There are no doubts about the zoonotic risk of swine flu. However, it is uncertain the importance of this respiratory pathogen from a production standpoint. Overall, influenza in North America is considered the second most important swine respiratory disease, and is usually included in vaccination plans of farms. Conversely, the impact of this disease in Europe is differently perceived between countries and even in different areas or companies within a country. A possible explanation for these different viewpoints could be the lack of a clear cause-effect association; that is, between the presence of the influenza virus and the presence of an evident clinical impact. In some cases this may be due to inadequate or insufficient diagnosis.
To assess the impact in production of influenza, two different situations need to be explored: 1) epidemic situations, often characterized by acute respiratory outbreaks with high morbidity, and 2) the endemic situations, generally characterized by a low incidence that may be related with varied scenarios; from an apparently subclinical circulation of the virus to respiratory outbreaks of recurrent appearance.
Epidemic situations often occur when an influenza strain is introduced into a susceptible herd. In these cases, the impact is variable and will be affected by different conditions (environment, facilities, management, concomitant diseases ...). In some cases, the epidemic situation might lead to serious problems in breeding farms. In a case presented in the “Swine days”, at the Autonomous University of Barcelona (Casanovas, 2012), an epidemic outbreak caused by a H3N2 strain in a Spanish breeding farm of 800 sows was described. The outbreak caused a significant increase in abortions and even deaths in sows. Specifically, 21 deaths and 14 abortions caused by fever. Noteworthy, this is an extreme case occurred in June 2011, when the high temperatures likely played a major role in the severity of the case. Remarkably, an epidemic influenza circulation can also cause a significant increase in the irregular return to estrus, which can reach +15% (Lorenzo, Avances, 2014). In other stages of production, the potential impact is less important as long as secondary infections can be easily controlled, due to the low mortality (<1%) and the compensatory growth observed in most of cases.
Regarding endemic situations, they occur when a virus remains for long periods in a farm. In these situations viral infection impact is difficult to assess, as there may be interactions with other pathogens (viral or bacterial). However, there are two detailed studies evaluating the impact of influenza viruses circulating endemically in nurseries (Gillespie 1999, Torremorell et al., 2009). Similar situations are described in both works: 1) recurrent respiratory signs in each production batch, 2) increased mortality of about 2% and 3) decrease in final weight at weaning, ranging from -10% in the first work to -30% in the second. In addition, secondary bacterial complications were observed in both studies, increasing the indirect costs per treatment.
In Europe, there is not many information available evaluating in such detail the impact of influenza in endemically infected farms. In a study conducted by the group of epidemiology and infectious diseases at the veterinary faculty of the Autonomous University of Barcelona (2012, unpublished data), the impact of endemic influenza in batches of pigs were evaluated during the fattening stage. Production data from 137 batches within the same company holdings were evaluated. Specifically, in 33 out of the 137 premises respiratory outbreaks were detected. Of these 33 cases, 20 were found to be positive for influenza virus by RT-PCR for oral fluids. The batches affected by influenza virus showed significant differences when compared with healthy lots in regard to: 1) the average production days (+3 days), 2) the total days of facilities occupation (days to complete emptying of facilities; +2,5 days) and 3) the mortality rate (+24.6% in relative terms)(Table 1). From a productive point of view, the holdings where clinical outbreaks were caused by influenza incurred a loss of approximately 0.66€ per produced pig when compared with batches that did not suffer respiratory outbreaks (see Table 2). Moreover, the cost of medication associated with respiratory problems should be added. Obviously the results obtained in this study are preliminary and it is worth to note that the evaluation was conducted retrospectively, but it can be useful as a first approximation to the real impact in the circulation of influenza virus at this productive period from an economic and productive point of view.
Table 1. Productive and economic analysis of influenza virus infection in fattening units.
Preliminary data obtained from 137 batches belonging to the same company.
|Deaths||Time in average until all animals are sent to slaughterhouse (days)||Time in average needed to reach slaughter weight (days)|
|Batches with influenza virus outbreaks confirmed in laboratory (n=20)||4.1%a||159.2a||137.2a|
|Batches with respiratory outbreaks negative to influenza (n=13)||4.08%a||158,5a.b||134.6b|
|Healthy Batches (n=104)||3.39%b||156.6b||134.2b|
a>b>c P-value < 0,05
Table 2. Calculation of cost per animal sold in the influenza positive feedlots
|Feed Consumption/ Animal||NO||-||-|
|Final weight / Animal||NO||-||-|
|Deaths||YES||Increase of deaths due to influenza outbreaks compared with healthy feedlots = +0.827%; relative increase = +24.6%||→ Cost of nursery piglet (Spain, 2012) = 42€
→ Benefit losses due to a death = 7€
→ Total cost of a death = 42+7 = 49€
→ Increase of deaths due to flu = 0.827 x Animals in the fattening unit
→ Sold animals = Animals in the fattening unit – Total deaths (4.1% in the herds affected by flu)
→ Cost per sold animal = (Total cost of death x Increase of deaths due to flu) /Sold animals = 0.42€/sold animal
|Time in average needed to reach slaughter weight||YES||Increase of days needed to reach slaugther weigh in average due to influenza outbreaks compared with healthy feedlots = + 3 days||Not taken into account as long as there are no differences in feed consumption and final weight|
|Time in average until all animals are sent to slaughterhouse||YES||Increase of time in average until all animals are sent to slaughterhouse due to influenza outbreaks compared with healthy feedlots = +2.6 days||Cost of one day take up of the facilities (Spain, 2012) = 0,095€
Cost per sold animal = 0.095€/day x 2.6 days = 0.247€
TOTAL COST INCREASE PER SOLD ANIMAL IN INFLUENZA AFFECTED HERDS = 0.667€
In conclusion, swine flu is a disease that may have a high economic and productive impact on a farm. It occurs infrequently but explosively in epidemic situations; while in endemic situations shows a recurrent and constant low-incidence impact. Unfortunately, there is little information available in the literature on the impact of influenza from a production standpoint. Today, there are many open topics, such as: 1) what is the impact of the disease in endemic situations at the reproductive level (irregular return to estrus / fertility), especially in gilts; or 2) what is the productive impact in farrowing units, especially when we know that suckling piglets can play a role as a reservoir of the virus in an endemically infected farm.