Acquired Immunodeficiency Syndromes

Tumors on chicken's leg from Marek's disease

The overcrowded, stressful, unsanitary conditions inherent to intensive poultry production not only directly increase the risk and spread of bird flu infection, but may predispose the birds to infections with immunosuppressive viruses that could further compromise their already dysfunctional immune systems. The relationship between immune-weakening poultry viruses and bird flu was first proposed by University of Hong Kong zoologist Frederick Leung and later expanded upon by anthropologist and agroecologist Ronald Nigh.¹⁹³⁹ Leung noted a speculative correlation between Hong Kong chicken farms that had suffered outbreaks of an immunodeficiency virus known as infectious bursal disease virus in 1996 and the subsequent initial outburst of H5N1 about six months later in 1997.¹⁹⁴⁰

The bursa is a specialized avian organ responsible primarily for the development of a bird’s immune system.¹⁹⁴¹ That’s how human antibody-producing “B-cells” got their name, since they were first discovered in chickens.¹⁹⁴² Just as HIV in humans replicates in white blood cells called T-helper cells, leading to their destruction and the body’s subsequent immunodeficiency, the infectious bursal disease (IBD) virus in birds infects B-cells, crippling the immune system and leaving survivors immunosuppressed for life.¹⁹⁴³ With a “severely impaired”¹⁹⁴⁴ ability to produce antibodies, surviving birds respond poorly to vaccinations¹⁹⁴⁵ and are susceptible to a wide variety of viral, bacterial, and parasitic diseases.¹⁹⁴⁶

Beginning in the 1980s—a couple of decades after the IBD virus was identified¹⁹⁴⁷—the United States started seeing a dramatic increase in chickens suffering from various respiratory infections. Vaccines no longer seemed to be working as effectively.¹⁹⁴⁸ Investigators discovered that a new hypervirulent strain had arisen in the most concentrated poultry production area in the world,¹⁹⁴⁹ the Delmarva peninsula, incorporating corners of Delaware, Maryland, and Virginia.¹⁹⁵⁰ The Delaware variant, as it was called, started its march across the world in the late ’80s¹⁹⁵¹ thanks in part to a “high concentration of poultry in close proximity.”¹⁹⁵² There’s even evidence that the IBD virus of domestic chickens has been detected in Emperor penguins in the Antarctic, considered an example of industrial animal agriculture’s “pathogen pollution” to the farthest reaches of the globe.¹⁹⁵³

There has also been a dramatic increase in the virulence of another viral affliction—Marek’s disease (MD), first described a century ago¹⁹⁵⁴ —since the 1960s.¹⁹⁵⁵ Besides tumors in the skin, muscles (meat), nerves, and abdominal organs of chickens, the Marek’s disease herpes virus also causes immunosuppression.¹⁹⁵⁶ A major 2005 scientific review describes the evolution of virulence in what is now a familiar story:

Poultry production up to the mid 1900s mainly comprised backyard farming with very low population densities of birds…with low growth rates and low egg production. In this environment, MD was not considered as a major disease even though outbreaks of MD were reported in different parts of the world. However, since the 1960s there have been major changes in poultry production practices. Today poultry production has become a major global industry operating in very high population densities under highly intensive management conditions aimed at higher rates of growth and productivity…. Until about 1960, when the poultry production was not on an intensive scale, both the virus and the hosts were able to achieve a state of balanced co-existence. However, the transformation of the poultry industry into the intensive production practices from the early 1960s saw a shift in this balance greatly in favor of the virus. The continuous availability of large populations of genetically susceptible naïve hosts, usually in an overcrowded environment, enabled the virus to spread rapidly, encouraging their rapid evolution towards greater virulence. This was evident when huge MD outbreaks swept through poultry flocks in the 1960s, wiping out large populations all around the world¹⁹⁵⁷

The first wave of evolution in the late 1950s shifted the virus from “mMDV” (mild Marek’s disease virus) to “vMDV” (virulent Marek’s disease virus). Due in part to continued and escalating industrial practices, “vMDV” became “vvMDV,” and presently the world is dealing with “vv+MDV.”¹⁹⁵⁸ The industry had created another monster.

Other immunosuppressant viruses include chicken infectious anemia virus (CIAV) and a virus that causes hemorrhagic enteritis in turkeys.¹⁹⁵⁹ CIAV was first described in 1979¹⁹⁶⁰ and has since spread throughout the world to become ubiquitous in egg and meat-type chickens worldwide.¹⁹⁶¹ CIAV destroys immune precursor cells, undermining the immune system before it can even develop.¹⁹⁶² Immunosuppression associated with CIAV is considered to be a factor in “many of the disease problems in flocks raised under the high-density conditions of modern poultry production.”¹⁹⁶³

These immunodeficiency viruses can interact with each other to synergistically further predispose the global chicken flock to infection. CIAV infection, for example, can boost the virulence of Marek’s virus,¹⁹⁶⁴ and co-infection between IBDV and CIAV can result in an even more profound vulnerability to additional infectious disease agents.¹⁹⁶⁵ A Tyson’s poultry scientist describes the U.S. poultry industry as being “in a constant battle with immunosuppressive diseases,”¹⁹⁶⁶ and a 2005 World Poultry “Global Disease Update” reports that “[t]he deleterious effects of infections which suppress the immune systems are underrated in many parts of the world.”¹⁹⁶⁷

The unhygienic conditions under which commercial poultry are confined conspire to spread these viruses. “The transformation in the poultry farming practices into a highly intensive industry has enormously changed the poultry house environment,” reads one Marek’s disease review. Infection with Marek’s disease occurs when a chicken inhales infected dust in a poultry shed saturated with virus flaking directly off the chickens’ skin.¹⁹⁶⁸ The emergence of new strains of IBDV has also been blamed in part on “improper cleaning and disinfection.”¹⁹⁶⁹ One reason why the industry doesn’t clean and disinfect sheds more frequently is that they want young breeding chickens to get infected with viruses like CIAV early in hopes that they will clear the infection before egg laying leads to progeny with “poorer performance.”¹⁹⁷⁰ Immunodeficiency diseases like Marek’s cost the poultry industry more than a billion dollars a year,¹⁹⁷¹ but cleaning up its act might cost even more. One animal science textbook explains that “compromise inevitably must be struck because animal agriculture is a business, and providing the best environment possible may be unprofitable.”¹⁹⁷²

None of these viruses affect humans directly, but with the threat of bird flu, anything that leads to immune suppression in chickens may now be an issue of human public health importance. The same factory farming conditions that facilitated the emergence of killer viruses like H5N1 are leading to the emergence—and spread—of immunodeficiency viruses that may themselves be partially responsible for H5N1 and may midwife even deadlier pandemic strains into the world.