OIE: Epidemiological Notes On South Carolina's H7N3 Outbreak

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Just over two weeks ago, in USDA/APHIS Confirm HPAI H7N3 In South Carolina Turkey Flock, we saw the first confirmed highly pathogenic avian influenza outbreak in the United States since the spring of 2017.

The initial report suggested that this HPAI H7 virus came about via an LPAI-to-HPAI mutation event (see graphic above).

There are two broad categories of avian influenza; LPAI (Low Pathogenic Avian Influenza) and HPAI (Highly Pathogenic Avian Influenza).

• LPAI viruses are quite common in wild birds, cause little illness, and only rarely death. They are not considered to be a serious threat to public health, although H5 & H7 strains have the potential to mutate into HPAI strains.

• HPAI viruses are more dangerous, can produce high morbidity and mortality in wild birds and in poultry, and can sometimes infect humans with serious result. The number of HPAI viruses that scientists have been tracking has increased markedly over the past 8 years, and now includes H5N1, H5N2, H5N6, H5N8, H7N9, and others.

Before the middle of the last decade, there was no uniform requirement to report or track LPAI infections in poultry. That changed in 2006 when the OIE made reporting of LPAI H5 & H7 viruses mandatory. The concern with these LPAI H5 and H7 viruses is that when they are not controlled - they have the potential to mutate into highly pathogenic strains. 

HPAI viruses have been generated in the laboratory by repeated passage of LPAI viruses through chickens (cite FAO) but exactly how and why this occurs naturally is poorly understood (see JVI Emergence of a Highly Pathogenic Avian Influenza Virus from a Low Pathogenic Progenitor).

On Friday the OIE released an update, indicating no additional outbreaks in South Carolina and providing the following epidemiological analysis.

24 April 2020 update 

All control area and enhanced weekly surveillance testing have reported negative results since 8 April 2020. Routine passive and active surveillance for the National Poultry Improvement Plan (NPIP) avian influenza programs is ongoing state-wide in South Carolina.

The USDA Animal Plant Health Inspection Service (APHIS) and South Carolina State Veterinarian’s Office, part of Clemson University Livestock Poultry Health (CULPH), continue conducting a comprehensive epidemiological investigation and enhanced surveillance in the area.  

Based on sequencing information for all H7N3 North Carolina/South Carolina (NC/SC) cases, data supports a single virus introduction followed by secondary spread. 

Mutation of the low pathogenic avian influenza (LPAI) virus to highly pathogenic avian influenza (HPAI) occurred in one house on a single premises. Sequencing information also supports that H7N3 LPAI NC/SC viruses are: 

1. distinct from other recent H7 events in poultry, and have not previously been detected in poultry
2. similar to other wild bird viruses, sharing a common HA ancestry with those from 2016-2018
3. North American wild bird-origin viruses 
4. NOT related to the H7N3 HPAI event in Mexico nor the Anhui lineage H7N9 viruses

The very good news in this report - aside from the fact that the virus has not spread to other farms - is that this appears to be a one-off, local event, and is not tied to the dangerous Asian H7 lineage, or to Mexico's recurrent outbreaks of H7N3 (see PLoS One: Rapid Evolution of Mexican H7N3 HPAI Viruses In Poultry).

This is a reminder that nature's laboratory continues to tinker with creating new strains of avian flu, and while most will end up brief footnotes in virological history, every once in awhile a genuine contender emerges. 

While we struggle mightily with a 1%-2% CFR Coronavirus pandemic, we've seen avian flu viruses with the potential for ten times greater lethality. Thankfully none have acquired the ability to transmit efficiently between humans.

Comforting, yes.  But the same could have been said about COVID-19 until about four months ago.

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