Epizootiological Study Research Articles

A Cross-Sectional Epizootiological Study and Risk Assessment of Foot-Related Lesions and Lameness in Intensive Dairy Sheep Farms.

Simple SummaryFoot-related lameness is one of the most significant welfare issues in farm animals. Contrary to dairy cows and meat sheep breeds, epizootiological data on foot-lesions and associated lameness in dairy sheep are scarce. In this study, data were collected from 30 representative intensive dairy sheep farms. Multivariate statistical analysis was used to produce a typology of intensive farming systems which resulted in the assignment of farms in two distinct clusters. Six hundred adult ewes were randomly selected from six flocks (three flocks per cluster) and a cross-sectional study was implemented to investigate the epizootiology and potential risk factors of foot-related lameness, foot-lesions and diseases. Ovine interdigital dermatitis and infectious footrot were the most common infectious foot diseases, while white line disease and hoof wall cracks were the most prevalent non-infectious lesions. Infectious footrot was the main cause of lameness and increased with age, whereas body condition score was associated with increased prevalence of ovine interdigital dermatitis. Comparisons between the clusters regarding foot-related lameness, foot-diseases and lesions at the animal, the limb, and the hoof level are presented, and relevant literature, mechanisms, hypotheses, and challenges of the field are discussed.Foot-related lameness, foot-diseases and lesions are emerging issues in dairy sheep; however, relevant epizootiological studies are scarce, and risk factors have not been elucidated. The objectives of this cross-sectional study were (i) to address this dearth of knowledge by investigating the epizootiology of lameness-related foot-lesions and diseases, and (ii) to assess the impact of potential risk factors on foot health, in intensive dairy sheep farms. Thirty farms were assigned in two representative clusters using a multivariate statistical analysis. Three farms per cluster and 100 multiparous milking ewes per farm (total n = 600) were selected and enrolled in the study. Foot-related lameness, ovine interdigital dermatitis (OID), infectious footrot (IFR), white line disease, hoof wall cracks, as well as health and welfare traits were recorded. Overall prevalence of foot-related lameness was 9.0% and was primarily associated with IFR; however, additional infectious and non-infectious foot diseases and lesions also contributed. Among infectious foot diseases, OID was the most prevalent (21.3%) followed by IFR (8.0%); WLD and hoof wall cracks were the most prevalent non-infectious foot-lesions (37.7% and 15.3%, respectively). IFR and OID prevalence increased with age (p < 0.05) and BCS (p < 0.01), respectively, suggesting that host-related factors and husbandry practices are important determinants of its occurrence.

Epizootiological Situation for Cryptosporidiosis in Industrially-Bred Pigs in the Northwestern Region of the Non-Black Soil Zone of the Russian Federation

The paper presents the results of an epizootiological study of pig cryptosporidiosis in the Northwestern region of the Russian Federation using the example of the Vologda Region. It has been established that cryptosporidiosis in pigs is accompanied by the signs of diarrhea, anorexia and dehydration; it inhibits and reduces body weight gain and causes the death of animals. Cryptosporidiosis has a pronounced seasonal dependence: the peaks are in autumn and spring. Sucker piglets are most susceptible to the infection during the first month of age, especially during the first two weeks of life. Often, Cryptosporidium infection occurs in association with other parasitic intestinal protozoa.

Myxosporean parasites of Ceratomyxa merlangi and Myxidium gadi in whiting Merlangius merlangus: a comparative epizootiological analysis based on samples from two localities off southern and northern coasts of the Black Sea.

Totally 690 and 423 whiting Merlangius merlangus samples were collected from two localities off Southern (Sinop) and Northern coasts (Balaklava Bay) of the Black Sea, respectively, and examined for myxosporeans in the period from May 2011 to March 2014. Ceratomyxa merlangi and Myxidium gadi were the only myxosporean parasites identified in the content of gall bladder and their infection indices of prevalence (%) and intensity were calculated for length classes and sex of fish as well as for the seasons at both sampling localities. Overall infection prevalence of C. merlangi was 22.6% in Sinop and 27.9% in Balaklava samples while those values of M. gadi were 18.4% and 28.6% in Sinop and Balaklava samples, respectively. Both parasite species were also found to be co-existed in whiting samples. This is the first comprehensive epizootiological study yielded comparable data on C. merlangi and M. gadi infections in whiting in the southern and northern coasts of the Black Sea.

Temporal changes in infections with some pathogens associated with gill disease in farmed Atlantic salmon (Salmo salar L)

Gill disease (GD) in farmed Atlantic salmon may be due to the impact of several agents that may interact. We followed the infections of four gill disease-associated agents during the first year of production in six farms; examining the correlation between their densities in the gills. These farms were located in an area with high risk of developing gill disease, and three of the farms were diagnosed with GD in the autumn.In the present study we show temporal changes in pathogens in salmon from all 6 farms, revealing recurring infection patterns by Desmozoon lepeophtherii and Candidatus Branchiomonas cysticola (C. B. cysticola), and large variation without an apparent pattern in densities of Ichthyobodo salmonis. Infections with the microsporidian D. lepeophtherii show a clear seasonal pattern, and high densities are associated with GD. Ca. B. cysticola appears not to be directly connected to the disease, albeit densities of this bacterium may coincide with high D. lepeophtherii densities. Amoebae (Paramoeba spp.) were only sporadically detected with low densities. Salmon with low condition had elevated densities of D. lepeophtherii and Ichthyobodo spp., while no such pattern was seen for the other pathogens.Challenge studies are needed to substantiate the role of D. lepeophtherii in GD, but realistic challenge model (i.e. bath) has so far not been developed. Based on the present molecular screening, we suggest that D. lepeophtherii infections both may be responsible for GD, and cause runting in farmed salmon populations. Statement of relevanceThis is an epizootiological study of Atlantic salmon and gill diseases. There is much known about pathogen in connection with GD outbreak, less is known about the occurrence of potential pathogen in healthy farmed salmon or how such infections develop over time.High densities of the microsporidian parasite Paranucleospora theridion in the gills coincide with gill disease, making P. theridion a possible primary source to this condition.Densities of Paranucleospora theridion, Candidatus Branchiomonas cysticola and Ichthyobodo salmonis may be correlated in farmed salmon populations with gill disease.P.theridion and I. salmonis densities are elevated in Atlantic salmon with low condition (‘runts’).

Desmozoon lepeophtherii (microsporidian) infections and pancreas disease (PD) outbreaks in farmed Atlantic salmon (Salmo salar L.)

An association between infections with the ubiquitous microsporidian Desmozoon lepeophtherii (D. lepeophtherii) and pancreas disease (PD) outbreaks has previously been suggested. We followed infections with D. lepeophtherii and salmonid alpha virus (SAV) in thirteen Atlantic salmon farms in western Norway during the first production year at sea. The purpose of the survey was to examine if the presence and density of infection with the microsporidian could be linked to severity of PD-infection. Since the microsporidian develops both in salmon and in salmon lice (Lepeophtheirus salmonis) and louse abundance varies geographically, we also examined the spatial and temporal distribution and density of D. lepeophtherii infections in farmed Atlantic salmon. The prevalence of D. lepeophtherii was 100% in all samples from all farms. D. lepeophtherii densities varied with season, peaking in autumn followed by a marked decrease in winter. Geographical differences between areas were observed in August, between the outer coast (salinity 23.5ppt.) and the Hardangerfjord (inner area) (16.1ppt.) with higher infection rates in the outer coastal areas. We likewise found that D. lepeophtherii density was significantly associated with the host condition factors; D. lepeophtherii density being significantly higher in salmon with low status. SAV infections occurred in six farms. Five of these developed — and were diagnosed with PD. No relationship was detected between D. lepeophtherii density, the detection of SAV or PD outbreaks over time. Statement of relevanceThis is an epizootiological study of Atlantic salmon, P. theridion and SAV. There is much known about the pathogen in connection with disease outbreaks, but less is known about the occurrence of potential pathogen in healthy farmed salmon or how such infections develop over time.P. theridion has been connected to SAV infection and PD outbreak. The result from this study does not support such connection.It appears that the SAV virus can persist in a farmed population at least 10months prior to a PD outbreak.There is a connection between the salmon condition and density of P. theridion where salmon with low condition has elevated density. This could identify P. theridion as the agent possibly causing this condition.

Epizootiological Study of the Occurrence of Canine Babesiosisin Southwestern Slovakia

Abstract This epizootiological study was carried out to investigate the occurrence of canine babesiosis in southwestern Slovakia. The study focused on the proportion of the species of ticks serving as babesia vectors of babesiosis in the ticks collected from selected locations close to Komárno, in southwestern Slovakia. Additionally, observations were made on the health and overall clinical signs in dogs suspected of having babesiosis. In 2014 we collected ticks from vegetation by the drag cloth (flagging) method and also directly from dogs. A totally of 622 ticks were collected: 491 Dermacentor reticulatus and 131 Ix-odes ricinus. Representative samples of ticks (n = 103) were examined by the molecular method and the presence of Babesia spp. DNA was identified in 12.5 % of the Ixodes ricinus ticks collected by flagging and in 9.5 % of the Ixodes ricinus ticks collected from the dogs. Babesia canis (KU681325) with 90 bp sequence, 100 % identical with Babesia canis isolates from dogs for example from: Turkey (KF499115), Rumania (HQ662634), Croatia (FJ209025), Poland (EU622792) and Russia (AY962186), was confirmed after sequencing in one Ixodes ricinus female obtained from a dog. This was the first confirmation of the occurrence of B. canis in Ixodes ricinus ticks in Slovakia. In 2.2 % of the Dermacentor reticulatus ticks obtained from vegetation by flagging, we were able to diagnose the DNA of Babesia canis. In 4.8 % of the Dermacentor reticulatus ticks collected from dogs, the presence of Babesia spp. was confirmed. Thirty three dogs with suspicion of babesiosis were observed in an ambulance by their health and clinical signs. The loss of appetite was observed in 22 patients (66.7 %), apathy in 19 cases (57.6 %), and fever in 19 cases. Closer specification indicating babesiosis was finding engorged ticks on the dog bodies which occurred in 21 cases (63.6 %), haematuria in 8 cases (24.2 %), anaemia in 4 cases (12.1 %), and tremor in 6 cases (18.2 %). Other non-specific signs, such as diarrhoea, vomitus, slowed-down movement, ataxia and lacrimation were also observed in less than 18 % of the examined dogs. On the basis of the specific clinical signs, blood was withdrawn from 33 dogs for preparation of blood smears and in 19 of them (57.6 %) babesia in erythrocytes were confirmed microscopically.

Canine hepatozoonosis in southeastern Bahia, Brazil.

In Brazil, canine hepatozoonosis is a tick-borne subclinical hemoparasitosis caused by a protozoa Hepatozoon canis and is highly prevalent in dogs in rural areas. An epizootiological study was conducted to investigate the prevalence of H. canis in the canine population of Ituberá, Bahia, and to analyze any associated risk factors. Blood samples were collected from 380 dogs and determined the presence of the protozoan by performing capillary blood smear and polymerase chain reaction (PCR). Epizootiological data were collected by asking dog owners to answer a structured questionnaire. H. canis gamonts were not detected in the blood smears. However, PCR detected H. canis in 163/380 (42.9%) dogs examined. Physical examination and anamnesis indicated 105 (64.4%) positive asymptomatic dogs. Hematological alterations were observed in 115 (70.5%) infected dogs. No clinical, hematological, or epizootiological variable was found to be significantly associated to the infection. In conclusion, the high prevalence of H. canis infection in local dogs may be because of the peri-urban features of this municipality. Moreover, to the best of our knowledge, this study the first study to report H. canis infection in the State of Bahia.

ESTUDIO EPIZOOTIOLÓGICO DE PENAEUS VANNAMEI EN TRES FINCAS CAMARONERAS DEL CARIBE COLOMBIANO

An epizootiological study of the mayor deseases affecting the penaeid shrimp Penaeus vannamei was performed from june 1994 throuh august, 1995 in tree farms located on the colombian Carebbean coast. The presence and prevalence of infectious agents such as bacteria, protozoa, viral inclusions bodies and changes histopatologicals was determinated by means of gross examination and histological analysis.. The relationship between these infection agents and specific disease was also studies. The presence of some viruses such a Baculovirus penaei, IHHNV, TSV and LOVV, were determinated by demostration of the cytopathical effects characteristic of these agent. Additionaly, a logistic regression analysis was used to determine the relationship between some clinical signs and the presence of the infectious agent; the risk factor of having the disease, was also determinated; thus, shrimp with tumors and deformities presented a risk almost two times higher having IHHNV disease, as determined by the presence of inclusion bodies Cowdry A (CAI’s) than those whitout deformities; in an analogous way, the risk factor of presenting alterations in the lymphoid organ (L.O) was higher in organisms that have exposed to the TSV desease, than in those were not. Bacterial infections an infestations by protozoa were also analyzed. Some differences concerning the presence and prevalence of some of the diseases according to the geographical location of the farms and the different climatic conditions in such the cultivation started, wer observed. Some physical-chemical variables such as salinity, where related to the presence and prevalence of some clinical signs.

Effects of Habitat Alteration on the Epizootiology ofMyxobolus cerebralis, the Causative Agent of Salmonid Whirling Disease

Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis , is a serious health threat to salmonid fish and its control remains problematic. The parasite has a 2-host life cycle involving a salmonid and the aquatic oligochaete Tubifex tubifex . A commonly used strategy to control parasites that requires an obligatory invertebrate host is to eliminate or reduce the host population size to a point where parasite transmission can no longer occur. Large numbers of T. tubifex are frequently found in degraded habitats that are characterized by an abundance of fine sediments, organic matter, and a lack of aquatic invertebrate diversity. If such environments are rehabilitated, then the normal flora and fauna should re-establish and the numbers of T. tubifex should decline due to their inability to compete with the re-established invertebrates. During an epizootiological study on Rock Creek, located in west-central Montana, 2 opportunities were available to examine the effects of habitat restoration on the transmission of M. cerebralis . The Puyear Ranch re-establishment project was a major endeavor conducted on the main channel of Rock Creek, a little more than midway upstream. Another significant restoration was conducted on Upper Willow Creek, a tributary of Rock Creek, located closer to the headwaters. Sentinel trout studies, along with examining T. tubifex for the parasite and measuring various water-quality parameters, revealed that the restoration of the Puyear Ranch locality had no significant effect on reducing the intensity of M. cerebralis in trout. This was likely due to the restored area being located mid-river, just downstream from a "hot spot" of infected T. tubifex . In comparison, there was a significant reduction in the intensity of M. cerebralis in sentinel fish after the Upper Willow Creek restoration project was completed. Unlike the Puyear Ranch locality, there was no hot spot of infected T. tubifex above the area rehabilitated on Upper Willow Creek. Further, the relative abundance of T. tubifex and M. cerebralis -infected worms was reduced. Although further study is needed, it appears that habitat rehabilitation can reduce the transmission of M. cerebralis . Since the triactinomyxon stage of the parasite released from T. tubifex (which infects trout) can float for many kilometers, the rehabilitation of a hot spot may reduce the infection of trout downstream where they inhabit a healthy environment with no M. cerebralis -infected T. tubifex in the vicinity. Thus, rehabilitation of a relatively small area could significantly affect the drainage for many kilometers beyond the improved habitat.

Real time polymerase chain reaction to diagnose Anaplasma marginale in cattle and deer (Ozotoceros bezoarticus leucogaster) of the Brazilian Pantanal

Epizootiological study of Anaplasma marginale in regions that contain various reservoir hosts, co-existence of rickettsia pathogens, and common vectors is a complicated task. To achieve diagnosis of this rickettsia in cattle and campeiro deer of Brazilian Pantanal, a comparison was made between a real time polymerase chain reaction (RT-PCR) with intercalating Sybr Green fluorochrome and primers based on msp5 gene of A. marginale; a conventional PCR (C-PCR); and parasitological examination using thin blood smear stained with Giemsa-MayGrunwald. Both PCRs showed good performance in the diagnosis of A. marginale in cattle, and were superior to the parasitological exam. The RT-PCR detected seven positive campeiro deer (16.3%). This rate was significantly higher compared to C-PCR, which identified one animal as positive (2.3%), and also compared to parasitological diagnosis, which did not find any positive animals. The dissociation temperature average of positive reactions in cattle (81.72 °C ± 0.20) was identical to dissociation temperature found in the cervids (81.72 °C ± 0.12), suggesting that both animal species were infected with A. marginale. We concluded that RT-PCR can be used for A. marginale diagnosis and in epizootiological studies of cattle and cervids; in spite of the small number of campeiro deer samples, the results indicated that this wildlife species has importance in the Anaplasma epizootiology in the Brazilian Pantanal.

Molecular detection of Theileria annae and Hepatozoon canis in foxes ( Vulpes vulpes) in Croatia

An epizootiological field study on tick-borne protozoan infections in foxes ( Vulpes vulpes) was carried out in different parts of Croatia. Spleen samples of 191 carcasses of red foxes killed in sanitary hunting, were examined for the presence of hematozoa by polymerase chain reaction (PCR) and subsequent sequencing. The investigation revealed four species of hematozoa in 57 foxes (30%), namely Theileria annae, Theileria sp. 3182/05 and Hepatozoon canis. T. annae was found in 10 foxes (5%), Theileria sp. 3182/05 in a single animal (1%), H. canis in 44 (23%) and Hepatozoon sp. was detected in two foxes (1%). T. annae and H. canis were distributed through all the studied regions, while Theileria sp. 3182/05 and Hepatozoon sp. were restricted to the Zagreb and Zagorje, and Istria regions, respectively. Detection of T. annae in all regions of Croatia indicates the presence of the natural cycle of the parasite and raises the possibility of other vectors other than the proposed Ixodes hexagonus.

Molecular epizootiology of infectious bursal disease (IBD) in Korea

We conducted a molecular epizootiological study of infectious bursal disease (IBD) in Korea by analyzing 85 IBD viruses (IBDVs) obtained from vaccinated or unvaccinated flocks between 1980 and 2007. Phylogenetic analysis of the partial nucleotide sequence of the hypervariable region of the VP2 gene (nucleotides 661-1020) and pathogenicity tests revealed more genetic and phenotypic diversity of IBDV in Korea than has been reported previously. We showed that very virulent IBDVs (vvIBDVs) were already present in Korea in 1986. Moreover, vvIBDVs were repeatedly detected in Korean poultry that had been vaccinated, which casts doubt on the IBD vaccine programs. We also identified novel putative antigenic variant (AV)-like IBDV isolates on the basis of their antigenic indices and the presence of amino acid changes (P222S or P222T-A321D) that are known to affect the antigenicity of VP2. These observations suggest that future studies examining the efficacy of conventional vaccines against atrophy of the bursa of Fabricius and vvIBDV shedding may be useful. Moreover, it will be of interest to determine the prevalence of putative Korean antigenic variants and whether these strains exert immunosuppressive effects in vaccinated birds.

Colony Collapse Disorder: A Descriptive Study

BackgroundOver the last two winters, there have been large-scale, unexplained losses of managed honey bee (Apis mellifera L.) colonies in the United States. In the absence of a known cause, this syndrome was named Colony Collapse Disorder (CCD) because the main trait was a rapid loss of adult worker bees. We initiated a descriptive epizootiological study in order to better characterize CCD and compare risk factor exposure between populations afflicted by and not afflicted by CCD.Methods and Principal FindingsOf 61 quantified variables (including adult bee physiology, pathogen loads, and pesticide levels), no single measure emerged as a most-likely cause of CCD. Bees in CCD colonies had higher pathogen loads and were co-infected with a greater number of pathogens than control populations, suggesting either an increased exposure to pathogens or a reduced resistance of bees toward pathogens. Levels of the synthetic acaricide coumaphos (used by beekeepers to control the parasitic mite Varroa destructor) were higher in control colonies than CCD-affected colonies.Conclusions/SignificanceThis is the first comprehensive survey of CCD-affected bee populations that suggests CCD involves an interaction between pathogens and other stress factors. We present evidence that this condition is contagious or the result of exposure to a common risk factor. Potentially important areas for future hypothesis-driven research, including the possible legacy effect of mite parasitism and the role of honey bee resistance to pesticides, are highlighted.

Diagnosis and sero-epizootiology of equine herpesvirus type 1 and type 4 infections in Japan using a type-specific ELISA.

Recently, a type-specific ELISA using equine herpesvirus type 1 (EHV-1) and type 4 (EHV-4) glycoprotein Gs (gGs) was developed by Crabb and Studdert [1993]. To investigate the dissemination of EHV-1 and -4 among horses in Japan, we applied their ELISA as suitable for discriminating between EHV-1 and -4 infections serologically. Type-specificity of the ELISA was confirmed by using paired sera of infected horses with either EHV-1 or -4. Application of the ELISA to sera collected before and after the winter season of 1995-1996 from 80 racehorses revealed that 30 horses showed significant antibody responses against EHV-1 and 9 against EHV-4, respectively. The results indicated that this ELISA using paired sera is useful for a diagnosis and an epizootiological study on EHV-1 and -4 infections.

Definitive identification of louping ill virus by RT-PCR and sequencing in field populations of Ixodes ricinus on the Lochindorb estate.

Rapid and precise virus detection procedures are an important component of any epizootiological study. An automated one tube reverse transcriptase and nested primer polymerase chain reaction (RT-PCR) followed by nucleotide sequencing of the cDNA product, was used for the rapid detection and identification of louping ill (LI) virus in field caught Ixodes ricinus and compared with a classical isolation method i.e. infectivity in cell culture. The results establish the genetic identity of LI virus on the Lochindorb Estate. There was a high correlation between the results obtained by RT-PCR and infectivity assays. RT-PCR and sequencing proved to be a rapid and accurate system for identifying LI virus in field specimens. Development of this system should improve the capacity to undertake detailed epizootiological studies of LI virus.

Epizootiology of the reindeer nose bot fly,Cephenemyia trompe(Modeer) (Diptera: Oestridae), in reindeer,Rangifer tarandus(L.), in Norway

First-instar larvae of the reindeer nose bot fly, Cephenemyia trompe (Modeer) (Diptera: Oestridae), were sampled with a rinsing and sieving technique from 571 semidomesticated reindeer, Rangifer tarandus (L.), from different districts in northern Norway in the infection years 1983, 1984, 1985, 1987, and 1988, and from 44 wild reindeer from southern Norway (infection year 1983). This is the first comprehensive epizootiological study of this parasite from Fennoscandia. The first instar was found between 10 September to 25 May, the second instar from 11 February to 17 May, and the third instar from 3 April to 28 June. Old third-instar larvae were sometimes found trapped in the sinuses of the host. The overall prevalence of infection was 65.2% (range 6.7 – 100%) and the abundance (= relative density) was 11.53 (range 6.7 – 62.7). Individual intensities ranged from 0 to 221. There were significant differences in abundance between some districts and years. Distance and timing of the spring migration of the host are thought to be the major factors causing variability in infection levels between districts, whereas the summer climate during infection greatly influenced the differences between years. The frequency distribution was highly overdispersed (aggregated) and could adequately be described by the negative binomial model (overall parameter, k = 0.29, range 0.03 – 2.64). Heterogeneity in host behaviour during infestation is hypothesized to create this parasite distribution. Two measures of aggregation, Morisita's index of aggregation (IM) and1/k, decreased with larval burden, indicating that factors restricting parasite numbers (negative feedback) start to operate at high infection levels.

Outbreaks of Salmonella Dublin infection among calves on a dairy farm applying Salmonella bacterins in Zambia.

In Zambia, a dairy farm keeping about 600 cows and self-contained calves had applied S. Dublin and S. Typhimurium bacterins to pregnant cows and calves in combination with all-in all-out pen system for rearing calves. Only relatively small scale outbreaks of S. Dublin infection occurred repeatedly in these years from 1989 to 1991 among fattening calves on the farm. The results obtained from the epizootiological study suggest that the preventive measures including the vaccination with Salmonella bacterins gave insufficient protection against S. Dublin infection to the calves, but they might have prevented large scale outbreak of the disease. This is the first report of the epizootiological study on outbreak of bovine S. Dublin infection on farm in Zambia.

Pyogranulomatous Typhlitis and Hepatitis of Market Turkeys

Pyogranulomatous lesions in the liver and cecum were observed in groups of 7-to-8-week-old market turkeys. Necropsy revealed multifocal lesions in the liver and the presence of cecal cores that, in some cases, distended the cecum to the point of rupture. Mortality was associated with the ruptured ceca. Two successive flocks also succumbed to these lesions. An epizootiological study was performed to determine the cause and/or initiation of lesions leading to the formation of cecal cores.

Epizootiology and distribution of transmissible sarcoma in Maryland softshell clams, Mya arenaria, 1984-1988.

Seasonal and geographic studies of transmissible sarcoma in Maryland softshell clams, Mya arenaria, were carried out from 1984 to 1988. Three major epizootics occurred in our sampling location during this time, resulting in prevalences as high as 90%, with comparable mortalities in other high prevalence areas. The disease invaded populations of large adult clams first, later spreading to the small juvenile clam populations. An apparent 2-year cycle was noted with varying seasonal effects. Affected sites tended to be in the main stem of Chesapeake Bay north of Tangier Sound, primarily in the areas where the major harvesting occurs. Several sites, mostly in upstream locations, were consistently free of disease. The epizootiological study supports the interpretation that the disease is infectious exclusively to this species. Regression analysis between sarcoma prevalence and contaminant levels in clam tissues showed a significant correlation (p = 0.0001) between chlordane levels and this disease. No correlations were found with other contaminants that were analyzed.

Epizootiological study of rickettsiosis of the Saint-Jacques scallop Pecten maximus

Rickettsia-like microorganisms were described in gill endothelial cells during mass mortalities of Saint-Jacques scallops Pecten maximus in Saint-Brieuc Bay (North Brittany, France) during winter 1985-86. In order to evaluate the epizootiological significance of rickettsiosis of scallops, an analytical and descriptive epizootiological study was undertaken. The study included the geographic distribution, incidence and severity of the disease for several northern European and French scallop populations. Since the center of French scallop production is in Saint-Brieuc Bay, epizootiological investigations were concentrated in this area. Dissemination appeared to be by horizontal transmission. The pathogenic significance of the rickettsia-like organism in mortalities of P. maximus is discussed.