym) Replaced terminology Diagnosis
PCV2 systemic disease
(PCV2-SD)
Postweaning multisystemic wasting syndrome (PMWS), porcine circovirosis, PCV2-associated systemic infection 1. Wasting, clinically evident weight loss, skin paleness, respiratory or digestive signs.
2. Moderate to severe lymphocyte depletion with granulomatous inflammation in lymphoid tissues.
3. Moderate to high amounts of PCV2 in the lesions.
PCV2 lung disease
(PCV2-LD)
PCV2-associated respiratory disease,
Proliferative and necrotizing pneumonia (PNP)
1. Respiratory distress, dyspnea.
2. Lympho-histiocytic to granulomatous or broncho-interstitial pneumonia, peribronchiolar fibroplasia, necrotizing and ulcerative bronchiolitis or proliferative necrotizing pneumonia in absence of PCV2-SD lymphoid lesions.
3. Moderate to high amount of virus in the lung.
Absence of lesions in lymphoid tissues (if they were present it would be PCV2-SD).
PCV2 enteric disease
(PCV2-ED)
PCV2-associated enteritis 1. Diarrhoea.
2. Granulomatous enteritis and lymphocyte depletion with granulomatous inflammation in the Peyer's patches (but not in other lymphoid tissues).
3. Moderate to high amount of PCV2 in the intestinal mucosa/Peyer's patches.
PCV2 reproductive disease
(PCV2-RD)
PCV2-associated reproductive failure Aborted and mummified piglets/foetuses:
1. Reproductive failure at late gestation.
2. Fibrous to necrotizing myocarditis in fetuses.
3. Moderate to high amount of PCV2 in the heart.
The use of the real-time quantitative PCR in tissues would be more sensitive for the detection of this disease in the foetuses.
Regular return-to-estrus:
1. PCV2 sero-conversion and/or PCR positivity against PCV2 around the return to oestrus.
PCV2 subclinical infection
(PCV2-SI)
None 1. Lower average daily weight gain in absence of evident clinical signs.
2. Absence or minimal histological lesions (mainly in lymphoid tissue)
3. Low viral load in few lymphoid tissues
The criteria no. 2 and 3 can potentially be replaced by PCV2 detection techniques such as PCR.
Porcine dermatitis and nephropaty syndrome
(PDNS)*
None 1. Dark red papules and macules on the skin, mainly on the hind limbs and the perineal area.
2. Haemorrhagic and necrotizing skin lesions and/or swollen and pale kidneys with generalized cortical petechia.
3. Systemic necrotizing vasculitis with necrotizing or fibrinous glomerulonephritis.

* The PDNS is still associated with the PCV2 albeit being considered an immune complex-mediated disease and its aetiology has not been proven clearly.

The PCV2-SD is considered a multifactorial disease that affects piglets in the weaner and fattening stages that present, mainly, a decreased rate of weight gain with a variable morbility and a high mortality. The magnitude of the disease was quickly evident due to its worldwide presentation in a short period of time, being a centre of attention due to its economic impact on the pig production. During these 16 years of conscious coexistence with the disease we have attained more knowledge about the PCV2-SD and the PCV2, that has been related, increasingly by the minute, with pathologies called, collectively, PCV2 associated diseases (or PCVDs = Porcine Circovirus Diseases) (Table 1). In fact, the commercialization of vaccines against the PCV2 from year 2004 in some countries and from 2006-07 in many parts of the world has contributed, to a great extent, to expand this list, because they have shown spectacular results that go beyond to that expectable in the improvement of the production parameters. So, albeit that initially, the PCV2-SD was the most relevant and known disease associated with the PCV2, currently, it is considered that the virus can have detrimental effects at a global level in the farm, and it can affect animals in all the production stages. In this sense, the awareness of the effect of the PCV2 subclinical infection (PCV2-SI) has gained a special relevance, because it is the most frequent situation in the farms albeit not producing evident clinical signs, but its production losses can be measured by means of the use of vaccination. Another recent pathology is the PCV2 reproductive disease (PCV2-RD), for which different reproductive problems, according to the moment of the infection (Table 2), have been described.

Table 2. Reproductive problems associated with the PCV2 according to the moment of the infection of the foetus during the gestation.

Time of the infection of the foetus during the gestation Consequence on reproductive parameters
1st-35th day Embryo death
Regular returns to oestrus
False pregnancy
Small litters
35th-70th day Mummified foetuses
Abortions
70th-115th day Mummified foetuses
Stillbirths
Piglets born weak
Delayed births
Abortions

Nowadays, the PCV2-SD is considered to be an easily controllable disease through the use of vaccines. In the same way, beneficial effects from the vaccination against the PCV2-SI and the PCV2-RD have been described. In fact, the use of vaccines is very widespread. In any case, when thinking about the use of the vaccine, we must evaluate if a beneficial effect is expected in our farm against the present PCVDs, and later on the profitability of its use must be evaluated. We must bear in mind that establishing the vaccination plan can have its complexity when the different possible PCVDs, other diseases present in the farm, and the rest of the vaccines to be administered to the animals have to be considered. The handling possibilities, the staff availability and the costs must also be borne in mind.

Nowadays the vaccine is considered an effective and profitable tool for the control of the PCV2-SD, and benefits in the improvement of the production parameters in the case of the PCV2-SI and the PCV2-RD have also been described. Even though, the rest of measures that help to counteract the disease as, for instance, the handling practices, the characteristics of the installations, the biosafety rules, the control of other concurrent diseases and the factors that can have an influence on the disease, and that are inherent to the animals, as genetics and the higher presence of PCV2-SD in castrated males, must not be forgotten.

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