Clinical case: Outbreaks of respiratory and reproductive disorder in a pig farm

Farm description

The outbreaks of respiratory and reproductive disorder occurred in February and October 2013 in a farrow to finish farm in Warmian-Masurian Voivodeship of Poland. The sow herd consisted of 400 sows of different breeds. The batches consisted of about 55 animals. Piglets were weaned at the age of 4 weeks and transferred to a separate nursery at 30 meters away. In farrowing rooms and nurseries strict all-in-all-out rule was applied. Every 3 months a group of 40 replacement gilts was purchased. They were coming from an independent gilt producer, free from PRRS. While in quarantine, the replacement gilts were vaccinated twice against influenza. The fattening site was located 8 kilometers away from sows buildings and nursery, and pigs of about 10 week of age were transferred there.

Piglets were vaccinated against Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae and PCV2. Sows were vaccinated against PPV, erysipelas, Escherichia coli and Actinobacillus pleuropneumoniae.

The biosecurity level of the farm was low. Sows and weaners were serviced by the same staff. The staff from the fattening site did not enter the sows and weaners site. There was a slaughterhouse 500 meters from the sow site.

Reproductive and respiratory health problems

The first visit on the farm was in February 2013. In the farrowing units there were several weak litters observed. Many piglets had pale or yellowish skin. In some cases all piglets from a litter were stillborn or mummified. In the nursery, respiratory symptoms, dyspnea, coughing were observed. Post mortem examination and laboratory diagnosis indicated infections with Actinobacillus pleuropneumoniae, Streptococcus suis, leptospira and Brachyspira hyodysenteriae.

In the fattening units there was chronic coughing and dyspnea observed. Antibiotic treatment was not effective. Mortality among fatteners was about 8%.

The clinical symptoms suggested PRRS so, serum samples from piglets, weaners of 6 and 9 weeks of age, and fatteners of 12, 15, 18 and 21 weeks of age were tested by ELISA for PRRSV seroconversion. From each age group 8 animals were sampled. The PRRSV seroconversion was detected in animals from all age groups which proved that the herd was infected with the virus (Table 1.1).

Table 1.1. Summary of the serological ELISA investigations for PRRSV.

As the serological results and clinical observation indicated that the PRRSV was circulating also in sows, it was decided to mass vaccinate all sows with modified live vaccine against PRRS. It was performed only once. Additionally, vaccination of replacement gilts 60 days prior to their introduction in the sow herd started. Also piglets at 10 days of life were vaccinated.

The health status in the farrowing units and nursery was gradually improving in the following months. However, PCR testing of a sample from the fetus aborted in the end of April 2013 detected presence of PRRSV so, apparently the virus was still circulating in the sow herd.

In June 2013 the piglet vaccination was stopped but the gilt vaccination continued.

In mid-September 2013 samples from 14 weaners of 7-8 weeks of age, and 6 fatteners of 12 weeks of age were tested for PRRSV antibodies by ELISA. All of the weaners and most of the fatteners tested negative (Table 1.2). Only one fattener tested positive. These results provided strong evidence that the sow herd was free from PRRSV circulation and produced piglets free from the virus. Also nursery seemed to be free from the virus. A single positive result from one fattener suggested that the virus was not completely eliminated from the farm.

Table 1.2. Summary of the serological ELISA investigations for PRRSV.

New outbreak of respiratory and reproductive disorders

In the end of September 2013 severe coughing started in fatteners. Eighteen serum samples from sows and fatteners were tested for influenza virus seroconversion. Nasal swabs from coughing fatteners were tested by PCR for influenza virus. All the results were negative.

By the end of October 2013 severe coughing started in nurseries. Also, fever above 40 ºC and inappetence was observed. In 2-3 days all animals from a given age group exhibited those symptoms. Sick pigs were treated with antipyretics and doxycycline. Mortality in weaners was about 3%. Unlike before, Glaser disease cases were also observed.

A week later, fever and coughing started in sows. Two sows and one boar died. Three sows aborted. Usually fever lasted for 3 days. The problems in sows seemed to roll over all animals and were observed in a period of about 3 weeks.

By the end of November and in December 2013 fever started also in fatteners. Average daily weight gain dropped from 830 gram down to 760 gram. The feed conversion rate increased to 3.2. Mortality in fatteners in this period was about 5%.

Table. 2. Percentage of stillborn piglets and mummies between August 2013 and March 2014.

Since the appearance of fever and coughing in sows, reproductive parameters deteriorated (Table 2). Percentage of stillborn increased from 5.5% in August and September, to 7.38 % in October, 6.33% in November and 7% in December 2013. Also proportion of mummies increased from below 1% in October to 2.77% in December. Increase in number of splaylegged piglets was also observed. From February 2014 the reproductive parameters, except for the percentage of the mummies, started to improve. Respiratory symptoms were also diminished.

Because of some similarity to the problems observed in February 2013, PRRSV was suspected for causing the reproductive disorder. In early February 2014, 8 samples of serum from 1, 4 and 7 weeks old piglets were tested by ELISA for the presence of PRRSV antibodies. Specific antibodies were detected in 6 out of 8 youngest piglets and in one pig from the two remaining age groups (Table 1.3). Although PCR for PRRSV was not performed on those samples, the serological results were highly indicative for the lack of PRRSV circulation in sows and weaners.

Later in February 2014, ELISA test for PRRSV antibodies was performed on 8, 24 weeks old fatteners. Seven of them reacted positive (Table 1.3). As they were born after the piglet vaccination with modified live vaccine against PRRSV was stopped, the result was a proof of PRRSV circulation in fatteners.

Table 1.3. Summary of the serological ELISA investigations for PRRSV.

Additionally, the same serum samples were tested by ELISA for influenza virus antibodies and all of them reacted positive. This result showed that influenza virus infected the farm after September 2013 when the HI and PCR tests for influenza virus gave negative results.

In the beginning of March 2014 samples of lungs from three piglets that died before weaning were collected, as well as oral fluid samples from pens of 7 weeks old weaners and 10, 13, 16, 19 and 24 weeks old fatteners. Two pens from each age group were sampled. Infuenza virus was detected by PCR in lung samples from piglets as well as in oral fluid samples from 7, 16 and 19 weeks old pigs (Table 3).

The same oral fluid samples, and serum samples from the pens where oral fluid was collected, were tested by PCR for PRRSV. PRRSV was detected only in one out of two oral fluid samples from 10 weeks old fatteners and in a pool of serum samples from the same pen of pigs. ELISA testing performed on sera from pigs from the same pen performed 2 weeks later gave all positive results (Table 3).

Table 3. Summary of PCR investigation for PRRSV (in serum and oral fluid) and influenza (in lungs and oral fluid) in March 2014. Serum samples were pooled (one pool per pen) for RNA extraction. POS – positive; NEG – negative. POS 1/2 means that one of two samples was positive in PCR.

Final diagnosis

The diagnostic investigation allowed exclusion of PRRSV as an etiologic agent of respiratory and reproductive failure that started in October 2013 and lasted until February 2014. Earlier PRRS outbreak that was diagnosed in February 2013 was successfully controlled using single mass vaccination with modified live vaccine in sows and vaccination of piglets until June 2013. Replacement gilt acclimation was probably the key to the success, and despite apparent elimination of PRRSV from sow herd and nursery, it has been continued. These elements were not enough to eliminate PRRSV from the farm. The virus persisted in fattening pigs where continuous flow of pigs was maintained. Interestingly, later analysis proved that only one, older, of two buildings for fatteners was infected. The new building, adjacent to the old one, that was populated with growers after the PRRSV elimination from nursery, remained virus free.

Before the occurrence of acute clinical symptoms in the second half of September 2013 the farm was free from influenza. So, the respiratory symptoms in this period were not linked to influenza virus. Later findings showing the presence of PRRSV in fatteners suggested that it was the main factor, or at least one of the factors of the disease, prior to influenza outbreak in nursery in October 2013.

In February 2014, the seroconversion to influenza virus was found in fatteners, and the virus was detected by PCR in lungs of dead piglets and oral fluid of weaners and fatteners. This suggests that influenza virus was likely the main cause of the observed respiratory and reproductive disorder. More acute disease in fatteners that in other groups of animals might be caused by co-infection of influenza and PRRS viruses.

Based on the collected evidence it was decided to introduce sow vaccination against influenza at 3 weeks before farrowing.

Conclusions

Influenza virus replicates in epithelial cells of the respiratory tract and induces high inflammatory response in lungs. In pregnant dams it can induce embryonic and fetal death at every time of gestation. Clinical diagnosis is not always straightforward and the detection of influenza virus by PCR in nasal swabs from sows with fever or lungs of dead animals is recommended. Oral fluid seems to be a good alternative material to serum, as it allows simultaneous detection of other pathogens present in the respiratory tract or the environment. Oral fluid collection is easier, less labor intensive and less stressful for animals than blood collection.

Unusually fast elimination of PRRSV from sows and nursery using limited vaccination protocol seems surprising but very likely the herd immunity against PRRSV was already built and this limited vaccination program helped to finally eliminate the virus.

The continued increased occurrence of mummies is thought to be related to non-infectious factors.