Prof.(Dr) R.N.Sreenivas Gowda*
Introduction
A subclinical infection is when a person or animal has a pathogen (like a virus or bacteria) in their body, but experiences few or no noticeable symptoms, existing between being completely healthy and having a full-blown illness. These infections are often detected only through lab tests, and the infected individual can still unknowingly spread the pathogen as an asymptomatic carrier.
What are clinical and subclinical infections?
Clinical infections produce observable symptoms and signs of illness in the host, and substantial mortality and morbidity. while subclinical infections (also known as asymptomatic or inapparent infections) with minimal or no visible signs or symptoms, are present in the body but cause poor growth. lingering mortality and poor performance in terms of egg or meat production.
Clinical = obvious symptoms; Subclinical = no or mild symptoms.
The subclinical infections in poultry
Subclinical infections in poultry are infections caused by pathogens (viruses, bacteria, parasites) that cause minimal or no obvious outward sickness (like sneezing, lethargy, or high mortality) but still impact bird health and farm economics, leading to poor growth, reduced feed efficiency, lower egg production, or increased susceptibility to other diseases, often detected only through lab tests. These “hidden” infections allow birds to act as asymptomatic carriers, spreading diseases like Avian Influenza or Chicken Infectious Anemia Virus (CIAV) unknowingly within flocks.
The Subclinical poultry infections are widespread in India and are a major cause of economic loss due to reduced performance, immunosuppression, and increased susceptibility to secondary infectious diseases.
- The sub clinical infections are like a hidden iceberg, Most of the infection appears as clinical 20% but, they rest deep without showing any apparent infection about 80% sub clinically .(fig,1,). No obvious signs: The infection doesn’t cause visible signs or symptoms of disease, or they are so mild they go unnoticed.
Fig: 1. Analogous to the largest, hidden portion of an iceberg, the major proportion of cases of coccidiosis are sub-clinical and unnoticed.
The common subclinical infections identified in India include:
- Infectious Bursal Disease (IBD) / Gumboro Disease: This viral infection is highly prevalent in India, often in a subclinical form that causes immunosuppression without high mortality. This immunosuppression leads to poor response to other vaccines (like Newcastle disease vaccine) and makes birds more susceptible to secondary bacterial infections, resulting in significant production losses.
- Chicken Infectious Anemia (CIA): The Chicken Anemia Virus (CAV) circulates widely in Indian poultry flocks, often without overt clinical signs in older birds. Subclinical CIA causes immunosuppression and poor growth performance, and can lead to secondary infections such as gangrenous dermatitis when co-infected with Clostridium perfringens or Staphylococcus aureus.
- Coccidiosis: Caused by Eimeria species parasites, coccidiosis is one of the most common and economically important diseases in poultry worldwide, including India. While severe cases cause bloody diarrhea and death, subclinical forms are very common and result in chronic damage to the intestinal mucosa, leading to malabsorption of nutrients, poor weight gain, and increased feed conversion ratios.
- Necrotic Enteritis: This bacterial disease, caused by Clostridium perfringens, is often subclinical. The subclinical form causes chronic damage to the intestinal lining, which impairs nutrient absorption and drastically reduces growth rate and feed efficiency, resulting in substantial economic losses.
- Salmonellosis: Many paratyphoid Salmonella infections are subclinical in most poultry, with adult birds often acting as asymptomatic carriers. These can pose a public health risk as they are a major cause of foodborne illness in humans.
- Mycoplasmosis: Mycoplasma gallisepticum (MG) infections often develop slowly and can be subclinical or result in mild respiratory signs and a drop in egg production in adult birds. The pathogenicity of MG is often enhanced when other infections or environmental stressors are present.
- Marek’s Disease: While it can cause tumors and paralysis, Marek’s disease virus infection can also be subclinical, with infected birds becoming lifelong carriers and shedding the virus through dander and dust.
- These subclinical infections are problematic because infected, apparently healthy birds continue to spread the pathogens, making disease control and eradication challenging. Good biosecurity, sanitation practices, and effective vaccination programs are crucial for management.
Economic impact of subclinical infections
Subclinical infections in poultry production have a significant economic impact, primarily through reduced performance (weight gain, feed efficiency, egg production), increased production costs (veterinary bills, antibiotic use), and indirect losses (carcass condemnation, secondary infections). These “hidden costs” often represent the vast majority of the financial burden of disease, significantly eroding profit margins.
Economic Impact
The economic losses from subclinical infections manifest in several ways:
- Reduced Body Weight Gain: Sub clinically infected birds often exhibit impaired growth rates and lower average market weights compared to healthy flocks. For example, studies on subclinical necrotic enteritis (SNE) estimated a 12% reduction in body weight in affected birds.
- Poor Feed Conversion Ratio (FCR): An increased FCR means more feed is required to produce a kilogram of meat or a dozen eggs, directly increasing production costs. SNE has been associated with a 10.9% increase in FCR.
- Increased Mortality Rates: While subclinical infections may not cause mass die-offs, they can lead to slightly higher baseline mortality rates and make birds more susceptible to other fatal diseases.
- Immunosuppression and Secondary Infections: Many subclinical infections, such as Infectious Bursal Disease (IBD) or Chicken Anemia Virus (CAV), cause immunosuppression. This compromises the bird’s immune system, leading to a higher incidence of secondary infections (e.g., E. coli or Mycoplasma) and reduced efficacy of vaccination programs, leading to further losses.
- Carcass Condemnation: Diseases can lead to lesions or other conditions that result in the condemnation of carcasses at processing plants, representing a total loss for those individual birds.
- Increased Veterinary and Treatment Costs: The need for increased medication, including antibiotics, to manage secondary infections or the subclinical condition itself, adds substantially to operating expenses.
- Reduced Egg Production: In layer and breeder flocks, subclinical infections can lead to a drop in egg production and quality.
Quantifiable Losses
Specific examples from research highlight the magnitude of these impacts:
- Infectious Bursal Disease (IBD): Flocks with chronic IBD infection showed a 14% decrease in financial return per 1,000 birds compared to unaffected flocks.
- Coccidiosis: The global cost of coccidiosis in chickens was estimated at around $13 billion USD annually, considering production losses and treatment costs. Losses are maximum due to reduced body weight gain and increased FCR.
- Subclinical Necrotic Enteritis (SNE): An analysis estimated losses ranging from approximately $878 to $1,481 USD per 20,000-bird flock due to reduced body weight and FCR.
Management Implications
Management of subclinical infections in poultry focuses on prevention and control strategies that minimize pathogen presence and strengthen bird immunity, as infected birds often show no obvious symptoms but suffer production losses and can act as disease reservoirs. Antibiotics were historically used but have been restricted due to concerns about antimicrobial resistance.
Key management strategies include:
1. Biosecurity and Husbandry
Strict biosecurity is crucial for preventing the introduction and spread of pathogens within and between flocks.
- “All-in, all-out” management: Cleaning and disinfecting houses thoroughly after depopulation and before new stock arrives.
- Hygiene: Maintaining clean, dry litter conditions, proper ventilation, and appropriate brooding temperatures to reduce parasite loads and microbial growth.
- Access Control: Limiting access to the farm, using dedicated clothing and footwear, and implementing handwashing and disinfection procedures for all workers and visitors.
- Water and Feed Quality: Ensuring constant access to clean, fresh water (e.g., using nipple drinkers) and high-quality feed.
2. Vaccination Programs
Vaccination is a primary method for controlling several subclinical infections, particularly viral diseases and coccidiosis.
- Coccidiosis: Live oocyst vaccines are administered to chicks to induce natural immunity, which is crucial for preventing the intestinal damage that often precedes secondary bacterial infections like necrotic enteritis.
- Viral Infections: Vaccines for diseases like Marek’s disease, Avian Encephalomyelitis, and some strains of Avian Influenza are used to prevent clinical signs, reduce viral shedding, and provide maternal antibodies to young chicks.
- Bacterial Infections: Inactivated or live attenuated vaccines are used against pathogens like Salmonella to reduce colonization and shedding.
3. Nutritional and Dietary Interventions
Dietary management can significantly influence gut health and disease resistance.
- Probiotics and Prebiotics: Supplementation with beneficial live microorganisms (e.g., Bacillus spp., Lactobacillus spp., yeasts) and non-digestible feed ingredients helps balance intestinal microflora, enhance immunity, and exclude pathogens through competitive exclusion.
- Butyrate: This short-chain fatty acid serves as a primary energy source for intestinal cells, helping to reinforce the gut barrier function and reduce inflammation.
- Phytochemicals/Herbal Medicines: Plant extracts (e.g., from garlic, oregano) containing compounds with antimicrobial or immunomodulatory properties are explored as alternatives to conventional drugs.
- Enzymes and Amino Acids: Using highly digestible protein sources and supplementing specific amino acids like threonine and methionine can improve nutrient absorption and support gut integrity, reducing the available substrate for pathogens like Clostridium perfringens.
4. Monitoring and Diagnostics
Regular monitoring is vital for detecting subclinical infections.
- Fecal Examinations: Routine examination of droppings helps identify parasitic oocysts or abnormal bacterial presence.
- Serological and PCR Testing: Blood tests (ELISA) and molecular diagnostics (PCR) are used to screen for exposure to specific pathogens and confirm infection status in breeder flocks.
- Consulting a Veterinarian: Proper diagnosis requires professional input. A veterinarian can help develop an effective control program and ensure compliance with regulations on the use of medications in food-producing animals.
By combining these strategies, producers can manage the risk associated with subclinical infections, improve animal welfare, and maintain flock productivity without relying heavily on therapeutic antimicrobials.
Conclusion
Subclinical infections in poultry, often from pathogens like coccidiosis or Clostridium perfringens (necrotic enteritis), Mycoplasmal infections and some viral diseases such as IBD,CIA, and MD significantly harm health and production by causing hidden damage to the gut, leading to reduced weight gain, poor feed conversion (FCR), and lower nutrient absorption, resulting in substantial economic losses for producers, even without obvious outward sickness. These infections weaken the immune system, making birds more vulnerable to other diseases and increasing stress, impacting overall flock performance and profitability
(*Former and Founder VC, KVAFSU, Bidar, Former Director IAH&VB, Bangalore, Former Prof &HOD. Dept. of Pathology. Veterinary College UAS, Bangalore.)