Dynamics of Mycoplasmal Conjunctivitis in a Northeastern House Finch Population, 1994-1999
Barry K. Hartup1, Jean M. Bickal2, George V. Kollias1 and David H. Ley3
1Division of Wildlife Health, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853, USA; 2136 Mercer St. , Trenton, New Jersey, 08611, USA; 3Department of Farm Animal Health and Resource Management, College of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina 27606, USA
Abstract: A field study was conducted to determine the prevalence of conjunctivitis, and later Mycoplasma gallisepticum (MG) infections, in house finches (Carpodacus mexicanus) from Mercer County, New Jersey (USA). We captured and examined 1597 individuals between November 1994 and August 1999. Clinical conjunctivitis (eyelid or conjunctival swelling, erythema, discharge) was observed in 11% (n = 183) of the birds examined. Monthly conjunctivitis prevalence ranged from 0% to 43% during the study. Conjunctivitis in house finches exhibited marked seasonal fluctuation, with peak disease prevalence during winter months, and lower disease prevalence during the breeding season (March through August). During the peak of the breeding seasons (April through June), disease was most prevalent in males not in breeding condition compared to males in breeding condition (p < 0.05). Disease prevalence was similar for non-breeding and breeding females during the same period. The prevalence of diseased juvenile house finches increased linearly between July and November in this population (p < 0.01); juvenile finches were 4.5 times as likely to be observed with conjunctivitis in November compared with recent fledglings from July. Clinical conjunctivitis and MG infections, determined by in vitro isolation and polymerase chain reaction testing, were both at low prevalence during the two breeding seasons when monitored by both methods, but did persist in this population. Fingerprints of 10 MG isolates using random amplification of polymorphic DNA techniques showed no apparent differences in banding patterns over the course of the study, suggesting persistence of a single MG strain in the study population. Infection with MG may constrain or delay the achievement of breeding condition in adult male house finches by shifting energy required for breeding toward an immune response. This may represent an additional causative factor contributing to recent population declines of eastern house finches apart from direct mortality due to disease.
Key words: Carpodacus mexicanus, Conjunctivitis, Epidemiology, House finch, Mycoplasma gallisepticum.
Introduction
Mycoplasmal conjunctivitis is a recently described infectious disease of house finches (Carpodacus mexicanus) in eastern North America. 1,2 Caused by Mycoplasma gallisepticum (MG), a common respiratory pathogen of domestic poultry, the disease was first described in early 1994 from Maryland and Virginia. 3 Several studies have described various facets of the epidemiology of this emergent disease and the potential for spread of the disease to other free-ranging avian hosts. 4-8 The arrival of mycoplasmal conjunctivitis is correlated with population declines as great as 60% across much of the range of the eastern house finch, and may represent a limiting factor to their abundance. 9 To date, no published field studies are available that document the prevalence of conjunctivitis over time in a population large enough to test the association of disease with individuals of differing sex or age classes, or breeding condition. These observations are invaluable for assessing the potential of mycoplasmal conjunctivitis to limit house finch populations.
The primary objective of this study was to document the occurrence of conjunctivitis in house finches common to residential feeding stations over several years, and to retrospectively analyze disease frequencies among various subclasses of house finches at different times of the year. This study also presents preliminary data on the prevalence of MG infections in the same house finch population beginning in February 1998, combined with molecular epidemiological analysis of the MG isolates obtained.
Materials and Methods
The study was conducted between November 1994 and August 1999 in Mercer County, New Jersey. Birds were captured with Potter traps10 under permits from the New Jersey Department of Fish, Game and Wildlife (Trenton, New Jersey, USA) and the United States Fish and Wildlife Service, Department of Interior (Washington D.C. , USA). Traps were distributed among bird feeders maintained year-round at two residential sites in the city of Trenton. All birds were identified by applying a numbered aluminum leg band (Bird Banding Laboratory, Laurel, Maryland, USA). All birds were given a physical examination that included close inspection of the eyes and adnexa for signs of conjunctivitis, such as eyelid or conjunctival swelling, erythema, and discharge (Fig. 1). The age of each bird was determined through plumage characteristics and extent of skull ossification,11 and classified as either a hatch-year juvenile (HY, within the calendar year of the bird’s hatching) or an after hatch-year adult (AHY, at least after January 1 following the calendar year of the bird’s hatching). Wing chord length (mm), weight (g), and a furcular fat score (0 to 5 scale, 0 = no visible fat deposits, 5 = extensive fat deposits) were also determined. Breeding condition of AHY house finches was also determined during the breeding season each year (March through August). Male breeding condition was assessed by determining the presence or absence of a cloacal protuberance (absent = not breeding, present = breeding). Female breeding condition was determined by the presence or absence of one of the following: a brood patch, palpable egg or cloacal protuberance. Beginning in February 1998, diagnostic conjunctival swab samples were obtained from house finches for MG culture and polymerase chain reaction (PCR) testing.
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| Figure 1a. Normal adult male house finch. |
Figure 1b. Adult male house finch with conjunctivitis. |
Conjunctival swabs taken in the field were immediately immersed in mycoplasma broth and held under refrigeration for 24-48 hrs. Samples were then shipped by overnight mail and incubated according to the protocol described by Hartup and Kollias. 12 Mycoplasma colonies appearing on agar media were identified by direct immunofluorescence. 13 Aliquots of broth cultures from all birds were tested for the presence of MG-specific DNA by PCR methods. 14 Ten MG isolates made during the study were later compared using a random amplification of polymorphic DNA fingerprinting technique (RAPD) using two primer systems. 15,16 The RAPD assay included DNA extracts from a historical house finch MG isolate and a MG vaccine strain used in commercial poultry (F strain) for comparison.
Disease frequency data is presented as the proportion of individuals with conjunctivitis among individuals sampled each month during the study. Wing chord lengths, body weight and the furcular fat scores of house finches with conjunctivitis were compared to those of healthy house finches using logistic regression. 17 Sex and the season of capture (breeding vs. winter) were included as potential confounding variables in each analysis (StatView 5 statistical software, SAS Institute, Inc. , Cary, North Carolina, USA). Potential associations between sex, age, season, breeding condition, and conjunctivitis were evaluated with Mantel-Haenszel chi-square tests or Fisher’s exact test (EpiInfo v. 6.04, 1997 version, Centers for Disease Control and Prevention, Atlanta, Georgia, USA). The association between breeding condition and conjunctivitis in house finches was assessed by using observations during the peak breeding months of April through June, when nearly all AHY house finches would be expected to be in breeding condition. 18 Monthly disease prevalence among HY house finches and the monthly proportions of breeding male house finches were analyzed for linear trends using chi-square methods. 19 Statistical significance was established at p < 0.05.
Results
Examinations were completed on 1597 house finches during the study period. Conjunctivitis was observed in 11% (183/1597) of the house finches at initial capture. Disease assessments from 652 recaptures of house finches were also used to calculate monthly disease prevalence estimates. House finches with conjunctivitis were observed during each month of the year. Monthly conjunctivitis prevalence ranged from 0% to 43% during the study (Fig. 2). Conjunctivitis in house finches exhibited seasonal fluctuations, with peak disease prevalence during winter months, and lower disease prevalence during the breeding season. The proportion of diseased AHY house finches was greater in winter than during the breeding season (p < 0.01). During the breeding season, disease was most prevalent in non-breeding males (p < 0.05) compared to males in breeding condition. Non-breeding males were nearly four times as likely to be diseased than breeding males between the months of April and June. There was a linear increase in the proportion of males in breeding condition over the three months (p < 0.01); the proportion of males in breeding condition was significantly lower in April compared to May or June. Disease prevalence was similar for non-breeding and breeding females during the breeding season.
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| Fig. 2. Monthly prevalence of conjunctivitis in house finches from Mercer County, New Jersey. Arrows indicate annual declines in prevalence during the peak of breeding activity within this population. (*) denotes months without samples. The prevalence estimates shown are based on a mean monthly sample of 47 + 6 house finches (+ S.E. ). |
Among AHY house finches, fat scores, weight and wing chord lengths were not different between diseased and non-diseased individuals. The proportion of diseased males and females was similar in each season.
A seasonal change in disease prevalence was also observed in juvenile HY house finches. The proportion of diseased HY house finches exhibited a linear increase between July and November (p < 0.01; December data was not included due to small sample size, n = 5). Juvenile house finches were 4.5 times as likely to be observed with conjunctivitis in November compared with recent fledglings from July. During both the breeding season and fall/early winter, the proportion of diseased juvenile HY house finches was similar to AHY house finches.
M. gallisepticum was isolated from 48 house finches after February 1998 (3 clinically normal, 45 with conjunctivitis). In addition, nine culture negative house finches (seven with conjunctivitis, two clinically normal) were positive for MG by PCR testing. Thus, we identified a total of 57 MG infected house finches from this population between February 1998 and August 1999. The monthly prevalence of MG infections ranged from 0% to 44% during this period (Fig. 3). Clinical conjunctivitis and MG infections were both at low prevalence during the two breeding seasons monitored by these methods. The prevalence of conjunctivitis was greater than that of MG infections during 11 of 17 months. The prevalence of MG infection provides the best estimate of new infections in the population, as MG isolation rates appear to decline as the disease progresses in individual finches. We also observed an unexpected decrease in conjunctivitis and MG infection prevalence during the month of December 1998 (n = 46 house finches sampled).
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| Figure 3. Monthly prevalence of conjunctivitis and MG infection in house finches from Mercer County New Jersey, February 1998 – August 1999. No birds were captured during April 1998, and no diagnostic samples were available in May 1998 (NS). |
DNA fingerprints of 10 house finch MG isolates showed no apparent differences in RAPD banding patterns over this recent period (Fig. 4). The DNA profiles were also similar to a 1994 isolate from North Carolina (USA), but were different from a common MG vaccine strain. These relationships were confirmed by using a second set of primers in the RAPD assay (Fan et al. , 1995; data not shown).
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| Figure 4. RAPD patterns of MG vaccine strain F (lane 1), a 1994 house finch isolate from North Carolina (lane 2), and isolates from Mercer County house finches made between August 1998 and February 1999 (lanes 3-12). DNA base pair size standards are shown at the far right (AmpliSize Molecular Ruler, Bio-Rad Laboratories, Hercules, California, USA). |
Discussion
The results of this study show dramatic seasonal fluctuations of mycoplasmal conjunctivitis among house finches from a large northeastern population. Conjunctivitis occurred in both sexes and all age classes of house finches, and most commonly during the winter months. Though mycoplasmal conjunctivitis was rare in breeding house finches and may have become locally extinct during mid-1997, we were able to confirm the persistence of the disease among breeding house finches and their offspring in 1998 and 1999. These trends in disease frequency are similar in overall pattern to those described by Dhondt et al. 5 and Hartup et al. 6 using a citizen-survey to describe disease trends throughout the northeastern USA.
Studies of house finches in captivity7 and data collected from nationwide abundance indices9 suggest that mycoplasmal conjunctivitis has had a profound impact on host survival and is linked with recent population declines in eastern house finches, respectively. An unpublished analysis of our banding data using capture-recapture methods revealed no differences in the survival probabilities of diseased versus normal appearing house finches, but was likely due to low recapture rates that limited the statistical power to detect a difference if present. Our field study, however, does suggest a second means by which MG infections in house finches could lead to population decline.
Infection with MG may limit or delay the achievement of breeding condition in adult male house finches by shifting energy required for breeding toward an immune response. We predict alterations to normal endocrine or behavioral profiles would negatively impact male fecundity or mate attraction and lower the productivity of affected populations. We observed a significantly greater frequency of disease among non-breeding males during the breeding season, and a lower than expected proportion of males in breeding condition in April, which are consistent with the above hypothesis. Unfortunately, the observed statistical associations do little to clarify the temporal, and hence, causal relationships involved. Diseased non-breeding males observed during the breeding season may have increased their risk of exposure to MG because they were unpaired and tended to form flocks, or spent greater time at bird feeders. 6
The estimates of MG infection prevalence parallel those of conjunctivitis present in this population. The greater frequency of MG infections compared to conjunctivitis observed over several months are likely due to the presence of birds in the early stages of infection or potentially in an infected, but disease-free carrier state (e.g. lacking conjunctivitis or upper respiratory disease). 7 Observations of conjunctivitis continued during the peak breeding months of 1998 and 1999, despite the apparent decline of MG infections in the population. This decrease may have coincided with arrival of uninfected, migrant house finches, or dispersal of winter flocks that may have lowered transmission rates among susceptible hosts and hence disease prevalence, or may be a function of small sample size that lowered the probability of detecting an infected house finch.
The DNA fingerprints of 10 of the isolates made during this study suggest persistence of a single strain of MG in Mercer County, New Jersey house finches. This strain retains considerable similarity to a historical house finch isolate, yet remains different from a MG vaccine strain, supporting the findings of Ley et al. 4 that the epidemic is being sustained by a single pathogenic strain of MG. We did not observe any marked alterations in disease prevalence suggestive of an altered or emergent strain of MG.
Acknowledgements
M. Zgola provided valuable laboratory assistance. This research was supported by grants from the American Wildlife Research Foundation, Inc. and the Frank M. Chapman Memorial Fund of the American Museum of Natural History.
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