Brucella RB51 From Raw Milk, 2017–2018: One Pennsylvania Cow, Two Strains, Two Patients

In December 2018, Pennsylvania's Department of Agriculture (PDA) and Department of Health (PADOH) learned of a New York patient with brucellosis caused by Brucella abortus RB51 — the live, weakened vaccine strain of B. abortus used to protect cattle against brucellosis. The patient had drunk raw (unpasteurized) milk from a Pennsylvania farm, dairy A; selling raw milk at retail is legal in Pennsylvania.

Brucellosis is a serious infection passed from animals to people, caused by Brucella bacteria. Fever is the most common sign, followed by bone and joint symptoms, sweating and general symptoms. Without proper treatment it can become chronic and potentially life-threatening.

Earlier cases

The investigation at dairy A

Dairy A sold only raw milk. It did not vaccinate cows born on the farm — 16 of its 30 — against RB51. The other 14 were born elsewhere, and their records were not good enough to show whether they had been vaccinated, so they might have been.

In December 2018, PDA took milk samples from every cow except those known to be unvaccinated, and from the bulk tank, which holds milk pooled from all the cows. Each sample was tested by PCR and culture, and isolates from the patient and the milk underwent whole-genome sequencing (WGS).

RB51 turned up in the bulk tank and in one cow — cow 122. Sequencing found that cow 122 was shedding two different RB51 strains, from different quarters of its udder:

• one matched the 2018 New York patient's isolate, 3 single nucleotide polymorphisms (SNPs) apart;
• the other, unexpectedly, matched the 2017 New Jersey patient's isolate, 1 SNP apart.

Two different RB51 strains from different quarters of one cow's udder had not been reported before.

Tracing the milk

Cow 122 had been given RB51 in 2011 and was bought by dairy A in 2016. From 2016 to 2018, dairy A sent out raw milk that might have carried RB51 to 19 states, and PADOH told those states' public health veterinarians.

PADOH worked with the raw milk cooperative that distributed dairy A's milk: it notified people who might have been exposed, sent notices through Epi-X to find cases, and gave the cooperative a letter on RB51 and preventive treatment for dairy A's customers. No other cases were found. Cow 122 was taken out of milk production, and repeated PCR tests of the bulk milk afterwards were negative for RB51.

What the report recommends

Raw milk also carries other serious infections, including Campylobacter, Shiga toxin–producing Escherichia coli and Salmonella. Outbreaks from raw milk in the United States increased from 2007 to 2012, and 66 of 81 reported outbreaks (81%) were in states where selling raw milk is legal. Pregnant women, children, older adults and people with weakened immune systems are at greatest risk.

• Pasteurize all milk — the way to remove the risk from germs in milk, RB51 included.
• Study shedding: little is known about whether dairy cows shed RB51 on and off or continuously, and more research is needed on this emerging threat to milk drinkers.
• Weigh vaccination: states can consider the United States Animal Health Association's recommendations on whether RB51 vaccination is needed where B. abortus is not endemic in wildlife.

Sources

Based on Gruber JF, Newman A, Egan C, et al., "Notes from the Field: Brucella abortus RB51 Infections Associated with Consumption of Raw Milk from Pennsylvania — 2017 and 2018," MMWR Morbidity and Mortality Weekly Report, volume 69, Centers for Disease Control and Prevention; a work of the United States government in the public domain.