We obtained serum samples from 40 guinea pigs from 3 different regions of Ecuador (Figure 1), 20 from Cuenca and 10 each from Guayaquil and the Manabí region. Cuenca is located in the Andes region, 2,500 m above sea level; it is one of the main producers of guinea pig meat. Guayaquil, the most populated city in Ecuador, is located at the head of the Gulf of Guayaquil on the Pacific Ocean. The Manabí region is located in western Ecuador on the Pacific Ocean coast. Serum samples were collected from adult guinea pigs that we purchased from local farms (Cuenca), where they had been raised as livestock, or from live animal markets (Guayaquil and Manabí). The samples were collected by heart puncture under general anesthesia (combination of ketamine and xylazine). Animals were euthanized after samples were obtained.

For antigens in the serologic analyses, we used whole viruses and recombinant hemagglutinin, nucleoprotein, and neuraminidase (subtypes N1 and N2) proteins produced in our laboratory as described (5,6). The whole viruses were A/Brisbane/59/2007 (H1N1) (Brisbane07), A/New Caledonia/20/1999 (H1N1) (Newcal99), A/Wisconsin/67/2005 (H3N2) (Wisconsin05), A/Vietnam/1203/2004 (H5N1) (Vietnam04), and B/Yamagata/16/1988 (Yamagata88). The hemagglutinins were A/California/04/2009 (Cal09), NewCal99, Vietnam04, Wisconsin05, and Yamagata88. The nucleoproteins were Brisbane07, A/Puerto Rico/08/1934, and B/Florida/04/2006. The subtype N1 neuraminidases were Cal09 and Vietnam04; the N2 subtype was A/Hong Kong/1/1968.

ELISA was done as described (7), with slight modifications for the use of guinea pig serum. The cutoff for serum considered positive for influenza virus was the value of the negative control (naive guinea pig serum) +3 SD (from 3 repetitions). For Western blot (WB) analyses (8), serum samples were pooled into groups of 5 (groups 1–4, 5–6, and 7–8 were from Cuenca, Guayaquil, and Manabí region, respectively). Pooled samples were also used for the hemagglutination inhibition (HI) assay, as described (9), using influenza strains Brisbane07, Wisconsin05, and Vietnam04. As positive controls, we used serum from guinea pigs that we had infected with Cal09, Brisbane07, NewCal99, Wisconsin05, Vietnam04, or Yamagata88.

ELISA results for the 40 serum samples showed that 20, 18, and 14 were positive for influenza subtypes H1, H3, and H5, respectively (Table 1). The samples were also tested for the presence of antibodies against the influenza virus nucleoprotein: results for 29 were positive. Samples with positive results to >1 hemagglutinin antigens and to nucleoprotein were also analyzed for the presence of antibodies against proteins of the neuraminidase subtypes N1 (Cal09, Vietnam04) and N2 (influenza A/Hong Kong/1/1968 [H3N2]) (Table 1). Serum samples were also tested for the presence of influenza B virus by using whole virus, recombinant hemagglutinin, and recombinant nucleoprotein as described above. Samples from Cuenca showed the highest overall positivity to the 3 antigens (Table 2).

WB analysis of pooled samples from the 3 areas confirmed the ELISA results that showed antibodies against the hemagglutinin and nucleoprotein antigens; however, samples from Manabí region (7,8) showed immunoreactivity to the H3 antigen only (Figure 2). WB results for serum samples from Cuenca and Guayaquil were positive for influenza B virus, but results were not positive for samples from Manabí when whole virus was used as antigen (Figure 2). HI activity was observed in all pooled samples against all 3 viruses tested. Titers were highest in samples from Cuenca (320 for Brisbane07, 640 for Wisconsin05, and 80 for Vietnam04) and lowest for samples from Manabí (80 for Brisbane07 and 40 for Wisconsin05 and Vietnam04). Titers for samples from Guayaquil were 160 for Brisbane07 and 80 for Wisconsin05 and Vietnam04.

We showed the presence of influenza virus antibodies with HI activity to different virus subtypes in guinea pigs marketed through local farms and live animal markets in different regions of Ecuador. Seroprevalence was similar for influenza A virus subtypes H1 (50%) and H3 (45%). A study of influenza among humans in Ecuador also showed similar seroprevalences for subtypes H1 (5.1%) and H3 (5.5%) (10). This10-fold difference between seroprevalence rates among humans and the guinea pigs in our study may be explained by animal husbandry practices that facilitate influenza transmission (e.g., the crowding of animals in cages).

The presence of antibodies to influenza B virus in guinea pigs is notable because infection with this virus is thought to be restricted to humans (11). Recent studies in our laboratory demonstrated that guinea pigs can also be infected with influenza B viruses and that they readily transmit the virus to naive guinea pigs (12). These studies support our finding that influenza B virus has the potential to be transmitted from humans to other species. Further studies are needed to isolate and characterize the type B influenza virus present in the population of guinea pigs to determine if there has been an adaptation to the new host or if the guinea pig is only a transient reservoir for the human virus.

We found that several guinea pigs had antibodies against influenza A (H5). Ecuador is purported to be free of avian influenza (13); however, reports from other countries in South America demonstrate the presence of avian subtypes H3, H5N2, H7N3, and H13N9 in wild birds (14). In addition, chickens, turkeys, and guinea pigs that are later sold in local farmers’ markets are raised together by some families, thereby facilitating contact of guinea pigs with avian influenza viruses. We tested only for seroreactivity to the H5 hemagglutinin and the N1 neuraminidase; therefore, further studies are needed to determine whether different avian origin influenza viruses are present in the guinea pig population.

We did not determine whether guinea pigs are an incidental host for influenza virus infection or, if instead, the virus has been adapted to these animals or if guinea pigs are a natural reservoir for some influenza viruses. To this end, virus isolation and characterization would be necessary to determine the virus strains circulating in this population. In the laboratory, guinea pigs are infected and efficiently transmit influenza viruses to naive hosts without showing any overt clinical signs of disease (1). Therefore, further studies are needed to address the specific role of guinea pigs raised as livestock in the ecology and epidemiology of influenza viruses in the region.